Vol. 66, No. 24 (2017)
2017-12-20
GENERAL
2017, 66 (24): 240502.
doi: 10.7498/aps.66.240502
Abstract +
Recently, the influences of the Dzyaloshinski-Moriya (DM) interaction on the performances of the basic thermo-dynamical quantities have attracted a lot of attention. A large number of investigations on the quantum coupling systems with DM interaction have been carried out. However, the specific effects of spin-orbit coupling with the performance on the quantum heat engine have not been taken into account in previous studies. DM interaction is a special kind spin-orbit coupling. To enrich the research of the quantum heat engines, the investigation about the effect of DM interaction on its thermodynamic characteristics should be included. In this study, we construct two entangled quantum engines based on spin-1/2 systems with different DM interactions, with the spin exchange constant and magnetic field fixed. The quantum Otto engine and the quantum Stirling engine are discussed in this article. By numerical calculation, we obtain the expressions for several thermodynamic quantities and plot the isoline maps of the variation of the basic thermodynamic quantities such as heat transfer, work with D1 and D2 and their efficiency in the two engines. The results indicate that the DM interaction plays an important role in the thermodynamic quantities for the quantum Otto engine and the quantum Stirling engine. In addition, the positive work condition is discussed with different DM interactions, with the spin exchange constant and magnetic field. Furthermore fixed, it is found that the efficiency of quantum Otto engine cycle is smaller than the Carnot efficiency while the quantum Stirling cycle can exceed the Carnot efficiency by using the regenerator. Finally, the second law of thermodynamics is shown to be valid in the two entangled quantum systems.
2017, 66 (24): 240301.
doi: 10.7498/aps.66.240301
Abstract +
In order to assure the security of the long-distance quantum communication, the maximum entangled state is necessary. However, the decoherence of the entanglement is inevitable because of the channel noise and the interference of the environment. Quantum entanglement concentration can be used to convert a non-maximum entangled state into a maximum one. In previous entanglement concentration proposals, we need the initial coefficients of non-maximum entangled state or repeat the entanglement concentration process to improve the possibility of success, which reduces the efficiency of the entanglement concentration. A more efficient entanglement concentration for phontonic polarization state is proposed in this paper, which is based on the interaction between circularly polarized light and quantum dot-cavity system. An auxiliary photon is introduced to connect two distant participants. To overcome the channel noise, the auxiliary photon transmits though two channels between the two participants. The photons interact with coupled quantum dot-cavity before and after the auxiliary photon transmission. Then the states of spins and auxiliary photon are measured, and the maximum phontonic polarization entangled state is obtained by single-photon operations according to the measurement results. The success possibility of the proposed scheme is 1 in ideal conditions, that is, the concentration can be realized deterministically. However, the cavity leakage is unavoidable, so the fidelity of the entanglement concentration is calculated by taking one of the measurement results for example. The results show that the influences of the initial coefficients of non-maximum entangled state on the fidelity can be ignored in most cases, which saves a mass of photons used to measure the initial coefficients of the non-maximum entangled state. The fidelities with varying coupling strengths and cavity leakages are also shown in the paper. In the case of weak coupling, the fidelity is low and varies sharply with cavity leakage. Fortunately, the fidelity will plateau in a strong coupling case, and reaches 99.8% with a coupling strength 0.7 for diverse cavity leakages. Much progress has been made in the study of the strong coupling between quantum dot and optical cavity, which can satisfy the requirement of our entanglement concentration. So the proposed scheme is feasible in the current experimental conditions. In general, our proposal still maintains high fidelity even considering the cavity leakage, and the initial information about partially entangled state and the repetition of the entanglement concentration process are not required. This not only improves the security of the quantum entanglement concentration, but also contributes to efficient quantum information processing with less quantum resources. These characteristics increase the universality and efficiency of the entanglement concentration, thus assuring the quality of the long-distance quantum entanglement.
2017, 66 (24): 240501.
doi: 10.7498/aps.66.240501
Abstract +
In real neuronal systems, information transition delay is an inevitable factor. However, between some neurons, neuronal information is transmitted instantaneously or the time delay is too small and can be neglected. Thus, differing from the conventional studies where all connections are considered to be delayed, here we mainly focus on the effect of partial time delay on stochastic resonance in a Watts-Strogatz small-world neuronal network. Meanwhile, in the same neuronal network, the electrical and chemical synapses usually coexist. Thus, effects of hybrid synapses are also considered. Firstly, in the absence of time delay, noise could induce stochastic resonance when the neuronal network contains much more excitatory synapses than inhibitory ones; while it cannot induce stochastic resonance vise verse. Interestingly, it is further revealed that when the ratio of excitatory synapse to inhibitory synapse is approximately 4:1, noise-induced stochastic resonance is more robust. Thus, to discuss the effects of other factors on noise-induced stochastic resonance, we set this ratio to be 4:1. In the absence of time delay, we also consider effects of chemical synapses with a ratio of excitatory synapse to inhibitory synapse of 4:1 on the noise-induced stochastic resonance. The obtained results show that the noise could always induce stochastic resonance no matter how the probability of chemical synapses varies. And the optimal noise intensity increases linearly with the probability of chemical synapses increasing. For partial time delay, it is surprisingly found that the stochastic resonance could appear multiple times with the variation of the time delay being just for small partial time delay probability. Moreover, chemical synapse is found to facilitate this effect of partial time delay. Finally, by analyzing the joint effects of partial time delay and noise intensity, it is found that the larger the time delay and the partial time delay probability are, the wider the optimal noise region corresponding to large response amplitude is.
ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2017, 66 (24): 244201.
doi: 10.7498/aps.66.244201
Abstract +
The aero-optical distortion caused by the compressibility of high-speed flow field has a great influence on the development of airborne optical detection system of (hypersonic) supersonic vehicles. The turbulent boundary layer is one of the most important aspects in the aero-optical study, and has become one of the hot research points in the field of aero-optical study. The nano-particle-based planar laser scattering technique is used to measure the density distribution of the supersonic (Ma=3.0) turbulent boundary layers, and the optical path difference, which is quite crucial for the aero-optical study, is obtained by ray-tracing method. The experimental result is verified by being compared with the theoretical result computed by the aero-optical scaling method of turbulent boundary layers. Five different light incident angles (α=60°, 75°, 90°, 105°, 120°) are selected and used to examine the influences of light incident angles on the supersonic turbulent layer, and the underlying flow physics is analyzed. Research shows that the light propagation path in the supersonic turbulent boundary layer contributes to the light incident angle dependence of aero-optics. The different propagation paths lead to the difference between the light propagation distance in the flow field and the correlation results of the corresponding density fluctuation. The oblique incidence of light results in the increase of the propagation distance in the flow field, and then the aero-optics turns worse. The greater the angle between the incident direction of light and the vertical direction of the wall, the more significant the aero-optics is, the difference increases at different times, the difficulty in correcting the aero-optics is also increased. In the supersonic turbulent boundary layer, a large number of vortices with a specific orientation lead to the anisotropy of the aero-optics in the turbulent boundary layer. By calculating the spatial two-point correlation of the density fluctuations at the streamwise plane (x-y plane), the cross-correlation result of density fluctuations at any light incidence angle (α=0°-180°) can be obtained. The local coherent structure scale is nearly 0.20 mm, which is basically consistent with the aero-optical effective scale (≈ 0.18 mm) computed from the formula proposed by Mani et al. When the light is inclined downstream, the direction of light propagation is consistent with the vortex structure in the flow field, and in this direction, the correlation coefficient of density fluctuation is larger, so the aero-optics is more serious. When the light beam is tilted upstream, the correlation coefficient is smaller, so the aero-optics is weaker.
2017, 66 (24): 244202.
doi: 10.7498/aps.66.244202
Abstract +
The laser coherent field imaging system emits multiple beams of laser from earth to space, and laser scans remote space target by passing through turbulence atmosphere. Multi-beam laser intensity fluctuation caused by atmosphere turbulence is a key factor affecting high-resolution imaging quality of the coherent field imaging system. Aiming at solving the problem of imaging quality degradation caused by laser intensity fluctuation error, we discuss the mechanism of laser intensity fluctuation error influencing the imaging quality of laser coherent field high-resolution imaging system. The theoretical model about the relationship between laser intensity fluctuation factor and imaging quality is proposed for the first time. Firstly, the laser echo field signal error induced by laser intensity amplitude fluctuation factor is deduced according to laser transmitting atmosphere theory. Then adopting multi-beam phase closure arithmetic, the phase closure coefficient error is derived from the laser intensity fluctuation factor and laser echo field signal. The mechanism of disturbed laser echo signal influencing phase closure coefficient is investigated in detail. In the following, based on reconstructed spectrum theory, the model of imaging frequency spectrum error propagation, caused by laser intensity fluctuation factor, is proposed. Finally, we reveal the mechanism of laser intensity amplitude fluctuation factor influencing reconstructed imaging frequency spectrum and imaging quality. The correctness and validity of the theoretical model are verified in simulation experiment. In the three-beam laser coherent field imaging simulation experiment, the imaging quality is evaluated by the Strehl ratio of the image. Experimental result shows that the Strehl ratio is only related to the light intensity fluctuation of one of the three beams of laser, and the greater the fluctuation of laser intensity, the more serious the degradation of imaging quality is. The research draws the conclusion that the reconstructed imaging frequency spectrum and image quality are mainly affected by the laser intensity fluctuation of the second beam in the three-beam phase closure algorithm, regardless of other two laser intensity fluctuations. Thus, in order to restrain the degradation of imaging quality caused by laser intensity fluctuation, we only need to keep stable the laser intensity of the second beam but not all of the laser beams. In this paper, we reveal the mechanism of laser intensity fluctuation affecting high-resolution imaging quality in the three-beam laser coherent field imaging system. The research provides a theoretical basis for analyzing imaging quality degradation from the laser intensity fluctuation caused by atmospheric turbulence, and reasonably assigning the light intensity stability of multi-beam laser emitter to improve the imaging quality in laser coherent field imaging system.
