Twin Markovian field correlation on four-level attosecond polarization beats

Based on the phase-conjugate polarization interference between two two-photon processes, we obtained an analytic closed form for the second-order or fourth-order Markovian stochastic correlation of the four-level attosecond sum-frequency polarization beat (FASPB) in the extremely Doppler-broadened limit. The homodyne-detected FASPB signal is shown to be particularly sensitive to the statistical properties of the Markovian stochastic light fields with arbitrary bandwidth. The different roles of the amplitude fluctuations and the phase fluctuations can be understood physically in the time-domain picture. The field correlation has a weak influence on the FASPB signal when the laser has narrow bandwidth. In contrast, when the laser has broadband linewidth, the FASPB signal shows resonant-nonresonant cross-correlation, and drastic difference for three Markovian stochastic fields. The maxima of the two two-photon signals are shifted from zero time delay to the opposite direction, and the signal exhibits damping oscillation when the laser frequency is off-resonant from the two-photon transition. A Doppler-free precision in the measurement of the energy-level sum can be achieved with an arbitrary bandwidth. As an attosecond ultrafast modulation process, it can be extended intrinsically to any sum frequency of energy levels.

TDR和FDR测定黄绵土土壤含水量的标定

时域反射仪(TDR)与频率反射仪(FDR)是目前最为先进,并且最具有发展前途的土壤含水量测量仪器。有便利、快捷、准确的优点。介绍了6种基于TDR或FDR原理的仪器对黄绵土含水量进行试验标定的方法与结果。标定结果表明:仪器的测量值与烘干法的测量值之间有一定的差异,CS616和Trime-T3烘干法测定结果接近,但是其余的4种仪器测定值均高于烘干法测定值,在应用这些仪器测定土壤含水量时需要进行标定。

施肥对干旱胁迫下夏玉米根系提水的调节作用研究

以耐旱性玉米品种郑单958号为材料,采用两室分根土培装置,通过时域反射计(TDR)对上下土层土壤含水量进行控制和观测,研究施肥对干旱胁迫条件下玉米根系提水作用的影响.结果表明,玉米根系在土层上干下湿条件下(即上下层土壤存在一定水势差时)存在明显提水作用;玉米根系提水量在整个生育期呈单峰变化,并以吐丝期最大;上层土壤施肥可以调节玉米根系提水作用强弱,整个生育期根系总提水量表现为NP配施>单施P>CK>单施N,NP配施处理全生育期单株提水量(1 948.6 g)分别是单施P处理、CK和单施N处理的1.5倍、3.1倍和3.5倍.玉米整个生育期根系总提水量与收获期不同层次根系干重和体积存在极显著正相关关系,也与其地上部分生物量和籽粒产量呈极显著或显著正相关关系.可见,玉米根系的提水作用强弱因生育期和施肥处理而变化,施肥主要通过影响根系生长来调节其提水作用;在一定水分环境条件下,玉米根系提水作用能促进作物生长,提高其籽粒产量.

光子晶体的微腔特性

为了设计一种高品质因子的光子晶体微腔和研究单缺陷光子晶体微腔谐振模波长随晶格常数的变化规律,使用时域有限差分法（difference time-domain method）和基于Baker算法的Padé近似方法计算了半导体材料上空气孔阵列光子晶体微腔的谐振模波长和品质因子.得到的新型光子晶体微腔的品质因子达246510,单缺陷光子晶体微腔模波长随晶格常数α和孔半径r的近似为线性变化关系

Application of step-scan FTIR to the research of quantum cascade lasers

The principle of step-scan Fourier transform infrared （FTIR） spectroscopy is introduced. Double modulation step-scan FTIR technique is used to obtain the quantum cascade laser＇s stacked emission spectra in the time domain. Optical property and thermal accumulation of devices due to large drive current are analyzed.

Applicatioo of Pade Approximation in Simulating Photonic Crystals

To save finite-difference time-domain(FDTD) computing time, several methods are proposed to convert the time domain FDTD output into frequency domain. The Padé approximation with Baker's algorithm and the program are introduced to simulate photonic crystal structures. For a simple pole system with frequency 160THz and quality factor of 5000,the intensity spectrum obtained by the Padé approximation from a 28-item sequence output is more exact than that obtained by fast Fourier transformation from a 220-item sequence output. The mode frequencies and quality factors are calculated at different wave vectors for the photonic crystal slab from a much shorter FDTD output than that required by the FFT method,and then the band diagrams are obatined. In addition,mode frequencies and Q-factors are calculated for photonic crystal microcavity.

