Vol. 65, No. 13 (2016)
2016-07-05
GENERAL
2016, 65 (13): 130201.
doi: 10.7498/aps.65.130201
Abstract +
Optics lattice clock is a hot topic in the researches of frequency standard and metrology. Neutral mercury atom is one of the most promising candidates for optical lattice clock. Due to its large atomic number, mercury atom is insensitive to black body radiation, which is the severe limitation for developing the optical lattice clocks. To realize the optical lattice clock of neutral mercury atoms, the first step is to implement laser-cooling and trapping of neutral mercury atoms. The cooling transition of mercury atom is 1S0-3P1 transition. The wavelength is 253.7 nm, the line width is 1.27 MHz, and the saturation intensity is 10.2 mW/cm2. Quantum projection noise (QPN) is an important parameter that affects optical lattice clock. Increasing the loading rate of magneto-optical trap (MOT) can help lower the QPN, thereby improving the performance of optical lattice clock. In this work, we calculate the scattering force of deep UV cooling laser, which is exerted on mercury atom in our single chamber MOT, and numerically simulate the one-dimensional motion of the atom in the MOT. It gives us the capture velocity under optimized parameters of the MOT. Then we calculate the loading rate of three-dimensional MOT by a high efficient random sampling method. According to the rate equation of MOT, the loading rate is proportional to the atom number of the steady state, which is the accessible parameter in the experiment. An experimental setup of MOT is established with a high vacuum system and a frequency quadrupled semiconductor laser system. The fluorescence imaging on an EMCCD gives the atom number in the MOT. We also calibrate the vapor density of background mercury gas in the vacuum, and measure the atom number in a steady MOT. We numerically simulate and experimentally study the influences on the atom number on the parameters of MOT, such as laser intensity, laser detuning and magnetic field gradient. The calculated results are in consistent with the experimental results. We also find the optimized parameters to maximize the loading rate. The numerical simulation also gives some results beyond current experimental condition, especially for laser intensity. From the simulations we obtain the optimized MOT parameter and relation for laser cooling of neutral mercury atom. Especially, current deep UV cooling laser with 20 mW of power does not have enough power to enhance the loading rate. These results are rather valuable for designing the next generation of optical lattice clocks for neutral mercury atoms.
2016, 65 (13): 130302.
doi: 10.7498/aps.65.130302
Abstract +
By using quantum packet transmission technology in large scale quantum communication networks, the throughput of transmission node, network link utilization, and the anti-interference performance of communication can be effectively improved. However, the fast transmission of quantum packets is closely related to the performance of router. The bottleneck of the router performance will seriously affect the scalability of the network and the transmission efficiency of the link. In order to reduce the number of quantum packet queues in nodes of the quantum communication network and to reduce the transmission delay of quantum packets, firstly, according to the classical computer communication network structure, in our paper, we divide the quantum communication network into quantum local area network, quantum metropolitan area network and quantum wide area network. Secondly, the quantum packet format and the quantum cluster format compatible with the packet format in the computer network are determined. Then, a quantum information packet transmission scheme based on the hierarchy is proposed, to realize the end-to-end transmission of quantum information. In our scheme, the quantum packets are divided into quantum packet header information and quantum data information. Quantum dense coding mode is used to transmit the quantum packet header information, while the quantum data information uses quantum teleportation to transmit. First, the quantum packets are sent to the router of the quantum local area network at source address, then the quantum LAN router relay the packets to the quantum metropolitan area network router, the router here makes the quantum packets into quantum cluster according to destination address. Quantum clusters are transmitted in the quantum metropolitan area network and quantum wide area network, ending in the quantum metropolitan area network routing. After the quantum clusters are decomposed, they are sent to the destination address through the local area network router of each quantum packet.We analyze the number of quantum entanglement pairs and the total transmission time in our scheme. The results show that the more the routers by the quantum packet and the quantum cluster are, the more the number of quantum entanglement pairs required by the transmission of a certain quantum packet is. When the number of routers is certain, the number of entanglement pairs required in the transmission process of quantum packet and quantum cluster depends on the number of quantum packets. Finally, the theoretical analysis and calculation are carried out by Matlab simulation, the results show that hierarchical quantum packet information transmission scheme can effectively reduce the transmission time of quantum packet information in the quantum communication network, and the reduced time is related to the quantum router performance and the number of quantum packets to send: the more the number of quantum packets to send, the longer the length of packet processing time needs is and the more obvious the advantage of our scheme is. Therefore, the proposed scheme in this paper is suitable for the construction of large scale quantum communication networks.
2016, 65 (13): 130501.
doi: 10.7498/aps.65.130501
Abstract +
Since wind has an intrinsically complex and stochastic nature, accurate wind power prediction is necessary for the safety and economics of wind energy utilization. Aiming at the prediction of very short-term wind power time series, a new optimized kernel extreme learning machine (O-KELM) method with evolutionary computation strategy is proposed on the basis of single-hidden layer feedforward neural networks. In comparison to the extreme learning machine (ELM) method, the number of the hidden layer nodes need not be given, and the unknown nonlinear feature mapping of the hidden layer is represented with a kernel function. In addition, the output weights of the networks can also be analytically determined by using regularization least square algorithm, hence the kernel extreme learning machine (KELM) method provides better generalization performance at a much faster learning speed. In the O-KELM, the structure and the parameters of the KELM are optimized by using three different optimization algorithms, i.e., genetic algorithm (GA), differential evolution (DE), and simulated annerling (SA), meanwhile, the output weights are obtained by a least squares algorithm just the same as by the ELM, but using Tikhonovs regularization in order to further improve the performance of the O-KELM. The utilized optimization algorithms of the O-KELM are respectively used to select the set of input variables, regularization coefficient as well as hyperparameter of kernel function. The proposed method is first applied to the direct six-step prediction for Mackey-Glass chaotic time series, under the same condition as the existing optimized ELM method. From the analysis of the simulation results it can be verified that the prediction accuracy of the proposed O-KELM method is increased by about one order of magnitude over that of the optimized ELM method. Furthermore, the DE-KELM algorithm can achieve the lowest root mean square error (RMSE). The O-KELM method is then applied to real-world wind power prediction instance, i.e., the Western Dataset from NERL. The 10-minute ahead single-step prediction as well as 20-minute ahead, 30-minute ahead, 40-minute ahead multi-step prediction for wind power time series are respectively implemented to evaluate the O-KELM method. Experimental results of each of the short-term wind power time series predictions at different time horizons confirm that the proposed O-KELM method tends to have better prediction accuracy than the optimized ELM method. Moreover, the GA-KELM algorithm outperforms other two O-KELM algorithms at future 10-minute, 20-minute, 40-minute ahead prediction in terms of the RMSE value. The DE-KELM algorithm outperforms other algorithms at future 30-minute ahead prediction in terms of the normalized mean square error (NMSE) and the RMSE value. The results from these applications demonstrate the effectiveness and feasibility of the proposed O-KLEM method. Therefore, the O-KELM method has a potential future in the field of wind power prediction.
2016, 65 (13): 130701.
doi: 10.7498/aps.65.130701
Abstract +
As cantilever-based vibration energy harvesters are easily fractured under large amplitude vibration excitation, in this paper we present a vibration energy harvester based on a pendulum-type structure with broadband and frequency-doubling characteristics. The harvester consists of two Terfenol-D/PMN-PT/Terfenol-D magnetoelectric transducers and a rotary pendulum embedded with six magnets. These six magnets are arranged into an optimum configuration and can produce a concentrated flux gradient which makes the magnetoelectric transducers generate a high power. While the two transducers are used to further improve the output power and power density of the harvester without increasing the volume of the harvester. The rotary pendulum of the harvester changes linear vibration into a back-and-forth swing of the rotary pendulum. When the rotary pendulum swings, the stress is hardly generated in the interior of the rotary pendulum. Therefore the rotary pendulum is not easily fractured under the large amplitude vibration. Therefore the proposed pendulum-based vibration energy harvester is suitable for scavenging the large amplitude ambient vibration energy. The swing equation of the rotary pendulum is established. The nonlinear dynamic equation of the rotary pendulum is solved by the Lindstedt-Poincar method. The frequency response characteristic and the mechano-magneto-electric transduction characteristic of the harvester at resonance are analyzed by combining the swing equation of the harvester with the magnetoelectric characteristics of the magnetoelectric transducers. The spectrum of the output voltage waveform of the harvester is discussed. The analytical and experimental results indicate that the harvester has broadband and frequency-doubling characteristics. The broadband characteristic of the harvester is derived from the nonlinear magnetic force between the magnets and magnetoelectric transducers. The voltage frequency-doubling characteristic is derived from the nonlinearity of the magnetic field produced by the magnets. It does not need frequency conversion mechanism for the proposed harvester, so the proposed harvester has some advantages, such as simple structure and easy manufacture. Under 1 g (1 g = 9.8 m/s2) RMS vibration acceleration excitation, the measured maximum RSM voltage and the resonant frequency of the prototype are 90.9 V and 16.9 Hz, respectively. The 3 dB bandwidth for the sweep-down condition is 4.8 Hz from 16.9 Hz to 21.7 Hz and that for the sweep-up condition is 2.1 Hz from 22.8 Hz to 24.9 Hz. Compared with other harvesters, the proposed harvester has a wide relative bandwidth. The load output power of the prototype reaches 3.569 mW across a 1.9 M optimal resistor at resonant frequency of 16.9 Hz with 1 g RMS vibration acceleration. The output RMS powers of the prototype across 1.9 M resistor are 0.156 mW, 0.6863 mW, 1.777 mW at 0.3 g, 0.5 g and 0.7 g with resonance, respectively. The proposed harvester can effectively improve the output powers at lower frequency vibrations for its two transducers, broadband and frequency-doubling characteristics.