2017, 66 (24): 244204.
doi: 10.7498/aps.66.244204
Abstract +
In this paper, we report a high-conversion-efficiency organic semiconductor distributed feedback laser. The gain layer of the laser device is made from poly (2-methoxy-5-(20-ethylhexyloxy) p-phenyl-enevinylene) (MEH-PPV), and the holographic polymer dispersed liquid crystal (HPDLC) grating is used as the external light feedback layer. Thus the parameters of the laser device can be modulated independently. The solution of MEH-PPV in xylene (6 mg·mL-1) is deposited on the bottom glass substrate by spin-coating (2000 r/min). The MEH-PPV layer thickness is controlled at (80±2) nm by the spin-coating rate and confirmed by the Dektak profilometer. The HPDLC is made by the photo-induced phase separation method. To determine the orientations of LC molecules, the diffraction efficiency of each sample is measured by a He-Ne laser. The diffraction efficiency is defined as the diffracted light intensity in the first order divided by the incident light intensity. If p light diffraction efficiency (ηp) is much larger (smaller) than s light diffraction efficiency (ηs), it can be thought of as a symbol of a fairly good alignment of LC along the grating vector (grating grooves). When the period of HPDLC grating is larger than 450 nm, ηp is greater than ηs, and the averaged orientation of liquid crystal molecules is aligned along the grating vector direction, i.e., orthogonal to the holographic plane. For feedback light propagating along the grating vector, the refractive index modulation is dependent on the difference between the polymer refractive index np and the ordinary refractive index no of phase-separated LC. These two values are very close to each other, thus the effective light feedback for lasing output is not high. However, when the period of HPDLC grating is smaller than 450 nm, ηs is greater than ηp, and the orientation of phase-separated LC is altered. The refractive index modulation of feedback light originates from the difference between the polymer refractive index np and the extraordinary refractive index ne of phase-separated LC, thus the refractive index modulation can be improved and the HPDLC layer can provide better light feedback. The lasing threshold is 0.70 μJ/pulse, and the conversion efficiency is 2.5% for the sample with a grating period of 593 nm. However, the lasing threshold is lowered to 0.18 μJ/pulse, and the conversion efficiency increases to 6.4% for the sample with a grating period of 395 nm. These results show that the output lasing performance can be improved by using small period grating, since it has bigger refractive index in the grating vector direction (the lasing feedback direction). The laser performance of sample with small grating period is improved in some aspects such as threshold energy, conversion efficiency to some extent compared with those reported previously.
EDITOR'S SUGGESTION
2017, 66 (24): 244205.
doi: 10.7498/aps.66.244205
Abstract +
Continuous variable (CV) quantum entanglement is a fundamental resource of CV quantum communication and quantum computation. It is useful in a wide variety of applications, including quantum teleportation, quantum dense coding, quantum key distribution, and high-precision quantum measurement. In this paper, we generate CV quantum entanglement at a telecommunication wavelength of 1342 nm by using a nondegenerate optical parametric amplifier (NOPA) with a type-Ⅱ periodically poled KTiOPO4 (PPKTP) crystal. A home-made continuous-wave single-frequency dual-wavelength (671 nm and 1342 nm) Nd:YVO4/LiB3O5 laser is achieved with output powers of 1.5 W (671 nm) and 1.3 W (1342 nm). Then a mode cleaner (MC1) with a fineness of 400 and linewidth of 0.75 MHz and a mode cleaner MC2 with a fineness of 400 and linewidth of 0.75 MHz are used to filter the noises of laser at 1342 nm and 671 nm, respectively. By using MCs, the intensity noise of laser reaches a shot noise level (SNL) for analysis frequencies higher than 1.0 MHz, and the phase noise of laser reaches an SNL for analysis frequencies higher than 1.3 MHz. Utilizing this kind of low noise single-frequency 671 nm laser as a pump, a doubly-resonant optical parametric oscillator with a threshold of 325 mW is realised. When the low noise single-frequency 1342 nm laser is injected as a signal and the relative phase between the pump and injected signal is locked to , the NOPA is operated at deamplification. After optimizing the temperature of the type-Ⅱ PPKTP crystal and at a pump power of 260 mW, Einstein-Podolsky-Rosen (EPR)-entangled beams with quantum correlation of 3.0 dB for both the amplitude and phase quadratures are experimentally generated. The strength of EPR-entangled beams is relatively low. It is maybe due to the low nonlinear conversion efficiency and large absorption of the type-Ⅱ PPKTP crystal at 671 nm and 1342 nm. The generated CV quantum entanglement at 1.34 m has lower transmission loss and smaller phase diffusion effect in a silica fiber. The research contributes to a high quality quantum source for the CV quantum communication based on existing telecommunication fiber networks.
2017, 66 (24): 244207.
doi: 10.7498/aps.66.244207
Abstract +
Optical chaos based on semiconductor laser (SL) has some vital applications such as optical chaos secure communication, high-speed physical random number generation, chaos lidar, etc. Among various schemes to drive an SL into chaos, the introduction of external cavity feedback is one of the most popular techniques, which can generate chaos signals with high dimension and complexity. For the chaos output from an external cavity feedback SL, a time-delay signature (TDS) and bandwidth are two key indexes to assess the chaos signal quality. In this work, according to the rate-equation model of an optical feedback SL, we theoretically investigate the characteristics of TDS and effective bandwidth (EWB) of chaotic output from a Gaussian apodized fiber Bragg grating (GAFBG) feedback SL (GAFBGF-SL). The results show that with the increase of feedback strength, the GAFBGF-SL experiences a quasi-periodic route to chaos. Through selecting the suitable feedback strength and the frequency detuning between the Bragg frequency of the GAFBG and the peak frequency of the free-running SL, the TDS of chaotic output from the GAFBGF-SL can be efficiently suppressed to a level below 0.02. Furthermore, by mapping the TDS and EWB in the parameter space of the feedback strength and the frequency detuning between the Bragg frequency of the GAFBG and the peak frequency of the free-running SL, the optimized parameter region, which is suitable for achieving chaotic signal with weak TDS and wide bandwidth, can be determined. We believe that this work will be helpful in acquiring the high quality chaotic signals and relevant applications.
2017, 66 (24): 244210.
doi: 10.7498/aps.66.244210
Abstract +
High energy laser beams propagating in the atmosphere are subjected to a variety of effects, such as the absorption and scattering of molecule and aerosol, atmospheric turbulence effects, thermal blooming effects, and the interaction between turbulence and thermal blooming. In general, these atmospheric propagation effects degrade laser beam quality and reduce the beam power concentration at the target. With adaptive optics compensation, the beam quality can be modified. But small-scale perturbation has developed and the phase compensation becomes unstable in some conditions. The performance of adaptive-optics system is degraded, which effects can be well explained by small-scale linear theory of thermal blooming. However previous theoretical studies of small-scale thermal blooming focused on the Kolmogorov turbulence. In the past decade, experimental evidence has shown significant deviations from Kolmogorov model in certain portions of the atmosphere. An generalized power-law of non-Kolmogorov turbulence model has been introduced, which becomes quite popular in the optical propagation community. Numerous theoretical and developmental efforts have been made based on non-Kolmogorov turbulence model in recent years. Thus it is very meaningful and imperative to explore the theoretical mechanism of high energy laser phase compensation with non-Kolmogorov turbulence.In this study, the Strehl ratio of the thermal blooming phase compensation is generalized with the non-Kolmogorov turbulence spectrum, and the analytical expression is obtained based on the linear theory of small-scale thermal blooming. The influence of the turbulence spectrum on the phase compensation of the high energy laser is analyzed. The results show that the turbulence spectrum has an important influence on the phase compensation of turbulent thermal blooming effect. Under the same turbulence Fresnel number condition, the compensation effect is worse when the spectral index is closer to 3 and the compensation effect is better when the spectral index is close to 4. Under the same atmospheric coherence length condition or under the same turbulence refractive index constant condition, the Strehl ratio decreases with the increase of the thermal blooming effect when the spectral index is close to 3 and the decline rate of the Strehl ratio is slower when the turbulence spectrum index is close to 4. This is because as the turbulence spectrum exponent increases, the logarithmic amplitude fluctuation slows down due to the interaction between turbulence and thermal blooming. These theoretical results can provide some scientific bases and theoretical guidance for the practical applications of high energy laser transmission.
