Optical detection of surface states in GaAs(110) and GaP(110)

We present results on the optical detection of surface states in GaAs(110)and GaP(110)by the method of the fractional change of external reflectivity. Optical transitions are observed at ～3.1 eV m GaAs(110) and ～3.4 eV in GaP. A comparison with existing theories suggests a rotational relaxation model for the surface, with partial relaxation for GaAs(110) and full relaxation for GaP(110).     

Time-resolved transmission of GaAs under intense laser excitation

Time-resolved transmission and reflectivity of bulk GaAs and GaAs on sapphire have been studied at λ = 1152 and 633 nm following excitation by an ≈8 ns pulse at 485 nm. As in the case of silicon, the spectral dependence of the transient absorption coefficient, from 1.1 eV to 2.5 eV, is inconsistent with the existence of a metallic molten state during pulsed laser annealing.     

二维缺陷GaAs电子结构和光学性质第一性原理研究

采用基于密度泛函理论的第一性原理方法计算了存在Ga空位缺陷和掺杂B原子的二维GaAs的能带结构、态密度和光学性质.计算结果表明空位缺陷二维GaAs显示出金属特性,B原子的引入使体系变为间接带隙半导体,禁带宽度为0.35 eV.态密度计算发现体系低能带主要由Ga的s态、p态、d态和As的s态、p态构成;高能带主要由Ga和As的s态、p态构成.掺杂B原子与存在空位缺陷的二维GaAs相比,静态介电常数相对较低,变为8.42,且易于吸收紫外光,在3.90～8.63 eV能量范围具有金属反射特性,反射率达到52%.     

Electronic structure of strontium titanate

The spectra of three sets of optical functions for SrTiO₃ crystals are determined in a broad energy range of fundamental absorption. The calculations are carried out using the experimental reflectivity spectrum in the range 1–35 eV and two theoretical permittivity spectra in the ranges 0–30 eV and 0–14 eV. The special features of these spectra have been determined. The theoretical spectra of the optical functions are compared with the spectra determined using the experimental reflectivity spectrum.     

Optical properties and electronic structure of europium chalcogenides

The normal incidence reflectivity of europium chalcogenide single crystals and of Gd doped EuO has been measured at room temperature in the spectral region from 250 μm to 12 eV and has been analyzed in terms of the optical constants. In addition, in a reduced spectral region from 0.5 to 6 eV, the optical constants have been evaluated by means of a polarimetric method, as well above as below the magnetic ordering temperature. To enhance the resolution of the magneto-optical transitions, a modulation technique has been applied with a magnetic field as modulating parameter. The Kramers-Kronig relation has been used to analyze the normal incidence reflectivity and the magnetoreflectance spectra in terms of, respectively, the optical constants and the changes in the real and imaginary part of the dielectric response function. For Gd doped EuO the Kramers-Kronig analysis has revealed plasmon and coupled plasmon-phonon modes. The interband transitions of the europium chalcogenides are discussed within the framework of recent APW and OPW energy band calculations. On the other hand we have derived an energy level scheme of the europium chalcogenides from our optical data.     

Magnetic field-modulated magnetoreflectance of europium chalcogenides

The optical constants of EuO and EuS single crystals have been determined at 300 K by means of a Kramers-Kronig analysis of the reflectivity for photon energies up to 12 eV. For EuS the optical constants have also been determined above and below the Curie temperature in the energy region from 1.5 to 5.7 eV. A first tentative assignment of optical structure to interband transitions has been attempted on the basis of recent orthogonal plane wave (OPW) and earlier augmented plane wave (APW) band structure calculations. For photon energies from 1.2 to 3.8 eV a low magnetic field-modulated magnetoreflectance has been measured using circularly polarized light. By use of the Kramers-Kronig relation for the differential reflectance spectra in conjunction with our data of the optical constants, a detailed analysis of the magnetoreflectance spectra of EuS was carried out for the first time.     

Optical properties of the red bronze K0.33MoO3

The optical reflectivity of the red bronze K_0.33MoO₃ has been measured on single crystals in the spectral energy range between 0.03 and 12 eV at temperatures from 4 K to 300 K using polarized light. The optical constants have been determined by means of a Kramers-Kronig analysis; the data are interpreted that this compound is a 0.5 eV energy gap semiconductor with very strong anisotropy in the infrared and visible energy range.     

Optical properties and valence mixing in CeN

The reflectivity spectrum of CeN shows a strong temperature dependence in the photon energy range up to 2.5 eV. The conduction electron contribution in terms of the optical constants has been analysed by means of the Drude theory.     

