Optical nonlinearity and bistability of CdS

Experimental data concerning the nonlinear optical behaviour of CdS are reported. At cryo-temperatures an increasing nonlinear absorption at the exciton resonance is observed, with the aid of which optical hysteresis is demonstrated. At room-temperature saturation of the band-edge absorption is found at intensities as low as 10 kW/cm². The switch-on time is determined to be smaller than 6 ps.     

Quantum mechanical treatment of the escape probability from traps in thermally stimulated processes

In thermally stimulated processes the calculation of the transition probability of a trapped charge carrier into an excited state is based on an atomistic model involving multiphonon processes. The displacement of lattice particles from equilibrium positions gives rise to electron-lattice interaction. As a result the temperature dependence is found to be of non-Arrhenius type except in some limiting cases. The difference of the probabilities for non-radiative transitions, as calculated by the quantum mechanical and by semi-classical methods, is discussed. Thermoluminescence proves to be an example for a non-radiative transition controlled phenomenon and the calculated glow curve differs appreciably from one model to the other. As a result the glow curves calculated in the quantum mechanical concept exhibit shapes which are broadened and shifted to lower temperatures, as compared to that calculated in the semi-classical concept. In turn, the activation energy, as determined by the initial rise method or the method of heating rate variation, is found to be much smaller in the quantum mechanical case.     

The pre-exponential factor of the escape probability in thermally stimulated processes

The pre-exponential factor of the transition probability of a trapped charge carrier into an excited state is evaluated from two different approaches. The semi-classical treatment based on a thermodynamical concept involves the calculation of the vibrational entropy change from the partition sum. The quantum-mechanical treatment deals with non-radiative transitions due to electron-phonon coupling. The factor is found to be explicitely dependent on both the temperature and the lattice vibration frequencies represented by a single Einstein oscillator. In addition, the results from different concepts show similar behaviour. As a consequence the effect of lattice vibrations on the position as well as on the shape of glow peaks in thermally stimulated measurements are investigated on a simple phenomenological base. The analysis of some experimental results can be performed in accordance with the theoretical predictions.     

The electronic structure of GaP (110) and Cu-Phthalocyanine overlayers studied by ellipsometry

Ellipsometric spectroscopy in the spectral range 1.4 eV<<4 eV is used to study cleaved GaP (110) surfaces after adsorption of hydrogen and oxygen and after deposition of Cu-Phthalocyanine (CuPc) molecules. The adsorption of gases and of CuPc induces band bending changes which are derived from changes of the optical constants in the space charge layer due to an electric field effect. For the clean surface, furthermore, information is obtained about electronic surface states. CuPc overlayers with thicknesses below one monolayer exhibit an optical absorption which suggests a chemisorption interaction through the macrocyclic-system of the Pc-molecules.     

Optical rectification in CdS

Optical rectification non-linear process in CdS single crystals at the Nd-YA1G and CO₂ frequencies is reported. V_OR signal measurements versus θ angle, between radiation E-field and crystal c-axis, give non-zero terms values of the optical rectification tensor. The frequency dispersion effect of this non-linear process is discussed.     

Optical nonlinearity of azodye-doped polymer waveguides

Results of investigations of the optical and photochromic properties of waveguides prepared by chemical diffusion of the azodyes methyl red and Sudan G into polycarbonate are reported. High optical nonlinearity of the layers is observed under conditions of self-action of λ=633 nm laser radiation, and the mechanism of this effect is established. Zh. Tekh. Fiz. 68, 124–126 (February 1998)     

Semiconducting black phosphorus

Black phosphorus is a narrow-gap semiconductor newly attracting attention because of recent success in growing single crystals at a high temperature under a high pressure. This review covers recent research work on the preparation, the crystal structure, the band structure, the electrical and optical properties, and the superconductivity of black phosphorus.     

