Vol. 67, No. 10 (2018)
ATOMIC AND MOLECULAR PHYSICS
Multiconfiguration time-dependent Hartree-Fock treatment of electron correlation in strong-field ionization of H2 molecules
2018, 67 (10): 103301. doi: 10.7498/aps.67.20172701
Electron correlation plays an important role in the multielectron interactions of many physical and chemical processes.The investigation of correlation effects in the non-perturbative electronic dynamics (e.g.non-sequential double ionization) when atoms and molecules are subjected to strong laser fields requires non-perturbative theoretical treatments. The direct numerical integration of the time-dependent Schrödinger equation successfully explains many experimental results,but it is computationally prohibitive for systems with more than two electrons.There is clearly a need for a theory which can treat correlation dynamics self-consistently in strong time-dependent electric fields.In this paper we develop a three-dimensional multiconfiguration time-dependent Hartree-Fock method,which can be applied to the non-perturbative electronic dynamics for diatomic molecules,and it can also investigate the effect of electron correlation in strong-field ionization of H2 molecules.This method adopts the prolate spheroidal coordinates (which can treat the two-center Coulomb potential accurately) and the finite-element method together with discrete-variable representation (which lowers the calculation burden from two-electron integrations).For the temporal propagation,we use the efficient short iterative Lanczos algorithm for the equation which governs the configuration expansion coefficients,while an eight-order Runge-Kutta (RK) method and an Bulirsch-Stoer (BS) extrapolation method,both with adaptive precision controls,are implemented to solve the nonlinear orbital equation.While both methods yield correct results,the BS method displays a better stability in the realtime propagation,while the RK method demands less computation.The alignment-dependent ionization probabilities of H2 molecules in intense extreme ultraviolet pulses are calculated.Comparisons between multi-configuration and single-configuration results show that electron correlation has little effect on the single ionization process,but it plays an important role in double ionization,leading to the decrease in the ionization probability.The double ionization probability from the single-configuration space 1σ is about three times larger that from 4σ1π.The ionization probability increases monotonically when the alignment angle increases from 0° to 90°, yielding a ratio of 2.6(1.5) between 90° and 0° for the double (single) ionization process.This method presents the basis for the future study of electron correlation in strong-field processes.
2018, 67 (10): 103401. doi: 10.7498/aps.67.20180324
The isotopic effect is a significant way to further understand the reaction mechanism without greatly changing the system. However, the isotopic effect of the H + Li2 reaction has received little attention in previous theoretical studies. Furthermore, as a deep potential well exists on the reaction path, obtaining convergent result is very time-consuming. So some approximate methods were used in previous theoretical calculations. However the Coriolis coupling effect plays an important role in the reaction, and thus whether these approximate methods are reasonable needs further testing. Based on the potential energy surface (PES) reported by Song et al., the dynamical calculations of H/D + Li2 LiH/LiD + Li reactions are carried out by time dependent quantum wave packet method with second order split operator in a collision energy range from 0 to 0.4 eV. In order to obtain the convergent results, lots of convergence tests are carried out and because the Coriolis coupling effect plays an important role in the reaction, all the number of projections of total angular momentum J are included in the present calculation. The dynamical properties such as reaction probability, integral cross section, differential cross section are calculated and compared with previous theoretical values. Large discrepancies are found between present results and the values obtained from Gao et al. especially at high collision energies. Owing to the fact that the same PES is applied to the calculation and Gao's results of total angular momentum J=0 accord well with the present values, we suppose that the parameters used in the calculation have little influence on the final results and the main discrepancies are attributed to the number of projections of total angular momentum which are cut off in Gao et al.'s calculation. In order to verify our speculation, the numbers of projections of total angular momentum which are 1, 5, 10, 15, 20, and 25, are considered in the calculation, respectively. The results indicate that the main discrepancy between present values and the results obtained from Gao et al. can be attributed to the number of projections of total angular momentum used in Gao et al.'s calculation that is not convergent, and that the present values are more accurate than previous theoretical studies for all the numbers of projections of total angular momentum which are included in the calculation. Furthermore, when the H atom is substituted by the heavy isotope D atom, the reaction probability and integral cross section become large. However, it does not generate large effect on the reaction mechanism. The forward and backward symmetry differential cross section signals indicate that the complex forming reaction mechanism dominates the reaction.
ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
A kind of optical diffractive element named photon sieve, which is essentially Fresnel zone plate in which the transmissive rings are replaced with a large number of randomly distributed isolated pinholes, can be used to focus X-ray and extreme ultraviolet lithography spectrum into spots with sizes smaller than the diameter of the smallest circular pinhole. However, both the traditional photon sieves and Fibonacci sieves have no more than two axial foci. In order to break this limitation, the Lucas sequence is introduced into the design of photon sieves, and thus producing four axial foci. With respect to the previous Fibonacci sequence, Lucas sequence has the same recursion relation as well as the same eigenvalue of golden mean =(1 + 5)/2. The only difference between them is the first two initial seeds. Based on Fresnel-Kichhoff diffraction theory, the simulation results show that there exist four focal spots with approximately equal intensity along the optical axis on condition that the hole diameters are set to be 1.16 times the underlying Fresnel zone width. Then in order to verify the validity of our proposed model, a Lucas sieve of diameter 12.11 mm and referred focal length 180 mm is fabricated by photolithography and its focusing properties are precisely measured by the in-line phase-shifting digital holography. In experiment, a quarter wave plate is used to realize two-step phase-shift interferences, and obtain the quad-focal length by auto-focusing algorithm in holography. Meanwhile, the quad-focal spots can also be calculated through the diffraction propagation of reconstructed object wave. Compared with the theoretical values, the measurement results indicate that the maximum deviation of quad-focal lengths is less than 0.9%, and the relative errors of the full width at half maximum of four Airy spots are all less than 5%. The experimental results agree well with the theoretical analysis results. Owing to the advantages of small volume, little weight and easy processing, Lucas sieve has great potential in X-ray microscopy, array imaging for living biological cell and especially in the next generation of synchrotron light sources.
2018, 67 (10): 104205. doi: 10.7498/aps.67.20172550
Using transfer matrix method and TFcalc thin film design software,the reflectance spectrum of distributed Bragg reflector (DBR) and vertical cavity surface emitting laser (VCSEL) are simulated.The reflectance spectra from the cavity and surface are compared with each other,thus providing the basis for white light source (WLS) optical reflectance spectrum of the VCSEL epitaxial wafer.When using WLS to characterize VCSEL wafer,it is necessary to combine the simulation results and the shape of optical reflectance spectrum to speculate the reflectance seen from the cavity.The influences of different cap layers on the reflectance of DBRs are discussed theoretically and experimentally.With a 1/4 GaAs cap layer,the reflectance reaches up to 97.8% seen from the cavity.This design can make the wavelength of the VCSEL etalon picked easily because of avoiding the influence of test noise. The active region has higher heat accumulation due to the small area and poor thermal conductivity.The characteristics of the gain spectrum of InGaAs/AlGaAs strained quantum well (QW) under different temperatures and the temperature distribution in VCSEL are simulated by Crosslight software.The gain-to-cavity wavelength detuning is used to improve the slope efficiency and the temperature stability.The temperature in active region ranges from 360 K to 370 K.The gain peak wavelength and the Fabry-Perot cavity wavelength are designed in the ranges of 829-832 nm and 845-847 nm,respectively.Epitaxial wafer with top-emitting VCSEL structure grown by metal-organic chemical vapor deposition is characterized.The room temperature photoluminescence peak is at 827.5 nm and the etalon cavity wavelength measured by optical reflectance is 847.7 nm,which are consistent with designed values. The oxide restricted VCSELs with 7.5 m oxide aperture are fabricated.The image of the infrared light source CCD shows that the oxide aperture is circular.A passivation layer of 120 nm SiO2 is finally deposited to insulate water vapor.The threshold current is 0.8 mA,and the maximum output power reaches up to 9 mW at 13.5 mA.The optical spectrum at 6.0 mA reveals multiple transverse modes.The center wavelength is 852.3 nm and the root mean square (RMS) spectrum width is 0.6 nm,meeting the high-speed Datacom standards.Shannon theory indicates that the maximum data rate is not only proportional to bandwidth but also related to signal-to-noise ratio (SNR).It is effective to reduce relative intensity noise and enhance the SNR by increasing output power.From the eye diagram of 25 Gbit/s on-off key VCSEL,it is demonstrated that fall time is 38.66 ps,rise time is 41.54 ps,SNR is 5.6,and jitter RMS is 1.57 ps.Clear eye opening is observed from eye diagram of 25 GBaud/s PAM-4 VCSEL,which indicates the qualified 50 Gbit/s high speed performance.
