Vol. 64, No. 12 (2015)
2015-06-20
GENERAL
2015, 64 (12): 120501.
doi: 10.7498/aps.64.120501
Abstract +
Based on the theory of fractional integration, direct transport behaviors of coupled Brownian motors with feedback control in viscoelastic media are investigated. The mathematical model of fractional overdamped coupled Brownian motors is established by adopting the power function as damping kernel function of general Langevin equation due to the power-law memory characteristics of cytosol in biological cells. Numerical solution is observed by fractional difference method and the influence of model parameters on cooperative direct transport of the coupled Brownian motors is discussed in detail by numerical simulation. The research shows that the memory of the fractional dynamical system can affect the direct transport phenomenon of the coupled Brownian motors through changing the on-off switching frequency of the ratchet potential with feedback control. To be more specific, in a proper range of the fractional order, the memory of the dynamical system can increase the on-off switching frequency of the ratchet potential, which can lead to the velocity increase of the direct transport. Furthermore, in the case of small fractional order, since the coupled Brownian motors move under the competition between the damping force with memory and the potential force with feedback control, the resultant force exerted on the coupled particles is always positive when the ratchet potential with feedback control is on although the fractional damping force is large, which leads to the result that the coupled Brownian motors move in the positive direction in the mass. On the contrary, in the case of large fractional order, the on-off switching frequency of potential with feedback control becomes small, as a result of which the main influential factor of the direct transport becomes the potential depth. Therefore the coupled Brownian motors are more likely to stay in the potential wells for a long time because the probability that describes the possibility that the coupled Brownian motors surmount the potential barriers becomes small. Finally, with the parameters of the fractional dynamical system (e.g. potential depth, noise intensity) fixed, the direct transport velocity of the coupled Brownian motors shows the generalized stochastic resonant phenomenon while the fractional order varies.
2015, 64 (12): 120502.
doi: 10.7498/aps.64.120502
Abstract +
The period-adding bifurcations in a discontinuous system with a variable gap are observed for two control parameters. Various period-adding bifurcations are found by simulations. The bifurcation diagram can be divided into two different zones: chaos and period. The period attractor takes up a considerable part of the parameter space, and all of them show stable period attractors. The periodic zone can also be divided into three different zones: stable period-5 attractor, period-adding bifurcations on the right side of period-5 attractor, and period-adding bifurcations on the right side of period-5 attractor. We choose various control parameters to plot the cobweb of period attractor, and find that it will exhibit a border-collision bifurcation and the period orbit loses its stability, once the position of iteration reaches discontinuous boundary. The discontinuous system has two kinds of border-collision bifurcations: one comes from the gap on the right side, and the other from the gap on the left side. The results show that the period-adding phenomena are due to the border-collision bifurcation at two boundaries of the forbidden area. In order to determine the condition of the period orbit existence, we also choose various control parameters to plot the cobweb of period attractor. The results show that the iteration sequence of period trajectory has a certain sequence with different iteration units. The period trajectory of period-adding bifurcation on the left side of period-5 attractor consists of period-4 and period-5 iteration units, forming period-9, period-13 and period-14 attractor. The period trajectory of period-adding bifurcation on the right side of period-5 attractor consists of period-6 and period-5 iteration units, forming period-11, period-16 and period-21 attractor. All attractors can be easily shown analytically, owing to the piecewise linear characteristics of the map. We analyze its underlying mechanisms from the viewpoint of border-collision bifurcations. The result shows that the period attractor can be determined by two border-collision bifurcations and the condition of stability. Based on the theoretical and iteration unit, the border-collision bifurcations, two border collision bifurcation curves are obtained analytically. The result shows that the theoretical and numerical results are in excellent agreement.
2015, 64 (12): 120601.
doi: 10.7498/aps.64.120601
Abstract +
Under the special environment of airborne platform, the theoretical model is built based on probability characteristic of laser ranging, and the probability density distribution functions of pulse-echo signal envelope under different signal-to-noise ratios (SNRs) are obtained. The theoretical results show that the decrease of SNR causes different distributions of ranging data through echo wave characteristic. The experimental data present Gauss distribution with larger SNR, or Rayleigh distribution with poor SNR, or Rice distribution with general condition. According to the constant-ratio timing method, experimental results verify the rationality of theoretical model under a number of ranging data. When the ranging accuracy defined by variance is not applicable, the concept of uncertainty of measurement is introduced. Combined with the theory of uncertainty, a new method of evaluating airborne laser ranging performance is put forward. This method could overcome the unity and irrationality of traditional evaluating method, and meanwhile, it could provide an important reference for evaluating and testing airborne optoelectronic system performance.
2015, 64 (12): 120602.
doi: 10.7498/aps.64.120602
Abstract +
Optical clocks are considered as promising candidates for redefining the second in the International System of Units. Compared with microwave clocks, optical clocks are powerful tools for the fundamental research such as the constancy of the fundamental constants, the validity of Einstein’s theory of general relativity, and the predictions of quantum electrodynamics. Recently two research groups have demonstrated the optical clocks with an unprecedented precision level of 10-18, which is two orders better than the present primary frequency standard. Using two Sr optical clocks and three Cs fountain clocks, SYRTE group has demonstrated the definition of second with optical clocks.#br#For redefining the second with optical clocks in the future, the optical clocks from the remote laboratories should have a high precision and the frequency of the optical clocks need to be transferred over a long distance, with extremely high precision. Unfortunately the conventional means of frequency transfer such as two-way satellite time and frequency transfer can reach a 10-16 level in one day which is far below the requirement for an optical clocks. Various methods have been developed to transfer optical frequency signal via optical fibers. Especially a research group from Germany has achieved a frequency transfer stability of 10-19 level in hundreds of seconds with a fiber length of 1840 km.#br#We demonstrate the recent development of optical frequency transfer over a 70-km fiber spool at National Time Service Center. The measurement shows that the compensation for the fiber noise is close to the limitation induced by the fiber delay for the Fourier frequency from 1 Hz to 250 Hz. The transfer stability (Allan deviation) of the fiber link is 1.2×10-15 in 1 s averaging time, and 1.4×10-18 in 10000 s. A preliminary test of the optical frequency transfer over a 100-km spooled fiber is achieved with a stability of roughly one order worse than the 71 km result, 5×10-15 in 1 s.#br#We demonstrate a new scheme of remote compensation for optical frequency transfer via fibers against conventional local compensation method. This new scheme has the advantage of great simplification of the local site, which can find applications in massive extension of star network. The key feature is that we transfer the mixture of the round-trip signal and local reference to the remote user’s end via an auxiliary fiber. At remote site, the fiber noise is measured and compensated by AOM2 accordingly.#br#Transfer stabilities of 13×10-15 in 1 s averaging time and 4.8×10-18 in 10000 s are achieved with the remote fiber noise compensation via a 25 km fiber spool. The demonstrated transfer stability is comparable to that obtained by the local fiber noise compensation method.#br#The future star fiber network of optical frequency transfer can benefit from this method, because the simpler local setup is required and even can be shared in the central site for multitudinous remote users.
2015, 64 (12): 120701.
doi: 10.7498/aps.64.120701
Abstract +
X-ray communication, which was firstly introduced by Keithe Gendreau in 2007, is potential to compete with conventional communication methods, such as microwave and laser communication, against space surroundings. As a result, a great deal of time and effort has been devoted to making the initial idea into reality in recent years. Eventually, the X-ray communication demonstration system based on the grid-controlled X-ray source and microchannel plate detector can deliver both audio and video information in a 6-meter vacuum tunnel. The point is how to evaluate this space X-ray demonstration system in a typical experimental way. The method is to design a specific board to measure the relationship between bit-error-rate and emitting power against various communicating distances. In addition, the data should be compared with the calculation and simulation results to estimate the referred theoretical model. The concept of using X-ray as signal carriers is confirmed by our first generation X-ray communication demonstration system. Specifically, the method is to use grid-controlled emission source as a transceiver while implementing the photon counting detector which can be regarded as an important orientation of future deep-space X-ray communication applications. As the key specification of any given communication system, bit-error-rate level should be informed first. In addition, the theoretical analysis by using Poisson noise model also has been implemented to support this novel communication concept. Previous experimental results indicated that the X-ray audio demonstration system requires a 10-4 bit-error-rate level with 25 kbps communication rate. The system bit-error-rate based on on-off keying (OOK) modulation is calculated and measured, which corresponds to the theoretical calculation commendably. Another point that should be taken into consideration is the emitting energy, which is the main restriction of current X-ray communication system. The designed experiment shows that the detected X-ray energy is 7×10-5 mW/m2. This relatively low power level not only restricts the bit rate of transceiver, but also increases the error fraction to some extent. Obviously, OOK modulation can meet the high communication rate and relatively low bit-error-rate requirement of current audio demo system. Current restriction has been pointed out and the potential improvement is also presented.
Crossing behavior of pedestrians and vehicles at unsignalized intersection based on dirty faces game
2015, 64 (12): 120201.
doi: 10.7498/aps.64.120201
Abstract +
The conflicts between pedestrians and vehicles at unsignalized intersections are rather dangerous. Besides the motion features, pedestrians and vehicle drivers also have behavioral features, so modelling the crossing behaviors of pedestrians and vehicles from the sociological aspect is necessary. Through an in-depth analysis of the interference mechanism between pedestrians and vehicles at unsignalized intersections, in this paper, a dirty faces game to analyze the crossing behaviors of pedestrians and vehicles as “rational man” is proposed. The formation process of common knowledge, the producing mechanism of dominant strategy and the kinetic process of dirty faces game are analyzed. Under the assumption of probability of the dominant strategy and distribution of crossing times, theoretical derivations of the benefits gained by pedestrians and vehicles as well as the conflict probability are given. Results show that a collision is more likely to occur when the theoretical time of pedestrian passing through is closer to that of the vehicle driver, and this is consistent with the actual situation. Furthermore, the influences of vehicle type, waiting time and heterogeneity of pedestrians on the conflict probability are also considered.
ELECTROMAGENTISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2015, 64 (12): 124206.
doi: 10.7498/aps.64.124206
Abstract +
Based on a novel multiplexing system of two distinct chaotic signals, the corresponding modified Lang-Kobayashi rate equations are established. The numerical investigations into the performance of chaos synchronization are carried out. In more detail, the influences of single-parameter mismatch, multi-parameter mismatch, feedback-strength discrepancy, and frequency detuning between the two master semiconductor lasers (MLs) on synchronization performance are investigated, respectively. Moreover, the security and spectrum characteristics are addressed briefly in this work. The numerical simulations show that by adopting parameter mismatch, i.e., choosing appropriate system parameters of the two MLs, the correlation between the two MLs becomes extremely low, while the matched master-slave laser pairs can achieve high-quality chaos synchronization, indicating that the condition of optical chaos multiplexing is satisfied; the parameter mismatch between the MLs has a significant influence on their synchronization quality, but no obvious influence on their synchronization quality of the matched master-slave laser pairs, which further demonstrates the validity and feasibility of the chaos multiplexing scheme. More importantly, in this paper, the multiplexed chaotic signals in the time and frequency domains in terms of autocorrelation function and power spectrum are analyzed, demonstrating that the present system could provide higher security than the single external-cavity semiconductor laser.
2015, 64 (12): 124101.
doi: 10.7498/aps.64.124101
Abstract +
Because the weather conditions in different sea areas are different, the evaporation duct occurring over a large sea surface is normally regional and range-dependent. This property results in the fact that the radio wave propagation within the environment of this type is distinct from that within the range-independent evaporation duct environment. Therefore, it is meaningful to perform the regional range-dependent evaporation duct inversion for accurately predicting radio wave propagation and improving radar performance. From among the variety of ways of detecting evaporation duct in practical application, we adopt the regional modified refractivity profile of evaporation duct predicted by the mesoscale numerical weather model MM5 as the prior information, and propose a posterior probability estimation model of the regional range-dependent evaporation duct on the basis of the radar sea clutter power. First, in this model we use the principal component analysis method to model the range-dependent property of evaporation duct, and on this basis, establish the inversion procedure of the range-dependent evaporation duct by using the radar sea clutter. Then, we obtain the relationship among prior probability distribution, posterior probability distribution, and likelihood function of the parameters of the modified refractivity profile by using the Bayesian theory, and finally realize the maximum posterior probability estimation of the evaporation duct parameters. By estimating the real regional range-dependent evaporation duct over East China Sea, it is indicated that the proposed model can perform the inversion of regional range-dependent evaporation duct with a higher precision.
2015, 64 (12): 124102.
doi: 10.7498/aps.64.124102
Abstract +
The phase profiles of the reflected circularly polarized waves can be freely manipulated by virtue of a co-polarization reflective metasurface. Based on the co-polarization reflective metasurface, a circularly polarized wave reflection focusing metasurface can be achieved, it can make the reflected waves focus at a focal spot under the normal incidence of circularly polarized plane waves. In this paper, a reflection focusing metasurface is designed. It is found that around the central frequency f=16 GHz, the reflected waves focus on a focal spot above the metasurface with a focal distance L=200 mm under the normal incidence of right-handed circularly polarized waves. However, in the case of normal incidence of left-handed circularly waves, the reflected waves focus on an imaginary focal spot below the metasurface with the focal distance L=-200 mm. The beam-width at the focal spot and focal depth are also calculated by using CST Microwave Studio. The simulation results indicate that the beam-width at the focal spot is approximately equal to the operating wavelength. Therefore, the circularly polarized wave reflection focusing metasurface has a good performance for focusing the reflected waves. In addition, the proposed focusing metasurface displays the advantages of the long focal depth and the broad operating bandwidth.
2015, 64 (12): 124201.
doi: 10.7498/aps.64.124201
Abstract +
Based on Fresnel diffraction theory and complex Gaussian function expansion of hard-edged aperture, the optical field formula of Bessel beam propagating through an elliptical annular aperture is derived, and the transverse intensity distribution of the beam is numerically simulated. The changes of the optical field and the propagation process of the diffracted beam behind the elliptical annular aperture are studied. In the experiment for the first time, a quasi non-diffracting beam is generated by an axicon and the patterns that are due to the beam diffraction by an elliptical annular aperture at different propagation distances are observed with a charge-coupled device camera. The theoretical analysis and experimental results both show that Bessel beam passing through an elliptical annular aperture can generate a hollow beam.
2015, 64 (12): 124202.
doi: 10.7498/aps.64.124202
Abstract +
According to the phase and amplitude modulation of the spatial light modulator (SLM) loading the phase distribution for generating arbitrary vector beams, we present a method of generating arbitrary vector beams based on the optical holography with angle multiplexing. First of all, we use the optical holography to record the special phase distribution on the SLM, and so an optical holographic grating is obtained. In the reproduction process, the two conjugate reference beams with the same incident angle illuminate the holographic grating and the superposition of the two reproduced beams is achieved, thus the arbitrary vector beams are obtained. This method can avoid the emergence of complex polarization distribution, and has advantages such as simple optical setup, convenient operation, and higher polarization purity of generated arbitrary vector beams. Good results of the arbitrary vector beams are also obtained by computer simulation.
2015, 64 (12): 124203.
doi: 10.7498/aps.64.124203
Abstract +
Coherent field imaging is based on the assumption of equal transmitting apertures spacing and equal spectrum of laser, and high-resolution image is reconstructed by iteratively computing the frequency spectrum. However, the inevitable transmitting aperture spacing error of laser is a key factor to affect the coherent field imaging quality in the application. Aiming at the problem of degrading imaging quality caused by the transmitting aperture spacing error, we discuss the mechanism of influence of aperture spacing error on imaging quality and propose a novel method of improving imaging quality from the perspective of suppressing the influence of transmitting aperture spacing error. Firstly, the mechanism of the influence of aperture spacing error on imaging quality and laser echo spectrum is analyzed in detail. Then we derive a frequency spectrum error propagation model. Based on the model, the iterative effect of frequency spectrum error is investigated and the trend of variation in imaging quality with frequency spectrum error is given. We propose a theoretical equation, in which the transmitting aperture spacing error has no influence on frequency spectrum nor imaging quality. To solve the above equation, an optimized method of linear programming is proposed and the optimized matrix of aperture spacing error is obtained. In practice, the influence of aperture spacing error on imaging quality can be largely counteracted by reasonably allocating aperture spacing error according to the optimized spacing error matrix. The correctness and validity of the theoretical model are verified by a simulation experiment. The results show that the Strehl ratio of imaging quality index can be improved by about 100% through using the proposed method, the greater the aperture spacing error, the higher the Strehl ratio of imaging quality index obtained by the method will be. In addition, the method is easy to use practically and less costly as well. Finally, it is concluded that the proposed method can easily and effectively counteract the degrading effect of aperture spacing error on imaging quality. The research can provide a theoretical basis for improving imaging quality and reasonably assigning transmitter aperture spacing accuracy of coherent field imaging telescope.
2015, 64 (12): 124204.
doi: 10.7498/aps.64.124204
Abstract +
In order to further improve the precision of remote sensing image registration, we propose a new registration scheme by combining the scale-invariant feature transform (SIFT) and the optimization of regional mutual information in this paper. Firstly, taking advantage of the randomness and ergodicity of chaotic sequence, we present a new chaos quantum-behaved particle swarm optimization (CQPSO) algorithm to solve the premature convergence problem of the quantum particle swarm optimization (QPSO) algorithm. By taking full account of the quantity differences among the values of different dimensions for the particle location information, small disturbances are generated as the Hadamard product of chaotic sequence and the particle location information. Before being added to the particle location information, the small disturbances are adjusted by an evolutionary parameter to ensure that each new particle location information is within the scope of reasonable evolution. The image registration scheme consists of two processes, namely the pre-registration process and fine coregistration process. The pre-registration process is implemented by the SIFT approach with a reliable outlier removal procedure. By the repetitive fine-tuning of several selected matched feature point coordinates, a series of registration parameters is estimated by a least square method and used to construct initial particle swarms. Next, the fine coregistration process is implemented to obtain the optimal match parameters by maximizing regional mutual information based on CQPSO. The proposed CQPSO algorithm is tested on several benchmark functions and compared with QPSO as well as standard PSO experimentally. Furthermore, comparative experiments are carried out on the registration of remote sensing images with different ground resolutions and the registration of remote sensing images at different phases by using four algorithms: the SIFT algorithm, SIFT combined with PSO algorithm, SIFT combined with QPSO algorithm, and SIFT combined with CQPSO algorithm. The regional mutual information, root mean square error, and the joint histogram are used to evaluate the performance of the algorithms. The experimental results verify the superiority of CQPSO and the effectiveness of the proposed registration scheme.
2015, 64 (12): 124207.
doi: 10.7498/aps.64.124207
Abstract +
Based on the scattering theory of Guo-Åberg-Crasemann (GAC), which has no artificial assumptions, high harmonic generation (HHG) is studied by using first-principles. The HHG spectra of different rare atoms are also calculated. Using the properties of ordinary Bessel functions and the Einstein photoelectric law in the strong-field case, we reveal a new cutoff law qcħω = (9 -4√2) Up + (2√2-1) Ip ≈ 3.34 Up + 1.83 Ip of HHG based on a mathematical deduction method and a graphical method, which accords well with the Popmintchev’s experimental result published on Science in 2012. This cutoff law also agrees well with our own calculation using the HHG transition rate formula derived from the GAC scattering theory. Thus, we have four pieces of independent evidence for the same cutoff law of HHG. The cutoff orders predicted by this theory are higher due to the absorption of the extra photons. These photons only participate in the photon-mode up-conversion and do nothing in the photoionization process.
2015, 64 (12): 124208.
doi: 10.7498/aps.64.124208
Abstract +
The lidar observations of aerosols under near range condition are distorted because of insufficient overlapping between the transmitting laser beam and the field of view of receiving telescope in non-coaxial lidar especially in multiwavelength lidar. However, the near-range atmosphere is closely related with human’s activity. So it is important to correct the near-range signal of multiwavelength lidar. This paper presents a novel method of correcting the near-range optical parameter of multiwavelength lidar based on aerosol particle size distribution measurements and Mie scattering theory. The near-range aerosol extinction profiles on fine day, cloudy day and foggy day are corrected. The results show that the method is convenient and feasible compared with the traditional methods. This method reduces the uncertainty of inversion and correction because the wavelength dependences of lidar ratio and the weather correlation of aerosol size distribution are taken into account. This method realizes the direct correction to near-range optical parameter of multiwavelength lidar. It is favorable and convenient to study physical and optical properties of boundary layer atmosphere by using this method. It will have a broad application and prospect.
