Laser induced photoacoustic grating effects in molecular crystals

A transient grating of singlet electronic excited states is produced in a pentacene-doped p-terphenyl molecular crystal by optical absorption from two crossed time-coincident picosecond excitation pulses at 532 nm. The diffraction properties of this volume grating are probed by a weak, variably delayed, Bragg-matched picosecond probe pulse. At high excitation intensities, a strong oscillatory behavior in the time-dependent scattering efficiency is observed superimposed on the exponential decay pattern of the excited-state grating. We attribute the oscillatory behavior to a thermal grating which induces coherent microwave acoustic phonons. These in turn modulate, at the sound frequency, the optical absorption properties of the pentacene molecules in the excited state. An acoustically induced amplitude grating effect is thus obtained, in contrast to conventional acousto-optic phase grating effects.     

Comparison of four-wave mixing techniques for studying orientational relaxation

Three four-wave mixing techniques for measuring orientational relaxation of molecules in solution using time-separated pulses are compared both theoretically and experimentally. This comparison is shown to be greatly facilitated by modifying the conventional third-order response functions to incorporate a portion due to the slowly varying reorientation of the transition dipoles. Ordinary Bragg diffraction of a probe field, in which a transient grating formed by excitation pulses of the same polarization is probed with parallel and perpendicular polarizations, has the advantage of yielding both the time dependence and the magnitude of the induced anisotropy. However, the accompanying acoustic grating interferes with the polarization-sensitive excited state concentration grating unless the probe wavelength falls within a very strong absorption band. The acoustic grating can be eliminated by using perpendicularly polarized pumping pulses (the "crossed grating" configuration). Polarization spectroscopy in which the generated field is collinear with the probe gives the same anisotropy decay kinetics as the crossed grating, but is more sensitive to interference from background birefringence. These three methods, together with time-resolved fluorescence polarization, have been used to examine rotational dynamics in the ground and excited states of rhodamine 6G, 9-aminoacridine, and benzophenone.     

On Multiple Scattering of Radiation by an Infinite Grating of Dielectric Circular Cylinders at Oblique Incidence

A rigorous analytical representation for the multiple scattering coefficients of the fields radiated by an infinite grating of dielectric circular cylinders excited by an obliquely incident plane electromagnetic wave is derived in terms of the “well-known scattering coefficients of an isolated dielectric cylinder at oblique incidence” and “Schlömilch series”. In addition, a generalized sum-integral grating equation is acquired for the multiple scattered amplitude of a cylinder at oblique incidence in the grating in terms of the scattering coefficients of the insulating dielectric circular cylinder at oblique incidence.     

Integrated two-state AWG-based multiwavelength laser

An integrated InP-InGaAsP two-state coupled-laser device for use in optical packet switching and signal processing is presented. The two states are identified by distinct lasing wavelengths. Single-mode lasing occurs in both states and the contrast ratio between the two states is 35 dB. Switching between states with optical pulses is demonstrated. The use of an arrayed waveguide grating (AWG) and ring laser configuration permits monolithic integration without the need for cleaved facets. How the AWG can be used to obtain partial isolation between multiple interconnected devices is also discussed.     

四路频分复用荧光共焦显微探测系统实验研究

报道了采用全息聚合物分散液晶(H-PDLC)布拉格光栅阵列斩波调制的四通道频分复用荧光共焦显微探测系统的实验研究。通过高衍射率H-PDLC布拉格光栅将单束激光分成四束激发光并进行不同频率的载频调制,将激发光聚焦到生物样品上产生荧光信号并采集后,再通过傅里叶变换、滤波和解调,最后将采集到的信号还原成四路荧光信号强度随时间变化的曲线。实验搭建了激发光中心波长为405 nm的四路频分复用荧光探测系统,并成功探测到鼠神经海马细胞样品中激发的四点荧光信号的图像以及强度变化信息。     