2018, 67 (10): 104206. doi: 10.7498/aps.67.20172532
High-resolution and high-sensitivity molecular spectroscopy is widely used in fundamental molecular physics, atmospheric studies, remote sensing, industrial process monitoring, and medical diagnostics. Accurate determination of the parameters of molecule absorption lines, such as line positions, line strengths, line widths and profiles, is essential to support these studies and applications. For example, in order to retrieve the column density of carbon dioxide with a precision of one part per million (ppm), we need laboratory data of line positions with a uncertainty lower than 0.3 MHz and line intensities with a relative accuracy better than 0.5%. Here we present precision spectroscopy of molecules using a laser locked with a high-finesse cavity. The cavity made of invar is thermo-stabilized to reduce the drifts of its length and the cavity mode frequencies. The frequency of the probe laser is locked on a longitudinal mode of the cavity by using the Pound-Drever-Hall method. Another beam from the probe laser, which is frequency shifted and on resonance with a nearby longitudinal mode of the cavity, is used for cavity ring-down spectrum (CRDS) measurement. The CRDS absorption spectrum is recorded by stepping the modulation frequency of a fiber electro-optic modulator in increment of the mode spacing of the cavity. Note that the cavity mode frequencies are shifted due to the dispersion introduced by the absorption lines. Prior to the CRDS measurements, the transmittance spectra of the cavity modes are recorded by scanning the probe laser frequencies over the resonance, which allows the determination of the cavity mode frequencies with an accuracy at a Hz level. Therefore, a dispersion spectrum is also obtained using the same setup by measuring the frequency shifts of cavity modes of the samples with and without absorption. The absolute frequency of the probe laser is determined by an optical frequency comb referring to a GPS-disciplined rubidium clock. The long term drift of beat frequency between the optical frequency comb and the probe laser is measured to be about 1.8 MHz per hour, which is consistent with the thermal expansion of the cavity under a temperature drift of 50 mK. The performance of the spectrometer is demonstrated by measuring the Doppler-broadened spectra of CO2 around 6470.42 cm-1. Precise spectroscopic parameters are derived from both the absorption and dispersion spectra recorded by the same spectrometer. The line position is determined with an accuracy of 0.18 MHz, which is over one order of magnitude better than those given in previous studies and spectral databases.
2018, 67 (10): 104209. doi: 10.7498/aps.67.20172604
Conventional Muller matrix imaging polarimeter (MMIP) with several rotating elements suffers mechanical complexity, vibration noise, heat generation, and other unwanted problems. To overcome those shortcomings, we present a snapshot Muller matrix imaging polarimeter (SMMIP) using a birefringent crystal with high extinction ratio. The snapshot imaging polarimeter allows a single image to be used to measure the polarization of a scene without electronic control units or moving mechanical components. This new polarimeter combines the technique of Muller matrix spectropolarimetry with a snapshot imaging polarimeter through using modified Savart polariscope (MSP-SMMIP). It contains both a generator and an analyzer module. Spatial polarization fringes are localized on a sample by incorporating modified Savart polariscope into a polarization generator module. These fringes modulate the Mueller matrix components of the sample, which are subsequently isolated with modified Savart polariscope in an analyzer module, and the analyzer and the imaging lens combine with 16 beams to create interference, resulting in spatial modulation on the two-dimensional CCD camera. Expressions for interference intensities, optical system analysis, theory of calibration and method of reconstruction are presented. Finally, the numerical simulation is used to demonstrate theoretical analysis and the feasibility of MSP-SMMIP. The layout is very easy to calibrate and the reference target is only a linear polarizer at 22.5°. Moreover, the remarkable advantages of the proposed instrument, compared with conventional Muller matrix imaging polarimeter, are that it is also simple, compact, snapshotted, and static (no moving parts). Therefore we believe that the proposed snapshot imaging polarimeter will be very useful in many applications, such as biomedical imaging and remote sensing.
2018, 67 (10): 104501. doi: 10.7498/aps.67.20172092
Dense granular impinging jets widely exist in natural flow phenomena and industrial processes, such as the rapid heating, cooling or drying, and gasification. It is important to investigate the factors influencing the flow patterns of dense granular impinging jets and reveal the evolution rules of the flow patterns. The dynamic behaviors of the dense granular impinging jets are experimentally studied by a high-speed camera and image processing software of Image J. The effects of the particle diameter, the granular jet velocity (u0) and the solid content of the granular jet (xp) on flow pattern of the granular impinging jet are investigated. Two flow regimes of the dense granular impinging jets, i.e., the liquid-like granular film and the scattering pattern, are identified. The results show that with the increase of the particle diameter and the granular jet velocity, both the solid content of the granular jet and the inter-particle collision frequency decrease, which results in the transition of granular sheet to scattering pattern. With the increase of granular jet velocity, the opening angle of the granular sheet from the side view increases, while the opening angle from the front view increases first and sharply decreases later. The results also show that with the increase of the granular jet velocity, the liquid-like granular film becomes unstable and a non-axisymmetric oscillation appears. And the amplitude and frequency of the liquid-like granular film increase with granular jet velocity increasing, and are significantly affected by particle diameter. The interesting behaviors of the liquid-like surface waves are observed on the granular sheet. The surface waves become remarkable with the increase of the granular jet velocity, and their propagating velocities normalized by the granular jet velocity vary from 0.7 to 0.9. The waves propagating on the granular sheet may emerge, which will reduce the frequencies of the surface waves and increase the surface wavelengths. The results also show that the oscillation frequency of the granular film nearly equals the pulsation frequency of the granular jet. It is indicated that the gas-solid interaction inside the nozzle increases with granular jet velocity increasing, and causes the instability of the granular jet, resulting in the non-axisymmetric oscillation on the granular sheet consequently. The results in this paper present significant knowledge of the dense granular impinging jets and also provide some principles for the applications in dense granular impinging jets in industrial processes.
2018, 67 (10): 104502. doi: 10.7498/aps.67.20172772
The mixture of scrap rubber particles and sands has been extensively used as geotechnical engineering recycled materials due to its environmental protection performance, light quality and excellent energy dissipation capability. The mechanical properties of the system can be modulated by the mixing ratio between soft and hard components. But the reasons for such a change on a particle scale are not yet clear. In this paper the elastic behaviors of glass-rubber mixed particles are studied by the sound velocity measurement and discrete element simulation. The velocity of compressional wave and the dynamic effective elastic modulus of mixed sample under hydrostatic stress are measured by time-of-flight method. It is found that the wave velocity is almost constant and the modulus decreases slightly with the proportion of rubber particles increasing to 20%. After that the wave velocity and modulus decrease rapidly and the system transforms from rigid-like behavior to soft-like behavior until the proportion of rubber particles reaches to 80%. When the proportion of rubber particles are more than 80%, the compressional wave velocity and the dynamic effective elastic modulus remain stable again. Such experimental results are consistent with discrete element method analyses which provide more in-depth insights into the micromechanics of the mixture. The simulation reveals that at low rubber fraction the main force chain structure is basically composed of glass particles without rubber particles, which accounts for the phenomenon that the velocity of the compressional wave is basically constant. When the glass particles and rubber particles co-construct the main force chain structure, the distribution of the normal contact force is relatively uniform at high rubber fraction. This can be regarded as the glass particles suspending in the rubber particles. An improved effective medium theory is proposed to describe the elastic behavior of the mixed particles system. It is considered that the deformation of the internal particles is relatively uniform for glass dominated mixture which satisfies the isostress hypothesis. A parallel spring model can be used to describe the nonlinear contact model of particles in such materials. On the other hand, rubber dominated mixture approximately satisfies the isostrain hypothesis, which can be described by a series spring model. The outcomes of such models are in agreement with the simulation results for rigid glass dominated mixture and soft rubber dominated mixture. This study is helpful in exploring the mechanisms that are responsible for the macroscale elastic behavior of mixed granular material from the microscopic point of view.