2017, 66 (24): 244203.
doi: 10.7498/aps.66.244203
Abstract +
At different temperatures, a semiconductor laser with a wavelength of 650 nm is used as probe light, and an Nd:YAG continuous laser with a wavelength of 532 nm is selected as pump light. The azo samples are placed between a pair of orthogonal polarizers with the vertical direction clockwise and counterclockwise 45 degrees, respectively. The polarization direction of the pump light is set to be the vertical direction. In order to reduce the effect of the stray light, a chopper is placed in the optical path of the probe light. The signal of photo-induced birefringence is recorded by a phase-locked amplifier (NF-LI5640). The photo-induced birefringences of the doped azo material, the azo polymer and the azo liquid crystal polymer are measured respectively, and the dynamic processes of photo-induced birefringence are fitted by a double e-index model. The experimental results show that with the influence of the pump light, photo-induced birefringences of the three types of azo materials rise rapidly at first and then gradually tend to reach their own saturation state because of the photo-induced cis and trans isomerism and the photo-induced molecular orientation properties of azo molecules. The photo-induced birefringence shows a tendency to increase at first and then decrease with the temperature increasing, which can be understood as a competitive mechanism. The photo-induced birefringence depends on the photo-induced orientation and irregular thermal motions of azo groups. In the range below the glass transition temperature of the samples, the increase of the temperature of samples contributes to the rearrangement of the azo molecules due to the influence of the pump light. When the temperature of the samples is higher than the glass transition temperature, molecular chains begin to move. The irregular thermal motions of azo components and polymer molecules are aggravated. This destroys the orientations of the polymer molecules and results in the drop of the photo-induced birefringence. Comparing the doped azo material with the azo polymer sample, the azo liquid crystal polymer sample exhibits not only a larger photo-birefringence, but also the photo-induced birefringence that does not change obviously after the pump light has been turned off, which means that the azo liquid crystal polymer sample has long optical storage properties. This shows that the azo liquid crystal polymer material is an ideal polarization-sensitive optical recording medium, which is expected to be used in the fields of optical storage, polarization holography and optical information processing.
2017, 66 (24): 244206.
doi: 10.7498/aps.66.244206
Abstract +
Time-delay signature (TDS) and effective bandwidth (EBW) are two key performance indexes to evaluate a chaos signal generated by a laser system including delay-time feedback. In this paper, we propose and simulate a technical scheme to optimize the TDS and EBW of chaotic signal generated by a slave vertical-cavity surface-emitting laser (S-VCSEL) under chaotic optical injection from a master vertical-cavity surface-emitting laser (M-VCSEL), which is subjected to double external-cavity feedback. First, based on the spin-flip model of a VCSEL subjected to two double external-cavity feedback, the time series of two orthogonal polarization components (referred to as X-component (X-PC) and Y-component (Y-PC), respectively) in the M-VCSEL can be obtained. Furthermore, with the help of self-correlation function (SF) analysis method, the TDSs of X-PC and Y-PC can be evaluated. The results show that through selecting suitable system operation parameters, X-PC and Y-PC in the M-VCSEL can simultaneously output chaotic signals with equivalently average intensity and weak TDS. Under optimized operation parameters, the peak values of the SF (σ) of the chaotic signal are 0.20 for X-PC and 0.16 for Y-PC, respectively, and the EBWs of the chaotic signal are 10.72 GHz for X-PC and 10.10 GHz for Y-PC, respectively. The chaotic signals output from the M-VCSEL under optimized operation parameters are injected into the S-VCSEL for further weakening TDS and enhancing EBW. Through examining the evolution rules of TDS and EBW of polarization-resolved chaotic signals in the parameter space composed of injection strength and frequency detuning, the ranges of optimizing injection parameters are determined for achieving two-channel chaotic signals with well suppressed TDS (σ 15 GHz).
2017, 66 (24): 244208.
doi: 10.7498/aps.66.244208
Abstract +
Self-stimulated Raman lasers have attracted more and more interest, because they have no need of additional Raman device, and they are compact in structure and also economical in cost. Self-stimulated Raman lasers are always emitted from crystalline mediums such as Nd3+:KGd(WO4)2, Nd3+:PbWO4 that are commonly used as laser host materials and proved to be available Raman-active mediums. The Nd3+ doped crystals possess high stimulated emission cross-section for laser emission and high Raman gain coefficients for Raman transitions, but the required pump powers (typically hundreds of milliwatts) are large in those experiments.The whispering-gallery mode (WGM) of silica microsphere cavity has achieved the highest Q factor (8×109) to date. The high Q factor and small mode volume make it possible to realize a resonant buildup of high circulating optical intensities, thereby drastically reducing the threshold powers for laser oscillation and stimulated nonlinear process. The coupler with optical fiber taper allows the excitation of WGMs with ultralow coupling loss, which significantly improves the overall efficiency to produce stimulated Raman laser. In this paper, we report the observation of ultralow threshold self-stimulated Raman laser operating in an Nd3+ doped silica microsphere, and the wavelength range can be extended to O-waveband 1143 nm.A high Q microsphere is fabricated with a thin Nd3+ doped silica layer covered by sol-gel method, in which smooth surface is formed by electrical arc-heating. An optical taper fiber is employed to couple the 808 nm laser into Nd3+ doped microsphere (NDSM) to form whispering gallery mode, which acts as the pump light. Based on 4f electron of neodymium ion transmission and optical oscillation in microsphere, the stimulated laser with a wavelength band of 1080 nm-1097 nm is excited. Due to high power density of the excited laser near the surface of orbit in microsphere, the first order self-stimulated Raman laser with a wavelength range of 1120-1143 nm is stimulated in the high Q microsphere. In a theoretical model, the formulas for calculating the output power and the threshold power of the oscillation laser and the self-stimulated Raman scattering are derived. In experiment, we succeed in getting single-mode and multi-mode laser oscillation due to the 4f layer electron transitions of Nd3+ ions, pumped by 808 nm laser. The results show that the NDSM emits a typical single-mode output laser at 1116.8 nm with a pump power of 8.33 dBm, also the relationship between the 1116.8 nm output power and the pump power with a threshold pump power of 3.5 mW. The multi-mode laser spectrum dependent on the microsphere morphology characteristics is observed, which varies by changing the couple position of the optical fiber taper with microsphere. The characteristics of the laser are discussed including the output power, threshold power, spectral line width, side-mode suppression ratio, etc. The NDSM will have many potential applications in new compact lasers. It is beneficial to wavelength converter and optical amplifier in O band.
2017, 66 (24): 244209.
doi: 10.7498/aps.66.244209
Abstract +
Extreme ultra-violet (XUV) light and soft X-ray are widely used to detect the microscopic structure and observe the ultra-fast physical process. It is found that high order harmonic with the frequency as high as that of the waterwindow waves and the pulse duration as short as attosecond can be obtained in the laser-plasma interaction. Due to these features, high order harmonic (HH) is a promising alternative to generating ultra-short XUV light and X-ray. Recently, HHs have been observed in the experiments. However, the frequency spectrum is not complete compared with the results predicted theoretically and numerically. It might relate to the damage of the grating target surface by a long laser repulse. In this article, the effect of target surface roughness on the high order generation (HHG) in the interaction between ultra-intense laser pulse and grating targets is investigated by surface current model and particle-in-cell simulations. We find that both the spatial and spectral domains of harmonics are modulated by the periodical structure of the grating due to the optical interference. The roughness on the surface significantly distorts the modulation effect and leads to different radiation angle and spectral distributions. For the ideal grating, only harmonics satisfying matching condition in a certain direction can be enhanced and the radiation power is restricted in the direction nearly parallel to the target surface. When the surface roughness of the grating target is considered, the matching condition is not valid and the harmonics are scattered into the direction away from the target surface. Comparing with the ideal grating target, most of the harmonic energy is concentrated in the low order harmonics and the intensities of the harmonics decrease rapidly with increasing HH order when surface roughness is considered. The results show good consistence with the phenomena observed in previous experiments and provide the technical reference for exploring the future applications of HHs and HHG.