2016, 65 (13): 130702.
doi: 10.7498/aps.65.130702
Abstract +
III-nitride materials have attracted considerable attention in the last decade due to their wide applications in solidstate light devices with their direct wide band-gaps and higher quantum efficiencies. InGaN/GaN multiple quantum well is important active region for light-emitting diode, which can be tuned according to indium composition in the InxGa1-xN alloy system. Owing to difficulty in fabricating bulk materials, GaN thin films are heteroepitaxially grown on latticemismatched and thermal-expansion-mismatched substrates, such as sapphire (Al2O3), Si and SiC, which subsequently results in a mass of threading dislocations and higher residual strains. On the one hand, dislocations and defects existing in GaN epifilms trap the carriers as scattering centers in the radiative recombination process between electrons and holes, and play an important role in drooping the internal quantum efficiency. On the other hand, higher built-in electric field induced by residual strains existing in GaN epifilm could make the emission wavelength red-shifted.It is common knowledge that temperature is one of the important factors in the growth process of epitaxial films, as a result, further research on thermal expansion behaviors is needed. Based on the above analysis, an in-depth study of thermal expansion behavior of wurtzite GaN epitaxial film is of vital importance both in theory and in application.In this study, we investigate the thermal expansion behaviors of wurtzite GaN epitaxial films by using temperaturedependent Raman scattering in a temperature range from 83 K to 503 K. According to the physical implication, Gruneisen parameter is almost a constant (Gruneisen parameters of all phonon modes are in a range between 1 to 2 for GaN) that characterizes the relationship between the phonon shift and the volume of a solid-state material. More importantly, Gruneisen parameter is relatively insensitive to temperature and suitable for building the connection between the phonon shift and thermal expansion coefficient. Therefore, the linear relationship between the phonon shift and temperature is built and utilized to calculate the thermal expansion coefficient according to the physical implication of the Gruneisen parameter. Conclusions can be obtained as follows. (1) The thermal expansion coefficient of GaN epifilm can be calculated in a certain temperature range by measuring the phonon modes of E2 (high), A1 (TO) and E1 (TO) through using temperature-dependent Raman scattering when the corresponding Gruneisen parameters are determined. (2) The calculated thermal expansion coefficients of GaN epifilm are consistent with the theoretical values.Conclusions and methods in this paper provide an effective quantitative analysis method to characterize the thermal expansion behaviors of other III-nitride epitaxial thin films, such as AlN, InN, AlGaN, InGaN, InAlN etc., which can be of benefit to reducing the dislocation density and improving the luminescence efficiency of light emitting diode. Therefore, research on thermal expansion behaviors of epifilms using temperature-dependent Raman scattering has a direction for further studying the latter-mismatch and thermal-expansion-mismatch between the epitaxial film and substrate.
2016, 65 (13): 130202.
doi: 10.7498/aps.65.130202
Abstract +
Based on the reconstruction of the polarization parameters in a channel modulating polarization imaging system, the polarization features of the target could be extracted effectively. Considering that the reconstruction of polarization parameters can provide important reference for target recognition, material analysis, remote sensing and bio-medical treatment, the research on accurate reconstruction of polarization parameters is now urgently required. In order to improve the accuracy of polarization parameter reconstruction, we first study the influence of sample frequency of interference fringes on the imaging process. For the same carrier frequency, conjugate spectra are separated and also the spectra are not aliasing for two adjacent spectral lines. It is concluded that to prevent the image spectrum from aliasing, the sample frequency should be at least 4 times the maximum fringe frequency of the polarization interference image. Then we study Stokes parameter reconstruction method when the spectral line positions of interference image are changed by assembling error. Since different Stokes parameters are amplitude modulated at different frequencies, we apply segment filters to split the frequency domain into different regions, and seek for the largest spectrum in corresponding regions. The largest spectrum in different regions can be used to determine the spectral line position of polarization carrier frequency, and the two-dimensional images of the target are rebuilt in sequence by the frequency shifting, spectral filtering, and Fourier inversion transforming. According to the above method, we could obtain an exact polarization rebuilding image when the line position of polarization carrier frequency is modified. Finally, we use the computer simulation and experiment to verify the feasibility and effectiveness of such a rebuilding method. The results demonstrate that the reconstruction of polarization parameters in channel modulating polarization imaging by this rebuilding method is better than by the traditional theoretical rebuilding method. In detail, the mean square error between the reconstruction and original input image could be suppressed to 0.001 while the peak-signal-to-noise ratio is improved and the structural similarity index measurement could be more than 0.9 by utilizing the new rebuilding method. It turns out that the reconstruction method with great superiority can provide a promising reference for further research of channel modulating polarization imaging system.
2016, 65 (13): 130301.
doi: 10.7498/aps.65.130301
Abstract +
As is well known, quantum optics has developed significantly in recent years and advanced several hot research topics, such as quantum communications, quantum sensing, quantum calculations, etc. Among these researches, it is important to understand the quantum information transmitting in optical fiber. For realizing longer transmission distance and better transmission quality, great effort has devoted to the researches of encoding and decoding at the transmitter and the receiver end. However, less attention was paid to the fading of signal in the transmission channel. In this work, we mainly focus on the transmission model of optical quantum transmission and the influences of loss, dispersion and nonlinear effect on fiber transmission of optical quantum information are also discussed.Quantum information transmission can be influenced by loss, dispersion and nonlinear effect in optical fiber, leading to transmission state evolution and energy transfer. Based on the transmission equation of single mode fiber and quantum theory of electromagnetic field, the fundamental mode field of single mode fiber is quantized. A quantum transmission equation is deduced from the classical optical transmission equation through quantizing the amplitude of electromagnetic field. Compared with classic wave theory, the photon transmission equation quantizing the slowly-varying amplitude in the coupled nonlinear Schrdinger equation is obtained. In the classic wave equation, light is interpreted as energy which propagates as waves. The photon transmission equation is obtained by quantizing the slowly-varying amplitude of light, that is, the particle nature of light. The energy propagates through alternative interaction between creation and annihilation operator on photons. The transmission equations show that photons will interact with the transmission medium during propagation and be influenced by dispersion, nonlinear effect, loss, etc. By giving a trail solution and introducing a perturbation term, the transmission equation is solved for the complicated case where the dispersion, loss and nonlinear effect are all involved. A dispersion equation that should be satisfied for nontrivial solution is then obtained. From this dispersion equation, the relation between photon power and perturbation frequency is calculated and analyzed. The change of photon power in generalized field with perturbation frequency is discussed, and the influences of fiber dispersion and nonlinearity on the solution are analyzed.Some conclusions are obtained by perturbed solution and analyses of single photon transmission equation in optical fiber. It is found that photon power decreases with the increase of perturbation frequency and reaches its maximum value for zero perturbation frequency. At the same time, the optical power is affected by the dispersion of the optical fiber. Photon power decreases with the GVD coefficient far from the zero dispersion point. It is also found that photon power decreases with the increase of nonlinear coefficient. This work may contribute to the research of the properties of quantum fiber transmission system.
ATOMIC AND MOLECULAR PHYSICS
2016, 65 (13): 133101.
doi: 10.7498/aps.65.133101
Abstract +
Due to the diversified atomic structures and electronic properties, two-dimensional monolayer nanocarbon materials (graphyne or graphdiyne) composed of sp and sp2 hybridization C atoms have received the widespread attention in recent years. The fundamental questions include how the sp orbital hybridization affects the electronic structure of graphyne. In order to investigate the structure dependent electronic structures of graphyne, the energetic stabilities and electronic structures of -graphyne and its derivatives (-N) with N carbon atoms on each edge of the hexagons are investigated by density functional theory (DFT) calculations in this work. In our DFT calculations we adopt generalized gradient approximation of Perdew, Burke, and Ernzerhof (GGA-PBE) using the CASTEP module implemented in Materials Studio. The studied -Ns consist of hexagon carbon rings connected by vertexes whose edges have various numbers of carbon atoms N= 1-10. The structure and energy analyses show that -Ns with even-numbered carbon chains have alternating single and triple C-C bonds, energetically more stable than those with odd-numbered carbon chains possessing continuous C-C double bonds. The calculated electronic structures indicate that -Ns can be either metallic (odd N) or semiconductive (even N), depending on the parity of number of hexagon edge atoms regardless of the edge length due to Jahn-Teller distortion effect. Some semiconducting -graphyne derivatives (-N, N= 2, 6, 10) are found to possess Dirac cones (DC) with small direct band gaps 10 meV and large electron velocities 0.255106-0.414106 m/s, ～30%-50% of that of graphene. We find that Dirac cones also appear in -3 and -4 when we shorten the double bonds and elongate the triple bonds in -3 and -4 respectively. These results show that the bond length change will affect the characteristics of band structure and suggests that the band structure characteristics may be influenced by Peierls distortion in a two-dimensional system. Our DFT studies indicate that introducing sp carbon atoms into the hexagon edges of graphene opens the way to switching between metallic and semiconductor/DC electronic structures via tuning the parity of the number of hexagon edge atoms without doping and defects in nanocarbon materials and nanoelectronic devices.
2016, 65 (13): 133102.
doi: 10.7498/aps.65.133102
Abstract +
The elimination of bisphenol A (BPA) from water solution is of great importance, since BPA can cause the functional abnormalities of human endocrine system. One feasible removal method is the adsorption by graphene oxide (GO). However, the interactions between BPA and GO at an atomic level are still unclear. In this study, molecular dynamics simulations are performed to investigate the adsorption of BPA on the GO surface. The results show that all BPA molecules are attached to both sides of GO. The adsorption conformations of BPA in the closest layer to GO surface mainly exhibit two patterns. One is that the benzene rings of BPA are parallel to the basal plane of GO to form - structures, and the other is the two hydroxyl groups of BPAs interacting with the oxygen-contained groups on GO to form hydrogen bonds. Exploration of the detailed interactions between BPA and GO indicates that the hydrophobic - stacking interaction is the dominant force in the adsorption of BPA on GO, while hydrogen bonding enhances the binding of BPA on GO surface. Eventually, potential of mean forces (PMF) of BPA and water molecules on GO are calculated by umbrella sampling. The binding energy of BPA on GO reaches 30 kJ/mol, six times as large as that of water on GO, which is only about 5 kJ/mol. Our simulations further confirm that GO owns strong adsorption capacity and GO can be used as sorbent to eliminate BPA in water solution.
ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2016, 65 (13): 134101.
doi: 10.7498/aps.65.134101
Abstract +
The increasing demand of unoccupied and unregulated bandwidth for wireless communication systems will inevitably lead to the extension of operation frequencies toward the lower THz frequency range. Since atmospheric transmission windows exist in the lower THz frequency range, it can be realized that carrier frequencies of 300 GHz and beyond will be used for communications once the technology for high bitrate data transmission is available. However, the free-space path-loss and the attenuation due to molecules in the atmosphere can significantly reduce the transmittable data rate in the lower THz frequency range.The main factor affecting the behavior of terahertz band is the absorption by water vapor, which not only attenuates the transmitted signal, but also disperses the signal. A new model of the terahertz wave atmospheric propagation of attenuation and dispersion is developed by using the radiation transmission theory and the empirical continuum absorption based on the HITRAN database. Theoretical aspects of absorption are presented, emphasizing those that deserve special attention as frequency increases. The THz wave atmospheric attenuation experimental results and self- and foreign-continuum coefficients obtained with the improved THz-time domain spectroscopy (THz-TDS) technique are analyzed by this model. The intensities and locations of the observed absorption lines are in good agreement with spectral databases. This model accounts for the group velocity dispersion and the total path loss that a wave in the THz band suffers when propagating 1 km distance. The channel capacity of the THz band is investigated by this model under different conditions including antenna gains, channel bandwidth and transmitter power. In order to keep the considerations as general as possible, the derivations are based on simple assumptions and equations. The special requirement for antenna is also discussed.Three communication channels (340 GHz, 410 GHz and 667 GHz) are obtained in terms of the spectrum. The four parameters of the three channels, i.e., available bandwidth, center frequency, dispersion and transmittable data rate, are summarized and quantized. The signals through the atmosphere for the three communication channels within the corresponding atmospheric windows are not easy to broaden due to the low group velocity dispersion; high data rates of up to 10 Gbps or beyond per 1 GHz bandwidth can be transmitted via these channels if the antennas with high gains are used.
2016, 65 (13): 134102.
doi: 10.7498/aps.65.134102
Abstract +
Cirrus clouds play an important role in the energy budget and the hydrological cycle of the atmosphere. It is still one of the largest uncertainties in the global climate change studies. This is mainly attributable to the measurement discrepancies of cirrus parameters, especially the microphysical parameters, which are constrained by the existing methods. With THz wavelengths on the order of the size of typical cirrus cloud particles and therefore being sensitive to cirrus clouds, THz region is expected to have a promising prospect concerning measuring cirrus microphysical parameters (ice water path and effective particle size). In order to evaluate the effects of cirrus microphysical parameters on THz transmission characteristics and the sensitivity of cirrus in THz region, the THz radiation spectra at the top of atmosphere in the clear sky and the cloudy situations are simulated and calculated based on the atmospheric radiative transfer simulator. The effects of cirrus particle shape, particle size and ice water path on THz transmission characteristics are obtained by analyzing the brightness temperature difference between the two situations, and the sensitivity parameters that quantitatively describ the effects. The results indicate that cirrus particle shape, particle size and ice water path have different effects on the THz wave propagation. The cirrus effect varies also with channel frequency. Overall, in the low frequency channels, cirrus effects are enhanced with the increases of particle size and ice water path; in the high frequency channels, cirrus effects are more complicated and vary with particle size and ice water path. The effects are first enhanced and then turned into saturation. The THz wave is sensitive to cirrus cloud ice water path and effective particle size, and THz wave may be the best waveband for remote sensing of cirrus microphysical parameters in theory. For thin clouds, the sensitivity parameters are approximately constant, indicating that the spectral brightness temperature at the top of the atmosphere almost shows linear relationship with ice water path, and the sensitivity parameters increase with frequency increasing. For thick clouds, the sensitivity of cirrus to ice water path decreases and gradually becomes saturated, and the higher the frequency, the more quickly it tends to saturation level. Compared with the microwave and infrared, THz wave can provide many detailed information about cirrus. The two-channel look-up table indicates that THz wave passive remote sensing of cirrus may be a stable and effective method. The results will be conducible to developing the technology of THz wave remote sensing of cirrus microphysical parameters. Moreover, it is also beneficial to improving the cirrus detection precision.
2016, 65 (13): 134203.
doi: 10.7498/aps.65.134203
Abstract +
In this paper, the noise filtering effect on a femtosecond laser source via a broadband passive cavity is analyzed in detail. The results show that a passive optical cavity not only can be used as a low-pass noise filter, but also can inter-convert the phase and amplitude fluctuations of a light beam after transmission or reflection. Therefore, by measuring the intensity noise of the light field under test after transmission and reflection from a passive cavity, its phase noise properties can be explored. Based on this theoretical model, an eight-mirror ring passive cavity with a finesse of 1500 and a free spectral range of 75 MHz is designed and built. With a commercial Ti:sapphire femtosecond laser as a source, its intensity noises after transmission and reflection from the above cavity are measured with home-made self-homodyne detection setup. Furthermore, with the help of the noise conversion model of the passive cavity, the phase noise of the femtosecond laser as well as its evolution through the cavity transmission and reflection is indirectly derived. The result shows that after transmission through the passive cavity, both the amplitude and phase noise of the femtosecond laser source are evidently suppressed and reach the shot noise limit at the analyzing frequency of 2 MHz.
2016, 65 (13): 134205.
doi: 10.7498/aps.65.134205
Abstract +
The demands for highly directive antennas are becoming more stringent, especially in microwave regions. Traditional ways to enhance the antenna gain such as reflectors, dielectric lenses, waveguide horns and microstrip antenna arrays suffer design complexity, high cost and power loss in the feeding network, so it is urgent to find a simple way to solve the problem. Fabry-Perot (F-P) antenna has a high directivity and low sidewall, owing to the resonance of the cavity in a cophasal and tapered field distribution along the lateral direction. However, the disadvantage of F-P antenna is obvious for the inherently narrow gain bandwidth which inhibits their many applications. In this paper, a broadband and high-gain shared-aperture F-P resonator magneto-electric (ME) microstrip antenna working at X band is designed and fabricated. In order to design a wideband metamaterial superstrate unit, the structure with two different frequency selective surface (FSS) layers is presented: the metal pattern at the top of the unit is a square patch and has a high reflection coefficient in the high frequency band, and at the bottom the metal pattern is a cross patch, it has a high reflection coefficient in the low frequency band, therefore, the whole unit should resonate in a broadband frequency range. Theoretical analysis and simulation result indicate that the unit has a linearly increasing phase response and a high reflection coefficient across a broadband range and it has the potential to construct a wideband F-P resonator antenna. In the proposed antenna, a novel wideband ME microstrip antenna is used as the feeding source. For the antenna covers the whole X band, the bandwidth of the F-P resonator superstrate should be further expanded. Simulated calculation results indicate that different sizes of two-layer FSSs have different reflection phases but the same coefficient, therefore a shared-aperture structure with three different sizes of FSSs is obtained. The arrangement utilizes the phase compensation property along different FSSs, and broadens the gain enhancement bandwidth effectively. When the superstrate is set to be approximately 15.5 mm above the ground plane of the ME antenna, the antenna possesses an impedance bandwidth of 44.7% for the reflection coefficient (S11) below -10 dB from 7.8 GHz to 12.3 GHz, covering the whole X band. From 7.9 GHz to 12.1 GHz, the antenna has an obvious gain enhancement, with a peak of 7 dB. Numerical and experimental results indicate that compared with the traditional F-P resonator structure, the shared-aperture metamaterial superstrate can effectively broaden the antenna gain enhancement bandwidth, and it has great application values for designing new broadband metamaterial superstrate high-gain antennas.
EDITOR'S SUGGESTION
2016, 65 (13): 134206.
doi: 10.7498/aps.65.134206
Abstract +
The integration of photonic crystal cavity with quantum dot paves the way for photonic-based quantum information processing. Photonic crystal cavity has a high-quality factor and small mode volume, which can be utilized to enhance the interaction between light and matter. Two degenerate fundamental modes with orthogonal polarizations exist in photonic crystal H1 cavity. Entangled photon pairs can be generated with a single quantum dot coupled to degenerate H1 cavity modes. Therefore a coupling system comprised of quantum dot and photonic crystal H1 cavity is a promising platform to implement quantum information processing. The excitations of cavity modes are mostly affected by the location of the single quantum dot, namely a dipole source. For the two degenerate photonic crystal H1 cavity modes, the location of the dipole source determines which mode is excited. In this paper, the effects of location and polarization of a dipole source on the excitation of photonic crystal H1 cavity are investigated with the finite-difference time-domain method, a numerical analysis technique for computing the electrodynamics. We first design a photonic crystal slab structure patterned with hexagonal lattice of air holes. Combining the light modulation by the period lattice in the slab plane and the total internal reflection in the perpendicular direction, photonic bandgap is generated, which inhibits the propagation of photon with certain frequencies. By removing one of the air holes from the photonic crystal slab, an H1 cavity is formed with two degenerate fundamental modes. One mode is x-polarized, and the other one is y-polarized. Next, a dipole source is used to excite the H1 cavity modes. When the dipole source is located at the left to the H1 cavity center, only y-polarized mode is excited. While locating the dipole source above the H1 cavity center, only x-polarized mode is excited. Therefore each degenerate mode of H1 cavity can be selectively excited with the diploe source located at different positions in the cavity. Following that, the H1 cavity modes excited with the dipole sources with different polarizations are also studied. The x-polarized dipole source can only excite the cavity mode with x-polarization, while the y-polarized dipole source can only excite the y-polarized cavity mode accordingly. It can be seen that the dipole source with specific polarization can only excite the modes with corresponding polarization. The effects of location and polarization of a dipole source on the excitation of a photonic crystal H1 cavity are important for understanding the fundamental physics of entangled photon generation with a coupled quantum dot and photonic crystal system.
2016, 65 (13): 134207.
doi: 10.7498/aps.65.134207
Abstract +
Photonic crystal is a kind of periodic optical nanostructure consisting of two or more materials with different dielectric constants, which has attracted great deal of attention because of its wide range of potential applications in the field of optics. Photonic crystal can be fabricated into one-, or two-, or three- dimensional one. Among them, the two-dimensional photonic crystal turns into a hot focus due to its fantastic optical and electrical properties and relatively simple fabrication technique. Since the tunable band gaps of two-dimensional photonic crystals are beneficial to designing the novel optical devices, to study their optical and electrical properties for controlling the electromagnetic wave is quite valuable in both theoretical and practical aspects. In this work, we propose a new type of two-dimensional function photonic crystal, which can tune the band gaps of photonic crystals. The two-dimensional function photonic crystal is different from the traditional photonic crystal composed of medium columns with spatially invariant dielectric constants, since the dielectric constants of medium column are the functions of space coordinates. Specifically, the photorefractive nonlinear optical effect or electro-optic effect is utilized to turn the dielectric constant of medium column into the function of space coordinates, which results in the formation of two-dimensional function photonic crystal. We use the plane-wave expansion method to derive the eigen-equations for the TE and TM mode. By the Fourier transform, we obtain the Fourier transform form (G) for the dielectric constant function (r) of two-dimensional function photonic crystal, which is more complicated than the Fourier transform in traditional two-dimensional photonic crystal. The calculation results indicate that when the dielectric constant of medium column is a constant, the Fourier transforms for both of them are the same, which implies that the traditional two-dimensional photonic crystal is a special case for the two-dimensional function photonic crystal. Based on the above theory, we calculate the band gap structure of two-dimensional function photonic crystal, especially investigate in detail the corresponding band gap structures of TE and TM modes. The function of dielectric constant can be described as (r) = kr + b, in which k and b are adjustable parameters. Through comparing the calculation results for both kinds of photonic crystals, we can find that the band structures of TE and TM modes in two-dimensional function photonic crystals are quite different from those in traditional two-dimensional photonic crystal. Adjusting parameter k, we can successfully change the number, locations and widths of band gaps, indicating that the band gap structure of two-dimensional function photonic crystal is tunable. These results provide an important design method and theoretical foundation for designing optical devices based on two-dimensional photonic crystal.