2018, 67 (10): 104202. doi: 10.7498/aps.67.20172688
In the last decade, the scattering medium has been gradually attacking attention from researchers. Among the proposed approaches, the transmission matrix (TM) is considered as an effect way to describe the scattering properties which relate to input optical and output optical fields. However, the acquired transmission matrix and its eigenvalues strongly depend on the experimental conditions, such as the numbers of input channels (limited numerical aperture and illumination area, or the pixel number of the spatial light modulator) and output channels. In other words, the actual transmission matrix of the scattering medium is the acquired transmission matrix with infinite numbers of the input and output channels. We propose an approach to obtaining the actual matrix by evaluating its eigenvalues. First, the matrix is expressed by the singular value decomposition to obtain its inverse matrix. Then first level optimization is to dispose of some extreme singular values to remove the ill-conditioned problem of the matrix, and then, as a second level optimization, the genetic algorithm is to remove the eigenvalues which have the negative contributions to the intensity of the selected focal point. Our experiments show that the gray value of the intensity and the signal-to-noise ratio (SNR) of the focal point after employing the phase pattern are 129 and 7.54, respectively. After the first level optimization, the gray value of the intensity and the SNR rise to 172 and 9.73, respectively. Then, they reach to 192 and 10.29, respectively, after adopting the genetic algorithm. After the second level optimizations, the intensity at the focal point increases 48.8% compared with the case with just the optimized phase pattern from the acquired TM, and the SNR increases by nearly 36.5%. The reason behind the increase of the intensity after the optimizations, we believe, is that the transmission matrix of the scattering medium reaches its actual matrix in certain conditions. The proposed approach opens the way to obtaining the actual transmission matrix by mathematic optimizations without increasing the experimental levels.
2018, 67 (10): 104203. doi: 10.7498/aps.67.20172467
Radiation pressure in an optomechanical system can be used to generate various quantum entanglements between the subsystems. Recently, one paid more attention to the study of quantum entanglement in an optomechanical system. Here in this work, we study the properties of output entanglement between two filtered output optical fields by the logarithmic negativity method in a double-cavity optomechanical system. Our calculations show that the decay rate of the mechanical resonator, the bandwidth of filter function, and non-equal-coupling will evidently affect the value of the output entanglement. In particular, under the parameters of equal-coupling and zero filter bandwidth, the output entanglement in the vicinity of resonant frequency (=0 in the rotating frame) will decease with mechanical decay rate increasing. But under the parameters of equal-coupling and non-zero filter bandwidth, the output entanglement will be suppressed if the center frequency of output field is in the vicinity of the resonant frequency. However, the output entanglement can be enhanced if we adopt a non-equal-coupling to counteract the suppression effect of the filter bandwidth. Furthermore, we find that there are three peaks in the whole center frequency domain of the output field if we adopt strong non-equal-coupling. This is because the normal mode of Hamiltonian Hint will split into three normal modes in this case. Our results can also be used in other parametrically coupled three-mode bosonic systems and may be applied to realizing the state transfer process and quantum teleportation in an optomechanical system.
Frequency locking of fiber laser to 1530.58 nm NH3 sub-Doppler saturation spectrum based on noise-immune cavity-enhanced optical heterodyne molecular spectroscopy technique
2018, 67 (10): 104207. doi: 10.7498/aps.67.20172541
Noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) is a powerful tool for trace gas detection, which is based on the combination of frequency modulation spectroscopy (FMS) for reduction of 1/f noise, especially residual intensity noise, and cavity enhanced absorption spectroscopy (CEAS) for prolonging the interaction length between the laser and the targeted gas. Because of the locking of modulation frequency in FMS to the free spectral range (FSR) of the cavity, NICE-OHMS is immune to the frequency-to-amplitude noise, which is a main limitation to CEAS. Moreover, due to the building of high power inside the cavity, NICE-OHMS can easily saturate the molecular absorption thus obtain sub-Doppler spectroscopy, which possess a high resolution and odd symmetry, and thus can act as a frequency discriminator for the locking of the laser frequency to the transition center. In this paper, a fiber laser based NICE-OHMS system is established and the laser frequency is locked to the sub-Doppler absorption line of NH3 by sub-Doppler NICE-OHMS. To avoid the complex design of high-Q-factor bandpass filter at radio frequency, the frequency νpdh, used for Pound-Drever-Hall (PDH) locking, is generated by the beat frequencies νfsr and νdvb, which are used for NICE-OHMS signal and DeVoe-Brewer (DVB) locking, respectively. The performances of PDH and DVB locking are analysed by the frequency distribution deduced from the error signals, which result in frequency deviations of 4.3 kHz and 0.38 kHz, respectively. Then, the CEAS signal and NICE-OHMS signal in the dispersive phase for the measurement of NH3 at 1.53 μm under 70 mTorr are obtained, which show signal-to-noise ratios of 3.3 dB and 45.5 dB, respectively. Due to the high power built in the cavity, the sub-Doppler structure in the NICE-OHMS signal is obtained in the center of the absorption tansition with a satruation degree of 0.22, which is evaluated by the amplitude ratio between sub-Doppler and Doppler-broadened signals. The linewidth (full width at half maximum) of the sub-Doppler signal of 2.05 MHz is obtained, which is calibrated by the time interval between carrier and sideband. The free-running drift of the laser frequency is estimated by the NICE-OHMS signal and results in 50 MHz over 3 h. While, with locking, the relative deviation of the laser frequency is reduced to 16.3 kHz. In order to evaluate the long term stability of the system, the frequency deviation over 3 h is measured. The Allen deviation analysis shows that the white noise is the main noise of the system in the integration time shorter than 10 s. And the frequency stability can reach to 1.6×10-12 in an integration time of 136 s.
2018, 67 (10): 104204. doi: 10.7498/aps.67.20180138
In order to suppress the higher order modes and improve beam quality in high power waveguide laser, based on gainguided index-antiguided theory, a new symmetric layered waveguide structure is designed, and an interval layer is proposed to be sandwiched between waveguide layer and cladding layer in traditional symmetric GG-IAG waveguide structure. As a result, while reducing the leakage loss of fundamental mode, the threshold gain coefficient differences between fundamental mode and higher order modes will be further increased. When the gain in waveguide layer is between threshold gain coefficient of fundamental mode and that of higher order mode, the fundamental mode will have a greater advantage in mode competition than others, so higher order modes can be suppressed and the laser can obtain a single mode output. In the meantime, the guided-mode principle of this waveguide structure is explained with the theory of wave optics in this paper, the eigen equation of each mode is derived from the wave equation, and the field distributions of fundamental mode and higher order mode are also given. Additionally, in this paper we give the solution process of the threshold gain coefficient of each mode in this waveguide structure. The mode leakage losses of fundamental mode and higher order mode, after adding the interval layer, are numerically calculated, and the parameter optimization process of the interval layer is also given in this paper. In addition, the field distributions of fundamental mode and higher order mode are numerically simulated. The calculation results show that comparing with the traditional symmetric GG-IAG planar waveguide, after adding the interval layer, the loss of fundamental mode can be greatly reduced, while ensuring that the leakage loss of higher order mode reaches a maximum value by reasonably controlling the parameters of interval layer. In this way, we can suppress higher order modes and improve laser efficiency. This paper provides a new idea for improving the beam quality of high power waveguide laser with a large mode area.