EDITOR'S SUGGESTION
2015, 64 (12): 124209.
doi: 10.7498/aps.64.124209
Abstract +
In this paper, Nd3+-doped GdTaO4 laser crystal for scintillator with high density is successfully grown by the Czochralski method, and the absorption spectra are measured along the a, b and c directions in a wide wavelength range of 260-2000 nm. The experimental energy levels for Nd3+ are analyzed and identified. The free-ions and crystal-field parameters are fitted by the experimental energy levels with the root mean square deviation of 12.66 cm-1, and 102 Stark energy levels for Nd3+ in GdTaO4 host crystal are assigned. The fitting results of free-ions and crystal-field parameters are compared with those already reported for Nd3+:GdxLu1-xTaO4 (x=0.85) crystal. It indicates that the fitting results of Stark energy levels agree well with the experimental spectra.
2015, 64 (12): 124212.
doi: 10.7498/aps.64.124212
Abstract +
In order to satisfy the need of visible light communication, compound parabolic concentrators are selected as the optical antennas because of their wide fields of view and high gains in small field of view. Their geometries and optical properties are introduced in order to design compound parabolic concentrators with different fields of view by TracePro. These compound parabolic concentrators are tested under different light source conditions. The distribution of the received power of the receiver which has been coupled with the compound parabolic concentrator, is obtained by a simulation. The obtained gain of compound parabolic concentrator proves that the compound parabolic concentrator works better when the light source has a Lambert radiation pattern than the case under a parallel light condition. The results illustrate that compound parabolic concentrator is suitable to serving as an optical antenna for visible light communication. And it also shows that the smaller the field of view, the greater the gain is. Under the condition of simulation in this paper, a compound parabolic concentrator with 10° field of view could realize a gain of 22.88, which is 31% lower than the theoretical gain because of the effect of its position relative to the light source. On this basis, the model of a visible light communication system is established in a room with a size of 5 m×5 m×3 m. By using a compound parabolic concentrator with a field of view of 60° as an optical antenna, the simulation results show that the average received power is increased by 4.29 dBm for the directed light from light emitting diodes, and by 4.77 dBm with the reflected light being included. And the average received power is increased by 11.2% when the reflected light is considered.
2015, 64 (12): 124301.
doi: 10.7498/aps.64.124301
Abstract +
In this paper, we extend the Rayleigh-Plesset model by considering the effect of a magnetic field on the nonlinear response of an oscillating spherical air bubble in water. Water molecules in motion, derived by a time varying ultrasound pressure field, suffer a torque from the magnetic field by Lorentz force. The rotational energy and the translational energy together constitute the kinetic energy of the water molecule. The work done by the pressure during the contraction and expansion of bubble is equal to the total kinetic energy of the water molecule in liquid. According to energy conservation, we establish a modified control equation of the bubble motion under the action of an applied external magnetic field. The integration of the nonlinear differential equation governing the bubble motion is performed analytically by using a regular expansion, and is solved numerically by using a fourth-order Runge-Kutta method. It is shown that the variation of ambient pressure changes the bubble dynamics when the magnetic field is off. The ambient pressure is increased due to the effect of external magnetic field. The pressure induced by magnetic field increases linearly with the increase of magnetic field intensity and the coefficient is about 103 times. The bubble expansion rate, maximum radius, and the velocity of the collapsing bubble decrease as the magnetic field increases. It is predicted that the applying of a magnetic field can widen the pressure range and modify bubble dynamics.
2015, 64 (12): 124601.
doi: 10.7498/aps.64.124601
Abstract +
Piecewise nonlinear constraint exists in various fields and it always affects the stability of a system. In order to realize the dynamic characteristic of the system constrained by these nonlinearity, we consider two kinds of typical piecewise nonlinear constraints under the dynamic conditions, and establish a dynamic model with double piecewise nonlinear constraint of elasticity and damping, according to the generalized dissipation Lagrange principle. An average method is used to solve the amplitude and frequency response of the system under a periodic external incentive. By a numerical simulation, we compare the time domain responses under different piecewise nonlinear elastic constraints. The results show that the stronger the piecewise nonlinear elastic constraint, the more obvious the piecewise nonlinear damping constraint is. We also compare the bifurcation responses under different piecewise nonlinear damping constraints, the results show that the chaos state will emerge in an enlarged scope with the increase of the piecewise nonlinear damping coefficient, and threaten the stability of the system. The dynamic evolution process of the system is shown by the phase diagrams and Poincaré sections under the corresponding constraint conditions. By comparing the amplitude-frequency characteristics of the system under different constraint conditions, we obtain the response characteristic of the system and its change rule with the piecewise nonlinear constraints. By comparing and analyzing the amplitude-frequency characteristics under the piecewise nonlinear elastic and piecewise nonlinear damping constraint, we obtain the law of system stability influenced by different nonlinear factors, and the interaction relationship between the two piecewise nonlinear constraints.
2015, 64 (12): 124702.
doi: 10.7498/aps.64.124702
Abstract +
Considering the compressibility of liquid, we investigate the dynamical behaviors of a cavitation bubble in an acoustic standing wave field by regarding water as a work medium. The motion state of the cavitation bubble at each position is simulated in the standing wave field, the influences of the primary Bjerknes force on the motion direction of the cavitation bubble at each position are also simulated with different relevant parameters. The results show that in the standing wave field, the motion state of the cavitation bubble is divided into three aspects: the cavitation bubble is of steady-state cavitation near the pressure antinode; the cavitation bubble is of transient cavitation at the position deviating from the pressure antinode; in the vicinity of the acoustic pressure node, the cavitation bubble has been moving to the acoustic pressure node due to the primary Bjerknes force, so the phenomenon of cavitation does not occur. In the standing wave field, when the acoustic pressure amplitude exceeds its upper limit, the primary Bjerknes force makes the cavitation bubble move to pressure node, it is not conducive to the occurrence of cavitation. When the acoustic frequency is smaller than the bubble resonant frequency, the primary Bjerknes force will make more cavitation bubbles move to acoustic pressure node with the increase of the acoustic pressure, so this is not conducive to the occurrence of cavitation. Especially, the height of the liquid level should not be a quarter of acoustic wavelength. For a given acoustic frequency, the larger the initial radius of cavitation bubble, the more favorable the occurrence of cavitation is. But when the initial radius of cavitation bubble exceeds the resonant radius of acoustic frequency, the bubble will be pushed to pressure node. That is to say, the acoustic pressure amplitude, the acoustic frequency, and the initial radius of cavitation bubble each have a corresponding limit. Moreover, the lower limit is conducive to the occurrence of the phenomenon of cavitation.
2015, 64 (12): 124703.
doi: 10.7498/aps.64.124703
Abstract +
The capillary-driven liquid flow in tubes connected to containers under a microgravity condition is systematically studied in a drop tower experimentally. The microgravity time lasts up to 3.6 s and the working liquids are mixtures of ethanol and deionized water with different ratios. Theoretically, based on the previous theory for tubes directly immersed in fluid, a modified formula is developed to describe the change tendency of the height of meniscus with microgravity time for such a container/tube system exposed to a microgravity environment. From the theoretical formula, the numerical results of meniscus height at different microgravity time can be obtained, utilizing the geometrical parameters of container/tube systems and the relevant physical quantities of Eth/H2O mixtures with different ratios. By comparing the numerical results with experimental results for different contact angles between working liquid and container in different container/tube systems, we show that the theoretical model is able to quantitatively predict the capillary-driven flow in tubes connected to containers, and the numerical results have good consistence with the experimental results. In addition, the experimental results also show that though the ratio of ethanol to deionized water can change the contact angle remarkably, it has little influence on the capillary flow if the geometrical parameters of the container/tube systems are the same. This is because not only the contact angle, but also the surface tension and viscosity coefficient of the working liquid change with the ratio of ethanol to deionized water. It is found that when the contact angle increases from 42° to 66°, the surface tension increases from 0.0328 N/m to 0.0443 N/m correspondingly, but the viscosity coefficient decreases from 2.11 cSt to1.49 cSt. As a result, the changes of surface tension and viscosity coefficient offset the influence of the change of contact angle, which can be explained by our theoretical model. Compared with the extensively studied system in which tubes are directly immersed into liquid, the container/tube system studied in this paper is more similar to many actual systems such as fluid transfer systems in the microgravity condition and in micro-fluidic devices. Therefore, this study is useful for predicting and analyzing the capillary flows of these actual systems.
2015, 64 (12): 124103.
doi: 10.7498/aps.64.124103
Abstract +
The quality factor and the resonant frequency of a resonant cavity are the key factors that need to be considered in the process of resonator design. The wall of cavity is composed of conductor materials which are effective tools to generate high-frequency oscillation. The microwave cavity is widely used. From the perspective of the circuit, it has almost all the properties of LC resonance unit, such as mode selection. Therefore, it is widely used in filters, matching circuits, and antenna design. In industrial applications, the demand for high-frequency resonant cavity is relatively large. A traditional method can increase the resonant frequency of the resonant cavity by reducing the size of the cavity or using the high-order modes. However, as both approaches have their limitations, the design results are not ideal. By combining theoretical calculation and simulation, the factors that affect the resonant frequency of the resonator are analyzed. The results show the relationship between material properties of the filling medium and the resonant frequency of the cavity. Theoretical calculations show that when the left-handed materials are used as filling materials in the cavity, the resonant frequency can be increased without changing the size of the cavity. The results of high frequency structure simulator also prove the above result. Therefore, the resonant frequency of the resonator cannot be limited by the cavity size. It can be seen from the data that compared with reducing the size of the resonator or using high-order modes, filling left-handed materials can improve resonant frequency to a greater extent. The obtained conclusion shows a further progress compared with the traditional theory and provides a theoretical basis for the exploration and design of novel resonators.
2015, 64 (12): 124104.
doi: 10.7498/aps.64.124104
Abstract +
The motions of charged particles in electromagnetic fields composed of two or more laser beams show a variety of forms due to the adjustable properties of electromagnetic fields. In this paper, we consider the periodic laser standing wave field composed of two laser beams with opposite propagating directions. The movement of electrons in the standing wave field shows a periodic behavior, accompanied with the obvious radiation, especially when electrons are captured by the laser standing wave field. This phenomenon has aroused much interest of us. Under the existing experimental conditions, the free electron beam with low energy from an electron gun or the relativistic electron beam generated from laser acceleration can be easily obtained and injected into the periodic standing wave field. In this paper, using the single-electron model and the classical radiation theory of charged particles, we study the motion and radiation processes of low and high energy electrons in the polarized laser standing wave field. The results show that when the direction of incident electrons with low-speed is perpendicular to the direction of the laser standing wave electric field, the one-dimensional nearly periodic motion of electrons evolves into a two-dimensional folded movement by gradually increasing the light intensity of the laser standing wave field, and the strong terahertz radiation at micrometer wavelength is produced. High energy electrons generate the high-frequency radiation with the wavelength at several nanometers when the incident direction of high energy electrons is perpendicular or parallel to the direction of the laser standing wave electric field. In the case of low-energy electron, the motion of electron, frequency and intensity of radiation are affected by the laser intensity. In the case of incident high-energy electrons, the laser intensity affects the intensity of electronic radiation, and the initial electron energy influences radiation frequency. The bigger the incident electrons energy, the higher the frequency of radiation is. #br#We can obtain electron beams with different energies by laser acceleration, and they can be promising small radiation sources for terahertz and X-ray by using the electron beam radiation in a laser standing wave field. These studies also provide a basis for experimental researches and the applications of electron radiation in a laser standing wave field.