QUANTUM MECHANICAL MODEL AND SIMULATION OF GaAs/AlGaAs QUANTUM WELL INFRARED PHOTO- DETECTOR-Ⅰ OPTICAL ASPECTS

ＩｎｔｒｏｄｕｃｔｉｏｎＩｎｆｒａｒｅｄｄｅｔｅｃｔｏｒｔｅｃｈｎｉｑｕｅｈａｓｂｅｅｎａｋｅｙｆａｃｔｏｒｉｎｔｈｅｄｅｖｅｌｏｐｍｅｎｔｏｆｔｈｅｉｎｆｒａｒｅｄｔｅｃｈｎｏｌｏｇｙｆｏｒｍｏｒｅｔｈａｎ 4 0 ｙｅａｒｓ .Ｓｉｎｃｅ 1970 ,ｓｅｍｉｃｏｎｄｕｃｔｏｒｓｌｉｋｅＩｎＳｂａｎｄＨｇＣｄＴｅｈａｖｅｂｅｅｎｔｈｅｐｒｉｎｃｉｐａｌｍａｔｅｒｉａｌｓｆｏｒｖａｒｉｏｕｓｉｎｆｒａｒｅｄｄｅｔｅｃｔｏｒａｐｐｌｉｃａｔｉｏｎｓ .Ｔｈｅｆｏｒｍａｔｏｆｔｈｅｉｎｆｒａｒｅｄｄｅｔｅｃｔｏｒｍｏｔｉｖａｔｅｄｂｙｓｍａｒｔｔｈｅｒｍａｌｉｍａｇｉｎｇｓｙｓｔｅｍｃｈａｎｇｅｄｆｒｏｍｓｉｎｇｌｅｅｌｅｍｅｎｔｄｅｖｉｃｅｔｏｆｏｃａｌｐｌａｎａｒｒａｙｓ(ＦＰＡｓ)ｉｎｔｈｅｍｉｄｄｌｅｏｆ 80’ｓ [1].Ｔｏｄａｙ’ｓｔｅｃｈｎｏｌｏｇｙｏｆｉｎｆｒａｒｅｄｄｅｔｅｃｔｏｒｃｏｎｃｅｎｔｒａｔｅｓｌａｒｇｅｌｙｏｎｆｏｃａｌｐｌａｎａｒｒａｙｓ ,ｅｘｐｅｃｉａｌｌｙｆｏｒｓｅｎｓｉｔｉｖｅ ,ｈｏｍｏｇｅｎｅｏｕｓａｎｄｌａｒｇｅｆｏｒｍａｔｓｃａｌｅｄｅｖｉｃｅｓ .ＨｇＣｄＴｅ...     

AlGaAs:Si中与DX中心有关的光生电子陷阱特性研究(英文)

A new electron trap state SD was found by DLTS measurement under light illumination in Si doped A.lxGa1-xAs. This new trap energy level ESD is shallower than the DX center energy in the gap and the concentration of SD is comparable to that of DX centers. The emission activation energy Ec=0.20±0.05eV and capture activation energy Ec= 0.17±0.05eV. The SD DLTS peak has never been detected previously because under dark and thermal equilibrium condition most of the electrons occupy the deeper DX states and most of SD states are empty. However, when the sample is illuminated by light, electrons are excited to the conduction band and then re-captured by SD since the deeper DX states have a slower electron capture rate, thus a new DLTS peak corresponding to SD appears. Constant temperature capacitance transient C-t and transient C-V measurements were also used to further confirm the existence of SD states.     

Concept of multiorder multimode resonant optical filters

The idea of implementing optical filters by coupling evanescent waves from several diffracted orders into multiple leaky waveguide modes is studied theoretically. Using a dielectric thin-film structure consisting of a coupling grating placed between adjacent waveguides, guided-mode resonance filters exhibiting multiple reflection peaks within a specified wavelength range can be obtained. These peaks originate in the resonant waveguide modes that are excited by the diffracted waves dispersed by the grating. It is shown that this device can be used to realize multiwavelength as well as wide-band spectral filters.     