2017, 66 (24): 244211.
doi: 10.7498/aps.66.244211
Abstract +
The ground-state structural optimization and the terahertz spectrum calculation of an organic electro-optical crystal of 4-N, N-dimethylamino-4'-N'-methyl-stilbazolium tosylate (DAST) are performed using dispersion-corrected density functional theory (DFT-D2). DAST consists of an organic pyridinium salt (cation), one of the most efficient non-linear optical active chromophores and a sulfonate (anion) for enhancing the stability of the noncentrosymmetric macroscopic crystal. Such an organic crystalline salt DAST exhibits highly electro-optical and nonlinear optical coefficients, and it is an efficient emitter of THz pulses. The steady ground-state structure of DAST is obtained by a step-by-step optimization method with gradually increasing the convergence accuracy. The calculated terahertz spectra in 0-4 THz are in good agreement with experimental measurements, implying the reasonability of DFT-D2 method. Moreover, the vibration displacement vector diagrams for DAST molecular structure are obtained using Cambridge sequential total energy package animation simulation function. The results indicate that the phonon modes of DAST crystal at 1.12 THz are attributed to the optical phonon modes of the anion and cation, and DAST cation (organic pyridinium salt) and anion (sulfonate) undergo translational vibrations in their respective (benzene ring) plane. In contrast the vibrations at 1.46 THz and 1.54 THz are mainly related to the vibration of the sulfonate, among which 1.46 THz vibration is caused by the rotation of the sulfonate along the a axis, while 1.54 THz is due to the motion of the whole sulfonate along the c axis. And the vibrations at 2.63 THz and 3.16 THz originate from the torsional vibrations of cations and the rotation of anions, respectively. The results presented in this work clearly illustrate the contributions of the anion and cation of DAST in the THz responses. The mode assignments provide important reference and guidance for further synthesis of new DAST derivatives with larger electro-optical coefficients. In particular, our results suggest that DFT method is a powerful theoretical tool for studying the THz photonics and it is helpful not only for better understanding the mechanisms of the THz responses of organic electro-optic crystals, but also for controlling their performances.
EDITOR'S SUGGESTION
2017, 66 (24): 244301.
doi: 10.7498/aps.66.244301
Abstract +
An acoustic focusing lens based on a coiling-up space structure with near-zero refractive index is studied. According to the direction selection mechanism for acoustic waves in a near-zero refractive index material, we adopt the coiling-up space structure as a basic unit for arrangement, and design a geometric structure with specific incident and outgoing interfaces which is used to manipulate the outgoing direction of transmitted wave. Thus, the focusing effects for plane acoustic wave and cylindrical acoustic wave are realized. Besides, the influences of rigid scatterers inside the lens on the focusing performance are also discussed in detail. Moreover, the shape and direction of the acoustic waveform can be manipulated accurately by changing the outgoing interface of the lens with the near-zero refractive index. The results show that the lens with a single and two circular surfaces could realize the focusing effects of the plane and cylindrical acoustic waves, respectively, and the rigid scatterers inside the lens have no effects on the focusing performance. In addition, the cylindrical acoustic wave could be transformed into the plane acoustic wave through the lens with the circular incident surface and the plane exit surface, and the inclined angle of the exit surface could be used to manipulate the propagation direction of the plane wave. The simulation results between the lenses composed of the coiling-up space structure and the effective medium are in good agreement with each other. This type of lens has the advantages of single cell structure, high focusing performance, and high robustness. This work provides theoretical guidance and experimental reference for designing a novel acoustic focusing lens with the near-zero refractive index, and offers a new idea for studying the manipulation of the acoustic waveforms.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2017, 66 (24): 246101.
doi: 10.7498/aps.66.246101
Abstract +
In this paper, magneto-electronic properties of zigzag graphene nanoribbons (ZGNR) doped with triangular boron nitride (BN) segments are investigated by using first-principles method based on density functional theory. It is shown that in the nonmagnetic state, the ZGNRs doped with triangular BN segments at different positions are metals. In the ferromagnetic state, with the impurities moving from one edge of the nanoribbon to the other edge, a transition is caused from a spin metal to a spin half-metal, and then to spin semiconductor, and as long as the impurity is not on the edge of the nanoribbon, the doped ZGNR is always spin half-metal. In the antiferromagnetic state, the ZGNR doped in the middle of the nanoribbon is spin metal, while the ZGNR doped on the edge of the nanoribbon has no antiferromagnetic state. The electronic structures of the ZGNRs doped with BN segments at different positions are explained by the difference in charge density. The binding energies of doped ZGNRs are negative, thus the structures of the doped ZGNRs are stable. As the impurity moves from position P1 to position P5, the binding energy decreases gradually. When the impurity is located at position P5, the binding energy of ZGNR is smallest, and the structure of ZGNR is most stable. When the impurity doped in the middle of the nanoribbon, the antiferromagnetic state is the ground state, while the impurity is doped on the edge of the nanoribbon, the ferromagnetic state is the ground state. These obtained results are of significance for developing electronic nanodevices based on graphene.
2017, 66 (24): 246801.
doi: 10.7498/aps.66.246801
Abstract +
With the development of modern industrial technology, tungsten products prepared from traditional tungsten powder cannot meet the demands of industry. However, the properties of tungsten products produced from ultra-fine tungsten powder have been greatly improved:they have high strength, high toughness, and low metal plasticity-brittle transition temperature. Hence, it is necessary to carry out theoretical research of the micro-adsorption dynamics during hydrogen reduction of W20O58, which is beneficial to synthetizing ultra-fine tungsten powder. In this article, to comprehend the crystal characteristics of W20O58 (010) surface and provide the theoretical reaction law for hydrogen reduction on W20O58 (010) surface, the absorption mechanism of H2 molecule on W20O58 (010) surface is studied by the first-principles calculation based on density functional theory in a plane wave pseudo-potential framework. The results show that the indirect band gap of W20O58 is 0.8 eV, indicating that it has metallic characteristic. The W20O58 (010) surface has different terminations, i.e., WO-terminated (010) surface and O-terminated (010) surface. After the geometrical optimization of the two surfaces, the W–O bond length and bond angle of W–O–W are both changed. In addition, six absorption configurations of H2 on W20O58 (010) surface, including WO-L-O1c, WO-V-O1c, WO-L-O2c, WO-V-O2c, O-L-O1c and O-V-O1c, are chosen to be investigated. The calculation results show that the WO-L-O1c, WO-V-O1c and WO-L-O2c absorption system are unstable, while the WO-V-O2c, O-L-O1c and O-V-O1c absorption configuration are stable. When H2 molecule is dissociated into two H atoms, the absorption energies of the three stable configurations are-1.164 eV,-1.021 eV and-3.11 eV, respectively. It is obvious that the O-V-O1c absorption configuration is the most stable one. The analysis of density of states reveals that the 1s state of H atom interacts with the 2p and 2s states of O atom. The outermost O1c atom of O-terminated (010) surface contains an unsaturated bond, which results in the formation of bonding between two H atoms and O1c atom. As a result, an H2O molecule is formed and an oxygen vacancy on the surface is generated after absorption reaction. By combining experimental observations with simulation calculations, the mechanism of hydrogen reduction of W20O58 can be revealed from a microscopic view.
2017, 66 (24): 246102.
doi: 10.7498/aps.66.246102
Abstract +
Based on Monte-Carlo method, the characteristics and physical mechanisms for deposited-energy spectra in sensitive volume (SV), single event upset cross sections, and on-orbit error rates in 65-32 nm silicon-on-insulator static random access memory (SOI SRAM) devices induced by space energetic ions are investigated. Space ions on geostationary earth orbit exhibit a flux peak at an energy point of about 200 MeV/n. In consequence, the single event response of nanometric SOI SRAMs under 200 MeV/n heavy ions is studied in detail. The results show that 200 MeV/n space ions exhibit the large straggling of deposited-energy in the device SV with thickness ranging from 60 nm to 40 nm, which causes the single event upsets to occur in the sub-LETmth region. The device SV can only partially collect the electron-hole pairs in the single ion track with a wide distribution of secondary electrons. As a result, the maximum and average deposited-energy in the SV decrease by 25% and 33.3%, respectively. Further, the single event upset probability decreases and the on-orbit error rate decreases by about 80%. With the downscaling of feature size, the per-bit saturated cross sections and on-orbit error rates of nanometric SOI SRAM devices decrease dramatically. The phenomenon of constant-increasing single event upset cross section with higher ion linear energy transfer (LET) is not observed, owing to the fact that (a) the density of electron-hole pairs in the track of 200 MeV/n space ion is relatively low and (b) the SOI device has thin sensitive volume, which results in the fact that the secondary-electron effect cannot upset nearby sensitive cells. Besides, it is found that the direct-ionization process of trapped protons leads to an increase of on-orbit error rate of 65 nm SOI SRAM by one to two orders of magnitude.