2015, 64 (12): 124205.
doi: 10.7498/aps.64.124205
Abstract +
Received signal processing and target reconstruction technique plays a key role in coherent field imaging, and directly influences the quality of the reconstructed image of a target. Based on all-phase fast Fourier transformation (FFT) spectrum analysis theory, a new processing and reconstruction method is proposed. By directly extracting the phase and amplitude information from all-phase FFT spectrum of the return signal, the proposed all-phase target reconstruction method is capable of inhibiting the frequency error caused by many factors, thus reconstructing the target image more precisely. The validity of the proposed method is proved by a coherent field imaging system in outdoor environments, and it has a better reconstruction performance than a traditional method. The resolution of the reconstructed target is close to a theoretical diffraction limit.
First-principles study of the electronic structures and absorption spectrum of Fe:Mg:LiNbO3 crystals
2015, 64 (12): 124210.
doi: 10.7498/aps.64.124210
Abstract +
In this paper, the electronic structures and absorption spectra of LiNbO3 (LN) and Fe:Mg:LiNbO3 crystals are studied by the first-principles under the generalized gradient approximation. The supercell structures of the LN crystal are established with 60 atoms, including four models: pure LN crystal, Fe:LiNbO3 crystal (Fe:LN), Fe:Mg:LiNbO3 crystal with Mg of 2 mol%-3 mol% (Fe:Mg(L):LN), and Fe:Mg:LiNbO3 crystal with Mg of 5.0 mol% (Fe:Mg(E):LN). The electronic structures show that the extrinsic defect levels (within forbidden band) of Fe:LN are contributed by Fe 3d orbital and O 2p orbital, and the band gap of Fe:LN (about 2.85 eV) is narrower than that of LN. For Fe:Mg:LN crystals, the band gap changes to 2.90 eV and 2.81 eV respectively for the Mg ion concentration less than and equal to the threshold (～5.0 mol%). The two absorption peaks at 2.3 eV and 2.6 eV are attributed to the Fe ions in crystal. Moreover, the intensities of these peaks vary with the concentration of Mg ion. It is revealed that the concentration of Mg ion influences the concentrations and the sites of Fe2+ and Fe3+ ions in crystal. From the absorption spectrum, the values of ratio Fe2+/Fe3+ in Fe:Mg(E):LN and Fe:Mg(L):LN can be obtained, and the ratio of first sample is smaller than that of the second one. With the one-center model, one can distinctly deduce that the photoconductivity of Fe:Mg(E):LN is relatively weak compared with that of Fe:Mg(L):LN, but this is inconsistent with many experimental results. One notices the contribution of O 2p orbital to extrinsic defect level in electronic structure. Therefore, it is reasonable to presume that the one-center model is not suitable enough for this condition. Based on the research work, we find that the formations of photoelectrons are related to orbital electron states of iron ions and oxygen atoms at extrinsic defect levels in Fe:LN and Fe:Mg:LN crystals.
2015, 64 (12): 124211.
doi: 10.7498/aps.64.124211
Abstract +
High speed electrooptic phase modulators play very important roles in the high-speed optical fiber communication system, microwave photonic system, and coherent optical communication system, due to their advantages of bias voltage free and linear modulation. As an intrinsic parameter, the half-wave voltage of an electrooptic phase modulator has been characterized by using an electrical spectrum method and an optical spectrum method in the last two decades. The optical spectrum method is generally limited by the line-width of the laser source and the resolution of the available optical spectrum analyzer, while the electrical spectrum method requires the conversion from phase modulation to intensity modulation before photodetection, since a phase modulator generates a phase modulated optical signal with constant envelope. The major difficulty in the electrical spectrum method lies in the extra calibration for the responsivity fluctuation in the photodetection. In this paper, a novel self-calibrated measurement of half-wave voltage of electrooptic phase modulators is carried out based on the optical heterodyning between the two-tone phase modulated sidebands and the frequency-shifted carrier. The method achieves a self-calibration measurement, and avoids the effect of the responsivity fluctuation in the photodetection by setting a specific frequency relationship between the two-tone microwave signals. Moreover, it extends the measuring frequency range to the double bandwidth of photodetection and spectrum analysis. Compared with the optical spectrum method, the proposed method achieves very high frequency resolution measurement, and simultaneously avoids the line-width influence of laser source by use of two-tone heterodyning. Compared with the traditional electrical spectrum method, our method works under no small-signal assumption nor photodetection calibration, and eliminates the limits of electrical driving amplitude and operating wavelength. Moreover, it decreases by at least half bandwidth requirement for the photodetector and spectrum analyzer. Our experimental demonstration shows that the measured half-wave voltages of the electrooptic phase modulator obtained by our method agree well with the data measured by the optical spectrum method, and the two-tone heterodyning method greatly improves the measurement range and frequency resolution. The proposed measurement method provides a very simple analysis method for the microwave characterization of high-speed electrooptic phase modulators, which is also a reference for other optoelectronic devices.
2015, 64 (12): 124302.
doi: 10.7498/aps.64.124302
Abstract +
The high energy density pulse input into brittle structural materials will propagate as a shock wave. It induces compression fracture and function failure. In this work, voids are introduced to significantly enhance the shock plastic deformability of brittle structural materials, so that brittle structural materials can effectively absorb the shock wave energy, and restrain the propagation of shock-induced cracks. A lattice-spring model is established to investigate the mechanism of shock plastic, and the processes of energy absorbing and crack expanding in porous brittle materials. The shock wave inside porous brittle material splits into an elastic wave and a deformation wave. The deformation wave is similar to the plastic wave in ductile metal, however, its deformation mechanism is of volume shrinkage induced by voids collapse, and slippage and rotation deformation of scattered tiny scraps comminuted by shear cracks. We calculate the shock wave energy based on particle velocities and longitudinal stresses on nine interfaces of the modeled brittle sample, and further obtain the absorbed energy density. The absorbed energy density curve is composed of two stages: the absorbing stage and the saturation stage. The absorbing stage corresponds to the deformation wave, and the saturation stage corresponds to the shock equilibrium state (Hugoniot state). The energy absorb abilities of the dense sample and porous samples with 5% and 10% porosities are compared based on calculation results. It shows that the ability of the porous brittle material to absorb high energy density pulse is much higher than that of the dense brittle material. The ability of porous brittle materials to restrain the propagation of the shock fracture is also explored. The goal of this design is to freeze the propagation of the shock fracture in the middle of the brittle sample, so that the other parts of the sample keep nearly intact during the shock. Inside the protected area, the designed functions of brittle materials can be accomplished without the disturbance of the shock fracture. This design is used under the short pulse loading condition: the rarefaction wave on the rear of the short pulse will catch up and unload the deformation wave if it moves slowly; the deformation wave and the shock fracture propagate synchronously; when the deformation wave is unloaded, the shock fracture will be frozen in the middle of the porous sample. Under the short pulse loading condition, compared with the dense brittle material, whose entire regions are destructed, the porous brittle material can restrain the propagation and impenetration of the shock fracture, with the cost of increasing the damage extent in part of the sample. This is helpful to avoid the entirely function failure of the brittle structural material.
2015, 64 (12): 124701.
doi: 10.7498/aps.64.124701
Abstract +
When a shock wave releases from a metal-vacuum interface, some high velocity metal particles will be ejected from the metal surface which generally produce some tiny grooves on the metal surface. This phenomenon is often called the “micro-ejecta”. In this paper, we numerically investigate the effect of the micro-structures of these tiny grooves on the property of the micro-ejecta. To verify the numerical simulation model, a strict Pb micro-ejecta experiment is carried out, where the breakout pressure is about 40 GPa and the Pb target surface roughness is Ra1.6. The dynamic processes of the micro-ejection caused by the real surface groove of experimental target and simplified isosceles groove (both have a depth of 5 μm and wavelength of 75 μm), are respectively simulated by a two-dimensional smooth particle hydrodynamics method, and the effects of surface groove micro-structure on the micro-ejecta properties are examined. The simulation results of the tip velocity and accumulated mass, obtained from the real surface groove model, are in good agreement with the corresponding experimental results measured via DISAR and Asay foil, implying that the numerical result is exact. The tip velocity and accumulated mass caused by the real surface groove are much larger than those caused by the simplified isosceles groove, and a second ejection phenomenon is found in the micro-ejecta process from the real surface groove model. The process can produce some faster ejecta than a single ejecta process and influence the density distribution of the micro-ejection. It indicates that the micro-ejecta process can also be affected by the micro-structure of the metal surface groove, besides perturbation wavelength and surface groove depth.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2015, 64 (12): 125201.
doi: 10.7498/aps.64.125201
Abstract +
Some dielectric structures on satellites would experience temperature variation in a relatively large range, giving rise to a considerable change in its conductivity and consequently resulting in a significant influence on the dielectric internal charging. However, due to the limitation to the model of conductivity versus temperature and the tool for three-dimensional (3D) simulation of internal charging, this temperature dependence has not attracted much attention. Therefore, the conductivity of a satellite used modified polyimide is measured in a temperature changeable vacuum environment under high electric field (in MV/m). Keithley 6517 B is used to capture the mild electrical current in a relatively long measuring time (several hundred seconds). According to the Arrhenius temperature dependence and considering the conductivity enhancement due to high electric field, good agreement is obtained between fitted data and measured results by setting the activation energy to be 0.40 eV. In addition, the radiation induced conductivity (RIC) is taken into account by using the Fowler model. The conductivity at room temperature is found to be comparable to the RIC from the condition with 2 mm aluminum shielding. Using the derived results, the internal charging simulation in three dimensions is carried out for a selected part of a structure in this material, where Geant4 is used to derive the distribution of charge deposition and radiation dose in three dimensions. The incident energetic electrons are assumed to follow the exponential distribution under geosynchronous orbit severe radiation condition where the flux of electrons with energy larger than 2 MeV is assumed to be 1.0×109 m-2·-1·sr-1. It is found that the internal charging will become more serious as the temperature decreases. The charging time is about 1 h at temperature 330 K, whereas this time is increased to 10 h for temperature below 250 K. The most serious charging domain appears around the boundary line of the grounding surface close to the radiation source, where the electric field strength exceeds 107 V/m under the condition of 2 mm aluminum board with temperature 250 K. So the dielectric breakdown discharge is most likely to occur within this domain. Above all, under the condition of the material intrinsic conductivity (mainly depending on temperature) comparable to the radiation induced conductivity, temperature will play an important role in internal charging. This model for temperature-dependent conductivity and the method of 3D simulation of internal charging have great significance in both further evaluating spacecraft internal charging and implementing well protective designs.