AlGaN/GaN HFET中的陷阱

提出了二维表面态和表面缺陷层构成的AlGaN/GaN HFET中的陷阱模型。自洽求解薛定谔方程和泊松方程得到异质结能带和沟道阱基态、激发态及二维表面态的波函数。发现表面高密度缺陷减薄了势垒层厚度,显著增强了热电子隧穿过程。从缺陷态发射电子和热电子隧穿构成的新陷阱模型出发,解释了HFET的瞬态电流、肖特基势垒的伏安特性和产生-复合噪声。最后讨论了改进材料生长和器件工艺来抑制陷阱效应,改善器件性能的途径。     

Performance of laser-pumped quantum counters

Narrow-band quantum counters with less than unit internal quantum efficiency are analyzed. The pump power dependent expressions for the transient and steady-state quantum efficiencies are derived. These quanties are discussed for "typical" trivalent and divalent rare-earth-doped insulator crystals, and the performance of two specific systems is presented. It is shown that to achieve high quantum efficiency in trivalent rare-earth-doped quantum counter systems, the pump source must be highly nonblackbody in spectral character, i.e., a laser, and resonant with the transition between the relevant excited states. Performance limiting factors for laser-pumped quantum counters are pointed out.     

新型光敏剂竹红菌甲素的强激光效应及电子激发态特性

有机功能分子在开发新型光电子材料和光动力疗法(PDT)光敏剂方面是一个重要的研究热点;竹红菌甲素(HA)分子属于激发态质子转移(ESIPT)型分子,具有丰富的激发态特性,且反应速度快,在新型激光染料和新型光电器件方面有一定的应用前景。观察了竹红菌甲素分子的激射现象,其发射激光的范围在620~800 nm;并利用激射效应的特点为强激光条件下的质子转移提供了进一步的依据。利用瞬态光栅技术研究了竹红菌甲素的电子激发态特性,把其瞬态光栅的超快过程归结为竹红菌甲素质子转移形成的过渡态TS*的衰减,并得出过渡态TS*瞬态光栅的寿命为10.5 ps。     

New grating experiments in the study of irreversible photochemical reactions

We have investigated the irreversible hydrogen abstraction of benzophenone in glassy polymers through holographic grating techniques. Recording the decay of the transient grating after pulsed excitation and the building up of a permanent grating during CW excitation allows the extraction of valuable information about the photochemical kinetics. Variation of the fringe spacing permits one to distinguish diffusive and reactive processes, and so transient grating experiments promise to be of great value in the study of diffusion-controlled reactions. As grating experiments are sensitive to changes in the absorption coefficient as well as the index of refraction, one can very sensitively detect density changes and deformations of the matrix which accompany the photochemical transformation in the sample.     

Modeling Wavefronts from Acoustic Phased Arrays by Computer

A computer simulator is described which allows the study in the near field of acoustic pressure waves generated by a linear phased array excited with short pulses. The simulator provides two-dimensional (2-D) and three-dimensional (3-D) representations of acoustic wavefronts and receiver sensitivity patterns generated by a pulsed phased array. Using the computer simulator, several phased arrays are studied to investigate transient effects in an area covering the near field of the corresponding unfocused arrays. Weak and strong focusing are compared. An algorithm for the optimal quantization of time delays, acoustic beampatterns and the structure of sidelobes and grating lobes are analyzed.     

Analysis of interleaved arrays of waveguide elements

An analysis is presented of multiple interleaved arrays of waveguide radiators operating at different frequencies. The approach is a generalization of the analysis of an infinite array at a single frequency. The presence of the low-frequency elements affects the radiation characteristics of the high-frequency elements and generates grating lobes which do not appear in a single-frequency array. Furthermore, power is coupled 'into the low-frequency waveguides when the high-frequency elements are excited. Numerical examples are presented which show that these effects are not severe.     