2018, 67 (10): 104208. doi: 10.7498/aps.67.20172606
In spectraldomain optical coherence tomography the sample is illuminated by a broadband light source, and the spectrum of the interference light between the light returned from the sample and a reference mirror is detected by a grating spectrometer. Conventionally, the grating spectrometer is comprised of a diffraction grating, a focusing lens, and a line-scan camera. According to the grating equation the diffraction angle from the grating is approximately linearly related to the optical wavelength. Thus the distribution function of the light spectrum at the line-scan camera is nonlinearly dependent on wavenumber. For the high-quality image reconstruction, the numerical resampling of the spectral interference data from wavelength-space to wavenumber-space is commonly required prior to the Fourier Transformation. The nonlinear detection of the spectral interferograms in wavenumber space also degrades the depth-dependent signal sensitivity in conventional linear-wavelength spectrometer based spectraldomain optical coherence tomography. Recently reported spectraldomain optical coherence tomography based on a linearwavenumber spectrometer does not need the resampling or interpolating of the nonlinearwavenumber interference spectral data, which greatly reduces the cost of computation and improves the imaging sensitivity. Various methods based on the different evaluation protocols for optimizing the design of the linear-wavenumber spectrometer have been reported. Here we report an effective optimization method for linear-wavenumber spectrometer used in a high-resolution spectral domain optical coherence tomography system. We take the reciprocal of the fullwidthhalfmaximum of the simulated point spread function as an evaluating criterion to optimize the structure parameters of the linearwavenumber spectrometer, including the refractive index and the vertex angle of the dispersive prism and the rotation angle between the diffraction grating and the dispersive prism. According to the optimization, an F2 equilateral dispersive prism is used to construct the optimized linearwavenumber spectrometer with a rotation angle of 21.8°. We construct an optimized linearwavenumber spectrometer and implement the spectrometer in a developed spectraldomain optical coherence tomography system as a detection unit. We evaluate the performances of the linear-wavenumber spectrometer both theoretically and experimentally. The experimentally measured axial resolution of the spectraldomain optical coherence tomography system based on the linear-wavenumber spectrometer is 8.52 μm, and the sensitivity is measured to be 91 dB with -6 dB sensitivity roll-off within a depth range of 1.2 mm. The experimentally measured sensitivity roll-off curve accords well with the theoretical sensitivity roll-off curve. Utilizing the general parallel computing capability of a GPU card, the highquality spectraldomain optical coherence tomography images of the human finger skin can be reconstructed in real time without any resampling or interpolating process.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
Electronic structure and spin/valley transport properties of monolayer MoS2 under the irradiation of the off-resonant circularly polarized light
2018, 67 (10): 107101. doi: 10.7498/aps.67.20180213
The new-type monolayer semiconductor material molybdenum disulfide (MoS2) is direct band gap semiconductor with a similar geometrical structure to graphene, and as it owns superior physical features such as spin/valley Hall effect, it should be more excellent than graphene from the viewpoint of device design and applications. The manipulation of the spin and valley transport in MoS2-based device has been an interesting subject in both experimental and theoretical researches. Experimentally, the photoninduced quantum spin and valley Hall effects may result in high on-off speed spin and/or valley switching based on MoS2. Theoretically, the off-resonant electromagnetic field induced Floquet effective energy should modulate effectively the electronic structure, spin/valley Hall conductance as well as the spin/valley polarization of the MoS2, through the virtual photon absorption and/or emission processes. Utilizing the low energy effective Hamilton model from the tight-binding approximation and Kubo linear response theorem, we theoretically investigate the electronic structure and spin/valley transport properties of the monolayer MoS2 under the irradiation of the off-resonant circularly polarized light in the present work. The band gaps around the K and K' point of the Brillouin region for monolayer MoS2 proves to increase linearly and decrease firstly and then increase, respectively with the increase of external off-resonant right-circularly polarized light induced effective coupling energy, and decrease firstly and then increase and increase linearly with the increase of left-circularly polarized light induced effective coupling energy, therefore, the interesting transition of semiconducting-semimetallic-semiconducting may be observable in monolayer MoS2. Furthermore, the spin and valley Hall conductance of the monolayer MoS2 for the case without off-resonant circularly polarized light are 0 and 2e2/h, respectively, and they will convert into -2e2/h and 0 when the absolute value of the off-resonant circularly polarized light induced effective coupling energy is in a range of 0.79-0.87 eV. Finally, the spin polarization for monolayer MoS2 increases up to a largest value and changes from positive to negative and/or negative to positive at the vicinity of the effective coupling energy ±0.79 eV of the off-resonant right/left circularly polarized light, while the valley polarization should increase firstly and then decrease with the off-resonant circularly polarized light, and goes up to 100% in the range of 0.79-0.87 eV of the absolute value for effective coupling energy. Therefore, the external off-resonant circularly polarized electromagnetic field should be an effective means in manipulating the electronic structure, spin/valley Hall conductance and spin/valley polarization of the monolayer MoS2, the two-dimensional MoS2 may be tuned into a brand bandgap material with excellent spin/valley and optoelectrical properties.
First-principles study of hydrogen storage properties of silicene under different Li adsorption components
2018, 67 (10): 107103. doi: 10.7498/aps.67.20172720
Alkali metal has predicted to be a promising candidate for decorating silicene surface to obtain the high hydrogen storage capacity, owing to their physical properties of lightweight, lower cohesive energy, and appropriate strength of the interaction with H2 molecules. However, though the high potential in hydrogen storage of alkali metal adatoms-decorated silicene under the fixed adatom adsorption component is well known, the evidence for the hydrogen storage capacity of alkali metal adatoms-decorated silicene under different adatom adsorption components remains largely unexplored, which may be of great significance to make the most advantages of alkali metal adatoms-decorated silicene in hydrogen storage aspects. Herein, according to the first-principles calculation corrected by the van der Waals effect, we take Li-decorated silicene for example and perform the detailed study of the geometry structure, the stability and the hydrogen storage capacity of silicene under different Li adsorption components (LixSi1-x), aiming to maximize the hydrogen storage performance of Li-decorated silicene. The results show that the preferred site of Li changes from the hollow site to the valley site as the Li component increases from 0.11 to 0.50, and binding energy of Li is always greater than the corresponding cohesive energy, showing the high stability of Li-decorated silicene and the feasibility of the method to obtain a higher hydrogen storage capacity by increasing the Li component. The hydrogen storage of silicene under different Li adsorption components is investigated by the sequential addition of H2 molecules nearby Li atoms in a stepwise manner. It can be observed that the hydrogen storage capacity of Li-decorated silicene increases and the average adsorption energy decreases with the increase of the Li component. The corresponding hydrogen storage capacities of Li0.11Si0.89, Li0.20Si0.80, Li0.33Si0.67, Li0.43Si0.57 can reach up to 2.54 wt%, 4.82 wt%, 6.00 wt% and 9.58 wt% with 0.58 eV/H2, 0.47 eV/H2, 0.54 eV/H2 and 0.41 eV/H2 average adsorption energy, respectively. When the Li component increases up to 0.50, Li atoms are saturated with a maximum hydrogen storage capacity of 11.46 wt% and an average adsorption energy of 0.34 eV/H2, which well meet the hydrogen storage standard set by the U.S. Department of Energy and mean that the hydrogen storage can be theoretically improved by increasing the Li adsorption component to a saturated level. Furthermore, we analyze the Mulliken charge population, the charge density difference and the density of states, showing that the charge-induced electrostatic interaction and the orbital hybridization are the key factors for the hydrogen adsorption of Li-decorated silicene. Our results may enhance our fundamental understanding of the hydrogen storage mechanism and explore the applications in areas of hydrogen storage for Li-decorated silicene, which are of great importance for the usage of hydrogen in the future.
Enhanced optical transmission by exciting hybrid states of Tamm and surface plasmon polaritons in single slit with multi-pair groove nanostructure
2018, 67 (10): 107104. doi: 10.7498/aps.67.20180117
In recent years, a metallic single slit nanostructure or slit array structure, due to simple structure and easy-to integration, has been used to construct a light source in the nanostructures based on the surface plasmon polaritons (SPPs). However, the problem of low transmission through an isolated subwavelength single slit nanostructure is still existent. The main reason is that the excitation efficiency of SPPs in the single slit nanostructure is not too high. Therefore, how to effectively enhance the optical transmission has become a research focus. In order to further improve the transmittance of the metallic single slit nanostructure, in this paper, we improve the single slit nanostructure imbedded in the metal silver thin film on a distributed Bragg reflector (DBR) proposed in previous literature. As a result, a novel method of designing a single slit on a DBR is proposed to effectively enhance the optical transmission in a single slit by improving the excitation efficiency of SPPs. Our proposed novel structure is made up of a subwavelength single nano-slit surrounded symmetrically by a pair of grooves on both sides of metal silver film on a distributed Bragg reflector. When the TM polarized light is illuminated from the DBR side of our proposed structure to the DBR-silver slit-grooves nanostructure, the Tamm plasmon polaritons (TPPs) at the interface between the silver film and the DBR and the SPPs in the slit on the entrance side of the silver film are excited at the DBR-silver film interface, and the SPPs in the slit and grooves pair on the exit side of the silver film are excited simultaneously. In our proposed structure, coupling between the TPPs and the SPPs leads to the hybrid state of Tamm and surface plasmon polaritons in the slit and grooves. Finally, taking advantage of constructive interference between SPPs excited by the grooves and exciting hybrid states of TPPs-SPPs in the slit, due to the local field enhancement effect of the TPPs mode and the coupling effect of constructive interference between the pair grooves and the nano-slit, the excitation efficiency of the SPPs can be increased significantly. Furthermore, the quasi Fabry-Pérot resonance effect in the nano-slit is taken into consideration, and the transmittance of our proposed structure is enhanced greatly. In the present paper, the finite element method is used to study the transmission properties of the single nano-slit embedded with paired grooves on the DBR-sliver nanostructure. After a series of parameters are optimized, the maximum transmittance through the single slit in DBR-silver slit-groove nanostructure can increase to 0.22, and this transmittance is expected to be about 22 times the transmittance (0.01) of the light through a single slit in a silver film on the TiO2 substrate (without DBR and grooves), which is higher than the maximum light transsmission 0.166 given in Ref.. The research results of this study have a certain application value in the fields of nano-light source design, photonic integrated circuits and optical signal transmission and so on.