2015, 64 (12): 125202.
doi: 10.7498/aps.64.125202
Abstract +
Magnetized target fusion (MTF) is an alternative approach to fusion between traditional inertial confinement fusion and magnetic confinement fusion. It involves three processes: the formation of target plasma, the translation of target plasma, and compression process of implosion. In this paper, the translation process is studied with a two-dimensional magneto-hydrodynamic code MPF-2D, and the result shows that it is necessary to add a proper magnetic field in the translation process of field reversed configuration in order to maintain its topological structure. The effects of initial magnetic pressure, translation magnetic field, and the gap between coils are studied in detail.
2015, 64 (12): 125203.
doi: 10.7498/aps.64.125203
Abstract +
The plastic DD filled capsule implosion experiment is performed on Shenguang III prototype laser facility. One-dimensional hydrodynamic numerical simulations show that the implosion compression ratio can be controlled by changing the capsule ablator thickness. In experiments, two types of capsules are studied and most of important implosion parameters are collected, such as neutron yield, X-ray bang-time, trajectory, and shape of hot core. The comparison between post-simulations and experimental results is performed. In our experiments, the neutron yield is 6.8×107 and YOC1D reaches 34% for low compression ratio implosion; the neutron yield is 6.3×106 and YOC1D is only 2.3% for middle compression ratio implosion. Meantime, the shape of hot core obtains an extra higher Legendre partial (P2 is 18% and P4 is 5%). On another side, the trajectory and bang-time are compared with simulations well.
EDITOR'S SUGGESTION
2015, 64 (12): 125204.
doi: 10.7498/aps.64.125204
Abstract +
The nanosecond laser produced plasma expansion in an external transverse magnetic field is explored by using optical imaging of plasma self-luminescence, optical spectrum and optical interferometry techniques. The plasma displays bifurcation and focusing phenomena in a transverse magnetic field, which is different from the scenarios without external magnetic field significantly. We set up a simplified magnetohydrodynamics model according to the feature of experimental parameters. The theoretical results of the temporal evolutions of the plasma density and the temperature are in good agreement with the experimental results, which confirms the important role of the magnetic diffusion in the plasma evolution.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2015, 64 (12): 126201.
doi: 10.7498/aps.64.126201
Abstract +
In the process of the generation of jet formed by the shaped charge explosive compression, the grain of the metal liner is refined from 30-80 μm down to sub-micron or nanometer level. There is a strong scientific significance for studying the mechanism of grain refinement and dynamic superplastic deformation at a micro level. The main contents of this study are as follows. Firstly, the models of nanocrystalline copper with the grain sizes of 7.17, 9.11, 12.55, 14.85, 18.38 and 22.48 nm are established using the Voronoi geometrical construction method, and these models are relaxed in 100 ps to the equilibrium state at 293 K. Then, the tensile deformation processes of nanocrystalline copper at various grain sizes are simulated by using the molecular dynamics method. The strain increases to 0.2 gradually at a strain rate of 2×109/s. Based on the data output, the stress-strain curves at different grain sizes are gained and the corresponding values of the averaged flow stress are calculated. The results show that the average flow stress exhibits the maximum at a grain size of 14.85 nm. Finally, the primary deformation process of nanocrystalline copper is displayed by analyzing the atomic configuration evolvement. When the grain size is 22.48 nm, the typical dislocation motion is found and there are a huge number of dislocations in the deformation process. However, the number of dislocations decreases sharply at the grain sizes of 14.85 nm and 9.11 nm, and the grain-boundary motion is visible at these small grain sizes. The most significant work is that the deformation mechanisms of nanocrystalline copper at different grain sizes are analyzed in detail. The results indicate that the dislocation motion dominates the deformation process when the grain sizes of nanocrystalline copper are larger than 14.85 nm. As the grain sizes decrease below 14.85 nm, the grain-boundary sliding and rotation become a dominant deformation mechanism. This change of deformation mechanism is the fundamental reason for softening, which is so-called reverse Hall-Petch relationship. On the basis of previous study and this molecular dynamics simulation, combining the grain coalition and the grain-boundary rotation, an ideal deformation mechanism model is established at small grain sizes, which provides the microcosmic deformation mechanism reference for the large strain deformation of the jet.
2015, 64 (12): 126202.
doi: 10.7498/aps.64.126202
Abstract +
The polyimide/potassium tantalite niobate (PI/KTa0.5Nb0.5O3) nanoparticle composite model is established by a multi-scale modeling method. The influences of KTa0.5Nb0.5O3 nanoparticles with different sizes (5.5, 8.0, 9.4, 10.5, 11.5 Å) on the structure, elastic modulus and interaction energy of the polyimidebased nanocomposites are investigated by the molecular dynamics simulation. The cell parameters, cohesive energy density, solubility parameter, Young’s modulus and Poisson’s ratio are calculated. Moreover, the bond energy and the number of atoms per unit surface area of the nanoparticles are analyzed to explore the internal mechanism of mechanical property improvement. The results demonstrate that the density of PI matrix is 1.24-1.35 g/cm3, the cohesive energy density of PI matrix is 2.025×108 J/m3, and the solubility parameter of PI matrix is 1.422×104 (J/m3)1/2, which are consist with the actual PI parameters. Meanwhile, the Young’s moduli of the PI and PI/KTa0.5Nb0.5O3 composites are respectively 2.914 GPa and 3.169 GPa, and the Poisson’s ratios are respectively 0.370 and 0.353, which illustrate that the mechanical properties of the PI could be significantly improved by introducing the KTa0.5Nb0.5O3 nanoparticles. At the same pressure, the increases of Young’s modulus with temperature are basically the same without and with doping the KTa0.5Nb0.5O3 nanoparticles into the PI matrix; and when the temperatures are different, the standard deviations of elastic moduli of the PI matrix and PI/KTa0.5Nb0.5O3 composite are almost the same. No matter what the pressures and the temperature are, the Young’s modulus of PI/KTa0.5Nb0.5O3 composite is always larger than that of PI matrix. These all indicate that the effect of KTa0.5Nb0.5O3 nanoparticle on elastic modulus has a similar variation rule under the selected pressure and temperature conditions. In addition, the bond energies of particle surface atoms are 8.62-54.37 kJ·mol-1, which shows that the binding force between particles and the matrix is mainly van der Waals force, and hydrogen bonds exist at the same time. When the doping concentration is fixed, the proportion of nanoparticles surface atoms increases significantly as the size decreases, the interaction between particles and the matrix becomes stronger, the Young’s modulus increases obviously and the size effect is more significant. Therefore, it is confirmed that the doping small size KTa0.5Nb0.5O3 nanoparticles into the polyimide matrix is an effective way to improve the mechanical properties of the composite.
2015, 64 (12): 126501.
doi: 10.7498/aps.64.126501
Abstract +
Carbon nanotube (CNT) fiber is a promising material due to its extensive potential in micro/nanoelectronics, where the thermal performance is of great importance. In this work, a well-developed steady-state electro-Raman-thermal technique is employed and extended to the ambient environment for measuring thermal conductivity of the CNTs fiber. In this technique, two ends of the CNT fiber are attached to heat sinks and a steady electrical current flows in a sample to induce Joule heating. The heat dissipates to the ambient air and goes through the sample to the heat sinks. With combined effects of natural heat convection and heat conduction, a steady temperature profile along the sample can be established. The middle point temperature of the fiber is probed by measuring the local Raman spectrum. It is because the Raman scattering (such as G peak) of CNT fiber is temperature dependent and thus it can be used as a temperature indicator for thermal property measurement. In calibration experiment, the temperature coefficient of the G peak of CNT fiber is first obtained. A modified one-dimensional heat conduction solution involving free convection effect is derived as #br#T(x) =((I2R)/(hLS))(1 -(e√(hS)/(kAc)x)+e-√(hS)/((kAc)x)/(e√(hS)/(kAc)L/2)+e-√(hS)/(kAc)L/2))+ T0. It can be found that the relationship between middle point temperature (T0) and applied Joule heating power (I2R) can be used to extract the thermal conductivity of the material (k) as long as the convection coefficient (h) is available. In this work, the convection coefficient is calculated by the model established by Peirs et al. The thermal conductivity of CNT fiber synthesized from floating catalyst method is measured to be 66.93 W/(m·K)± 11.49 W/(m·K). This value is a little bit larger than that of other CNT fibers synthesized by the acid spun method or the dry-spinning method, which can be explained by the different sample structures induced from different synthesize method. This value is two orders of magnitude smaller than that of individual carbon nanotube, and two orders of magnitude larger than that of CNTs packed bed, showing that heat conduction in CNT based bulk material is determined mainly by a large number of thermal interfaces between CNTs contacts rather than the intrinsic thermal property of CNT.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2015, 64 (12): 127301.
doi: 10.7498/aps.64.127301
Abstract +
Supercapacitor is an energy storage device which obtains energy from the electrochemical double layer or the redox-type reactions at or beyond the surface of the electrode, which can meet the demands for high power and long cycle life. However, the electrode still has low energy density for supercapacitor device. The design of electrode material is essential for obtaining high capacity. We employ density functional theory based on the first principle to calculate the electronic structures and derive the capacitance of N-doping graphene. We find that the quantum capacitance can be substantially improved by N doping. The physical mechanism of such phenomena is discussed in this paper.
Influence of annealing temperature on the distribution of particle sizes of quantum dots doped glass
2015, 64 (12): 127302.
doi: 10.7498/aps.64.127302
Abstract +
In order to understand the influence of annealing temperature on PbSe quantum dots doped glass produced by a melt-annealing technique, experiments are carried out to compare the influences of different nucleation durations, crystallization temperatures and time on the particle size, distribution and absorption spectrum. Under the condition of the same nucleation temperatures and different crystallization temperatures, the transmission electron microscope images of all samples show that a certain quantity of PbSe crystals are crystallized in the glass. While the particle sizes and densities are slightly different. The calculated distribution of the particle sizes quantitatively indicates that the particle size will be enlarged with the increase of crystallization temperature and the crystal particle density. The measured absorption spectrum shows that the peak value of absorption spectrum increases gradually with increasing the crystallization temperature. At the same time, the peak value shows a red-shift phenomenon. While under the relatively low crystallization temperature, the infrared absorption peak cannot be obtained in spite that some crystals have grown inside the glass. The absorption spectrum is covered up by the background signals because of the relatively smaller particle size and density. This work will be benefit of producing different size quantum dots with a certain density, and realizing stronger absorption and emission in multiband.