Modulation of Photonic Bandgap and Localized States by Dielectric Constant Contrast and Filling Factor in Photonic Crystals

The band structure of 2D photonic crystals (PCs) and localized states resulting from defects are analyzed by finite-difference time-domain (FDTD) technique and Pade approximation. The effect of dielectric constant contrast and filling factor on photonic bandgap (PBG) for perfect PCs and localized states in PCs with point defects are investigated. The resonant frequencies and quality factors are calculated for PCs with different defects. The numerical results show that it is possible to modulate the location, width and number of PBGs and frequencies of the localized states only by changing the dielectric constant contrast and filling factor.

Potential light source for photonic integrated circuits based on the equilateral triangle resonator

The semiconductor microlasers based on the equilateral triangle resonator (ETR) can be fabricated from the edge-emitting laser wafer by dry-etching technique, and the directional emission can be obtained by connecting an output waveguide to one of the vertices of the ETR. We investigate the mode characteristics, especially the mode quality factor, for the ETR with imperfect vertices, which is inevitable in the real technique process. The numerical simulations show that the confined modes can still have a high quality factor in the ETR with imperfect vertices. We can expect that the microlasers is a suitable light source for photonic integrated circuits.

Flat Surface Plasmon Polariton Bands in Bragg Grating Waveguide for Slow Light

The formations of the surface plasmonpolariton (SPP) bands in metal/air/metal (MAM) sub-wavelength plasmonic grating waveguide (PGW) are proposed. The band gaps originating from the highly localized resonances inside the grooves can be simply estimated from the round trip phase condition. Due to the overlap of the localized SPPs between the neighboring grooves, a Bloch mode forms in the bandgap and can be engineered to build a very flat dispersion for slow light. A chirped PGW with groove depth varying is also demonstrated to trap light, which is validated by finite-difference time-domain (FDTD) simulations with both continuous and pulse excitations.

Broadband short-range surface plasmon structures for absorption enhancement in organic photovoltaics

We theoretically demonstrate a polarization-independent nanopatterned ultra-thin metallic structure supporting short-range surface plasmon polariton (SRSPP) modes to improve the performance of organic solar cells. The physical mechanism and the mode distribution of the SRSPP excited in the cell device were analyzed, and reveal that the SRSPP-assisted broadband absorption enhancement peak could be tuned by tailoring the parameters of the nanopatterned metallic structure. Three-dimensional finite-difference time domain calculations show that this plasmonic structure can enhance the optical absorption of polymer-based photovoltaics by 39% to 112%, depending on the nature of the active layer (corresponding to an enhancement in short-circuit current density by 47% to 130%). These results are promising for the design of organic photovoltaics with enhanced performance.

Electron trapping materials for use in a picosecond infrared streak camera

We describe our research on the employment of an infrared upconversion screen made of electron trapping material (ETM) in combination with the high sensitivity of the S-20 photocathode responsive to visible radiation to produce a streak camera arrangement capable of viewing and recording infrared incident pulses. The ETM-based upconversion screen converts 800-1600 nm infrared radiation to visible light which is viewed or recorded by the S-20 photocathode. The peak values of the upconversion efficiency are located at 1165 nm for CaS:Eu, Sm and 1060 nm for CaS:Ce, Sm. The present experiment showed time resolution was 12.3 ps for a CaS:Eu, Sm screen and 8.4 ps for a CaS:Ce, Sm screen. The minimum detectability is 4.8 x 10(-9) J/mm(2) (minimum detectability of the coupled visible streak camera is 8.3x10(-10) J/mm(2)). Other parameters, such as spatial resolution and dynamic range, have also been measured and analyzed. The results show ETM can be used in the measurement of infrared ultrafast phenomena up to picosecond time domain. In consideration of the limited number of trapped electrons in ETM, the infrared-sensitive streak camera consisting of an ETM-based upconversion screen is suitable to operate in the single shot mode. (C) 1999 American Institute of Physics. [S0034-6748(99)00112-4].