2017, 66 (24): 246301.
doi: 10.7498/aps.66.246301
Abstract +
The effects of torsional deformation on the structural stability, the electronic structures and the optical properties, including adsorption energy, band gap, absorption coefficient and reflectivity of O atom adsorbed graphene are studied by using the first-principles calculations. Our results indicate that the C atom closest to O atom is pulled up, causing the graphene plane to be distorted after the O atom has been adsorbed. The adsorption energy calculations show that due to the adsorption of O atom, the structural stability of graphene system decreases, but the degree of torsion has a weak effect on the structural stability. The analysis of band structure shows that the adsorption of O atom causes the graphene to convert into a semiconductor from a metal. Torsional deformation makes it change from a semiconductor to a metal, and to a semiconductor. The O atom adsorption system with a torsion angle of 12° has an indirect band gap but the band gaps of other systems are all direct bandgaps. Compared with the intrinsic graphene torsion system, the adsorbed O atom system has an electronic structure that is less sensitive to torsional deformation. When the torsion angle changes from 10° to 16°, the bandgap is always stable at around 0.11 eV. And the adsorption system always corresponds to a narrow bandgap semiconductor in this torsion angle range. For optical properties, comparing with the O atoms adsorbed on graphene with the 0° torsion angle, the peaks of the absorption coefficient and the reflectivity of the system are reduced, and have a transform of red shift into blue shift in a torsion angle ranging from 2° to 20°.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2017, 66 (24): 247203.
doi: 10.7498/aps.66.247203
Abstract +
Gallium nitride (GaN) and its family of materials (including GaN, InN, AlN and their alloys) are known as the third generation of semiconductor, which has important applications in optoelectronics and microelectronics. In the structure of GaN-based high electron mobility transistor (HEMT) device, there is a relatively large conduction band offset in the AlGaN/GaN heterojunction structure, and it can produce a strong spontaneous and piezoelectric polarization effect in the vicinity of the heterojunction, which can also accumulate high concentrations of two-dimensional electron gas (2DEG) under the condition of no need of intentionally doping at the interface. The surface of Heterojunction AlGaN/GaN interface will form a 2DEG channel, and the 2DEG in potential well is controlled by the gate voltage, also the 2DEG layer is very close to the surface, which is sensitive to the state of the surface. When the surface state changes, it can cause a change in the 2DEG density, thus the concentration of 2DEG can be adjusted by changing the surface states, thereby changing the current between the source and drain. GaN-based HEMT serves as a gate control device, which has a high concentration of 2DEG and is sensitive to the surface state at the AlGaN/GaN heterojunction. According to the basic structure and advantages of the GaN-based HEMT device, the ferroelectric thin film PZT is deposited on the metal gate serving as a light sensitive layer. When the light is incident on the gate, the photo-sensing layer PZT generates the photovoltaic effect, which causes the surface charge of the photosensitive layer to change, and also causes the 2DEG to change, so the input current changes. In this paper, firstly, a new “M/F/M/S” structure is proposed by introducing a photosensitive material PZT on a GaN-based HEMT gate electrode and combining the PZT of a ferroelectric thin film with photovoltaic effect. Secondly, the HEMT device is fabricated on the AlGaN/GaN epitaxial wafer of sapphire substrate, and the photosensitive unit PZT is prepared on the gate, and thus the HEMT device with photosensitive is realized. Finally, the carrier concentration in the channel is regulated by the photovoltaic effect of PZT and 365 nm UV and visible light are detected through changing the source-drain current. The comparative tests under the conditions with and without a photosensitive gate HEMT device show that when the voltage Vgs is smaller, the saturation drain-source current Ids measured under the irradiation of visible light in the former condition is not reduced compared with that in the latter condition, and the increment of Ids measured in the former condition is 5.2 mA larger than in the latter condition. Therefore it can be seen that the PZT can act on the gate GaN-based HEMT device under the irradiation of visible and ultraviolet light and adjust the channel current.
2017, 66 (24): 247301.
doi: 10.7498/aps.66.247301
Abstract +
In recent years, spintronic devices have attracted more and more attention because of their good characteristics. The spin-orbit coupling of HgCdTe is one of the most important parts in the study of narrow gap semiconductors. The magneotransport properties of the Hg0.9Cd0.1Te bulk material with an inverted band structure have been hardly reported so far. The spin-orbit coupling strength of HgCdTe is closely related to the band gap. The strength of the spin-orbit coupling increases with the width of the band gap decreasing. Thus, Hg0.9Cd0.1Te should have strong spin-orbit coupling. Meanwhile it should be one of the most suitable materials to fabricate spintronic devices. The main propose of our experiments is to prove this inference. Inside the sample, Rashba spin-orbit interaction (SOI) strongly influences the spin-splitting due to the lack of structural inversion symmetry. In other words, Rashba SOI is the main part of the zero field spin splitting △0. The band structure of Hg1-xCdxTe can be precisely tuned by changing the composition of Cd which keeps an inverted band order when 0 x Γ8 band lying below the Γ6 band (or equivalently a positive band gap) when x0.165. In this paper, the p-type HgCdTe bulk material with Cd component of 0.1 is grown by single crystal. Anodic oxidation is used to induce an inversion layer on the HgCdTe bulk, and indium is used to facilitate Ohmic contacts. The magnetoresistance is measured in the van der Pauw configuration, and the magnetic field is applied perpendicularly to the film. All measurements are carried out in an Oxford Instruments He cryogenic system. At 1.5 K and zero gate voltage, the carrier density n is 1.3×1016 m-2. Clear Shubnikov-de Haas (SdH) oscillation in ρxx and quantum Hall plateaus of Rxy are observed in the Hg0.9Cd0.1Te bulk material with an inverted band structure is investigated in magnetotransport experiment. This indicates that our sample is a good transistor. Fast Fourier transformation is used to deduce the zero-field spin-splitting △0 which is about 26.55 meV. By studying the beating patterns in SdH oscillations we find that the effective g-factor is about-11.54. Both the large zero field spin splitting and the negative effective g-factor suggest that Hg0.9Cd0.1Te has really strong spin-orbit coupling. The investigation of SOI in Hg0.9Cd0.1Te can increase our knowledge of Hg-based narrow-gap semiconductors and benefit the field of spintronics.
2017, 66 (24): 247701.
doi: 10.7498/aps.66.247701
Abstract +
We obtain the energy eigenvalues and wave functions of the single layer molybdenum disulfide by using an effective Hamiltonian. Moreover, the density of states and high electron-electron screening length up to 108 cm-1 are also evaluated based on the dielectric function of MoS2. It is shown that the quasi-linear energy bands split off due to the spin-orbit couplings. Plasmons in such a system are investigated theoretically within diagrammatic self-consistent field theory. In the random phase approximation, it is found that two plasma spectra can be produced via intra band transitions induced in conduction bands in monolayer MoS2 because of splitting off. The plasma spectrum frequency increases with increasing wave-vector q and electron density. It is found that the two plasmon modes induced by the spin intra-subband transitions are acoustic-like and depend strongly on wave-vector q. We find that the plasma spectrum is very different from those of graphene and two-dimensional electron gas due to the quasi-linear dispersion and spin-orbit couplings in single layer MoS2. Moreover, the plasmon frequency can be effectively controlled through changing the doping electron density. Our results exhibit some interesting features which can be utilized to realize the plasmonic devices based on the single layer MoS2.
2017, 66 (24): 247201.
doi: 10.7498/aps.66.247201
Abstract +
The polysilicon thin film piezoresistors are widely used in semiconductor pressure sensors. The polysilicon thin film has good piezoresistance properties, which are determined by the grain structure and doping concentration. The gauge factor of the polysilicon thin film is usually modified according to the relationship between gauge factor and doping concentration. The polysilicon thin films are classified into common polysilicon thin films and polysilicon nanofilms according to their thickness. The common polysilicon thin film thickness is more than 0.3 μm, which has good temperature characteristic, but its piezoresistance coefficient is small. However, the polysilicon nanofilm thickness is less than 0.1 μm, which has good temperature characteristic and high piezoresistance coefficient. The existing piezoresistance theory of the common polysilicon thin film cannot explain reasonably the experimental result of polysilicon nanofilm piezoresistance. Therefore, the tunneling piezoresistance model and an algorithm for the p-type polysilicon nanofilm piezoresistance coefficient were established in 2006. However, this algorithm presents an incomplete fundamental piezoresistance coefficient. In order to improve the polysilicon thin film piezoresistance theory, based on the tunneling piezoresistance model and the mechanism of silicon and the valence band hole conductivity mass with the change of stress, a novel algorithm for the piezoresistance coefficient of the p-type polysilicon thin film is presented. The theoretical formulas for three fundamental piezoresistance coefficients π11, π12 and π44 of the grain neutral and grain boundary regions, are presented respectively. With these formulas for the coefficients, the longitudinal and transverse piezoresistance coefficients for arbitrary crystal direction texture polysilicon can be obtained. According to the structure characteristics, the gauge factors of the p-type polysilicon nanofilm and the common polysilicon thin film are calculated, and then the longitudinal and transverse gauge factors are plotted each as a function of doping concentration, which are compared with the experimental results. According to the experimental results of the polysilicon nanofilm, the grain size is L=30 nm, the grain crystal directions are randomly distributed. The trap density in grain boundary region is Nt=1013 cm-2, the Young's modulus of elastic diaphragm is Y=1.69×1011 Pa, the Poisson ratio of elastic diaphragm is ν=0.062, the grain boundary width is δ=1 nm, and the thickness is 80 nm. The comparison indicates that the gauge factor average error between calculation and experiment is 0.5 times less than the average experimental difference between the maximum and the minimum for each doping concentration. For the common polysilicon thin film, according to the experimental results, its grain size L is 135 nm, thickness is 400 nm, the orientations of crystal grain neutral region are[311],[111] and[110] in the ratio of 49:31:20, i.e., 〈311〉:〈111〉:〈110〉=49:31:20, and the gauge factor calculated result is also good agreement with the experimental result. Therefore, the proposed algorithm is comprehensive and accurate, which is applicable to the p-type common polysilicon film and the polysilicon nanofilm.