2015, 64 (12): 127303.
doi: 10.7498/aps.64.127303
Abstract +
Voltage and current degrade the AlGaN/GaN high electron mobility transistors (HEMTs) under on-state stress. To determine which one dominates the degradation, two on-state stresses which have equal power are exerted on AlGaN/GaN HEMTs: high voltage and low current on sample A, low voltage and high current on sample B. In the former stress, drain-source voltage (VDS) is 28 V, drain-source current (IDS) is 75 mA/mm. In the latter stress, VDS is 14 V and IDS is 150 mA/mm. The package temperatures of samples A and B are kept at 150 ℃. The samples are measured every 24 hours, with an extra measurement at the 8th hour in the first 24 hours (note that the time refers to the stressing time). There is an interval of 4 hours between the stressing and the measurement. The device parameters include drain-source current-voltage (IDS-VDS) characteristics, large-signal parasitic source resistance (RS), large-signal parasitic drain resistance (RD), and transfer characteristics between IDS and gate-source voltage (VGS). The emission microscope (EMMI) is used to study the leakage current after experiment. The IDS-VDS characteristics of sample B are kept constant after being stressed, while that of device A shifts downward after being stressed. RS of sample A, RS of sample B, and RD of sample B increase slightly, RD of sample A increases more obviously with most change happening in the first 8 hours. IDS-VGS characteristics of sample B kept constant, IDS-VGS characteristics of sample A shift downward. The changes of threshold voltage (VGS(th)) is obtained from the transfer characteristics, and it is similar to the changes of transfer characteristics. The VGS(th) magnitude (absolute value) of sample A decreases obviously while that of sample B decreases slightly. The measurements show that the device under low voltage and high current stress degrades little and the device under high voltage and low current stress degrades more obviously. The EMMI images show that the leakage of sample A is greater than that of sample B. The analyses of the parameter change, experiment setting and EMMI image indicate that the voltage, rather than the current, dominates the degradation for AlGaN/GaN HEMTs. The influences of hot electron effect, gate electron injection, and self-heating are recoverable, and they vanish in the interval between the stressing and the measurements. The permanent degradation of device parameter is caused by the inverse piezoelectric effect induced by high electrical field between the gate and the drain. Besides, it is found that sudden failure without precursor is easy to happen to the device under low voltage and high current stress. The microscope image of damaged area shows that the failure is due to hot spot induced by high current density.
2015, 64 (12): 127304.
doi: 10.7498/aps.64.127304
Abstract +
The specific contact resistivity and reflectivity of Ni/Ag/Ti/Au contact with p-GaN are studied. It is found that the thickness of Ag, anneal time and deposition temperature have a great effect on the performance of Ni/Ag/Ti/Au electrode. Its optical reflectivity is measured by reflectivity spectrophotometer, and its specific contact resistivity is calculated by circular transmission line model. It is observed that the contact reflectivity values of Ni (1 nm)/Ag/Ti (100 nm)/Au (100 nm), when the thickness values of Ag are 25 nm and 50 nm, are low: their values are 68.5% and 82.1% at 450 nm, respectively, and they start to increase with increasing the Ag thickness, then reach their saturated values when Ag thickness is beyond 200 nm. When the anneal time changes from 1 min to 10 min in oxygen atmosphere, the specific contact resistivity decreases at 300 ℃, decreases further and then increases at 400-600 ℃. After annealing at temperatures at 300 ℃ and 400 ℃ in oxygen atmosphere, the contact reflectivity value of Ni/Ag/Ti/Au remains almost unchanged, even when anneal time increases from 1 min to 10 min. However, The contact reflectivity of Ni/Ag/Ti/Au decreases significantly after annealing at a temperature higher than 400 ℃ and it becomes smaller with longer annealing time. After 400 ℃ annealing in oxygen atmosphere for 3 min, the specific contact resistivity reaches 3.6×10-3 Ω·cm2. Additionally, the deposition temperature of Ni/Ag is investigated. It is noticed that the specific contact resistivity decreases and the reflectivity increases with increasing the deposition temperature from room temperature to 120 ℃. The reflectivity rises to 90.1% at 450 nm and the specific contact resistivity reaches 6.4×10-3Ω·cm2 for the Ni/Ag/Ti/Au electrode at a deposition temperature of 120 ℃. However, the effects of improving the electrical and optical characteristics weaken when deposition temperature changes from 120 ℃ to 140 ℃. With a overall consideration, Ni (1 nm)/Ag (200 nm)/Ti (100 nm)/Au (100 nm) is made at a deposition temperature of 120 ℃, and then anneals at 400 ℃ for 3 min in oxygen atmosphere to achieve the optimized electrode. The vertical light emitting diode with this Ni/Ag/Ti/Au electrode is fabricated. Its working voltage is 2.95 V and the light output power is 387.1 mW at 350 mA. The electro-optical conversion efficiency reaches 37.5%.
2015, 64 (12): 127305.
doi: 10.7498/aps.64.127305
Abstract +
In this paper, the influences of the growth time of low-temperature (LT) GaN nucleation layer on the crystal quality and optical properties of GaN film are investigated. It is found that the optimal LT nucleation layer growth time can effectively reduce the crystal defects and is favorable to forming the annihilation of dislocations. GaN films are grown on c-plane sapphire substrates by metal-organic chemical vapor deposition. Crystal quality and optical properties are characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM), high-resolution X-ray diffraction (HRXRD), and photoluminescence spectra, respectively. In the AFM images, the island density decreases as growth time increases, while the size of island becomes larger and the uniformity of island size deteriorates as growth time increases, leading to the phenomenon that the number of interfaces formed during the nucleation island coalescence, first decrease and then increase as detected by SEM, which also induces the screw dislocation density and edge dislocation density to first decrease and then increase as measured by HRXRD. This first-decrease-and-then-increase variation trend is consistent with the first-increase-and-then-decrease variation trend of the ratio of the band edge emission peak intensity to the yellow luminescence peak intensity tested by photoluminescence, which is confirmed by HRXRD. It is shown that GaN islands with different sizes and densities could lead to different mechanisms of dislocation evolution, thereby forming GaN epitaxial layers with different dislocation densities and optical properties. Through controlling the nucleation time, GaN films with the smallest dislocation density could be obtained.
2015, 64 (12): 127701.
doi: 10.7498/aps.64.127701
Abstract +
CaCu3Ti4O12 ceramic has drawn much attention due to its stable colossal dielectric permittivity and pronounced nonlinear electrical characteristics. In this work, the effects of direct current degradation on the dielectric response and electrical property of CaCu3Ti4O12 ceramic aged for 60 h under 3.5 kV/cm are investigated. The results of J-E characteristic analysis show that the breakdown field E1mA decreases from 216 V/mm to 144 V/mm and nonlinear coefficient η decreases from 4.1 to 2.1. The barrier heights of CaCu3Ti4O12 ceramics are calculated to be in a range of 293-368 K, based on the J-E curves, which decrease from 0.57 eV to 0.31 eV. It is found that the dielectric constant and dielectric loss at low frequencies are significantly increased. Based on Debye function, it is indicated that the dielectric loss is composed of direct current conductance loss and relaxation loss, especially the direct current conductance loss is enhanced by the direct current degradation. At 233 K, two relaxation peaks whose activation energies are 0.10 eV and 0.50 eV can be found, which are considered to be related to grain and domain boundary and not vary with direct current degradation. Electric modulus spectra are used to characterize the role of direct current degradation in the relaxation process of CaCu3Ti4O12 ceramic. The results show that the variation of interfacial space charges caused by direct current degradation obeys the Maxwell-Wagner polarization. It may be a key factor to lead to the increase of dielectric permittivity below 10 Hz, and a new corresponding relaxation peak θ can be observed in the modulus plot at low frequency. In the impedance spectra in 323-473 K, the relaxation peaks of grain boundary shift toward high frequency after direct current degradation. The results from the complex impedance plane show that the resistance of the grain boundary decreases by about two orders of magnitude and its activation energy drops off from 1.23 eV to 0.72 eV, while the resistance of grain decreases a little and its activation energy has no obvious variation. Therefore, it is proposed that direct current degradation should play an important role in grain boundary and affect its electrical property and dielectric response. An RC circuit model is proposed to elucidate the correlation between dielectric relaxation and electrical property of CaCu3Ti4O12 ceramic.
2015, 64 (12): 127801.
doi: 10.7498/aps.64.127801
Abstract +
Organic nonlinear optical materials have attracted considerable attention in recent years because of their potential applications in photonic devices and optical information processing. Recent studies have shown that annulene derivatives exhibit good second-order nonlinear optical properties, but their third-order nonlinear optical properties are studied little. In this paper, the values of molecular static linear polarizability α and second hyperpolarizability γ of substituted annulenes have been investigated with different levels of HF, B3LYP, BHandHLYP and CAM-B3LYP at different basis sets, respectively. Their ultraviolet spectra have also been calculated by using the TD-B3LYP method. It is found that the quality of the basis set is important for the hyperpolarizability calculations, and diffuse functions are important to obtain accurate results for the second hyperpolarizability. We also study the structure-optical property relationship for annulene. It is found that annulene molecular structure has a significant influence on third-order nonlinear optical response. Increasing the conjugation length and introducing push-pull electronic groups can enhance the second hyperpolarizability. But the introduction of push-pull electronic groups can enhance the hyperpolarizability more remarkably than increasing the conjugation length dose, which may be due to the fact that the introduction of push-pull electronic groups can provide a large number of polarizable electrons whereas increasing the conjugation length can only enhance the electron delocalization. Meanwhile the push-pull electronic group substituted annulenes can also exhibit high transparency in visible region. Thus, this work has a good reference for designing nonlinear optical material with high, nonlinear optical coefficient and good transparency. In addition, for the same push-pull electronic groups, the higher conjugation degree and the longer πup -conjugated bridge result in the decrease of HOMO-LUMO energy gap and transition energy which benefits the enhancement of nonlinear optical response. Our results demonstrate that annulene derivative shows both high transparency and large second hyperpolarizability, and thus becomes a promising candidate for third-order nonlinear optical material. In addition, the dynamic (hyper) polarizabilities of considered annulene molecules are calculated by using CAM-B3LYP method. It is found that in near-infrared region, with the increase of frequency of incident light, α (ω; ω), γ (-ω; ω, 0, 0) and γ (-2ω; ω, ω, 0) are all increased, and the near-resonance enhancement effect occurs. Under the condition of far resonance, α (ω; ω), γ (-ω; ω, 0, 0) and γ (-2ω; ω, ω, 0) change little. This dispersion effect may be helpful for the experimental study and applications as well.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
EDITOR'S SUGGESTION
2015, 64 (12): 128101.