Optical limiting properties of CdS nanowires

Intensity-dependent nonlinear optical transmission studies of cadmium sulfide (CdS) nanowires (∼ 50-100 nm diameter) suspended in dimethylformamide have been carried out in the visible region using the Z-scan technique with 7 ns pulses from the second harmonic of an Nd:YAG laser. The optical limiting threshold of CdS nanowires suspension was determined to be 1.3 J cm^− 2, with normalized transmittance of 0.47, which is relatively lower when compared with those of many popular metal nanowire suspensions reported in the literature. Based on an effective three-photon absorption model, nonlinear absorption and nonlinear scattering were identified as the dominant processes for the measured reduced transmittance.     

半导体粒子CdS的光学性质研究

采用DDA(discrete dipole approximation)方法对用于光催化制氢的半导体粒子的光学性质进行了研究,讨论了形状、尺寸等因素对放置在水中的CdS吸收效率的影响,选取的粒子形状有球形,圆柱形,正四面体,正三棱柱,长方体,正六角棱柱,发现粒子形状对局部表面等离子体共振峰的数目和共振波长有重要影响,而...     

Optical properties of citrate-stabilized CdS nanoparticles

Citrate-stabilized CdS nanoparticles of size 4 nm are obtained by varying the sulfide:citrate ion concentration in a simple aqueous synthesis method. The optical absorption and photoluminescence properties of the nanoparticles are studied. The size of the crystallites is found to be less affected by sulfide:citrate ratio. At lower concentrations of S²⁻, trap state emission is favoured and at higher concentrations excitonic transition is predominant as shown by optical absorption and photoluminescence spectra. Effective surface capping and optimum concentration of S²⁻ leads to the quenching of surface-defect-related emission. Increase in citrate ion concentration is found to increase the intensity of photoluminescence band arising from trap state emission revealing the role of sulfide:citrate ratio on surface modification of CdS nanocrystals. The nanoparticles are hexagonal as shown by the X-ray diffraction and selected area electron diffraction pattern.     

Thirring optical solitons with Kerr law nonlinearity

The dynamics of Thirring solitons in birefringent fibers with Kerr law nonlinearity is addressed in this paper. An exact 1-soliton solution is obtained. Bright, dark and singular soliton solutions are described.     

CdS在室温热光非线性状态下的动态光谱研究

提供了一种研究半导体在光学非性状态下的动态光谱方法.将此方法应用到半导体CdS上,观察到在热光非线性状态下,其透过光谱(在吸收带边缘附近)随时间的变化(在ms数量级上).计算机模拟结果与实验相符,并得出了在CdS上获得室温光学双稳态的条件.     

Optical nonlinearity in hydrogenated amorphous silicon-selenium film

The nonlinear absorption and nonlinear refractive index of hydrogenated amorphous silicon-selenium (a-Si,Se:H) film prepared by a plasma-enhanced chemical deposition process was measured using a z-scan technique. Intensity dependent transmission was observed and attributed to reverse saturation absorption (optical limiting). The thermal contribution to nonlinear refractive index is discussed. The nonlinear response time was measured by using the pump and probe technique. Reverse saturating absorption was utilized to demonstrate all optical switching.     

Second-order optical nonlinearity of surface-capped CdS nanoparticles and effect of surface modification

Surface-capped CdS nanoparticles (5 nm mean diameter) with hexametaphosphate and CTAB (hexadecyltrimethyl-ammonium bromide) in chloroform were prepared by colloidal chemistry and phase transfer method. The second-order nonlinear optical response was measured by incoherent hyper-Rayleigh scattering (HRS) technique. Results show that the surface-capped CdS nanoparticles have a large first-order hyperpolarizability β value of 1.30×10⁻²⁶ esu per particle. Surface capping molecule modification was performed by adding pyridine to a chloroform sol of the surface-capped CdS nanoparticles where pyridine can be bound to the CdS nanoparticle surface through Cd–N chemical bonds. A reduction in HRS signal intensity was observed when increasing pyridine concentration. This is explained in terms of a two-level model approximation derived from molecular chromophores, when considering the influence of the formation of Cd–N coordinating bonds on the charge-transfer transition energy of surface molecule-like scatterers of Cd–S polar bonds.     