Distributed Temperature Sensing System Based on Rayleigh Scattering BOTDA

A distributed temperature sensor based on Rayleigh scattering Brillouin optical time domain analysis (Rayleigh-BOTDA) is proposed in this paper. The sensor uses Rayleigh backscattering effect of microwave modulated pulse base sidebands as probe wave and a high sensitive photon counting detector for Brillouin signal intensity detection. Compared with a conventional BOTDA system, the Rayleigh-BOTDA effectively suppresses polarization-induced signal fluctuation resulting in improved signal intensity. The experimental scheme presented is simplified by using a single laser with one-end access. The temperature accuracy of the new sensing system was demonstrated as 1 degrees C on spatial resolution of 3 m.

基于局域网的多水下机器人仿真系统设计与实现

多水下机器人仿真系统是一个能够对多水下机器人系统的体系结构、协调控制、路径规划、学习算法等进行演示验证的分布式实时数字仿真系统,是开展多水下机器人技术研究的基础和有效手段.讨论了应用基于局域网的分布式仿真技术来解决多水下机器人系统仿真的问题,并详细说明了仿真系统的硬件组成和软件总体设计.

Fourier有限差分法深度偏移与正演模拟

With the development of seismic exploration, the target becomes more and more complex, which leads to a higher demand for the accuracy and efficiency in 3D exploration. Fourier finite-difference (FFD) method is one of the most valuable methods in complex structure exploration, which keeps the ability of finite-differenc method in dealing with laterally varing media and inherits the predominance of the phase-screen method in stablility and efficiency. In this thesis, the accuracy of the FFD operator is highly improved by using simulated annealing algorithm. This method takes the extrapolation step and band width into account, which is more suitable to various band width and discrete scale than the commonely-used optimized method based on velocity contrast alone. In this thesis, the FFD method is extended to viscoacoustic modeling. Based on one-way wave equation, the presented method is implemented in frequency domain; thus, it is more efficient than two-way methods, and is more convenient than time domain methods in handling attenuation and dispersion effects. The proposed method can handle large velocity contrast and has a high efficiency, which is helpful to further research on earth absorption and seismic resolution. Starting from the frequency dispersion of the acoustic VTI wave equation, this thesis extends the FFD migration method to the acoustic VTI media. Compared with the convetional FFD method, the presented method has a similar computational efficiency, and keeps the abilities of dealing with large velocity contrasts and steep dips. The numerical experiments based on the SEG salt model show that the presented method is a practical migration method for complex acoustical VTI media, because it can handle both large velocity contrasts and large anisotropy variations, and its accuracy is relatively high even in strong anisotropic media. In 3D case, the two-way splitting technique of FFD operator causes artificial azimuthal anisotropy. These artifacts become apparent with increasing dip angles and velocity contrasts, which prevent the application of the FFD method in 3D complex media. The current methods proposed to reduce the azimuthal anisotropy significantly increase the computational cost. In this thesis, the alternating-direction-implicit plus interpolation scheme is incorporated into the 3D FFD method to reduce the azimuthal anisotropy. By subtly utilizing the Fourier based scheme of the FFD method, the improved fast algorithm takes approximately no extra computation time. The resulting operator keeps both the accuracy and the efficiency of the FFD method, which is helpful to the inhancements of both the accuracy and the efficiency for prestack depth migration. The general comparison is presented between the FFD operator and the generalized-screen operator, which is valuable to choose the suitable method in practice. The percentage relative error curves and migration impulse responses show that the generalized-screen operator is much sensiutive to the velocity contrasts than the FFD operator. The FFD operator can handle various velocity contrasts, while the generalized-screen operator can only handle some range of the velocity contrasts. Both in large and weak velocity contrasts, the higher order term of the generalized-screen operator has little effect on improving accuracy. The FFD operator is more suitable to large velocity contrasts, while the generalized-screen operator is more suitable to middle velocity contrasts. Both the one-way implicit finite-difference migration and the two-way explicit finite-differenc modeling have been implemented, and then they are compared with the corresponding FFD methods respectively. This work gives a reference to the choosen of proper method. The FFD migration is illustrated to be more attractive in accuracy, efficiency and frequency dispertion than the widely-used implicit finite-difference migration. The FFD modeling can handle relatively coarse grids than the commonly-used explicit finite-differenc modeling, thus it is much faster in 3D modeling, especially for large-scale complex media.