doi: 10.7498/aps.64.128101
Abstract +
In this paper, a series of high-quality hydrogen-doped diamonds is successfully synthesized in Ni70Mn25Co5-C system by using Fe(C5H5)2 as hydrogen source at pressures ranging from 5.5 GPa to 6.0 GPa and temperatures of 1280-1400 ℃. We find that both pressure and temperature conditions strengthen with adding the Fe(C5H5)2. Scanning electron microscope micrographs show that the obtained diamonds at low levels of Fe(C5H5)2 additive have smooth surfaces. However, many defects are found and some pores appear on the diamond surface with increasing the Fe(C5H5)2 additive in the system. From the obtained Fourier transform infrared (IR) spectrum, we notice that there is no significant change of nitrogen concentration in the synthesized diamond with the Fe(C5H5)2 additive lower than 0.3 wt%, while the nitrogen concentration gradually decreases with the further increase of Fe(C5H5)2 additive. In the system with 0.5 wt% Fe(C5H5)2 additive, the nitrogen concentration in synthesized diamond is only half that of system without Fe(C5H5)2 additive. Meanwhile, the hydrogen associated IR peaks of 2850 cm-1 and 2920 cm-1 are gradually enhanced with the increase of Fe(C5H5)2 additive in the system, indicating that most of the hydrogen atoms in the synthesized diamond are incorporated into the crystal structure as sp3-CH2-symmetric (2850 cm-1) and sp3 CH2-antisymmetric (2920 cm-1) vibrations. From the obtained Raman spectrum, we find the incorporation of hydrogen impurity leads to a significant shift of the Raman peak towards higher frequencies from 1333.90 cm-1 to 1334.42 cm-1 with increasing the concentration of Fe(C5H5)2 additive from 0.1 wt% to 0.5 wt%, thereby giving rise to some compressive stress in the diamond crystal lattice. This is the first time that the gem-grade hydrogen-doped diamond single crystal, with size up to 3.5 mm has been successfully synthesized by using new hydrogen source Fe(C5H5)2 additive. We believe that our work can provide a new method to study the influence of hydrogen impurity on diamond synthesis and it will help us to further understand the genesis of natural diamond in the future.
2015, 64 (12): 128102.
doi: 10.7498/aps.64.128102
Abstract +
According to low temperature Ge buffer layer and selective area epitaxy technology, we selectively grow Ge film on patterned Si/SiO2 substrate using ultra-high vacuum chemical vapor deposition. By using X-ray diffraction (XRD), scanning electron microscope, atomic force microscopy and Raman scattering spectrum, we obtain its crystal quality and the laws of stress and other parameters varying with shape size. The results show that threading dislocation density decreases with shape size decreasing. Moreover, the tensile strain of Ge layer first increases and then turns stable with the increase of shape size, which can be attributed to the formation of (113) facet during Ge selective area growth. The formation of (113) facet reduces the strain energy of epitaxial material system, and the reduction of strain energy per unit volume decreases with increasing the shape size. The root-mean-square surface roughness of the Ge epilayer with a thickness of 380 nm is about 0.2 nm, the full-width-at-half maximum of the Ge peak of the XRD profile is about 678". It is indicated that the selective area epitaxial Ge layer is of good quality and will be a promising material for Si-based optoelectronic integration.
2015, 64 (12): 128701.
doi: 10.7498/aps.64.128701
Abstract +
Automatic recognition of epilepsy electroencephalography (EEG) signal has become a research focus because of its high efficiency, and many algorithms have been put forward to achieve it. As one of the classic algorithms of boosting algorithm, AdaBoost algorithm has been widely used in face detection and target tracking fields, but the algorithm also has a disadvantage that is its degradation. In order to solve this problem, this paper puts forward three measures to optimize the algorithm by filtering the weak classifiers whose recognition rates are low, introducing the smoothing factor and a weighted correction function. In order to verify the robustness of optimized algorithm, we choose three main parameters, i.e., the number of weak classifier, which is denoted by T; the base of logarithmic function, which is denoted by α; the threshold of weight, which is denoted by β. The experimental results of optimized AdaBoost show that it has good robustness and high recognition rate. #br#In this paper, we divide the whole process into three steps. The first step is to use the Butterworth digital low-pass filter in which the cutoff frequency of pass band is 40 Hz to filter noise whose frequency is above 40 Hz. The second step is to do feature extraction with the help of wavelet packet decomposition. The third step is to compute the sum of absolute value which are the wavelet packet coefficients of fourth layer, the wavelet package entropy and the sum of signal amplitude square and combine them together to form the feature vector of each EEG. Because the wavelet package entropy is far less than the sum of absolute value and the sum of signal amplitude square, in order to make sure that the entropy reacts in the third step, we use one thousandth of the sum of absolute wavelet packet coefficients, one hundredth of the sum of signal amplitude square and the wavelet package entropy as the weighted feature vector. Finally, we succeed in distinguishing EEGs between epilepsy and normal by using the optimized AdaBoost whose input is the weighted feature vector. The result shows that the presented method has a high recognition rate, it can identify 96.11% epilepsy EEGs and 99.51% normal EEGs, thus it provides an effective solution for the correct diagnosis of epilepsy.
2015, 64 (12): 128501.
doi: 10.7498/aps.64.128501
Abstract +
Superconducting quantum interference device (SQUID) amplifier is best known for its low input impedance, low noise and low power consumption. Nowadays it is widely used for detecting the weak signals. Compared with other methods, the Nb/Al-AlOx/Nb structure Josephson junction based SQUID has the advantages of high transition temperature, high voltage flux modulation index and good heat recycle ability, wide critical voltage range, so it is a very good option for making SQUID amplifier. In this work, we fabricate the overdamped Josephson junction and washer dc SQUID, and test the I-V characteristics at He3 3 K stage temperature and calculate the current resolution of SQUID. The result of SQUID modulation property is good. The magnification becomes larger after increasing the input line number of loops, and the system noise becomes smaller after the join of the LC filter. This work is very important for designing and manufacturing transition edge sensor readout circuits.
ATOMIC AND MOLECULAR PHYSICS
2015, 64 (12): 123301.
doi: 10.7498/aps.64.123301
Abstract +
In this paper the laser induced thermal grating spectroscopy thermometry technique is investigated. Two coherent, pulsed pump lasers are crossed in NO2/N2 mixture to induce an interference pattern, owing to the resonant absorption and the subsequently quenching effect. The heat released into the bulk gas can modulate the local refractive index (temperature grating). Simultaneously, the sound wave induced by the electric field forms the standing wave (acoustic grating). These two effects mentioned above produce a thermal grating, and a continuous probe laser satisfying the Bragg scattering condition, generates a coherent signal in the crossed region. The spatial and spectral filtering signal is detected with a photomultiplier tube, and displayed with a digital oscilloscope. The signal carries plenty of flow field information. The gas temperature is obtained through frequency analysis. In order to increase the precision of temperature measurement, we calibrate the grating spacing at a known temperature in a pressurized gas cell. Then the temperature in a range of 300-500 K is measured by the laser induced thermal grating spectroscopy technique, and the thermocouple temperatures are recorded at the same detecting point simultaneously. Both of them agree well with each other, though some discrepancies are still existent. The difference is explained according to the heat radiation loss. We also use this technique to measure the gas sound speed directly, which is crucial to studying the gas behaviors at high pressures and the interaction between molecules. In a certain temperature range, the measurement result and the theoretical curve are nearly consistent, which shows the high precision and multi-parameter measurement ability of laser induced thermal grating spectroscopy. The factors influencing the signal waveform are analyzed, too, and the results demonstrate that the signal duration, the signal intensity, and the oscillation peaks increase with pressure increasing. As a consequent, the accuracy of measurement can be improved. Also, other gas dynamic parameters, such as the thermal diffusion rate and the heat conductivity, can also be measured by using this technique. The unique advantage of laser induced thermal grating spectroscopy thermometry technique provides us with a powerful diagnostic tool used in high pressure condition.
2015, 64 (12): 123101.
doi: 10.7498/aps.64.123101
Abstract +
GeO molecule, which plays an important role in fabricating integrated optics and semiconductor components, has received much attention. However, the electronic state density of the molecule is very large, and the electric structures and transitional properties of the molecule have not been well investigated. In this work, the 18 Λ -S states correlated to the lowest dissociation limit (Ge(3Pg)+O(3Pg)) are calculated by a complete active space self-consistent field (CASSCF) method, through using the previous Hatree-Fock molecular orbitals as the starting orbitals. Furthermore, we take all configurations in the configuration interaction expansions of the CASSCF wave functions as a reference configuration, and calculate the energies of the 18Λ-S states by a high-level multireference configuration interaction method. The core-valence correlation effect of the 3d orbit of Ge atom, the scalar relativistic effect, and the Davidson correction are taken into consideration in the calculations. On the basis of the calculated potential energy curves of the bound and quasibound electronic states, the spectroscopic constants (Re, Te, ωe, ωeχe, and Be), vibrational energy levels, vibrational wave functions, and Franck-Condon factors (FCFs) are obtained by solving the radical Schrödinger equation. The computed spectroscopic constants of these electronic states are well consistent with previously available experimental results. We calculate the electric dipole moments of electronic states with different bound lengths, and analyze the influences of the variation of electron configuration on the electric dipole moment. The calculated potential energy curves indicate that the adiabatic transition energies of A1Π, 11Σ-, D1Δ, a3Π, a’3Σ+, d3Δ, and e3Σ- sates are located in a range of 26000-37000 cm-1, and the spin-orbit coupling of the states can obviously affect the corresponding vibrational wave functions. With the help of calculated spin-orbit coupling matrix elements, the perturbations of the nearby states to a3Π and A1Π are discussed in detail. Our calculation results indicate that the spin-orbit coupling between A1Π and e3Σ- states has an evident perturbation on the v’> 4 vibrational levels of A1Π, and the v’≥ 0 vibrational levels of a3Π state are perturbed by the crossing states a’3Σ+, d3Δ, e3Σ-, 11Σ-, and D1Δ. On the basis of computed transition dipole moments and FCFs of A1Π-X1Σ+ and A’1Σ+-X1Σ+ transitions, the radiative lifetimes of the six lowest vibrational levels of the two singlet excited states are computed.