砷化镓光电阴极光谱响应与吸收率关系分析

为了研究砷化镓(GaAs)光电阴极光谱响应与吸收率曲线间的关系,采用分子束外延法(MBE)和金属有机化合物化学气相沉积法(MOCVD)制备了两类GaAs光电阴极,并测试得到了样品吸收率和光谱响应实验曲线.对每个样品的这两条曲线在同一坐标系中做最大值归一化处理,将归一的光谱响应曲线与归一的吸收率曲线做除法,得到了类似光电阴极表面势垒的形状.结果表明,两种方法制备的光电阴极光谱响应曲线相比吸收率曲线都发生了红移,MBE样品偏移量稍大于MOCVD样品.短波吸收率不截止,光谱响应截止于500 nm左右;可见光波段上,光谱响应曲线的峰值位置相比吸收率曲线红移了几百meV;近红外区域,光谱响应曲线的截止位置相比吸收率曲线红移了几个meV.MOCVD样品中杂质对带隙的影响更小,光谱响应相比吸收率发生的能量偏移更小.这些结论对提高GaAs光电阴极光电发射性能有指导意义.     

LaFeAsO单晶各向异性光学性质的第一性原理研究

使用密度泛函第一性原理研究了高温超导体LaFeAsO各向异性的光学性质。在描述光学性质的计算原理和计算方法的基础上, 计算了LaFeAsO的态密度、光电导谱、反射谱以及电子能量损失谱。光电导谱中, x方向与z方向有着很大差别, 在沿x方向的第一个带间吸收峰出现在1.3 eV处, 沿z方向出现在1.5 eV处; 在反射谱与电子能量损失谱中, x方向与z方向的特征峰位置在能量较高处都是相互吻合的。分析认为, 主要是电子在Fe原子之间的各个态间的跃迁所引起。考虑到温度效应对其光学性质的影响, 在计算光学矩阵元时, 加入Lorentz展宽δ=0.10 eV。本文的研究结果, 可为实验制备以及材料性质的研究提供有价值的参考。     

GaAs/SiO2纳米晶镶嵌薄膜的拉曼光谱与吸收光谱研究

采用射频磁控共溅射与高真空退火相结合的方法,分别在单晶硅片和光学石英玻璃片上制备了ＧａＡｓ／ＳｉＯ２纳米晶镶嵌薄膜样品。激光拉曼光谱的测量结果表明,退火态样品（４００℃,６０ｍｉｎ）的拉曼光谱特征峰呈现宽化和红移,红移量为９．５ｃｍ＾－１,对应薄膜中ＧａＡｓ纳米晶粒平均粒径约为３ｎｍ。样品的室浊吸收光谱测量结果表明,由于受量子限域效应的主导作用,与ＧａＡｓ块状单晶相比,样品光学吸收边呈现出明显的蓝     

Polarization dependence of optical transitions in GaP(1 1 0) and GaAs(1 1 0) surfaces studied with surface differential reflectivity

We present results on the polarization dependence of Surface Differential Reflectivity in GaP(1 1 0) and GaAs(GaAs(1 1 0) surfaces. GaP(1 1 0) shows a transition at 2.8 eV which is excited only with the light electric vector parallel to the [1 1 0] direction; for the optical transition above 3.0 eV there is not any marked polarization dependence. GaAs(1 1 0) shows optical transitions between 2.6 and 4.0 eV which have a weak dependence on light polarization.     

Towards X-ray induced transient grating methods for nanometer scale dynamics: Diffraction on transient structures induced by extreme ultraviolet radiation from FLASH

Using the high brilliance femtosecond soft X-ray pulses from the Free-Electron LASer at Hamburg (FLASH) the X-ray induced transient optical reflectivity change of GaAs has been established as a versatile method for femtosecond X-ray/optical cross-correlation [1]. As the underlying physical mechanism is the X-ray induced dynamics within solids, we present in this work a feasibility study how transient grating methods could be used to study nanometer scale dynamics in materials, such as the radical diffusion parameters in photoresist materials for EUV lithography.     

Multilayer mirror for x rays below 190 eV

La/B(4)C multilayers have been fabricated by magnetron sputtering for use as x-ray mirrors at energies below 190 eV, particularly for detection of boron K and alpha x rays at 183 eV, their performance has been compared with that of Mo/B(4)C multilayers, which are currently the best-performing multilayers for this energy range. Transmission electron microscopy and synchrotron soft-x-ray reflectometry were used to study the structural quality of the multilayers and their performance as x-ray mirrors. The results show a significant improvement of the peak reflectivity and the spectral purity, indicating that La/B(4)C has a high potential to replace Mo/B(4)C in many x-ray optical applications below 190 eV.     

The blue bronze K0.3MoO3: A new one-dimensional conductor

The optical reflectivity of the blue bronze K_0.3MoO₃ has been measured on single crystals for photon energies between 0.03 and 12 eV at temperatures from 10 to 300 K using polarized light. The data are interpreted that this compound is a one-dimensional conductor for temperatures above 180 K and that the metal-semiconductor transition at 180 K is due to a Peierls type transition, leading to a gap of 0.15 eV in the density of states.     