2018, 67 (10): 107701. doi: 10.7498/aps.67.20172263
Wurtzite ZnO has long been considered to be a promising candidate material for photovoltaic application due to its high power conversion efficiency. More interestingly and very recently, some research results suggested that the ferroelectric property of the photovoltaic material introduced by chemical elements doping can promote its power conversion efficiency significantly. Therefore, in order to understand the effect of Ba doping on the electronic structure and the ferroelectric properties of ZnO and to reveal the potentially optoelectronic properties of Zn1-xBaxO, the energy band structure, the density of states, and the polarizability and the relative dielectric constant of the bulk Ba-doped ZnO supercell system, in which the Zn atoms are partly and uniformly substituted by the Ba atoms, are investigated by using the first-principles method based on the density functional theory and other physical theory. The norm-conserving pseudopotentials and the plane-wave basis set with a cut-off energy of 600 eV are used in the calculation. The generalized gradient approximation refined by Perdew and Zunger (GGA-PBE), the local density approximation (LDA) and the local density approximation added Hubbard energy (LDA+U) are employed for determining the exchange-correlation energy respectively. Brillouin zone is set to be within 4×4×5K point mesh generated by the Monkhorst-Pack scheme. The self-consistent convergence of total energy is at 2.0×10-6 eV/atom. Additionally, in order to obtain a stable and accurate calculation result, the cell structure is optimized prior to calculation. The calculated results suggest that the bulk Ba-doped ZnO semiconductor system is still a semiconductor with a direct wide band gap. The band gap of Zn1-xBaxO increases gradually with Ba atom doping percentage increasing from 12.5% to 87.5%. Consequently, the ferroelectric polarization properties and the dielectric properties of the bulk Ba-doped wurtzite ZnO materials are tailored by doping Ba atoms. It indicates that the polarizability of Zn1-xBaxO system increases with Ba doping atomic percentage increasing, especially, the polarizability reaches to a maximum when the atomic percentage of doping is 75%. Meanwhile, the relative dielectric constant inversely decreases with Ba atomic percentage increasing. This is attributed to the effective contribution of Ba atoms to the density of state at the bottom of the valence band. The diagonalized components of polarizability imply that there are possible micro-domains in the supercell while applying externally electric field to it. And the supercell presents a nearly isotropic polarizability macroscopically due to the strong interaction among the electric dipole moments existing in the different domains.
2018, 67 (10): 107702. doi: 10.7498/aps.67.20180287
In this work, the AgNbO3 piezoelectric nanomaterials are hydrothermally synthesized, and they have an average particle size of~1 m, which is obtained from scanning electron microscopy pattern. The AgNbO3 nanomaterial possesses an orthorhombic crystal structure with an mm2 point group symmetry, indicated by the X-ray powder diffraction analysis result. The piezo-electrochemical coupling of AgNbO3 is characterized, and its physical mechanism is discussed. Under an external mechanical vibration, the surfaces of the piezoelectric AgNbO3 nanomaterials will generate a large number of positive and negative electric charges. Due to the existence of spontaneous polarization, these positive and negative electrical carriers are respectively distributed on the top surface and bottom surface of AgNbO3 and can further induce the generation of some strong oxidation middle active species such as hydroxyl radicals in solution on the basis of some special chemical redox reactions, realizing the piezo-electrochemical coupling. Therefore, we can consider the piezo-electrochemical coupling as the product of the piezoelectric effect and the electrochemical redox effect. Utilizing the strong piezo-electrochemical coupling, a practical application in mechano-catalysis is further developed to decompose dye solution under a driven vibration. After experiencing~60 min vibration with AgNbO3 nanomaterial as mechano-catalyst,~70% rhodamine B (~5 mg/L) is decomposed. Prior to the vibration, the rhodamine B solution with the addition of AgNbO3 catalyst is slowly stirred for 30 min to ensure the establishment of the physical adsorptiondesorption equilibrium between catalyst and dye. It is difficult to directly exert a mechanical stress on the micro/nanoparticles. Here, an ultrasonic source with a vibration frequency of~40 kHz is employed to exert a stress to compress and stretch the AgNbO3 particles through utilizing micro-bubble collapse forces during ultrasonic cavitations, which needs the AgNbO3 particle size to be roughly identical with the diameter (~m) of micro-bubble. Our mechanocatalytic dye decomposition experiment is conducted at room-temperature and in a dark environment to avoid the influence of photocatalysis. The slight increase of temperature of the dye solution in the ultrasonic vibration process has no obvious influence on the dye decomposition efficiency, which has been confirmed from our experiment. Through a technology of fluorescence spectrum trapping, the intermediate active product in the piezo-electrochemical coupling process-the strongly oxidized hydroxyl radicals, is successfully observed. With the increase of vibration time, the number of hydroxyl radicals obviously increases, which proves that the piezo-electrochemical coupling plays a key role in our mechano-catalytic process. After using AgNbO3 catalyst in cyclic decomposition of rhodamine B 5 times, no obvious reduction in the piezo-electrochemical coupling performance occurs. The AgNbO3 nanomaterial possesses an efficient piezo-electrochemical coupling for mechano-catalysis, and it has the advantages of high decomposition efficiency and reusability, and potential applications in vibration decomposing dye.
2018, 67 (10): 106401. doi: 10.7498/aps.67.20180264
The nature of glass and glass transition are considered to be one of the most fundamental research problems in condensed matter physics. Colloidal suspension provides a novel model system for studying glass and glass transition, since the structures and dynamics of a colloidal system can be quantitatively probed by video microscopy. Traditional systems for studying glass transition typically are single-component systems composed of either isotropic or anisotropic colloidal particles. Recently, glass transition of mixture of isotropic and anisotropic colloids has attracted great attention, such as the observation of rotational glass and translational glass, and the establishment of the two-step glass transition. Similarly, computer simulations have also shown that mixture of isotropic and anisotropic colloidal particles could manifest interesting, new glassy behaviors. However, the experimental study of the glass transition in such a colloidal mixture is still rare. In this paper, we experimentally investigate the glass transition of a binary mixture of colloidal ellipsoids and spheres. The colloidal spheres are polystyrene microspheres with a diameter of 1.6 m, and the ellipsoids are prepared by physically stretching from polystyrene microspheres of 2.5 m in diameter. The major and minor axes of the as-prepared ellipsoid are 2.0 m and 1.2 m, respectively. The mixture is confined between two glass slides to make a quasi-two-dimensional sample. To prevent the mixture from crystallizing, the mixing ratio of ellipsoids and spheres is chosen to be 1/4 in number, which is similar to the mixing ratio used in the classical Kob-Anderson model of binary sphere mixture. We systemically increase the area fraction of colloidal mixture to drive the glass transition. We then employ bright-field video microscopy to record the motion of the particles in the colloidal suspension at a single particle level, and the trajectories of individual particles are obtained by standard particle tracking algorithm. Through the analysis of radial distribution function, Voronoi diagram and local order parameter, we find that the ellipsoids can effectively inhibit the spheres from crystalizing, and the structure of the system remains disordered when increasing the area fraction. For dynamics, mean square displacement and self-intermediate scattering function are calculated. We find that the dynamic process of the system slows down substantially when increasing the area fraction, and the relaxation time of the system increases rapidly and diverges close to the glass transition point predicted by the mode coupling theory. Moreover, we analyze the fast particles that participate in cooperative rearrangement regions (CRRs) in the system, and find that the shapes, sizes and positions of CRRs are closely related to the locations of the ellipsoids in the system.