SPECIAL ISSUE — Liquid crystal photonics and applications
2015, 64 (12): 120702.
doi: 10.7498/aps.64.120702
Abstract +
The transmission characteristics of full-filled photonic liquid crystal fibers (PLCFs) which are filled with five kinds of liquid crystals (LCs) are experimentally studied and theoretically analyzed. The influences of temperature and external electric field on the transmission characteristics of PLCFs are also discussed in this paper. The transmission spectra of PLCFs show obvious bandgaps, and the number and the central wavelengths of the bandgaps depend on the average value of the refractive indices of LCs. By changing the temperature from 20 ℃ to 80 ℃, a blue shift in the bandgap is observed, and the maximum tuning range of the bandgap is 41 nm. Then, with the voltage turning from 0 V to 250 V, the output power of the transmission spectrum decreases, while the central wavelength of the bandgap is almost unchanged. Finally, the transmission spectrum keeps a good stability, even if the polarization state of the input light changes.
2015, 64 (12): 124213.
doi: 10.7498/aps.64.124213
Abstract +
In conventional phase-only holographic display, the phase-only computer generated hologram is usually calculated based on the fast Fourier transform (FFT) algorithm, in which the Nyquist theory should be satisfied. However, due to the pixel structure of the liquid crystal spatial light modulator and a fixed spatial sampling rate, the size of the reconstructed image is limited by the space-bandwidth product of the liquid crystal phase modulator. The traditional solution is to use convolution algorithm or double-step Fresnel diffraction algorithm to calculate the Fresnel hologram, but FFT has to be calculated many times in both of the methods, thereby increasing the burden of hologram computation. Therefore, in this paper we propose a method to calculate the phase-only hologram based on setting a virtual hologram plane. This virtual hologram plane is set based on the principle of lens imaging. So the calculation of the hologram can be divided into two steps: the first step is to calculate the Fresnel diffraction from the object plane to the virtual hologram plane, and the second step is to calculate the hologram from the virtual hologram plane by being multiplied with a quadratic phase term. In this way, the hologram can be calculated from the original object with any sampling rate we need by adjusting the corresponding parameters of distance. By this method one can calculate the Fresnel diffraction between hologram plane and object plane with variable sampling rates, without considering the space-bandwidth product of the liquid crystal phase modulator, and this algorithm uses only one FFT calculation, which can speed up the calculation of hologram compared with the convolution based method (using three FFTs in calculation) and the double-step Fresnel method (using two FFTs in calculation). Both the computer simulation and the optical experiments demonstrate that the object can be reconstructed with different sizes in the holographic display system. In the optical experiment, the zero-order diffraction can be removed by placing a filter on the back focal plane of the imaging lens and the speckle noise can also be eliminated in order to improve the reconstruction quality by displaying multiple phase-only holograms at a high speed. The proposed method in this paper shows a potential application in zoom-able liquid crystal spatial light modulator based holographic display system.
2015, 64 (12): 124214.
doi: 10.7498/aps.64.124214
Abstract +
This review is intended to be a fundamental lecture. It focuses on systematically introducing the reader to the physical and optical background to certain basic concepts in nanoplasmonics, before devoting attention to the many new developments at the frontiers of modern photonics, such as tuneable nanoplasmonics. There is a special discussion of the advantages and applications of liquid crystals in this area. First, in optics according to the special requirements of an optical surface wave propagating alone a smooth boundary the concept of surface plasmon polariton (SPP) has been introduced from physics. After discussing the influences from more rough surfaces upon the SPP and the response from larger metallic particles to the optical electro-magnetic waves the results from interaction between the optical waves and metallic particles with dimensions much small than the wavelength of the optical waves-the exist of the local surface plasmon polariton, i.e. the base of nanoplasmonics, has been confirmed. Secondly, this review describes many new and interesting aspects from this important branch at the frontiers of modern photonics-nanoplasmonics, which are supported by metamaterials consisting of metallic particles with various shapes and nano-scale size from modern manufacture technologies and more powerful and functional software. Many device system based upon these aspects have broken through the limitations of classical optics and developed in many special new directions, for example the quantum coincidence of lasers-Spaser (surface plasmon amplification by stimulated emission of radiation) etc. Finally, we address tuneable nanoplasmonics, which is a very important topic that has warranted great attention. by reason of liquid crystals’ many special advantages in optical responses-for example their larger optical birefringence, which can be easily modulated by applying electric and/or magnetic fields etc.-the application of liquid crystals in tuneable nanoplasmonic devices is a more practical research direction. This review introduces recent developments in this area, and also discusses various challenges and possible research topics.
2015, 64 (12): 124215.
doi: 10.7498/aps.64.124215
Abstract +
Holographic three-dimensional (3D) display is a true 3D display technique, which can provide realistic image of a real object or a scene because holography has the ability to reconstruct both the intensity and phase information, i.e., the wave front of the object or scene. Therefore, it could allow the observers to perceive the light as it is scattered by the real object itself without any special eyewear, which is quite different from other 3D display techniques, such as stereoscopic displays and volumetric 3D displays. In this paper, the achievements and developments of the latest new holographic 3D displays are presented. Holographic 3D displays can be divided into static holographic 3D displays and dynamic holographic 3D displays. Here, we briefly introduce the principle of holographic 3D display technique and static holographic 3D displays, and focus on dynamic holographic 3D displays. Large-size, high-resolution and color static holographic 3D displays have already been successfully fabricated and applied in some areas, such as holographic 3D maps and holographic 3D images. However, dynamic holographic 3D displays based on both optical materials and spatial light modulators (SLMs) are still under research, which is a challenge to their applications. Some holographic researchers study the holographic 3D displays based on the SLMs for large-size and large view angle display, but it is difficult to realize them because of limitations of SLMs and there still needs much effort to solve these problems in SLMs. Other holographic researchers work on dynamic holographic materials, such as inorganic crystals, photorefractive polymer, photochromic material etc. The response time and diffraction efficiency are key factors to these materials. Compared with other holographic media, liquid crystals with super-fast response time (about 1 ms) have been reported, which makes it possible to realize video refresh-rate holographic displays. The achievements of dynamic holography, which are helpful for holographic 3D video applications, are presented. Recently, real-time dynamic holographic display has been obtained in super-fast response liquid crystal films, which makes it possible that large-size, high-definition, color holographic 3D video displayers are developed by using these liquid crystal films in the future.
2015, 64 (12): 126101.
doi: 10.7498/aps.64.126101
Abstract +
Blue phase liquid crystal display (BPLCD) is emerging as next-generation display, because of its fast response speed and very wide viewing angle. However, it has some weak points to be improved. The light leakage at the dark state affects the contrast ratio, and needs to be analyzed and improved. Considering the double-twist structure of blue phase liquid crystal (BPLC) and the simple twist structure of cholesteric liquid crystal (ChLC), the two twist structures are similar. The transmittances and reflectances of planar and focal conic texture of cholesteric liquid crystal and blue phase II liquid crystal are simulated by finite-difference time domain (FDTD) method. The FDTD method is based on the Maxwell’s equation, and can calculate the optical rotatory power directly. The effective optical rotatory powers of the three liquid crystal states are proposed and compared using the light leakages at the cell with crossed and parallel polarizers. The results show that the transmittance of BPLC with crossed polarizers is lower than that of planar texture and larger than that of focal conic texture of ChLC. The optical rotation of BPLC is not the same at any point in one periodic cross section in the light path because the liquid crystal arrangement is complex, the effective optical rotatory power is defined as the average value of the optical rotatory powers at all points. Comparing with the optical rotatory powers of planar and focal conic textures of ChLC, the optical rotatory power of BPLC is less than that of planar texture and larger than that of focal conic texture. Moreover, the Bragg reflections are also simulated, the results show that blue phase liquid crystal is similar to planar state cholesteric liquid crystal, only the reflection intensity is smaller, and no obvious Bragg reflection appears in focal conic state cholesteric liquid crystal. Considering the optical rotation and Bragg reflection, the light leakage and reflective light of BPLCD cannot be ignored if the helix pitch is not less enough, however, these of focal conic texture of ChLC are very small compared with those of BPLC, as a result, the focal conic texture of ChLC can replace blue phase liquid crystal to obtain the good dark state and high contrast ratio.
2015, 64 (12): 126102.
doi: 10.7498/aps.64.126102
Abstract +
It is critical to improve the response speed of a liquid crystal wavefront corrector in order to increase the bandwidth of a liquid crystal adaptive optics system. The design of liquid crystal molecules with small rotational viscosity becomes a basic method of increasing the response speed of a liquid crystal wavefront corrector. Various phases of liquid crystal from molecular dynamics simulation are given in this paper, and the detailed computational methods of order parameter and rotational viscosity are also presented. Rotational viscosities of liquid crystals are compared based on the molecular dynamics of mixtures. The data fluctuation is reduced effectively through several simulations and the multiple analysis of original data. A detailed process of molecular dynamics of mixtures is given in this paper and the result is greatly satisfactory. We believe that one can perform a better molecular design using this process and obtain a better understanding of molecular interactions of LCs.
2015, 64 (12): 126103.
doi: 10.7498/aps.64.126103
Abstract +
Copper is an alternative material for aluminum electrode to meet the stringent requirement for high mobility and low resistance-capacitance (RC) delay of amorphous indium-gallium-zinc oxide (a-IGZO) thin film transistor (TFT) for next generation of display technology due to its intrinsic high conductivity. However, low bonding strength between copper layer and insulator/glass and easy diffusion into active layer restrict its application in the field of TFT. In this work, a 30 nm thin film of molybdenum is introduced into copper electrode to form a copper-molybdenum source/drain electrode of a-IGZO TFT, which not only inhibits the diffusion of copper, but also enhances the interfacial adhesion between electrode and substrate. The obtained Cu-Mo TFT possesses a high mobility of ～9.26 cm2·V-1·s-1 and a low subthreshold swing of 0.11 V/Decade. Moreover, it has shorter current transfer length(～0.2 μm), lower contact resistance (～1072 Ω), and effective contact resistance (～1×10-4Ω·cm2) than the pure copper electrode. Cu-Mo electrode with low contact resistance and high adhesion to substrates paves the way to the application of copper in high conductivity interconnection of a-IGZO TFT.