基于小波分析的光栅信号质量评估方法

依据谐波分析法给出的评定信号质量指标的准则，提出了基于小波分析的光栅信号质量评估方法，并讨论了此方法的特征与优点。与谐波分析法相比较，小波分析在时域和频域同时具有良好的局部化性质，每次小波变换所取的都是原信号的最大可能近似值，信号的失真度小，通过变更尺度，可以对信号更细微的瞬态特征进行研究，并最大可能地除去噪声，较快地得到理想的质量评定效果(通常只需3至5次变换即可)。在此基础上，给出了相应的小波分析评定光栅信号质量的实验方法。     

密集型多重体全息光栅波长解复用技术

阐述了光折变多重体全息光栅作为滤波器实现波长解复用的原理,分析了滤波器的波长和角度选择特性,对器件设计中要考虑的问题进行了讨论。提出了实现多重体全息光栅波长解复用的实验方案和实用化要解决的问题。     

Fabrication of micro-moiré gratings on a strain sensor structure for deformation analysis with micro-moiré technique

In this study, the electron beam lithography technique is applied to fabricate moiré grating on the tiny structure of a strain gauge sensor. The grating fabrication technique is discussed in detail. The grating pattern is controlled by a pattern generator, which makes it possible to write a graph in the assigned region of the resist using the electron beam, and as a result a moiré grating is left on the surface of the sample. By the aid of the fabricated grating with electron moiré technique under the scanning electron microscope (SEM), the residual deformation of strain gauge sensor is measured after a direct voltage is imposed to it. The successful results verify the feasibility of the moiré grating fabrication using electron beam lithography, and the grating has a good potential for further application.     

Influence of a nonuniform coupling coefficient on the static andlarge signal dynamic behavior of Bragg-detuned DFB lasers

A time domain model is used to study the large signal dynamics of general Bragg-detuned cavities. Such structures, in which the grating pitch is spatially nonuniform so that a distributed phase shift is produced along the cavity, are often called corrugation-pitch-modulated distributed feedback lasers (CPM-DFB). Furthermore, by introducing a longitudinally dependent coupling coefficient κ, a corrugation-pitch-modulated distributed-coupling-coefficient DFB laser (CPM-DCC-DFB) with quite different spectral properties can be realized. We present in this paper a comparison between these two kinds of components. It is shown that an adequate profile of the coupling coefficient (stronger coupling at the center of the cavity) in a CPM-DCC-DFB structure can help to achieve better steady-state singlemode operation with respect to conventional CPM-DFB lasers (uniform coupling coefficient). However, in CPM-DCC-DFB lasers, side modes are liable to be excited during the turn-on transient interval, thus imposing a limitation to the modulation depth requirement     

Diffraction of an electromagnetic beam by infinite planar periodicmetal-strip grating

An hybrid angular spectrum method-Galerkin moment procedure has been developed to analyze the diffraction of an electromagnetic beam incident obliquely on an infinite planar periodic metal-strip grating. The integral expression obtained for the scattered field is expanded by means of an asymptotic method for determining the diffracted far-field. Some numerical examples show the decomposition of the incident beam in several beams corresponding to the propagating Floquet harmonics excited in the grating. The propagation directions of these beams show deviations with respect to the directions predicted by the Floquet theorem. The profile of the scattered far-field is analyzed as a function of the geometrical and electrical parameters     

Radiation of circularly polarized microwaves by a plane periodic structure of Ω elements

Transformation of the electromagnetic-wave polarization by a 2D grating of metal Ω elements is studied theoretically and experimentally. The electric dipole moment and magnetic moment excited by a linearly polarized incident wave in a single Ω element are determined. The parameters of an Ω element that are optimal for radiation of a circularly polarized wave are calculated. A 2D grating consisting of Ω elements with optimal parameters is fabricated, and the obtained theoretical results are verified experimentally.