2018, 67 (10): 106402. doi: 10.7498/aps.67.20172721
The viscosity of high-temperature metallic melt, which is an important index for evaluating dynamics of liquid melt, is one of the basic physical properties. It not only influences the mold-filling capacity of melting metal in traditional casting techniques, but also exhibits more distinct influence on the fabrication of advanced material, such as metallic glass. According to the variation tendency of viscosity with temperature in alloy melt, the fragility of superheated melt could be obtained, which has proved to correlate with the ability of alloy to form glass. Besides, the viscosity of alloy well above the liquidus temperature also plays a key role in probing into the characteristic of liquid-liquid phase transition, the fragile-to-strong transition phenomenon, how the potential energy landscape evolves during cooling, etc. It has been generally accepted that the viscosity of metallic melt at high temperatures increases with temperature decreasing and could be fitted by an Arrhenius curve in the whole temperature range. However, recently more and more studies show that the viscosity of metallic melt cannot be fitted by only one Arrhenius curve. Instead, there exists at least one specific temperature below which the viscosity data begins to deviate from the Arrhenius curve at high temperature during cooling. These data could be described by another Arrhenius curve. In order to in depth understand this phenomenon, in this paper we summarize the viscosity data of different metallic melts in the literature. On the basis of introducing the method of detecting high-temperature melt viscosity, we discuss comprehensively the changing tendency of viscosity with temperature and the characteristics of abnormal viscosity changes in pure metal, binary and multivariate alloys well above the liquidus temperature. It is found that the abnormal viscosity changes generally occur in alloys that could form the types of intermetallic compounds. The abnormal viscosity change in metallic melt is accompanied with exothermic or endothermic effect, depending on alloy system, and reflects the existence of liquid-liquid transition well above the liquidus temperature. Besides, such an abnormal change of viscosity influences the ability to form metallic glass liquids. Although the abnormal dynamic change of metallic melt hints the existence of complexity of structural change in liquid during cooling, what is the key factor underlying this phenomenon remains a mystery. By combining the advanced experimental techniques such as high-energy X-ray diffraction and neutron scattering with the computer simulation method, this problem may be understood further. Besides, the relation between viscosity abnormity and the phase diagram is another problem that deserves to be noticed in the future.
2018, 67 (10): 106701. doi: 10.7498/aps.67.20180236
The halide perovskite solar cells employing CH3NH3PbX3 (X=Cl-, Br-, I-) and CH3NH3PbI3-xClx as light absorbers each have shown a rapid rise in power conversion efficiency (PCE) from 3.8% to 22.1% in recent years. The excellent photovoltaic performance is attributed to good optical and electrical properties such as appropriate bandgap, large absorption coefficient, high carrier mobility, long carrier lifetime and long carrier diffusion length. However, the physical mechanism of high PCE for halide perovskite solar cells is still unclear. The Gaussian 09 software is utilized to optimize the geometries of isolated CH3NH3+ and CH3NH3 at a B3 LYP/6-311++G(d, p) level, and the Multiwfn software is used to visualize the electrostatic potentials (ESPs) of CH3NH3+ and CH3NH3. Based on the ESPs of CH3NH3+ and CH3NH3, it is found that the CH3NH3+ has a strong electrophilic character, however, the NH3- side and CH3- side of CH3NH3 have weak nucleophilic and electrophilic character, respectively. So the electrostatic characteristics of CH3NH3+ and CH3NH3 are significantly different. The strong electrostatic repulsive interaction between two neighboring CH3NH3+ radicals plays an important role in structural phase transition of CH3NH3PbI3 material. At room temperature, the CH3NH3+ in the inorganic cage is activated and disordered, and has a strong electrophilic character. Due to these characteristics of CH3NH3+, the interfacial electrons at TiO2/CH3NH3PbI3 heterojunction are combined with CH3NH3+ to form CH3NH3 in the inorganic[PbI3]- framework. The CH3NH3 at the heterojunction under the built-in electric field is more easily oriented than CH3NH3+. Two initial geometrical configurations for CH3NH3+:CH3NH3 and CH3NH3:CH3NH3 dimers are optimized by using Gaussian 09 at an MP2/Aug-cc-PVTZ level. On the basis of the electrostatic characteristic of CH3NH3+:CH3NH3 dimer, the interfacial electrons at TiO2/CH3NH3PbI3 heterojunction are easily injected into the CH3NH3PbI3 material, which leads to the strong polarization of CH3NH3PbI3 material at the heterojunction. From the ESP of optimized CH3NH3:CH3NH3 dimer, it is found that the weak electrostatic field of the inorganic framework, parallel to C-N axis, is induced by the CH3NH3 orientational order, which is made for improving the photogenerated electron-hole pair separation and carrier transport. The TiO2/CH3NH3PbI3 heterojunction has more advantage than traditional p-n junction because of no consumption of carrier for CH3NH3PbI3 material in the process of forming built-in electric field. The physical mechanism is the origin of high PCE for CH3NH3PbI3 solar cells. According to the experimental results and first-principle calculations, we can draw an important conclusion that the electrostatic characteristics of organic CH3NH3+ cations in the inorganic[PbI3]- framework result in the high performances of halide perovskite solar cells.
2018, 67 (10): 107801. doi: 10.7498/aps.67.20172716
As an important branch of metamaterial-based devices, metamaterial absorber (MA) has aroused great interest and made great progress in the past several years. By manipulating the magnetic resonance and the electric resonance simultaneously, the effective impedance of MA will match the free space impedance, thus resulting in a perfect absorption of incident waves. Due to the advantages of thin thickness, flexible design and tunable property, MA has been extensively studied at various frequencies, e. g. microwave frequency, THz, infrared frequency, and optical frequency. Infrared MA, having important applications in infrared stealth, infrared detection, radiative cooling, and sensors, receives more and more attention, especially for those absorbers based on easy-fabricated one-dimensional grating structure. However, such a grating-based absorber is usually workable in narrow band and effective only for transverse magnetic (TM) wave.In this paper, a dual-band broadband absorber is proposed based on the easy-fabricated grating structure. The basic unit of the proposed absorber consists of eight gradient subunits, each of which is composed of vertically cascaded two pairs of metal-dielectric bilayers. The as-designed absorber has perfect absorption for both TM and transverse electric (TE) waves. More importantly, the absorption band is different for different polarized wave, which provides more choices and greater flexibility for application. Full-wave simulation shows that the absorption of TM wave is above 90% from 1.68 μm to 2 μm, while the absorption of TE wave is very small (no more than 6%). The absorption of TE wave is above 90% from 3.8 to 3.9 μm, while the absorption of TM wave is very small (no more than 5%). In order to reveal the working principle of the proposed absorber, the electric-field distributions of the whole structure are calculated at different frequency, which demonstrates that the broadband absorption is achieved by exciting multiple resonant coupling. Furthermore, we investigate the performance of the proposed absorber in oblique incidence, and find that the designed absorber can exhibit a good absorption within a broad incident angle ranging from 0 to 60 degrees. It is worth noting that there is an absorption fracture band in the absorption spectrum of TM waves, which is because no resonance occurs in all subunits, resulting in almost no absorption.In conclusion, we have proposed a dual-band broadband absorber that demonstrates independent absorption of the TM waves and the waves in different bands, which has potential applications in thermal detectors and thermal emitters. The proposed scheme can be extended to microwave, THz, and even visible light band.