Optical gate realization with thin CdS films

A spray deposited thin (6 m) CdS film laser induced optical device (LIOD) was studied experimentally and theoretically. The all optical bistable (all-OB) and the hybrid bistable (HB) features at 210 K were investigated. It is shown for the first time that the observation of equal contrasted all-OB and HB loops is possible in thin CdS films. The slope of the all-OB loops were calculated with a new theoretical model which is established on the thermal enhanced shift of the absorption edge and the transmitted light equilibrium condition. A good agreement with the experiments is found. Furthermore, it is pointed out that the electron life time determines the fashion and the contrast of the HB loops. In this context the proof is furnished that the maximal possible HB loops is observed. Finally, the influence of Joule's heat on OB and HB is demonstrated.     

Fe掺杂对CdS光学特性的影响

采用低压金属有机化学气相沉积技术,在固定源流量的条件下,通过调节衬底温度(270~360℃)生长了不同Fe掺杂浓度的CdS薄膜。光谱测量表明低铁掺杂对CdS晶格振动的影响较小,但对光致发光性质影响较为明显。样品的光致发光谱包括两部分:2.5eV附近带-带跃迁的发光以及2.0~2.4eV之间与缺陷相关的发光。随着铁含量的增加,带-带跃迁逐渐被抑制,发光光谱被缺陷相关的发光主导,同时薄膜的电导也由n型转为p型,说明Fe离子掺入在薄膜引入了受主杂质。通过不同激发密度下的光致发光光谱测量,我们将2.0~2.4eV的发光归结为铁受主相关的D-A对发射,并根据掺杂浓度和发光峰位置估算了Fe受主的能级位置。     

CdS半导体超微粒样品光谱性质的研究

纳米尺度的半导体超微粒在线性和非线性光学方面表现出来的奇特性质使其成为研究和开发新的功能材料的热点。成键为 S、P轨道的 - 族金属硫化物半导体 ,电子结构和晶体结构均较为简单 ,结晶性能好 -界面无序结构少 ,在纳米尺度上其结构仍与体相材料近似 ,因此 ,成为研究量子尺寸效应的理想材料。本文对采用化学微乳液法合成的 Cd S半导体超微粒的光谱性质进行了研究 ,实验结果显示 ,Cd S超微粒样品的吸收光谱的带边与体相相比发生了蓝移 ;从吸收光谱中可以看到 ,Cd S超微粒样品的吸收边随粒径减少而向短波方向移动。拉曼光谱的电子—— L O声子耦合在粒子尺寸 2 .4- 2 .9nm范围内与粒子尺寸无关     

Optical switching in a coupler with saturable nonlinearity

Abstact We report in this paper a numerical study of optical switching in couplers with saturable nonlinearity. Regions that characterize different coupling behaviour are revealed. It is shown that, in the presence of saturation, a Gaussian pulse should be used instead of solitons for negligible pulse splitting between the coupler arms. Nearly complete switching is possible depending on the input optical power and the index saturation parameter.     

Optical nonlinearity of nanocrystalline Au/ZnO composite films

The third-order nonlinear optical susceptibilities, chi(3), of composite films consisting of nanocrystalline Au and ZnO particles were investigated by use of a degenerate four-wave mixing scheme. The maximum value of chi(3), measured at a laser wavelength of 532 nm and a pulse duration of 70 ps, was approximately 2 x 10(-6) esu. Also, this chi(3) value was achieved with small absorption (the surface-plasmon peak was at the 615-nm wavelength). Our composite materials showed no discernible degradation after they were subjected to irradiation for a total of 3 x 10(7) high-intensity pulses (24 Mw/cm2, 70-ps pulse duration at 500 Hz) during 16 h of testing.