2017, 66 (24): 247202.
doi: 10.7498/aps.66.247202
Abstract +
The electronic and the electrical properties of the Sr doped CaMnO3 oxide for Ca site are studied by the density funtional theory calculation method. The Sr doped CaMnO3 oxide bulk samples are prepared by the citrate acid sol-gel method as well as the ceramic preparation method, and the thermoelectric transport properties are analyzed. The results show that the Sr doped CaMnO3 oxide still has the indirect band gap yet with the band gap energy slightly decreasing from 0.756 eV to 0.711 eV. The effective mass of carrier near Fermi level is modified and the carrier density near Fermi level is also increased. The ability to release electrons of Sr is stronger than that of the Ca, and the Sr acts as n-type donor doping specy within the CaMnO3 compound. The electrical resistivity values remarkably decrease for the Sr doped CaMnO3 oxide materials. The Seebeck coefficient increases slightly to a certain extent compared with that of the intrinsic CaMnO3. The resistivity values for the Ca1-xSrxMnO3 (x=0.06, 0.12) samples at 373 K decrease to 25% and 21% of the un-doped intrinsic CaMnO3 sample, respectively. The Seebeck coefficients for the Ca1-xSrxMnO3 (x=0.06, 0.12) samples at 373 K increase to as high as 112.9% and 111.1% of the Seebeck coefficient for un-doped intrinsic sample, respectively. The thermoelectric performance is effectively enhanced by Sr doping for the CaMnO3 oxide material.
2017, 66 (24): 247302.
doi: 10.7498/aps.66.247302
Abstract +
The relatively low breakdown voltage (BV) seriously restricts the high power application of GaN based high electron mobility transistors (HEMTs). In this work, a novel AlGaN/GaN HEMT with buried p-n junctions is investigated to improve the breakdown characteristics by introducing six equidistant p-GaN islands buried buffer layer (PIBL) into the n-GaN epitaxial layer. The p-GaN islands act as reversed p-n junctions, which produces new electric field peaks at the edges of p-GaN islands, then realizing a much high breakdown voltage, and the reversed p-n junctions can help to suppress punch-through effect in buffer layer. Furthermore, the characteristics of proposed device are analyzed in detail from the aspects of off-state I-V characteristics, equipotential contour distribution, off-state electric field distribution, offstate carrier distribution and output characteristics. Simulated equipotential contour distribution shows that under the condition of high-voltage blocking state, multiple reverse p-n junctions introduced by the buried p-GaN islands produce five new electric field peaks, realizing a more uniform equipotential contour distribution especially at the edges of the buried p-islands. Then off-state electric field distribution demonstrates that p-GaN islands modulate the surface and bulk electric fields, which makes the voltage distributed in a larger area, therefore presenting a much higher breakdown voltage. It can be seen from off-state carrier distribution that the electrons in the buffer layer fully depleted in PIBL HEMT effectively suppress the buffer leakage current, thus alleviating the buffer-leakage-induced impact ionization leading to a high breakdown BV of over 1700 V with gate-to-drain length of 10μm, which is nearly 3 times larger than BV of 580 V in conventional AlGaN/GaN HEMT. Although, the introduction of p-type buried layer narrows the current path and causes an improved on-resistance, simulation shows that the specific on-resistance (Ron,sp) of PIBL HEMT is only about 1.47 mΩ·cm2, while the BV of the PIBL device is over 1700 V, and the obtained figure of merit (FOM=BV2/Ron,sp) reaches as high as 1966 MW·cm-2. The optimization of device structure reveals that when the distance between p-GaN layer and AlGaN layer (t) is 0.2μm, a thinner buried p-GaN island (tp) should help to realize a more significant electric field modulation, and PIBL HEMT can achieve a maximum BV of 1789 V with a tp=0.1μm. Compared with the traditional AlGaN/GaN HEMT, the PIBL HEMT reveals a higher breakdown voltage, meanwhile ensuring low Ron,sp, which makes this structure a promising candidate in the applications of high power electronic devices.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
EDITOR'S SUGGESTION
2017, 66 (24): 248201.
doi: 10.7498/aps.66.248201
Abstract +
Study of the relationship between conformation and photophysics of individual -conjugated polymer chain is one of the most important problems in polymer nanoscience and nanotechnology, which will facilitate the application of conjugated polymer in a range of electronic devices such as organic field-effect transistors, light-emitting diodes, and solar cells. Single-molecule spectroscopy has emerged as a powerful tool to unravel structure and dynamic heterogeneities that are hidden in ensemble average. Identification of the emitting segments through fluorescence of single conjugated polymer molecules and their dependence on the conformation can help reveal the mechanism and the extent of energy transfer process in a single polymer chain. In this paper, the photophysical properties of individual poly[2, 7-(9, 9-dioctylfluorene)-alt-4, 7-bis(thiophen-2-yl) benzo-2, 1, 3-thiadiazole] (PFO-DBT) conjugated polymer molecules are measured based on the defocused wide-field microscopy of single molecules. The single PFO-DBT molecules are prepared on cleaned glass coverslips by spin-coating solution of poly[methyl methacrylate] (PMMA) containing 110-9 mol/L PFO-DBT molecules in chloroform and toluene, respectively. Defocused imaging of single conjugated polymer molecule is performed based on a wide-field fluorescence microscope system. The change of defocused patterns of individual polymer chain maps the angular distribution of emitted chromophore and thus the emitting dipole orientation. Fluorescence trajectory and corresponding emission dipole moments of single conjugated polymer molecules are analyzed to identify the emitting conjugated segments. It is found that single PFO-DBT conjugated polymer molecules prepared by chloroform solvent show extended conformation. The intrachain energy transfer is dominant in the single conjugated polymer molecules that take extended conformation, which leads to photophysical properties of multiple chromophores. In contrast, single PFO-DBT conjugated polymer molecules prepared by toluene solvent hold folded conformation, which exhibit emission from single chromophore due to efficient interchain energy transfer. The emitting chromophore is not constant in a single PFO-DBT conjugated polymer molecule with folded conformation. About 35% of the single conjugated molecules prepared with toluene show only one constant emitting chromophore before photobleaching. However, about 65% of single conjugated polymer molecules prepared with toluene show two or more sequencely emitting chromophores. It can be concluded that the energy transfer properties of single PFO-DBT conjugated polymer molecule is greatly dependent on the conformation, which can be reflected in its photophysical properties. The study on the influence of single conjugated polymer conformation on energy transfer efficiency can provide the reference for the preparation and performance of optoelectronic devices and molecular devices based on conjugated polymer.
2017, 66 (24): 248202.
doi: 10.7498/aps.66.248202
Abstract +
Supercapacitor is a new-type energy storage device with the promising application prospect, and its development mainly relies on the development of electrode materials. In this work, a series of nickel-cobalt (Ni-Co) layered double hydroxides is synthesized via a simple hydrothermal method by using nickel and cobalt salts with four different anions (including sulfate, chlorate, acetate and nitrate) serving as nickel and cobalt sources. According to the types of salts, the obtained samples are named Ni-Co(SO4), Ni-Co(Cl), Ni-Co(Ac) and Ni-Co(NO3), respectively. The morphology and structure of Ni-Co layered double hydroxide are characterized by X-ray diffraction and scanning electron microscopy (SEM), respectively, and the electrochemical properties of the sample are investigated by CHI660D electrochemical workstation in 2 M KOH aqueous solution. The results demonstrate that the types of nickel and cobalt salts not only affect the morphology and structure of Ni-Co layered double hydroxide, but also significantly influence the electrochemical properties of the sample. The SEM images show that the Ni-Co layered double hydroxide synthesized with nickel sulfate and cobalt sulfate (Ni-Co(SO4)) possesses loose layer structure, which can provide abundant active sites and benefit the diffusion of electrolyte. The electrochemical test results show that the specific capacitances of Ni-Co(SO4), Ni-Co(Cl), Ni-Co(Ac) and Ni-Co(NO3) under a current density of 1 A/g at a potential window of 0.45 V, are 1551.1 F/g, 440.7 F/g, 337.8 F/g and 141.6 F/g respectively. As the current density increases from 1 A/g to 7 A/g, the capacitive retention rates of Ni-Co(SO4), Ni-Co(Cl), Ni-Co(Ac) and Ni-Co(NO3) are kept at 60.1%, 21.7%, 4.6% and 6.0%, respectively. The results of alternating current (AC) impedance test display that the electron transfer resistance follows an increasing trend:R[Ni-Co(SO4)] R[Ni-Co(Cl)] R[Ni-Co(Ac)] R[Ni-Co(NO3)]. The small electron transfer resistance is conducive to excellent capacitance at the high current density. Therefore, the excellent capacitive performance of the sample Ni-Co(SO4) is ascribed to the loose layer structure and low electron transfer resistance. In addition, the cycling stabilities of the samples are investigated by constant current charge-discharge test. The capacitive value of the sample Ni-Co(SO4) declines by 16% for 1000 cycles at a current density of 7 A/g. The capacitance decrease can be ascribed to the damage to the layered structure and the increase of the electron transfer resistance in the multiple constant current charge-discharge processes as shown in the results of SEM and AC impedance before and after cycle. This study provides a foundation for exploiting and utilizing high-performance nickel-cobalt layered double hydroxides as electrode material of supercapacitor.