2016, 65 (13): 134301.
doi: 10.7498/aps.65.134301
Abstract +
In a shallow water waveguide, the low-frequency acoustic field can be viewed as a sum of normal modes. Warping transform provides an effective tool to filter the normal modes from the received signal of a single hydrophone, which can be used for source ranging and geoacoustic inversion. However, it should be noted that the conventional warping operator h(t) = t2+tr2 is only valid for a signal consisting of reflection dominated modes, where r represents the source range. In a waveguide with a strong thermocline or a surface channel where refracted modes dominate the received sound field, the dispersive characteristics of the waveguide become different and the performance of the warping operator h(t) = t2+tr2 will be significantly degraded. In this paper, the dispersive characteristics and warping transform of the refractive normal modes in a waveguide with a linearly decreased sound speed profile are discussed. The formulae for the horizontal wavenumber, the phase in frequency domain and the instantaneous phase in time domain of the refractive mode are deduced. Based on these formulae, the time warping and frequency warping operators verified by the simulated data are presented. Through time-axis stretching or compression, the time warping operator h(t) =tr-t2, where tr= r/c(h) and c(h) represents the bottom sound speed, can transform the refracted modes into single-tone components of frequencies determined by source range, sound speed gradient of water, bottom sound speed and mode number. The frequency warping operator h(f) = Df3, where D is a constant, can transform the refracted modes into separable impulsive sequences through frequency-axis stretching or compression and the time delay of the impulsive sequences changes linearly with the source range. As the warped modes are separated in time domain or frequency domain, these two operators can be used for filtering the refracted normal modes from the received signal. The theories in this paper are also applicable for refractive modes in the waveguide with a linearly increased sound speed profile or a linear variation of the square of the index of refraction (n2-linear sound speed profile).
2016, 65 (13): 134302.
doi: 10.7498/aps.65.134302
Abstract +
Vibrational relaxation time is a parameter describing the macroscopic behavior of vibrational energy transition rate between molecular internal degrees of freedom (DOF) and external DOF in excitable gas, which determines the relaxation frequency of the maximum point in acoustic absorption spectrum. To measure the vibrational relaxation time, the traditional methods are used to obtain the acoustic absorption spectra by changing the ambient pressure at several operating frequencies. However, these traditional methods are not suitable for real-time measurement due to the complexity of equipment implementation and the non-ideality of test gas under high pressure. In order to solve those problems, we have developed an algorithm [2013 Meas. Sci. Technol. 24 055002] to capture the primary vibrational relaxation processes only based on the measurements of sound absorption and sound speed at two operating frequencies and a single pressure. But the algorithm only can reconstruct the absorption maximum and it cannot capture the relaxation time with high precision. To measure the frequency dependence of the complex effective specific heat of the relaxing gas, an algorithm synthesizing relaxation processes is given by Petculescu and Lueptow [2005 Phys. Rev. Lett. 94 238301]. In its derivation process, relaxational angular frequency was set to be the inverse ratio to relaxation time. However, the relaxational angular frequency was measured in the adiabatic process of transmission thermodynamic, while the relaxation time was obtained in the thermodynamic isothermal process, the derivation confused the two thermodynamic processes, making the algorithm unable to capture the relaxation frequency with high precision. In order to estimate the relaxation time with higher accuracy, in this paper we first obtain the theoretical relationship among the relaxation times under the three types of thermodynamics conditions, i. e., isothermal, adiabatic constant pressure and adiabatic constant volume. Then we correct the relaxation time derivation and propose our corrected algorithm to reconstruct the relaxation frequencies and relaxation times under the conditions of isothermal, adiabatic constant pressure and adiabatic constant volume. In experiments and simulations, the relaxation times and relaxation frequencies reconstructed by our corrected algorithm for various gas compositions including carbon dioxide, methane, chlorine, nitrogen, and oxygen around room temperature are consistent with the experimental data.
2016, 65 (13): 134303.
doi: 10.7498/aps.65.134303
Abstract +
With the advances of ship noise reduction technology, the working frequency of the passive sonar must be reduced in order to detect a target. For the conventional array, it requires a large array aperture, comparable to the wavelength, in order to achieve an acceptable angular resolution. Arrays of small physical size with high angular resolution are thus attractive for low-frequency direction-of-arrival estimation of underwater sound source. In this paper, we consider a 33 uniform rectangular array which consists of vector sensors with inter-sensor spacing much smaller than the wavelength. A broadband super-directive beamforming method is proposed for this vector sensor array, which extracts multi-pole modes of different orders from the spatial differentials of the sound field. By normalizing the amplitudes of the multi-pole modes, frequency invariant mode functions can be obtained, which are used to build the desired beam pattern, despite the Rayleigh limit on the achievable angular resolution. Vector sensors are used to replace the pressure difference operation, thus to achieve a desirable beam pattern, the order of spatial differential will be reduced. In other words, for the same array configuration, using the vector sensors provides higher directivity than using the pressure sensor. To concentrate on the sources, and to minimize all hindrances from around circumference, a suitable beam pattern is constructed as an example to analysis. To verify the algorithm, a prototype is built and tested in a water tank. Comparisons are carried out between the actually synthesized beam patterns and the theoretical ones. The experimental results show good agreement with the theoretical results, and that the directivity increases with the multi-pole mode order increasing, at the expense of lower robustness. The performances for different values of ka are also investigated, where k is the wave number and a denotes the inter-sensor spacing. Simulation results show that when the inter-sensor spacing is no more than one-sixth of the incident wave length, the error introduced by the approximations for muti-pole mode extraction can be neglected. It should be noted that this result of the inter-sensor spacing still applicable when considering array gain, showing that the array is insensitive to uncorrelated noise while preserving a relatively high array gain. Finally, the influence of the underwater acoustic waveguide on the array performance is analyzed. Simulations and experimental tests show that due to the small array aperture, the waveguide effects on the array performance are limited.
2016, 65 (13): 134701.
doi: 10.7498/aps.65.134701
Abstract +
Superhydrophobic surfaces with micro- and nano-scale structures are conducible to maintaining a gas layer where prominent slippage effect exists. It has been demonstrated that the drag reduction of superhydrophobic surface increases with growing the fraction of the gas-water interface and the rising of the thickness of gas layer. Whereas a large thick gas layer on the superhydrophobic surface collapses easily under tangential water flow. Here, we present a new method to maintain large-scale gas layer by creating hydrophilic patterns at the superhydrophobic surface, on which the binding force of air on the solid surface can be caused by wettability difference. Through testing the states of gas layer trapped on surfaces with wettability differences equal to 54.8, 84.7, 103.6 and 144.0 in apparent contact angle, respectively, the conditions of maintaining gas layer are mainly considered. We demonstrate that the critical velocity, over which the gas layer begins to collapse under the tangential water flow, is positively correlated with the thickness of the gas layer and the wettability difference between the superhydrophobic area and hydrophilic area, however, this is negatively correlated with the width of the gas layer in the crosswise direction. It is noteworthy that even a centimeter-scale gas layer can be kept steady in ～0.9 m/s through this method. Furthermore, an obvious slip velocity up to ～25% of bulk velocity is observed at the gas-water interface, through measuring the velocity profile above the 0.6 cm-long, 0.5 cm-wide and 0.15 cm-thick gas layer by using the PIV technology. We anticipate that this novel method of gas entrapment under water will effectively widen the applications of superhydrophobic surfaces for drag reduction.
2016, 65 (13): 134201.
doi: 10.7498/aps.65.134201
Abstract +
A real-time polarization imaging system employing the Wollaston prism and a single charge-coupled device(CCD) chip covering a wavelength range of 400 nm-650 nm is proposed to avoid the false polarization information from dynamic scenes in non-real-time polarization detection imaging method. An architecture consisting of telescope lens, collimation lens, Wollaston prism, the imaging lens and a single CCD chip is employed in the system. The telescope lens is used to focus the incoming light on an intermediate image. And after collimation, the beam is angularly separated by the Wollaston prism. Two beams corresponding to ordinary light and extraordinary light are subsequently focused on the CCD plane via the imaging lens. The telescope lens is designed to have a telecentric structure in the imaging space, and the invert of which is used as the collimation lens, the completely symmetrical structure design is used to reduce the influence of aberrations. More abundant details from this system can be obtained by using matched image post-processing strategy, which is beneficial to high-quality target detection with enhanced working distance and improved environment adaptability. After joint-designing and optimization, the system modulation transfer function (MTF) value at cut-off frequency is higher than 0.55, and the root-mean-square (RMS) radius of the system is less than 5.3 m, which is smaller than the pixel size of the CCD detector. Additionally, the lateral chromatic aberration of the system is much smaller than the diameter of airy disk, and the absolute values of all kinds of aberrations are kept smaller than 0.02 at the same time. The calculation results show that all the aberrations are mostly corrected. The system imaging is numerically modeled and analyzed, and it is demonstrated that two intensity images with perpendicular polarization states appear adjacently on the CCD plane simultaneously in the imaging simulation. One image is formed with the fraction of the backscattered light polarized parallelly to the incident light, and the other with light polarized orthogonally to the incidence, indicating that the expected design is accomplished. Compared with the traditional amplitude-split polarization imaging system, the proposed real-time polarization imaging system shows that the improved performance for real-time detection with promoted power efficiency, spatial resolution, and the light crosstalk in focal plane is well handled. Moreover, the joint design of the whole system can compensate for the distortion aberration in the vertical direction of the CCD detector, which means that a further improvement of image quality can be expected. The proposed system has a promising perspective in the fields of underwater imaging detection, astronomical observation, remote sensing, biological tissues inspection, and environmental monitoring.