Electronic structure of uranium monochalcogenides and uranium monopnictides

Single crystals of the uranium chalcogenides US, USe, UTe and the pnictides UAs and USb have been cleaved under high and ultrahigh vacuum conditions and the near normal incidence reflectivity has been measured in situ from 0.03 to 12 eV. In the case of US the spectral range of the measurement was further extended down to 0.0017 eV. The various optical functions (?₁, ?₂, σ, E_loss, n_eff) have been deduced by a Kramers-Kronig analysis. Typical features of all spectra are weak intraband contributions and the extension of strong interband transitions to very low photon energies. Highly damped plasmons are found at 4.25 eV for US, 3.5 eV for USe nad 2.2 eV for UTe. For the pnictides UAs and USb the corresponding values are 2 eV and 1.6 eV, respectively.The apparent contradiction between the presence of a large number of conduction electrons and only very weak intraband contributions is resolved within the spectral exclusion model of Keller. In this self-consistent cellular multiple scattering calculation the coupling of the narrow 5f states and the much wider 6d states produces a dip in the density of d states near E_F where the f electrons form a resonance state. Quantitative agreement is obtained between theory and experiment for US. For all investigated compounds empirical energy level schemes are derived.The reflectivity measurements down to 1.7 meV for US reveal the existence of a further excitation at 39 meV which is assigned to a transverse optical phonon mode. This unique observation of an optical phonon in a direct reflectivity measurement of a metal supports the picture of a small density of conduction electrons at E_F. Further arguments in favor of the spectral exclusion model are presented and the question of the localization of thr 5f electrons in various uranium compounds is discussed.     

Optical properties of AlxGa1-xAs/GaAs MQW studied by infrared ellipsometry and fourier transform infrared spectroscopy

The optical response of Al_xGa_1-xAs/GaAs MQW from near-infrared to far-infrared has been studied. Using infrared spectroscopic ellipsometry, we measured the optical response of the samples in the energy range 0.5-2.0eV, and oscillations have been observed. A multilayer film calculation indicates that this phenomenon results from interference. By Fourier transform infrared spectroscopy, the absorptance of double-well MQW has been measured in the energy range 0.08-0.25eV. The energy levels of subbands and the oscillator strengths were calculated from self-consistent interface potential theory. The calculation results are consistent with the measurements.     

Electro-absorption and refraction in Fabry-Perot quantum well modulators: a general discussion

Fabry-Perot resonators have long been advocated to improve the limited contrast ratio of multiple quantum well optical modulators used in photonic switches based on self electrooptic effect devices (SEEDs) and in other array based optical interconnection schemes. Using data on field dependent GaAs/AlGaAs quantum well absorption and refraction, we have modelled the reflectivity, modulation depth and contrast ratio of resonant modulators. Our results are generally valid for any quantum well modulator and demonstrate 23the important role played by electro-refraction even in regions of strong absorption. Resonators give large contrast ratios but there are trade-offs in the maximum reflectivity change achievable with Fabry-Perot resonators compared to simple modulators. The model gives the optimum number of quantum wells and reflectivity values required to make a resonator at any wavelength for a given quantum well structure. Understanding the limits of Fabry-Perot quantum well modulator performance is important for their application in symmetric self electrooptic effectiveness for photonic switching where modulation and detection properties are both used and for optical interconnection systems.     

透射式GaAs光电阴极抗光反射膜的研究

本文计算了在透射式GaAs光电阴极的窗玻璃和GaAlAs缓冲层之间引入Si₃N₄或SiO₂膜使光电阴极的入射光反射率所产生的变化,并测量了Si₃N₄膜、SiO₂膜和SiO₂/Si₃N₄复合膜对阴极反射率的影响。理论计算和测量表明,Si₃N₄膜和SiO₂/Si₃N₄复合膜可有效地降低光电阴极对入射光的反射率。当Si₃N₄膜的厚度约为100nm时,可使光电阴极的反射率降低近60%。本文结果为科学地选择透射式GaAs光电阴极所需的的抗光反射膜提供了依据。     

The band structure and electronic properties of sulphur nitride polymer

A semiempirical calculation of the energy band structure of (SN)_x has been made on a tight-binding model with three p orbitals per atom. An important feature is that the Fermi level crosses two overlapping conduction bands. Measurements are reported of the optical transmission spectrum between 0.2 and 4.0 eV in thin films, the free carrier reflectivity in thick films, and the hydrostatic pressure dependence of the conductivity to 15 kbar. The calculated band structure accounts for experimental results connected with interband transitions (optical absorption) and intraband effects (metallic conductivity, reflectivity, specific heat).