2017, 66 (24): 248801.
doi: 10.7498/aps.66.248801
Abstract +
Solar thermophotovoltaic (STPV) generator is a popular energy converter due to providing low noise, low thermal mechanical stress and portability. It has the ability to exceed the efficiency of pure solar photovoltaic system. An idealized STPV generator is a reversible heat-engine, offering a theoretical efficiency of over 80%, but the actual conversion efficiency of STPV generator is still low due to the mistuned spectral property between the thermal selective emitter and the TPV cell. One key issue in developing the STPV generator with high performance is the spectral matching between the thermal radiation spectrum of radiator and the spectral response of photovoltaic cell in visible and near-infrared region, which usually lies between the visible and the near-infrared region. High-temperature spectral emissivity of rare earth oxide is of special interest, because the radiation has a narrow band of wavelengths in the near infrared and infrared region from 900 to 3000 nm. In this work, the thermal-selective film Er2O3 emitter is fabricated by post-oxidation of Er film deposited on Si substrate through using electron-beam gun evaporation. Based on the X-ray diffraction results, the Er2O3 film is of cubic phase structure and well-crystallized when the oxidation temperature is 700℃, and the Si substrate has no obvious influence on the crystal structure of Er2O3 film. According to the X-ray photoelectron spectroscopy results of the Er2O3 film after thermal oxidation at 700℃, the atomic ratio of Er/O is stoichiometric. In order to obtain the selective emission characteristic of the Er2O3 film, a measurement system is designed. The system consists of two major portions, i.e., one is a near infrared spectrometer purchased from Ocean Optics, the other is a high-temperature emission characteristic tester which can provide oxyhydrogen flame to heat the sample by using an electronic impulse ignition to torch the hydrogen-oxygen mixture. The oxyhydrogen flame passes through the nozzle and sprays vertically on the surface of the thermal-selective emitter sample. The facula of the oxyhydrogen flame convergence is very small (facula diameter:~0.8 cm), and the highest temperature achieved is about 2500℃. The measurement condition of selective emission performance of the Er2O3 film emitter coincides with the application characteristic of STPV generator. The emission performance result of the film emitter at 700℃ shows a typical gray-body emission characteristic. The measurements carried out at 900 and 1100℃ show that the Er2O3 film has a distinct emittance spectrum at 1550 nm corresponding to Er3+, and the intensity of the selective emission peak strengthens with the measuring temperature or film thickness increasing. The thermal-selective film Er2O3 emitter is found to have emission spectrum suitable for efficient matching with the infrared response of GaSb photovoltaic cell.
2017, 66 (24): 248101.
doi: 10.7498/aps.66.248101
Abstract +
Due to the weak van der Waals interaction between GaN epitaxial layer and graphene substrate, GaN grown on graphene has attracted considerable attention in recent years, benefited from the possibility to grow epitaxial material without any necessity to satisfy the requirement for the lattice matching between the epitaxial materials and underlying materials, and the unique facility of transferring GaN epitaxy to other substrates. However, clusters formed in GaN grown on graphene lead to poor crystalline quality, deteriorating the applications of GaN epilayer on graphene. It is observed that preferential nucleation occurs primarily at the sites of defects and along the step edges of graphene. In order to study the effects of NH3/H2 ratio on the graphene/sapphire template and properties of GaN epilayer, the growth of GaN by metal organic chemical vapor deposition on the graphene/sapphire template pretreated with the mixed gas of NH3 and H2 is investigated.Prior to the deposition of GaN, five samples with different NH3/H2 flow ratios (0, 0.2, 0.5, 1 and 2, respectively) are pretreated at 1030℃ while the H2 flow rate is fixed at 3.6 mol/min. The surface topographies and Raman spectra of the pretreated graphene are investigated, and the chemical reaction mechanism is studied. It is found that the graphene is etched at the wrinkle firstly and then along the direction of wrinkles where there is bigger contact interface with NH3 and H2, and graphene decomposition is enhanced with the increase of NH3/H2 flow ratio. The pretreatment mechanisms of different mixed gases are also discussed. Owing to the weak bond energy, NH3 is easier to decompose than H2. The reaction between graphene and H, NH2 which are produced by the decomposition of NH3, enhances the etching of graphene.Finally GaN film is deposited on graphene/sapphire template pretreated by different NH3/H2 flow ratios. The quality of GaN was improved on graphene pretreated by appropriate NH3/H2 flow ratio and verified through highresolution X-ray diffraction.The lowest (002) and (102) full widths at half maximum (FWHM) of GaN obtained on graphene/sapphire template are 587 arcsec and 707 arcsec respectively, while the root-mean-square (RMS) of GaN is 0.37 nm. The stress of GaN is characterized by Raman spectra at room temperature. The co-presence of characteristic peaks of sapphire, graphene and GaN suggests that GaN has deposited on graphene/sapphire template. The E2-high Raman peak is used to estimate the residual stress in GaN material as described elsewhere. The E2-high peak of GaN grown on graphene is around 566.7 cm-1, while the value of strain-free GaN is 566.2 cm-1. Thus, there is subtle compressive stress in the GaN grown on graphene, which can be calculated from the relationship:△ωγ=4.3·σχχ cm-1·GPa-1, giving a value of 0.11 GPa of GaN obtained on graphene/sapphire template.This study provides an effective pretreatment technique to improve the crystal quality of GaN epilayer deposited on graphene/sapphire template, which gives guidance in well crystallizing three-dimensional materials on two-dimensional materials.
2017, 66 (24): 248401.
doi: 10.7498/aps.66.248401
Abstract +
Magnetic resonance imaging (MRI) has been a primary diagnostic technique due to its high imaging quality, noninvasion and non-radiation capacity. However, the application of conventional whole body MRI is restricted by its massive size, high installation and management cost. Dedicated MRI overcomes the shortcomings of whole body MRI and has great importance in medical diagnosis. The challenge is that the design of superconducting magnet for extremity MRI is largely constrained by physiological structure of human body. As a result, a limited longitudinal length with high field homogeneity in a 160 mm diameter sphere volume (DSV) is required for superconducting magnet of extremity MRI. In this article, a non-shielded 1.5 T extremity dedicated superconducting magnet is designed by using both 0-1 integer programming and genetic algorithm and fabricated with a comprehensive consideration of superconductivity wire consumption, central magnetic field intensity and imaging region homogeneity. The NbTi superconducting wire is chosen for coil winding, and copper-to-superconducting ratio of the wire is 1.3. The sizes of cross-section of the bare wire and the insulated wire are 0.75 mm×1.20 mm and 0.83 mm×1.28 mm respectively, and the critical currents at 4.2 K and 5 T are both about 935 A.According to the size constraint of the magnet, we first calculate the current carrying zone of the superconducting coils and divide it into grid elements with parallel current. The size of each grid element is equal to that of the superconducting wire, and the distribution of non-rectangular coils is obtained by using 0-1 integer programming. In order to obtain a higher homogeneity of magnetic field, a reverse current zone is manually created in the wide blank area of the feasible current carrying zone. Using the results above, we then optimize the distribution of coils and build a rectangular model which facilitates the fabrication by using genetic algorithm. The inductance of the magnet is 1.8094 H, the operating current is 402.09 A, the stored energy is 146.27 kJ and the peak magnetic field of current carrying zone is 5.48 T. The calculated peak-to-peak homogeneity in 160 mm DSV is about 22 ppm. Taking into consideration the factors such as mechanical error and cold shrinkage, the estimated homogeneity would reach 60 ppm (peak-to-peak) with passive shimming.The 1.5 T extremity dedicated superconducting magnet is successfully fabricated through a series of processes such as winding, curing, assembly and welding. The prototype magnet has a room temperature bore of 280 mm in diameter and a total length of 520 mm, and the volume of liquid helium vessel is about 30 liters. To reduce the evaporation of liquid helium, a 1.5 Watt two-stage Gifford-McMahon refrigerator is employed to cool the system and maintain the evaporation rate of Helium at zero level. The range of 5 Gauss line of the magnet is 3.2 m in the radial direction and 2.6 m in the axial direction. Moreover, the magnet is magnetized to 1.5 T after being conditioned three times and the measured homogeneity in 160 mm DSV achieves 55 ppm (peak-to-peak) and 3.4 ppm (Vrms) after passive shimming using silicon steel pieces.