2016, 65 (13): 134202.
doi: 10.7498/aps.65.134202
Abstract +
Polarization imaging technology is a powerful tool in remote sensing, bioscience, or other scientific areas science, and it can extract the polarization information of target effectively. As a novel polarization imaging technology, the channel modulating polarization imaging has been widely investigated in recent years, owing to its prominent advantages of compact, snapshot and full-Stokes acquirable. In the polarization imaging system based on Savart Plate, a half wave plate with its optical axis at an angle of 22.5 is used to rotate the vibration direction of each incident light to 45. Thus, the amplitude of the light could be equally divided by the second Savart Plate polariscope. Finally, the different components of light will interfere with each other on the focal plane and the target polarization information will be modulated in the interference pattern. Since the intensity distribution of interference pattern is sensitive to the orientation of the half wave plate, a small mismatch angle of half wave plate will lead to a wrong polarization image. In order to solve this problem, we investigate the relationship between the mismatch angle and the image intensity grabbed by focal plane array (FPA) and propose an error eliminating method to improve the accuracy of polarization imaging. We analyze how the mismatch angle affects the light intensity and deduce the expression of the image obtained by the FPA. According to the expression, the raw image we grabbed directly by the FPA is a superposition of the modulated Stokes images with different carrier frequencies. Compared with the ideal expression, the expression we obtained shows that the channels of Stokes parameter S1 and S2,3 each contain a constant factor which is related to the mismatch angle . On the basis of this expression, we propose a method to measure the mismatch angle by imaging a target twice, one is behind a polarizer that is oriented at 0 and the other at 45. Then we can calibrate the system by calculating the mismatch angle through the demodulated images. To image a target with the calibrated system, we just demodulate the raw image obtained by the FPA and then divide the reconstructed stokes images by the constant factor which is determined by . For a mismatch angle of 0.5, a computer simulation is conducted. The result shows that through the compensation method, the accuracies of S1, S2 and S3 can be increased by 0.06%, 3.49% and 3.49%, respectively.
2016, 65 (13): 134204.
doi: 10.7498/aps.65.134204
Abstract +
In this study, an 894 nm high temperature vertical-cavity surface-emitting laser (VCSEL) is reported. Furthermore, a Cs chip-scale atomic clock (CSAC) system experiment based on this VCSEL is carried out.To achieve low threshold/power consumption under high temperature condition, the VCSEL epitaxial structure is optimized. Especially, the so-called gain cavity-mode detuning technology is utilized to improve the temperature sensitivity of the device output characteristics. The relationship between the structure of quantum well and the gain is simulated by using the commercial software PICS3D. In order to achieve high gain and low threshold properties, the thickness of the quantum well is optimized. Based on the theory of transmission matrix, the VCSEL cavity mode (etalon) is calculated. Finally, a -15 nm quantum well gain-cavity mode offset is utilized to achieve relatively stable cavity mode gain, which can guarantee the temperature-insensitivity of the VCSEL output characteristics.The output performance of the VCSEL device we fabricated is investigated experimentally. The VCSEL lightcurrent (L-I) characteristic is tested under different temperatures. It is found that benefiting from the gain-cavity mode offset design, the threshold can be maintained at 0.200.23 mA when the temperature increases from 20 ℃ to 90 ℃. Meantime, the output power of more than 100 W is achieved at different temperature levels. By comparing with the results at room temperature, No dramatic degradation of the VCSEL high temperature L-I characteristics is found, which means that the VCSEL output characteristic is relatively temperature-insensitive. The wavelength of the VCSEL is 890.4 nm at a temperature of 20 ℃. When the temperature increases up to 85.6 ℃, the VCSEL wavelength is red-shifted to 894.6 nm (Cs D1 line), corresponding to a red shift ratio of 0.064 nm/℃. According to the polarization requirement of CSAC applications, the polarization properties of the VCSEL are studied and the results are as follows: under an injected current of 1 mA and operation temperature of 20 ℃, Pmax = 278.2 W and Pmin = 5.9 W, corresponding to a polarization ratio of 47:1; at a temperature of 85.6 ℃, Pmax = 239.2 W and Pmin = 4 W, corresponding to a polarization ratio of 59:8:1.Using the VCSEL reported in this paper as a laser source, the CSAC experiment is carried out. At 4.596 GHz of modulated frequency, the output laser of the VCSEL is collimated and interacts with Cs atoms. Finally the closed-loop frequency locking atomic clock is demonstrated. The Cs laser absorption spectrum for laser frequency stabilization, as well as the CPT signal for Cs CSAC microwave frequency stabilization is obtained.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2016, 65 (13): 136101.
doi: 10.7498/aps.65.136101
Abstract +
Using the firs-principles method based on density functional theory, we study the stability and doping performance of double h-BN/Graphene structure, here the exchange correlation potential is expressed through the local density approximation and the interactions between ions and electrons are described by the projective-augmented wave method. Because double layer h-BN/Graphene represents a kind of epitaxial semiconductor system, which can be applied to tunnel pressure sensor, the research is very meaningful. In order to improve the application of this special double layer structures, we often carry out the dopings of some atoms. Unlike previous research work, in which the dopings of the metals Au, Co, Mn and other atoms were took into account, we now mainly consider the dopings of the active metal atoms, such as the dopings of Li, Na, and K atoms. The band structure, electronic density of states, as well as the charge density and stability are studied on the double h-BN/Graphene structure after alkali metal doping. At the same time, bonding and electronic properties of double h-BN/Graphene are discussed under the different biaxial strain conditions. The results show that for the dopings of Li and K atoms, the structure deformation is very large, and the band structure of double h-BN/Graphene can show a small band gap at the K point in the first Brillouin zone, taking on a linear dispersion relation the same as that of the perfect graphene. We can tune the band gap by applying external strain and dopings of atoms, and find a new level appearing near the Fermi level after doping, which is mainly due to the contribution of N atoms. In addition, there exists charge transfer between Na atom and N and C atoms, and the material is converted into metal. We find obvious charge overlapping in the vicinity of Na atoms, these charge overlaps appearing around the Na and C atoms indicate the existence of covalent bond and this covalent bond also appears around the Na atoms and N atoms. We prove the existence of the chemical bonds by adopting the Bader charge analysis, which suggests that the C atoms in the lower graphene layer obtain 0.11 e and dopant atoms around the three N atoms obtain 0.68 e. We infer that the increasing of Na atom doping can increase the charge transfer, so the method of changing the substrate to increase the graphene layer charge density is very conducive to the application of graphene in electronic devices. Because the double h-BN/Graphene has been successfully synthesized, our calculations provide a theoretical basis for the further development and application of technology. We can expect that Na atom doped double h-BN/Graphene can be well applied to the future electronic devices.
2016, 65 (13): 136401.
doi: 10.7498/aps.65.136401
Abstract +
Eutectic solidification is very important for exploring new materials in which the periodic multiphase structures may have a remarkable or enhanced functionality. An asymptotic solution of the solute diffusion equation with flow terms for the rod eutectic in the weak convective melt in directional solidification is obtained by using the asymptotic method, and the effect of weak convection on the rod eutectic growth is studied. The so-called weak convection is defined in this paper as the condition in which the intensity of convection flow ahead of the solid liquid interface is relatively small. The relationships between the intensity of convection flow, the growth velocity, the rod spacing and the average interface undercooling can be derived. The result shows that the weak convection has a significant effect on the rod eutectic growth in directional solidification. The average interface undercooling is related to not only the rod spacing and the growth velocity, but also the intensity of convection flow. When specifically focusing on the effect of the intensity of convection flow on the average undercooling in directional solidification, the growth velocity is kept the same. For a certain growth velocity, the average interface undercooling of the rod eutectic decreases as the intensity of convection flow increases, especially at low growth velocity. The rod spacing, which is formed by solidified melt of eutectic or near-eutectic composition, plays a very important role in determining the properties of final products. In this study, by minimizing the average interface undercooling it is found that the rod spacing is a function of growth velocity and the intensity of convection flow. It is shown that for the small growth velocity, the rod spacing increases as the intensity of convection flow increases; for the large growth velocity, the rod spacing increases very slowly as the intensity of convection flow increases. In other words, the smaller the growth velocity, the greater the effect of the weak convection flow on the rod spacing. Our analytical result is compared with the results from other models, and it is also used to calculate the practical case such as the rod spacing of the typical eutectic alloy, Al-Cu eutectic, under the condition of weak forced convection induced by the accelerated crucible rotation technique. It is shown that the rod spacing increases as the rotation rate or the radial position increases, which is consistent with the experimental results obtained by Junze et al.
2016, 65 (13): 136801.
doi: 10.7498/aps.65.136801
Abstract +
Suspended two-dimensional (2D) materials have been widely used to improve the device performances in comparison with the case of supported 2D materials. To realize such a purpose, 2D materials are mainly suspended on the holes of substrates, which are usually used to support 2D materials. The holes beneath the 2D materials are usually full of air. The air layer with the thickness identical to the hole depth will affect the spectral features of the reflection and photoluminescence spectra of suspended 2D materials because there exist multiple optical interferences in the air/2D-flakes/air/Si multilayer structures. However, it is not clear that how the spectral features depend on the hole depth. In this paper, the reflection spectra of suspended multilayer graphene and MoS2flakes as well as the photoluminescence spectra of suspended multilayer MoS2flakes are systematically studied. The reflection spectra of suspended multilayer graphene flakes exhibit obvious oscillations, showing the optical characteristic with periodic oscillations in wavenumber. The oscillation period decreases with increasing the hole depth (or the thickness of the air layer), but is independent of the thickness of suspended graphene flakes. This can be well explained by the model based on multiple optical interferences in the air/graphenes/air/Si multilayer structures, which have been successfully utilized to understand the Raman intensity of ultrathin 2D flakes and substrate beneath the ultrathin 2D flakes dependent on the thickness of 2D flakes, the thickness of SiO2 layer, the laser wavelength and the numerical aperture of objective. The theoretical simulation shows that the oscillation is obviously observable only when the hole depth reaches up to the value on the order of microns. For suspended multilayer MoS2flakes, the reflection and photoluminescence spectra show similar periodic oscillations in wavenumber and the oscillation period is also dependent on the hole depth. The hole depth is measured by the surface profiler. It is found that the calculated oscillation period based on the measured hole depth and multiple optical interference model is usually larger than the experimental one, which is attributed to the existence of the dielectric layer in the holes. The dielectric layer may be the residues after the hole etching process, which have much smaller refractive indexes than substrates and 2D flakes. This results in an increase of the effective hole depth, which becomes larger than the one measured by the surface profiler. The observation of oscillation period in the reflection and photoluminescence spectra of different flakes of 2D materials demonstrates that the periodic oscillation is a general optical characteristic for optical spectra of suspended 2D materials. In principle, the electroluminescence spectra of suspended 2D materials may also exhibit similar periodic oscillations in wavenumber. These findings may be helpful for understanding the optical spectra of various suspended 2D materials and monitoring the existence of the residues in the holes of substrate after the etching process.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2016, 65 (13): 137302.