2018, 67 (10): 107102. doi: 10.7498/aps.67.20172290
Using hybrid density functional theory, we investigate the structural, electronic and optical properties of pristine GaN and Fe-doped GaN with a Fe concentration of 12.5%. Specifically, we first analyze the crystal lattice constant, band structure, and density of states, respectively. Then we predict the dielectric function, absorption coefficient, refractive index, reflectivity, energy-loss spectrum and extinction coefficient. Finally, we analyze the influences of the doping of Fe element on the photoelectric property of Fe doped systems. The calculated lattice constants for perfect GaN are a=b=3.19 Å, c=5.18 Å, which are in good agreement with the experimental values. Furthermore, we find that the doping of Fe element has little effect on the structural properties of GaN. The Band gap of pristine GaN is predicted to be 3.41 eV, very close to the experimental value of 3.39 eV. The band gap of Fe doped GaN (12.5%) significantly decreases to 3.06 eV. By comparing the densities of states of the systems with and without Fe doping, it is found that Fe-3 d state is mainly responsible for the decrease of band gap. The calculated static dielectric constant of perfect GaN is 5.74, and it increases to 6.20 after incorporating the Fe element. The results about the imaginary part of dielectric function show that two equal-strength perfect GaN peaks are observed to be at 6.81 eV and 10.85 eV. The first peak is closely related to the direction transition from the valence band top to the conduction band bottom. Furthermore, it is also observed that a peak is located at 4.04 eV in the low energy, which can be understood as resulting from the electron transition inside the valence band. The optical absorption edge of the intrinsic GaN is 3.25 eV, corresponding to the transition energy. The reason why this energy is smaller than the bandgap is because the electronic band gap equals the sum of optical bandgap and exciton energy. However, the maximum absorption coefficients of these two systems both occur at 13.80 eV in energy. The refractive index for intrinsic system is 2.39, and it increases to 2.48 after doping the Fe element. It is found from the energy-loss spectrum that the maximum energy-loss is at 20.02 eV for a perfect system, while it is at 18.96 eV for a doped system. Additionally, we obtain the reliable reflectivity and excitation coefficient. In conclusion, our calculated results provide a well theoretical basis for the theoretical research on the co-doping of Fe element and other elements. The analyses on the Fe-doped GaN high-voltage photoconductive switch materials and devices also provide a powerful theoretical basis and experimental support in the future research.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
Modes characteristics analysis of THz waveguides based on three graphene-coated dielectric nanowires
2018, 67 (10): 108101. doi: 10.7498/aps.67.20180036
In this paper, the real parts of the effective refractive indexes and the propagating lengths of five low-order modes of the terahertz waveguides based on three graphene-coated dielectric nanowires are analyzed by using the multipole method. The formation of these five lowest order modes can be attributed to the five combinations between the two lowest order modes supported when three nanowires exist alone. Therefore they are named Mode 1, Mode 2, Mode 3, Mode 4, and Mode 5 in sequence. The results show that the mode characteristics of the waveguide can be effectively tuned by changing the operating frequency, the radius of the intermediate nanowire, the gap distance between the nanowires and the Fermi energy of graphene. As the operating frequency increases from 30 THz to 40 THz, the real part of each of the effective refractive indexes increases and the propagation length decreases, and the crossover phenomenon occurs in the process of change. In addition, the real parts of the effective refractive indexes and the propagation lengths of Modes 3 and 4 are basically the same. When the radius of the middle nanowire increases from 25 nm to 75 nm, the real parts of the effective refractive indexes of Modes 1 and 2 increase, and the propagation length of Mode 1 decreases and then increases. Besides the real parts of the effective refractive indexes and the propagation lengths of Modes 3 and 4 are basically not affected by the change of radius, and the values of these two modes are basically the same. For Mode 5, the real part of the effective refractive index and propagation length slowly increase. When the spacing between the nanowires increases from 10 nm to 50 nm, Modes 3 and 4 are basically unaffected by the change of spacing, and the values of these two modes are basically the same. The real parts of the effective refractive indexes of the other modes decrease and the propagation lengths increase and eventually stabilize, and the crossover phenomenon occurs in the process of change. As the Fermi energy of graphene increases from 0.4 eV to 1.2 eV, the real part of the effective refractive index decreases and the propagation length increases. The calculation shows that the result obtained by the multipole method is exactly the same as that obtained by the finite element method. To date, no one has analyzed the terahertz waveguides based on three graphene-coated dielectric nanowires. This work can provide a theoretical basis for the design, fabrication and application of terahertz waveguide based on graphene-coated dielectric nanowires. Such waveguides have potential applications in the field of mode-division multiplexing.
2018, 67 (10): 108501. doi: 10.7498/aps.67.20172643
The Johnson noise thermometer is used to precisely measure Boltzmann constant by comparing the thermal noise caused by charge movement and the quantized voltage reference noise synthesized by the quantum voltage noise source (QVNS). The QVNS signal is synthesized based on quantized voltage pulses produced by two channels of superconducting Josephson junction arrays, which are designed for cross-correlation electronics. The Nb/NbxSi1-x/Nb Josephson junction is used as a core device of QVNS chip in this work for its non-hysteretic current-voltage (I-V) characteristics and conveniently adjustable barrier parameters.In this paper, we present the design consideration, fabrication process, and measurement results of the QVNS chip. The QVNS chip contains two Josephson junction arrays, each consists of four 6 μm×12 μm junctions and is embedded in a 50 Ω coplanar waveguide transmission line. The random noise in signals from the two driven channels is eliminated by cross-correlation, and then an accurate quantum noise is obtained. Test chips with different areas of Josephson junctions are also designed on the same mask, aiming at estimating the variation range of Ic. The typical fabrication process for voltage standard chips in our laboratory is used for preparing the QVNS chip.The sample is measured at 4.2 K. The DC I-V curve shows that the critical current Ic is 6.1 mA. The I-V characteristics of the junctions under 5 GHz microwave radiation are measured. For a series array of four junctions, a 41.44 μV one-stage Shapiro step is observed. Calculation shows that the error between the measurement and theoretical value of 41.36 μV is about 1.9‰, which means that the QVNS chip performs well under microwave radiation and can be used for synthesizing the AC quantum voltage reference noise.A single-frequency 100 kHz sinusoidal waveform is synthesized by the QVNS chip under pulse driven signal. A spectrum of the synthesized sinusoidal waveform shows a single peak, which means that the digital pulse signal is perfectly filtered by Josephson junction arrays and the synthesized signals possess quantum accuracy. The results indicate that our chip has good dynamic response and works well in synthesizing a single-frequency AC quantum voltage signal. This work can provide core devices for the noise thermometry system and support the precise measurement of Boltzmann constant as well as redefinition of Kelvin in future. As a next step, the design and package will be further improved, and the probe module will be optimized to reduce the measurement uncertainty.
Molecular dynamics simulations on DNA flexibility: a comparative study of Amber bsc1 and bsc0 force fields
2018, 67 (10): 108701. doi: 10.7498/aps.67.20180326
The structural flexibility of DNA plays a key role in many biological processes of DNA, such as protein-DNA interactions, DNA packaging in viruses and nucleosome positioning on genomic DNA. Some experimental techniques have been employed to investigate the structural flexibility of DNA with the combination of elastic models, but these experiments could only provide the macroscopic properties of DNA, and thus, it is still difficult to understand the corresponding microscopic mechanisms. Recently, all-atom molecular dynamics (MD) simulation has emerged as a useful tool to investigate not only the macroscopic properties of DNA, but also the microscopic description of the flexibility of DNA at an atomic level. The most important issue in all-atom MD simulations of DNA is to choose an appropriate force field for simulating DNA. Very recently, a new force field for DNA has been developed based on the last generation force field of Amber bsc0, which was named Amber bsc1. In this work, all-atom MD simulations are employed to study the flexibility of a 30-bp DNA with the force fields of Amber bsc1 and Amber bsc0 in a comparative way. Our aim of the research is to examine the improvement of the new development of force field (Amber bsc1) in the macroscopic and microscopic properties of DNA, in comparison with the corresponding experimental measurements. All the MD simulations are performed with Gromacs 4.6 and lasted with a simulation time of 600 ns. The MD trajectories are analyzed with Curves+ for the last 500 ns, since the system reaches equilibrium approximately after ~100 ns. Our results show that the new force field (Amber bsc1) can lead to the improvements in the macroscopic parameters of DNA flexibility, i.e., stretch modulus S and twist-stretch coupling D become closer to experimental measurements, while bending persistence lengths lp and torsional persistence lengths C from the two force fields (bsc1 and bsc0) are both in good agreement with experimental data. Our microscopic analyses show that the microscopic structure parameters of DNA from the MD simulation with the Amber bsc1 force field are closer to the experimental values than those with the Amber bsc0 force field, except for slide, and the obvious improvements are observed in some microscopic parameters such as twist and inclination. Our further analyses show that the improvements in macroscopic flexibility from the Amber bsc1 force field are tightly related to the microscopic parameters and their fluctuations. This study would be helpful in understanding the performances of Amber bsc1 and bsc0 force fields in the description of DNA flexibility at both macroscopic and microscopic level.