2017, 66 (24): 248901.
doi: 10.7498/aps.66.248901
Abstract +
The optimization of aviation networks is of great significance for optimizing the allocation of resources, improving transport efficiency, and enhancing the competitiveness among airline companies. There have been a lot of researches which combine the theory of complex network and the actual situations to analyze the air transportation system. The present work provides a certain theoretical basis for the plan of airline schedule. Firstly, we regard an airport as a node, flight frequency as a link weight, and build a heterogeneous network. Through empirical analysis, we find that the aviation network has small-world and scale-free properties. In addition, considering that the instant network consists of current flights changing over time, time-varying is another important characteristic of aviation network. Also, a spatiotemporal correspondence between the flight frequency and route geometric distance is demonstrated to be τij~rij-C. Secondly, by Monte Carlo simulation, we know that the time-ordered topologies influence the optimal navigation structure and make it different from those from traditional static models. Specially, we can obtain a unique restriction between C and optimal structural exponent α, which unveils a new optimization principle in route design and schedule arrangement. Applying these features to the cost-minimized optimization model, a method to evaluate the optimization of network is proposed, by which we can directly predict the overall optimal distribution of flight distances and corresponding flight frequencies only based on the information about the passenger flow assignment. Thirdly, China aviation network data from 2001 to 2010 are used for empirical study. It is found that the predictions consist with the actual data. Compared with traditional optimization methods, it can simplify the computational complexity, and therefore it takes full advantage of the structural convenience and provides a new perspective for the overall scheduling of air transportation system. In this case, companies are able to estimate route adjustments easily to see whether they are reasonable and analyze the development trend of network to provide suggestions for future optimization.
ATOMIC AND MOLECULAR PHYSICS
2017, 66 (24): 243201.
doi: 10.7498/aps.66.243201
Abstract +
Chipless radio frequency identification tags have been widely used in many areas, such as vehicle recognition and identification of goods. Near-field measurement of a chipless radio frequency identification tag is important for offering the precise spatial information of the backscattered field of tag. In this paper, we demonstrate the angle discrimination of a line-shape chipless radio-frequency identification tag via the near-field measurements of scattered electric fields in two orthogonal directions. Two laser beams with different frequencies counter propagate and pass through a roomtemperature caesium vapor. A Rydberg ladder-type system is formed in the experiment, which includes three levels, namely 6S1/2, 6P3/2, 51D5/2. The electromagnetically induced transparency of transmission of probe light, which is locked to the transition of 6S1/2↔ 6P3/2, is observed when the frequency of coupling light varies nearby the transition of 6P3/2↔ 51D5/2. When the 5.366 GHz microwave electric field that is resonant with the transition between two adjacent Rydberg states 51D5/2↔ 52P3/2 is applied to the caesium vapor cell by using a standard-gain horn antenna, the transmission signal of probe laser splits into two peaks, which is known as Autler-Townes splitting. The splitting between the transmission peaks is proportional to the microwave electric field strength at the position of laser beam. The spatial distribution of backscattered microwave electric field of the chipless radio-frequency identification tag is obtained through varying the position of the laser beam. The spatial resolution of near-field measurement approximately equals λMW/12, where λMW is the wavelength of the measured microwave electric field. The distributions of the electric field strength in two orthogonal directions show the clarity difference while the angle of radio-frequency identification tag is changed. The scattered electric field strength of the identification tag is strongest when the angle of line-shape tag is the same as that of the polarization of the horn antenna. Moreover, the scattered field strength of identification tag in the incident field direction of the horn antenna increases as the measured position and the identification tag get closer to each other. The scattered electric field distributions in the vertical direction are almost constant at the different angles between the incident electric filed and identification tag. The fluctuation of spatial distribution of the scattered electric field strength is attributed to the Fabry-Pérot effect of microwave electric field in the vapor cell. And the geometry of vapor cell results in the minor asymmetric distribution of scattered field. The simulation results from the electromagnetic simulation software are accordant with the experimental results. The novel approach to near-field measurement of identification tag will contribute to studying and designing the chipless radio-frequency identification tag and complex circuits.
2017, 66 (24): 243401.
doi: 10.7498/aps.66.243401
Abstract +
Nitrogen dioxide molecule plays an important role in modeling atmospheric process. It is a toxic gas and considered as an atmospheric pollutant due to its involvement in reactions that produce ground-level ozone. The electron scattering of NO2 molecule has been extensively studied, specifically at intermediate and high energies. The discrepancies between previous theoretical studies and experimental data at low impact energies (below 4 eV) suggest that the in-depth research should be carried out. The target optimized equilibrium geometry is computed at the highly accurate coupled cluster singles, doubles and perturbative triples[CCSD(T)] level in this study. The ab initio R-matrix method is employed to study the integral and momentum transfer cross sections of low-energy electron scattering from NO2 radical up to 10 eV. Two models including static-exchange and close-coupling approximation are used to reveal the dynamic interaction. The electronic excitation cross sections are computed from ground state to seven electronically allowed excited states. All target states whose vertical excitation energies are below 20 eV are included in the close-coupling expansions of the scattering system. In our CC model, six electrons are in the core orbitals 1a1, 2a1 and 1b2, and the remaining 17 electrons are free to occupy the 4a1, 5a1, 6a1, 7a1, 1b1, 2b1, 3b2, 4b2, and 1a2 orbitals. The aug-cc-pVTZ dunning basis sets are used to optimize the target structure and electron scattering. A Born closure procedure is used to account for the contribution of partial waves higher than l=4 to obtain cross sections. Two shape resonances found at 0.76 eV and 1.82 eV in this study are lower than previous theoretical calculations, but the comparisons with other theoretical calculations and experimental data show that the present R-matrix study not only agrees well with the experiments but also corrects the overestimations of total cross sections of some other theoretical data in the very low energy regions. To study the influence of electron correlations, 21, 82 and 107 target electronic configurations are used in the close coupling model calculations, respectively. The comparisons of integrated cross sections indicate that it is very important to include more target electronic configurations to obtain the converged scattering cross sections, which reveals the importance of electron correlations.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2017, 66 (24): 245201.
doi: 10.7498/aps.66.245201
Abstract +
High energy X-ray sources based on laser-wakefield accelerated electron beams have several important advantages, including high photon energy and small source size, and have many important applications such as high resolution radiography in non-destructive testing. Firstly, the thickness of electron converter is optimized with the targets Ta, W and Pb each with an optimal thickness of 2 mm. We calibrate the intrinsic spatial resolution of CsI needle-like scintillation screen, bismuth germanium oxide (BGO) scintillation array and DRZ scintillation screen with an X-ray tube. And the spatial resolution of CsI needle-like scintillation screen is as high as 8.7 lp/mm. The energy deposition responses of these three detectors to high X-ray are also simulated. Experiments show that the features of a two-layer object can be resolved up to an area density of 33.0 g/cm2 by using the high X-ray source generated by injecting laser-wakefield accelerated electron beam into a Ta convertor target. Experiment that compares X-ray radiography, mixed radiography of X-ray and electron, and electron radiography, is also carried out. Since low X-ray yield and low detection efficiency are two serious problems in high energy X-ray radiography based on laser-wakefield accelerated electron beams, we propose and prove a method of improving image signal intensity greatly at the cost of image contrast by adopting the mixed radiography of X-ray and electron.
2017, 66 (24): 245202.
doi: 10.7498/aps.66.245202
Abstract +
Steady state operation is essential for Tokamak-based fusion reactor, in which the plasma current has to be fully sustained and controlled by non-inductive methods. Lower-hybrid current drive is the most effective radio-frequency current drive method, which, however, has the drawback that the driven current profile is difficult to control. Electron cyclotron current drive has the ability to deposit power and drive current in a highly localized and robustly controllable way, while the efficiency of electron cyclotron current drive is known to be significantly lower than that of lower-hybrid current drive. Due to those complementary features, the combinative usage of lower-hybrid wave and electron cyclotron wave has been proposed. The current driven by simultaneously using the waves might be significantly larger than the sum of the currents driven by the waves individually in the same plasma conditions, which is the so-called synergy effect. While the lower-hybrid current drive and the electron cyclotron current drive are both affected by the trapping effect, which implies that the synergy effect between lower-hybrid current drive and the electron cyclotron current drive may also closely related to the trapping effect. In this paper, the effects of trapping on the synergy of lower-hybrid current drive and the electron cyclotron current drive are investigated by solving the bounce-averaged quasi-linear equation with different trapping angles. The diffusions induced by the lower-hybrid wave and the electron cyclotron wave are considered simultaneously. The resulting steady-state electron distribution function as a balance between the collisions and the wave-induced diffusions is obtained numerically by the CQL3D code, which is then integrated to calculate the driven plasma current. The velocity-space fluxes are analyzed for understanding the mechanism and the physics of the synergy process. It is found that the currents driven by the waves decrease as trapping angle increases. The synergy factors also decrease as trapping angle increases, which means that the current drive processes in the synergy case are more sensitive to the trapping effect than in the single wave case. The current driven by electron cyclotron wave drops rapidly with the increase of trapping angle, while the existence of lower-hybrid wave is helpful in decelerating the dropping. The lower-hybrid wave reduces the dependency of the electron cyclotron current drive on the trapping effect. The decouple effect turns stronger as the resonance region of the lower-hybrid wave becomes wider. Increasing the power of the electron cyclotron wave leads to more accelerated electrons and more electrons with relatively high parallel velocities, which results in stronger synergy effect and less dependence on trapping.