doi: 10.7498/aps.65.137302
Abstract +
Owing to their potential applications in topological quantum computation and because of their fundamental interest, Majorana fermions are currently attracting increasing attention. Numerous theoretical and experimental studies exactly show that the quantum dot (QD) structure is a good candidate for the detection of Majorana bound state (MBSs). QD system has many unique transport properties and interesting quantum phenomena, such as quantum interference effect, Fano effect, etc. In addition, compared with a single QD, a coupled QD structure has many adjustable parameters, and thus has more important theoretical and practical value, which provides an excellent platform to detect MBSs. In addition, QD coupled with normal metallic conductor and with superconducting electrode structure exhibits interesting transport properties. One of these properties is the so-called Andreev reflection (AR). Especially, in the subgap regime, the current almost entirely originates from the anomalous Andreev channel; such spectroscopy can thus directly probe any in-gap state. In the present paper, we consider a T-shaped double QD structure with side-coupled to MBSs and investigate the transport properties through the system by adding a normal and a superconducting lead. We calculate the AR conductance through the system in the subgap transport. Here we focus on the effects of MBSs on AR through the system. We find that the AR conductance presents a resonant peak around zero Fermi energy when only one QD (QD1) connects to metal and superconducting leads. As a consequence of quantum interference, when using another QD2 side-attached to QD1, a pair of new Fano-type resonant peaks appear and is distributed aside the zero point and the Fano antiresonant point is at the energy level of the QD2. If an MBS is introduced to couple to QD2, the AR conductance shows several new features. First, a pair of new Fano-type resonance curves appears and the original ones also persist except for the position shifting. In addition, the AR conductance value at the zero Fermi energy point is exactly equal to 1/2G0(G0=2e2/h) in the presence of QD-MBS coupling and zero inter-MBS coupling, which is not dependent on the inert-dot coupling nor the energy levels of QD nor the strength of the QD-MBS coupling. This feature is different from which the T-shaped DQD structure side-coupled to a traditional fermions, showing the robust properties of the Majorana fermions. We also show that in the Andreev reflection conductance curves appear resonance zone changes into antiresonance near zero Fermi energy by adjusting the coupling strength between the double quantum dots in the system without MBSs, while the antiresonance disappears and new resonance peaks appear if an MBS is introduced to couple to QD2. We hope that these results will be helpful for understanding the quantum interference in MBS-assisted AR and may find significant applications, especially in quantum computation.
2016, 65 (13): 137303.
doi: 10.7498/aps.65.137303
Abstract +
The nanostructured materials have been revealed to have exclusive physical and chemical properties due to their quantum-size effects, small-size effects and a large fraction of grain boundaries. Especially, the grain boundaries play an important role in the electrical resistivity of nanostructured metal. We use the four-point probe method to measure the values of electrical resistivity () of the nanostructured aluminum samples and the coarse-grained bulk aluminum samples at temperature (T) ranging from 8 K to 300 K to explore the relationship between the electrical resistivity and temperature. The aluminum nanoparticles produced by the flow-levitation method through electromagnetic induction heating are compacted into nanostructured samples in vacuum by the hot pressing and sintering technology. The microstructures of all nanostructured aluminum samples are analyzed by X-ray diffraction (XRD), transmission electron microscope (TEM), scanning electron microscope with the energy-dispersive spectrometer (SEM-EDS). The densities of all nanostructured aluminum samples are measured by using the Archimedes method (the medium is absolute alcohol). The experimental results show that the shape of aluminum nanoparticles is found to keep spherical from the SEM images and the relative density of all nanostructured aluminum samples is about 93% of the coarse-grained bulk aluminum. The XRD spectra state that the face-centered cubic (fcc) phase dominates the samples and no diffraction peak related to impurities appears in the XRD spectrum for each of all nanostructured aluminum samples. Amorphous alumina layers (about 2 nm thick) are found to surround the aluminum nanoparticles and hence connect the grains in the nanostructured aluminum as shown in the high-resolution TEM images. Owing to the scattering of grain boundaries on electrons and the phonon-electron scattering at grain boundaries, the electrical resistivity is far larger in the nanostructured aluminum than in the coarse-grained bulk aluminum and the relationship between the electrical resistivity and temperature for nanostructured aluminum shows a different feature from that for the coarse-grained bulk aluminum. Although the temperature dependent electrical resistivity ((T)) is a function of T4 at low temperatures for the coarse-grained bulk aluminum, it varies with the temperature not only according to the relation T4, but also according to the relation T3 for the nanostructured aluminum. The residual resistivity (0) of the nanostructured aluminum sample is about 5.510-4m, 5-6 orders magnitude larger than that of the coarse-grained bulk aluminum (2.0110-10m) due to the scattering of both the grain boundaries and amorphous alumina on electrons therein.
EDITOR'S SUGGESTION
2016, 65 (13): 137301.
doi: 10.7498/aps.65.137301
Abstract +
A novel plasmonic lens (PL) with simple nano-structure is proposed to realize the subwavelength focusing. The proposed PL is composed of the gold film with only five dielectric-filled nanoslits. The exit surface of the gold film is processed into quadric shape that can be parabolic, elliptical or hyperbolic cylinders. The film is fabricated to form five uniformly spaced nanoslits with different widths and depths. All five slits are symmetrically arranged with respect to the center of lens and filled with a dielectric medium (i.e., SiO2). Under the illumination of TM polarized beams, the surface plasmon polaritons (SPPs) are excited at the entrance surface of the PL, then pass through the SiO2-filled slits while acquiring specific phase retardations, and are finally coupled to the light waves in the free space. Each light wave originating from the slit can be regarded as an individual point source, and the constructive interference of light waves from slits gives rise to the beam focusing at the focal plane of the PL. We investigate the phase modulation mechanism of the PL and find that the focusing performance relies on the shape of exit surface, filling medium and geometric parameters of the slits. A suitable phase modulation can be achieved by adjusting the structure parameters of the PL with a specific exit surface shape. Three kinds of quadratic cylindrical PLs, i.e., parabolic, elliptical and hyperbolic cylindrical ones with continuous or stepped exit surface are designed to realize the focusing of TM polarized subwavelength beams in visible spectrum. The finite difference time domain (FDTD) method is employed to compute the light field and to investigate the focusing characteristics of the proposed PL. The performance measurements include the focal length, depth of focus (DOF) and full-width half-maximum (FWHM). The simulation results confirm that the proposed PL with a 2-m-diameter aperture can achieve the subwavelength focusing at a focal length of micron scale. The attainable smallest FWHM of the focal spot is 0.4050 (0 denoting the wavelength of the incident light) which is well beyond the diffraction limit. It is also worth mentioning that the step-like cylindrical PL can yield a sharper focal spot than the continuous cylindrical PL. For example, the FWHM of focal spot produced by the stepped elliptical cylindrical PL is about 92% of that produced by the continuous elliptical cylindrical PL. The proposed PL has the advantages of simple and compact structure with much smaller lateral dimension and easy integration with other photonic devices. Our study helps design the easy-to-fabricate PLs and facilitates applications of plasmonic devices in the fields such as optical micro manipulation, super-resolution imaging, optical storage and biochemical sensing.
2016, 65 (13): 137401.
doi: 10.7498/aps.65.137401
Abstract +
The interaction among particles in front of solid-liquid interface during solidification plays a role in determining the trajectories, distribution and sizes of particles, which eventually determines the properties of material. By using the interaction to control the migration of particles, impurity particles can be removed from the melt. A method of using an external high magnetic field to simulate the migration of Fe in Cu melt is proposed. Static high magnetic field (0.1 Tesla and 12 Tesla) and gradient high magnetic field (-92.1 T2/m) are subjected to the solid-liquid mushy zone of Cu-30 wt%Fe alloy. The case without high magnetic field is also investigated for comparison. Both macro- and microstructure of the samples are observed by optical microscope. The results indicate that primary Fe dendrites in Cu-Fe alloy are transformed into spherical Fe-rich particles after solidification in mushy zone, and high magnetic field is capable of changing the migration, distribution and arrangement of Fe-rich particles. In the absence of a static high magnetic field, Fe particles are distributed in Cu melt homogeneously. With increasing the magnetic flux density of imposed static high magnetic field, Fe-rich particles gradually migrate upwards. The migration direction is opposite to the direction of the gravity, and there are no Fe-rich particles kept on the bottom of the samples imposed by magnetic field. In the presence of negative high gradient magnetic field, however, the Fe-rich particles migrate downward and the direction is along the direction of the gravity. A model is built up to clarify the body force of Fe-rich particles and to analyze their movement while they are affected by high magnetic field. The results show that the migration behaviors of Fe-rich particles are related to the viscous dragging force, the interaction force between magnetic dipoles, and the magnetization force induced by gradient high magnetic field. The displacement of Fe particles is closely dependent on the body force. Through the analysis the experimental results are well explained. The diameters of Fe-rich particles are statically summarized under different high magnetic field conditions and in different zones. With increasing magnetic flux density of static high magnetic field, the aggregation of particles is increased. The magnetic field gradient, however, reduces the aggregation of particles. This might be as a result of the competitive coagulation between Stokes sedimentation and Marangoni migration in Cu melt. Microstructure of the samples indicates that Fe-rich particles tend to align along the direction of high magnetic field and the degree of alignment is likely to be related to external magnetic field strength, resistance force, effective time, and initial condition of particles, etc. As they are parallel to the direction of high magnetic field, the energy of the system is minimum, suggesting that the system is stable. The present study shed light on how to remove strong magnetic impurity from Cu melt.