2018, 67 (10): 108702. doi: 10.7498/aps.67.20180168
Objective image quality assessment (IQA) plays a very important role in transmission, encoding, and quality of service (QoS) of the image and video data. However, the existing IQA methods often do not consider image content features and their visual perception, so there is a certain gap between the objective IQA sores and the subjective perception. To solve this problem, in the study, we propose an objective IQA method based on the visual perception of image content, which combines the complexity characteristics of image content, and the properties of masking, contrast sensitivity and luminance perception nonlinearity of human visual system (HVS). In the proposed method, the image is first transformed using a nonlinear model of luminance perception to obtain the intensity perception image. Then, the intensity information is summed using the contrast sensitivity values of HVS and the average contrast values of the local image as a weighting factor of the intensity. The summed data information is taken as the content of human perceiving image, and an image perception model is constructed. Finally, the reference images and distorted images are perceived by simulating the HVS with this model. Moreover, the difference in intensity between two perceived images is calculated. Based on the intensity difference and peak signal-to -noise ratio model, an objective IQA model is constructed. Further, the simulation with 47 reference images and 1549 test images in the LIVE, TID2008, and CSIQ databases is conducted. Moreover, the experimental results are compared with those of four typical objective IQA models, namely SSIM, VSNR, FSIM, and PSNRHVS. In addition, we explore the factors that affect the IQA accuracy and a way to improve assessment accuracy by combining HVS characteristics, through analyzing the correlation between IQA results of the proposed model and the subjective mean opinion scores (MOSs) provided in the three image databases from the following two aspects. Namely, (1) all reference images in three image databases are distorted by multiple types, and the distorted images of each reference image are taken as a test sequence. Then, the proposed model is used to evaluate each test sequence to obtain the IQA scores. By analyzing the correlation between the IQA scores of each test sequence and the subjective MOSs and comparing them with the assessment results of SSIM, we explore the influence of the image content complexity on the objective IQA accuracy. (2) The test images which are distorted by each type and many distortion degrees are used as another sequence, and they are evaluated by the proposed IQA model. By analyzing the correlation between the subjective MOSs and the IQA results of each test sequence, and comparing them with assessment results of SSIM, we discuss the influence of image distortion mode on the IQA accuracy. The experimental results show that the coefficient values of Pearson linear correlation and Spearman rank order correlation between the objective IQA scores obtained by the proposed method and the subjective MOSs have been averagely improved by 9.5402% and 3.2852%, respectively, in comparison with IQA results from the SSIM method. Also, they are enhanced more significantly than those fom the PSNRHVS and VSNR methods. In summary, it is shown that the proposed IQA method is an effective and feasible method of objectively assessing the image quality; moreover, it is shown that in the objective assessment of image quality it is very helpful to improve the consistency of subjective and objective assessment of image quality by considering the content perception and complexity analysis of the images.
Concentrating characteristics of Fresnel lens with prism secondary concentrator and optimization of high concentrating photovoltaic module with triple-junction cell
2018, 67 (10): 108801. doi: 10.7498/aps.67.20172778
At present, Fresnel lens is commonly used as a concentrator in high concentrating photovoltaic (HCPV) module, and the triple-junction cell is currently one of the most common multi-junction cells used in it. The triple-junction cell is composed of three p-n junctions in series. Each sub-cell in it absorbs different-wavelength light. The solar cell efficiency of Ⅲ-V multi-junction high concentrating photovoltaic increases up to 46%, which the corresponding module efficiency is quite different from. The output power of the solar cell is related to not only the illumination energy, but also the spectral distribution and the uniformity of the illumination. The loss caused by the non-ideal concentration of the concentrator in the module is as high as 20%. After sunlight enters the lens, the direction of transmission of a monochromatic light is different, because a lens has different refractive index for different-frequency light. So the light disperses when leaving the lens, and thus the colors are arranged in a certain order to form a spectrum. Owing to the dispersion and the differences in refractive index among different spectral bands, the illumination distributions of the three spectral bands are different and non-uniform on the focal plane of lens. The divergence of light will obviously weaken the non-uniformity of the illumination on the solar cell surface. So the divergence angle of the light source has a greater influence on the cell performance because the non-uniformity of illumination has a negative influence on the performance of the cell.In this paper, according to the establishment of optical model and three-dimensional cell circuit network model under non-uniform illumination, taking Ⅲ-V triple-junction cells for example, we study the concentrating characteristics and photovoltaic characteristics of HCPV module with Fresnel lens concentrator and prism secondary concentrator. The results show that due to the non-parallel incident light and dispersion of the Fresnel lens, the concentrating spots of short-wave light, medium-wave light and long-wave light are divergent and their illuminations are non-uniform, resulting in the spectral response mismatch loss of the three sub-cells in the triple-junction cell, and the photovoltaic performance of the HCPV module also declines. The results show that the secondary optics element is obviously effective in reducing the non-uniformity of the illumination and the temperature which the Fresnel lens creates. However, each waveband of light has a different spot size at the same position, similar to the Fresnel lens without the secondary optics element. So the varieties of cell performance at different positions are similar too. And, by optimizing the focusing characteristics of the three wave bands along the optical axis, the power output of the HCPV module can increase more than 10%. The simulation results are verified experimentally.
Compressed sensing based fast method of solving the electromagnetic scattering problems for threedimensional conductor targets
2018, 67 (10): 100201. doi: 10.7498/aps.67.20172543
The method of moments is one of the most commonly used algorithms for analyzing the electromagnetic scattering problems of conductor targets. However, it is difficult to solve the matrix equation when analyzing the electromagnetic scattering problem of the electric large target. In recent years, the theory of the compressed sensing was introduced into the method of moments to improve the computation efficiency. The random selected impedance matrix is used as a measurement matrix, and the excitation voltage is used as a measurement value when using compressed sensing theory. The recovery algorithm is used to solve the induced current of target. The method can avoid the inverse problem of matrix equation and improve the computational efficiency of the method of moments, but it can be applied only to 2-dimensional or 2.5-dimensional target. The application of compressed sensing needs to know the sparse basis of the current in advance, but the induced current of three-dimensional target which is expressed by an Rao-Wilton-Glisson basis function is not sparse on the commonly used sparse basis, such as discrete cosine transform basis and discrete wavelet basis. To solve this problem, a method of combining compressed sensing with characteristic basis functions is proposed to analyze the electromagnetic scattering problem of three-dimensional conductor target in this paper. The characteristic basis function method is an improved method of moments. The target is divided into several subdomains, the main characteristic basis functions are comprised of current bases arising from the self-interactions within the subdomain, and the secondary characteristic basis functions are obtained from the mutual coupling effects of the rest of the subdomains. Then a reduction matrix is constructed to reduce the order of matrix equation, and the current can be expressed by the characteristic basis function and its weighting coefficient. In the method presented in this paper, the weighting coefficient is considered as a sparse vector to be solved when the characteristic basis function is used as sparse basis. The number of weighting coefficients is less than the number of unknown ones, so it can be obtained from the compressed sensing recovery algorithm. At the same time, the generalized orthogonal matching pursuit algorithm is used as the recovery algorithm to speed up the recovery process. Finally, the proposed method is used to calculate the radar cross sections of a PEC sphere, nine discrete PEC targets and a simple missile model. The numerical results validate the accuracy and efficiency of the method.
2018, 67 (10): 100701. doi: 10.7498/aps.67.20172691
The interference images with fixed spectral resolution can be obtained by using the existing static polarization-difference imaging system because the optical path of the system cannot be changed flexibly. However, for different detection targets, the spectral resolution of the system determined by the optical path difference must be appropriate. To satisfy a variety of application requirements, a novel dual channel polarization-difference interference imaging system (DPDⅡS), based on the lateral shear of the wide-field-of-view Savart polariscope (WSP) and the modulated Savart polariscope (MSP), is presented. The two-dimensional space images of a target and orthogonal interference images can be obtained by adjusting the MSP under different lateral displacements simultaneously. In addition, the remarkable characteristics of the system avoid spilling over rays and optimizing the system optical path effectively. In this paper, by using the Jones matrix, the system structure is demonstrated and the theoretical principle of DPDⅡS is analyzed in detail. The amplitudes of the four beams from the MSP and the interference intensity expressions of the coherent light are derived. Then the splitting characteristics of the Savart polariscope (SP) and WSP are presented. It is concluded that the WSP has better shear ability than SP and the WSP can optimize the optical path effectively compared with Wollaston prism in the DPDⅡS. The change ranges of the optical path difference and lateral displacement produced by the MSP for structure angles =/3, /4, /6 are analyzed in detail. The reconstructed orthogonal interferograms and the experimental interferograms under 632.8 nm monochromatic light for dMSP=1.00, 1.10, 1.20, 1.30 mm are obtained. A comparison between the experimental interference images and the simulated images proves that the interference fringes with different resolutions can be obtained simultaneously by adjusting the MSP. Meanwhile, the light intensities of the double optical paths are approximately equal and the same optical path difference is generated for the dual channel with the movement of MSP. The experimental results are consistent with the theoretical analyses. The spatial images of parallel and vertical components are detected under 632.8 nm polychromatic light. Then the total intensity image and the polarization-difference image are obtained through data processing. The conclusion that the polarization difference intensity image has a high resolution compared with the polarization intensity image is presented. The study has reference significance and practical value for the dual channel polarization interference imaging system.