2016, 65 (13): 137701.
doi: 10.7498/aps.65.137701
Abstract +
Decrease in remnant polarization at lower temperature, or low temperature degradation of polarization, in some ferroelectrics has attracted much attention. To investigate the mechanism of the decrease, phenomenological theory of ferroelectrics and the relevant mechanism of dipole in alternating electric field are used to develop a model of hysteresis-frozen effect of dipole in electric hysteresis loop measurement. Within the frame of Landau-Ginzburg-Devonshire theory, Ising model is used to derive the relationship among remnant polarization, coercive field, and saturated polarization strength. Then, two aspects are investigated: response of a dipole and thermodynamic properties of ferroelectric. Response of a dipole in an electric field is often described by relaxation time, on the assumption that Debye equation is satisfied. Potential barrier in the Debye equation is the Gibbs free energy barrier from one ferroelectric state, +P, to another ferroelectric state, -P. Increase in the Gibbs free energy barrier with temperature decreasing will prolong the relaxation time. As ferroelectrics can be taken as a capacitor, first order response function is used to introduce a hysteresis factor with measuring frequency and relaxation time into the expression of remnant polarization. In the aspect of thermodynamic properties of ferroelectric, the variation of compositions is a significant reason. In numerical simulation based on the derived formula the remnant polarization exhibits a frequency related peak, and shift of the peak depends on some other reasons: the increase of soft-mode coefficient in phase transition shifts the peak towards high temperature; the increases of coercive field, temperature-polarization coefficient (a concept defined in the present paper to indicate increase in polarization with increasing temperature) and saturated electric field shift the peak toward low temperature. Compared with the reported experimental results of BaTiO3/BiScO3 compound ceramics, the results show a good coincidence with numerical simulations. The parameter values of numerical simulation indicate that a large shift toward high temperature in peak of remnant polarization with increasing BiScO3 composition ratio is due to the increase in soft-mode coefficient with only small decrease in the Curie temperature. The soft-mode coefficient and temperature-polarization coefficient are closely related to polarization characteristic, ferroelectric, dielectric and mechanical properties. Therefore, the decrease in remnant polarization at low temperatures, ascribed to the hysteresis of dipole to a constant measuring frequency, may have an influence on changes in various properties, but freezing effect of dipole at low temperature can help ferroelectrics to save data longer.
2016, 65 (13): 137801.
doi: 10.7498/aps.65.137801
Abstract +
Gold film (40-nm-thick) sputtered on the glass substrate was decorated by using the sol-gel copolymer templated mesoporous TiO2 thin film (275-nm-thick) to fabricate the plasmon waveguide (PW). The Raman spectroscopy based on the Au/TiO2 PW is studied theoretically and experimentally. The surface morphology of the mesoprous TiO2 thin film and the cross-section of the PW chip are obtained by scanning electron microscopy (SEM) and the porosity (P) of mesoporous TiO2 thin film is determined to be about 0.589 by fitting the calculated waveguide coupling dips to the measured resonance wavelengths based on Fresnel equations. The angular distributions of Raman power from the molecular dipole located in the core layer of the waveguide are theoretically investigated based on the optical reciprocity theorem. The calculated results suggest that the Raman light radiated into the substrate consists of the directional Raman signal propagating at the resonant angle and the non-directional Raman signal whose radiation angles are smaller than the critical angle of total reflection. The directional Raman signal could be detected with the aid of the prism coupler, while the non-directional Raman signal can be detected directly on the back of the sensor chip. Furthermore, the angular distribution of the backscattered Raman signal is divergent and it is unaffected by the use of the prism coupler. The highest power of the directional Raman signal is much larger than that of the non-directional Raman signal and the backscattered Raman signal. The Raman spectroscopy based on the PW is studied by experiment with CV molecules adsorbed into the mesoporous TiO2 thin film. The Raman spectrum is obtained with the 532 nm laser radiating directly onto the waveguide surface. The experimental results show that the Raman signal including the directional Raman signal, non-directional Raman signal and the backscattered Raman signal can be detected with the PW chip. Besides, the directional Raman signal can only be detected by using the prism coupler, while the non-directional Raman signal can be detected directly on the back of the chip. Then the results also show that the peak intensity of the directional Raman signal is twice higher than that of the non-directional Raman signal. The further measurements reveal that the backscattered Raman signal hardly changes under the condition with or without the prism coupler. The experimental results mentioned above are in accordance with the theoretical calculations. The Raman spectroscopy based on PW in this work has potential value in further developing the Raman sensing technique.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2016, 65 (13): 138401.
doi: 10.7498/aps.65.138401
Abstract +
Since the rotorcraft can easily be recognized by using the micro-Doppler (m-D) signature of rotor blades, the m-D effect induced by micro-motion dynamics plays an important role in target recognition and classification. However, the existing researches on the rotor blades pay little attention to the mechanism of the time-domain and time-frequency-domain flash phenomena. To comprehensively explain the flash phenomena from physics, the modeling of the rotor blades and the mechanism of the flash phenomena are studied in this paper. Firstly, for the rotor blades, the target cannot be represented as a rigid, homogeneous line nor several points. Taking the scattering coefficients and the interval of adjacent scattering points (the scattering point distribution on the blade) into consideration, the scattering point model of the rotor blade echo is established, and the influence of the scattering point distribution on the radar echo is analyzed as well. The detailed mathematic analysis and comparison results show that the conventional integral model of the rotor blade is only a special case of the scattering point model. Furthermore, In the case where the scattering point model is approximately equivalent to the conventional integral model, the critical interval of adjacent scattering points is deduced by mathematic analysis. Secondly, on the basis of the proposed model above, the physical mechanism of the time-domain and time-frequency-domain flash phenomena is studied from the viewpoint of the electromagnetic (EM) scattering. On the one hand, considering the EM scattering and scattering point distribution, the mechanism of the time-domain flashes is analyzed. Ideally, when the rotor blade is at the vertical position relative to the radar line of sight, i.e., at the flash time, the blade has the strongest echo. At this moment, the radar echo consists of echoes of all scattering points, thus inducing the time-domain flashes. At the non-flash time, the scattering points at the tip of blade and hub of rotor have stronger scattering intensities, so the echo is much weaker than that at the flash time. On the other hand, the time-frequency analysis and the cross range resolution are simultaneously used to analyze the mechanism of the time-frequency-domain flashes in the m-D signature. The m-D signature of the rotor blades consists of three parts: the time-frequency-domain flashes, the sinusoidal Doppler curves, and the zero-frequency band. At the flashes time, the Doppler frequency of adjacent scattering points cannot be distinguished, thus the m-D signature has the frequency band caused by all scattering points, i.e., the time-frequency-domain flashes appear. At the non-flash time, the sinusoidal Doppler curves and the zero-frequency band are caused by the scattering points at the tip of blade induced by the scattering points at the hub of rotor respectively. Finally, the simulation results about the scattering point model with the different intervals of adjacent scattering points show that the effectiveness of the proposed model and the correctness of theoretical analysis.
2016, 65 (13): 138402.
doi: 10.7498/aps.65.138402
Abstract +
High power microwave (HPM) source is attractive in generating gigawatt (GW) class microwaves based on the beam-wave interaction. Generally, HPM source with a high beam-wave conversion efficiency has a higher impedance. To improve the single-tube output power of HPM source, reducing the impedance of the device and increasing its power capacity are necessary. In this paper, a new low-impedance HPM source is proposed and proved to be capable of generating two phase-locked high power microwaves, which makes it promising to realize a higher combined power in a single HPM device.The new low-impedance HPM device consists of a two-cavity TKA (denoting the outer sub-source in the following) and a multiwave Cerenkov generator (referring to the inner sub-source below) inserted in the TKA inner conductor. These two sub-sources are connected in parallel and share a common magnetic field. A dual-concentric annular cathode is used in this microwave source, which is capable of emitting two concentric annular electron beams and driving the internal and external sub-source simultaneously. The advantages of this device are reducing the impedance and improving the injection electric power. When a voltage pulse is applied to the diode, part of microwaves generated in the inner subsource will leak into the outer sub-source (i.e., TKA) through the A-K gap. By amplifying the leakage microwaves, the TKA will be easily locked by the inner sub-source. Considering the fact that the microwave source consists of two sub-sources, the power capacity will also be greatly improved.As a result, particle-in-cell simulation indicates that when the diode voltage is 687 kV and the axial magnetic field is 0.8 T, two microwave beams that have a nearly identical frequency of 9.72 GHz and output powers of 1.20 GW and 2.58 GW respectively, are generated. The corresponding power conversion efficiencies are 28% and 30%, respectively. The frequency difference between these two microwaves fluctuates within 3 MHz and their phase difference is not in excess of 3. When the diode voltage changes from 665 kV to 709 kV, frequency difference between the two sub-sources fluctuates within 3 MHz and their phase difference fluctuation is within 5 in one voltage burst; the phase difference changes 10 in this voltage range. The impedance of this HPM source is as low as 36 .To sum up, the new HPM source proposed in this paper has a lower impedance and higher power capacity. The phase difference between the inner sub-source and the outer sub-source is very stable and favorable for the coherent power combination, which indicates that the new HPM source promises to realize a higher output power in a single-tube device.
2016, 65 (13): 138501.
doi: 10.7498/aps.65.138501
Abstract +
There is an increasing interest in grapheme photodetector for its applications, because graphene has rich optical and electronic properties, including zero band gap, high mobility and special optical absorption properties. A design of microcavity-enhanced photodetector based on ultra-thin graphene is proposed in this paper: detector absorption can be effectively improved by confining the light field in the microcavity. Through studying the light field resonant condition in the microcavity and enhanced mechanism of detector responsivity under resonant mode, the light absorption model of a microcavity-enhanced graphene photodetector under standing wave effect is established; it is analyzed that the influences of microcavity mirror reflectivity and length on detector performance are increased by light field. Further the optimal structure parameters and performance evaluations of microcavity-enhanced graphene photodetector at different incident wavelengths are demonstrated. Theoretical analysis shows that under the standing wave effect the effective absorption coefficient of monolayer graphene at the antinode is one multiple enlargement compared with no cavity; the microcavity length and topbottom mirror reflectivity directly affect the optical total phase during light folding back at one time in the microcavity, and the shift of the total optical phase changes the full width at half maximum (FWHM) of the responsivity of the microcavity-enhanced graphene photodetector. Through coordinating the relations among the microcavity length and reflectivities of two mirrors and the incident wavelength, it can be realized that the photodetector has a good characteristic of wavelength selectivity. At a nominal operating wavelength of 850 nm, the presented microcavity-enhanced graphene photodetector can reach a responsivity of 0.5 A/W, 32-fold increase compared with monolayer graphene photodetector with no cavity and FWHM can reach 10 nm, indicating that the designed photodetector has a high responsivity and a good charactoristic of narrowband. As for the application in the practical engineering, through adopting bias on the two sides of graphene in the cavity to speed up the migration velocity of the photon-generated carrier, more photon-generated carriers are produced to increase the photodetector responsivity. However, the increased level of photodetector responsivity will be impeded acctually on account of the high contact resistance between graphene and electrode, and the measured value will not equal the theoretical value, so the quantitative analysis on the value of the bias should be carried out. Through combining the microcavity with graphene the incident light can be confined to reflect multiple times between two mirrors in the microcavity to improve the graphene absorption, and then make the microcavity-enhanced graphene photodetector responsivity improved. Our approach can be used to improve the optical response of graphene photodetector, and provides a way to solve the trade-off between photodetector responsivity and response speed.
2016, 65 (13): 138101.
doi: 10.7498/aps.65.138101
Abstract +
The purpose of this work is to prepare the high-performance transparent conductive thin films of fluorine-doped tin oxide (FTO) by using a simple technological process. The FTO thin films are formed in the period of calcination process combined with the advantages of sol-gel method and chemical vapor deposition method, which not only avoids the shortcomings of film cracking in sol-gel coating process, but also reduces the cumbersome traditional dip-coating method and spin-coating method on glass substrates, largely simplifying process and cutting costs. The FTO thin films are deposited onto glass substrates by the sol-gel-evaporation method with SnCl45H2O as a tin source, and SnF2 as a fluorine source. The effects of F-doping content and the structure of the film on the properties of FTO film are mainly studied. The prepared films are characterized by IR, DTA-TG, XRD, TEM, SEM, etc. The results show that the maximum performance index (TC) of the FTO film, the lowest surface resistance of 14.7 cm-1, and the average light transmittance of 74.4% when F/Sn=14 mol% are achieved under the conditions of the reaction temperature of 50 ℃, the reaction time of 5 h, sintering or evaporation temperature of 600 ℃ for 2 h. It is indicated that part of O is replaced by F, and SnO2-xFx crystal structure is formed. It reveals that the crystal structure is polycrystalline and has a preferential orientation along the (110) direction and the spacing between the lattice fringes is about 0.33 nm in the FTO film. And the particles in the FTO film present a tetragonal rutile phase with an average size of 20 nm and a film thickness of 1.22 m. Fractal dimension of image by dealing with SEM image of FTO film shows that the surface resistance decreases with the decreasing of fractal dimension, which in fact critically demonstrates the lower barrier. The lower the barrier, the smoother the surface of the thin films is. So the fluorine concentration is the main factor affecting the properties of FTO thin film. Too much or too less fluorine is not conducive to the growths of SnO2-xFx crystals. And then the three-dimensional information such as structure, particle shape and size of the FTO thin film is also the factor influencing the FTO film properties. The analysis of SEM shows that the surface morphology of the thin film is in the pyramid-shaped structure, which is beneficial to improving the utilization of photons, and well used in the optoelectronic devices.
2016, 65 (13): 138801.
doi: 10.7498/aps.65.138801
Abstract +
Heterojunction with intrinsic thin-layer (HIT) solar cells attract attention due to their high open circuit voltage and stable performance. However, short circuit current density is difficult to improve due to light losses of transparent conductive oxide and hydrogenated amorphous silicon passivation (a-Si:H) layer and low absorption coefficient of crystalline silicon (c-Si). Silicon germanium alloy (Si/Si1-xGex) quantum wells and quantum dots are capable of improving low light utilization by strong optical absorption in the infrared region. In this article, opto-MoS2of the HIT solar cells integrated with Si/Si1-xGex quantum wells (HIT-QW) as a surface absorber are investigated by numerical simulation with Technology Computer Aided Design (TCAD). The influences of germanium content on the MoS2of HIT solar cells with long carrier lifetimes of Si1-xGex layers (p*) and defect-free a-Si:H/c-Si interface are investigated at first. The simulation results indicate that optical utilization in the infrared region is enhanced with the increase of germanium fraction, while open circuit voltage degrades due to the decreasing of the energy band gap of Si1-xGex, radiative recombination and auger recombination mechanism in the Si/Si1-xGex quantum wells. And the conversion efficiency reaches a maximum value at a germanium fraction of 0.25 then drops distinctly. When the germanium fraction increases from 0 to 0.25, the short circuit current density increases from 34.3 mA/cm2 to 34.8 mA/cm2, while the open circuit voltage declines from 749 mV to 733 mV. Hence, the conversion efficiency increases from 21.5% to 21.7% due to the fact that the enhancement of short circuit current density compensates for the reduction of open circuit voltage. When the germanium content increases to more than 50%, a serious open circuit voltage loss of more than 130 mV associated with the energy band gap loss of Si1-xGex arises in the HIT-QW solar cells, which indicates that the dominating carrier transport mechanism changes from shockley diffusion to recombination in the Si/Si1-xGex quantum wells. Subsequently, the influences of interface defects at a-Si:H/c-Si interface and bulk recombination centers in the Si/Si1-xGex quantum wells are discussed. Both interface holes at a-Si:H/c-Si interface and bulk holes in Si1-xGex quantum wells can be recombined through the interface defects at a-Si:H/c-Si interface and bulk recombination centers in the Si/Si1-xGex quantum wells, respectively, which restricts the position of hole fermi level in the open circuit condition. When the germanium fraction increases, the influence of interface defects at a-Si:H/c-Si interface becomes weak on the degradation of open circuit voltage compared with the significant influence of the bulk trap centers. Moreover, p* of longer than 510-5 s is necessary for the retention of electrical performance of HIT-QW solar cells by the simulation. Based on this research, high-efficiency HIT solar cells can be achieved by incorporating high-quality Si/Si0.75Ge0.25 quantum wells, which also requires the impactful passivation of a-Si:H/c-Si interface.
THE PHYSICS OF ELEMENTARY PARTICLES AND FIELDS
EDITOR'S SUGGESTION
2016, 65 (13): 131101.
doi: 10.7498/aps.65.131101
Abstract +
Sum rules for the dynamic structure factors are powerful tools to explore the collective behaviors in many-body systems at zero temperature as well as at finite temperatures. The recent remarkable realization of synthetic spin-orbit (SO) coupling in quantum gases is opening up new perspective to study the intriguing SO effects with ultracold atoms. So far, a specific type of SO coupling, which is generated by a pair of Raman laser beams, has been experimentally achieved in Bose-Einstein condensates of 87Rb and degenerate Fermi gases of 40K and 6Li. In the presence of SO coupling, the dynamic structure factors for the density fluctuation and spin fluctuation satisfy different sum rules. In particular, in the two-component quantum gases with inter-species Raman coupling, the f-sum rule for the spin fluctuation has an additional term proportional to the transverse spin polarization. Due to the coupling between the momentum and spin, the first moment of the dynamic structure factor does not necessarily possess the inversion symmetry, which is in strong contrast to the conventional system without SO coupling. Such an asymmetric behavior could be observed in both Fermi gases and Bose gases with Raman coupling. As a demonstration, we focus on the uniform case at zero temperature in this work. For the non-interacting Fermi gases, the asymmetric first moment appears only when the Raman detuning is finite. The asymmetric amplitude is quite limited, and it vanishes at both zero detuning and infinite detuning. For the weakly interacting Bose gases, the first moment is asymmetric in momentum space even at zero detuning, when the ground state spontaneously breaks the Z2 symmetry in the plane-wave condensation phase. Using the Bogoliubov method, the dynamic structure factor and its first moment are explicitly calculated for various interaction parameters. We find that the asymmetric behavior in the spin channel could be much more significant than in the density channel, and the asymmetric amplitude is enhanced as the interaction strength increases. Experimentally, the dynamic structure factors can be directly measured through the two photon Bragg scattering. Numeric simulations show that to observe the deviation of inversion symmetry in the first moment, the resolution of the Bragg spectroscopy should reach a required value. For the typical parameters of the rubidium atomic gas, the required resolution is about 10-2Er with Er being the recoil energy. Our predictions can be tested in the future experiment.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
2016, 65 (13): 139501.
doi: 10.7498/aps.65.139501
Abstract +
Image definition will be influenced by alignment errors of mirrors in an optical system consisting of hyperbolic, parabolic or ellipse mirrors. The major factors of alignment errors are gravity, wind loads and heat exchange for some optical systems like ground-based telescopes, while vibration and temperature gradient for systems like space telescopes. Larger telescopes are more sensitive to these error sources, which becomes the concerns of researchers. So the alignment errors of mirrors must be corrected in time to keep systems working in best condition. In order to solve the problem, many methods are proposed based on the detection of wave-front errors using wave-front sensors like Hartmann-Shack. However, wave-front sensors may not be used or cause optical systems to be more complicated. For example, multi-fields must be tested when telescope is working. On the one hand, if a wave-front sensor is used, it must be moved around imaging plane, on the other hand, if more wave-front sensors are used, system must be more complicated. So a new method is discussed for alignment error correction by evaluating the quality of spot diagrams based on the using of stochastic parallel gradient descent (SPGD) algorithm. The method considers the performance metric like spot diagram radius as a function of control parameters and then uses the SPGD optimization algorithm to improve the performance metric. The control parameters include positions of mirrors. The iteration process must be used in the right way to control position parameters. If it is not considered, a problem may come up that positions of spot diagrams may be influenced by the iteration. Furthermore, spot diagrams will probably disappear from detectors. Then the radii of spot diagrams are not correct. So a better way is put forward by the combination of de-center and tilt of mirrors. The way ensures that the position error produced by de-center and tilt are compensated for. A formula is provided in this paper to give the relationship between them. Based on the analysis, an optical system is designed to verify the conclusion. The SPGD algorithm is achieved by computer programming and the position of the mirror is controlled by a hexapod. Firstly, the problem is verified that the spot diagram will disappear from the detector with a normal iteration process. Then the new way is implemented. In the iteration process, the spot diagram is always in the center of the detectors. In order to prove the feasibility of the method, three different alignment errors are tested and all of them each give an Airy disk finally. The experiment can provide reference for engineering practice.
