Photoconductivity in polyacetylene

The results of steady state photoconductivity experiments on cis- and trans-(CH)_x are interpreted in terms of charged solitons, photogenerated either directly (threshold hv = 4Δ/π) or indirectly through coupling of the lattice to electron-hole pair excitations (hv ? 2Δ). The transient photocurrent, after laser pulse excitation, decays as a power law, I(t) ? t^?0.6, suggesting dispersive transport of the photogenerated carriers.     

Influence of excitation wavelength on the reflectivity of photogenerated plasmas in germanium

High-density carriers were produced in germanium using the fundamental (1.06 μm) and second harmonic (0.53 μm) output of a Q-switched glass:Nd³⁺ laser. Differences in the plasma reflectivity at 10.6 μm are attributed to different amounts of lattice heating. In particular, the more substantial damping of the plasma resonance associated with the 0.53 μm excitation is attributed to a higher electron-phonon collision frequency. The results are discussed with respect to diagnostics of photogenerated semiconductor plasmas and the effeciency of semiconductor switches.     

Time resolved spectroscopy of CdSe,using monopulsed picosecond excitation

We have studied at 4.2 K, the time resolved luminescence of CdSe platelets, using two photons absorption and monopulsed excitation. From the analysis of the A-LO excitonic lineshape, we have obtained the relaxation kinetics of the exciton temperature during the first 1.3 nsec. We deduce the loss rate d<E>/dt of the mean exciton energy. We report also at higher excitation, the occurence of new lines in the luminescence spectrum.     

Time resolved ADMR applied to the triplet state of the primary donor of bacterial photosynthetic reaction centers

The population and depopulation kinetics of the triplet sublevels of reaction centers ofRhodobacter (Rb.) sphaeroides strains R26 and GA were determined at 9 K by time resolved absorption detected magnetic resonance (ADMR) using the RF pulse method. The comparison of the saturation and pulse methods of time resolved ADMR proofs the latter to be superior. The kinetic behavior of the 2|E| signal was investigated for the first time. It exhibits an unexpected slow decaying component with a rate comparable to thez-level decay. Using a simple kinetic 4-level model for the triplet dynamics we conclude that this slow component as well as the sign of the ADMR signal of the 2|E| transition can be explained by a selective spin lattice relaxation channel connecting the triplet sublevelsx andz.     

Laser hyperacoustic spectroscopy of single crystal germanium

Using a photothermal laser deflection technique the profiles of laser-induced hyperacoustic pulses in single crystal germanium were studied at a subnanosecond time resolution. It is shown that the hyperacoustic pulses are excited due to an electron-deformation interaction of photogenerated carriers with the crystal lattice, which is much more effective than the thermoelastic mechanism of the acoustic wave generation. Evolution of the hyperacoustic pulse profiles related to the diffraction and acoustic absorption effects was studied. An analysis of the hyperacoustic signal profiles allowed us to estimate the coefficient of ambipolar diffusion of the photogenerated charge carriers and the coefficient of hyperacoustic wave damping. It is established that the front of the electron-hole plasma laser-excited in germanium at room temperature propagates at a supersonic velocity.     

卤化银材料光作用过程的时间分辨谱测量

卤化银材料是重要的光信息材料，对于信息科学与技术的发展具有非常重要的作用。光作用过程的时间特性在很大程度上决定了卤化银材料性能的优劣。本文采用微波吸收技术来测量卤化银材料光作用过程的时间特性。卤化银材料样品被放入微波谐振腔中，在准分子激光的作用下，卤化银材料中生成了光生载流子，光生载流子的产生改变了卤化银材料的介电函数。因此，介电函数的变化体现光生载流子时间特性的信息，同时，介电函数的变化引起微波谐振腔品质因数的改变，品质因数的改变使腔输出了一个反射波，反射波包含卤化银材料光作用时间特性的信息，通过测量反射波、我们能获得卤化银材材料光作用过程的时间分辨谱并研究光作用过程的时间特性。本文获得了卤化银材料光作用过程的时间分辨谱，并对黑白、彩色和X－光片的光作用时间特性进行了分析。     

Pump power dependence of the electron-hole plasma luminescence in Ga-doped ZnO film

Time resolved two-photon absorption induced electron-hole plasma (EHP) luminescence of Ga-doped ZnO thin film was measured by an ultrafast optical Kerr gate (OKG) in femtosecond time regime. Experimental results showed that the buildup time of the EHP luminescence was strongly dependent on the excitation fluence. The dependence of the buildup time of EHP on excitation fluence probably arose mainly from the relaxation of the hot carriers due to the carrier-carrier interaction, which increased with the increase of excitation fluence.     

Charge-carrier dynamics in polythiophene films studied by in-situ measurement of flash-photolysis time-resolved microwave conductivity (FP-TRMC) and transient optical spectroscopy (TOS)

An in-situ measurement system for flash-photolysis time-resolved microwave conductivity (FP-TRMC) and transient optical spectroscopy (TOS) has been developed to perform simultaneous measurements of photo-induced changes in conductivity and charge-carrier density in an organic thin film. The electric field in the resonant cavity designed for the present system was analysed by electrostatic simulation. Using the present system and the simulated electric field, the photoconductivity and transient absorption in a regioregular poly(3-hexyl thiophene) film were measured using one particular geometry under photon excitation energies of 6.39, 4.98, 3.48, and 2.34?eV. The dynamics of photogenerated charge carriers is discussed in terms of the excitation energy and incident photon intensity. The transient absorption spectrum induced by 3.48?eV light is presented and compared with the TRMC transient.     

Thermal emission of semiconductors under nonequilibrium conditions

The thermal emission of semiconductors under the fundamental absorption edge at excitation of nonequilibrium charge carriers in them has been investigated. A broad spectrum of various actions upon a semiconductor—photoexcitation, contact injection, magnetoconcentration effect—has been used for the first time for modulation of the emissivity of crystals under isothermal conditions. Spectral, field, temperature and coordinate dependencies of the thermal emission as well as its transient characteristics at a pulsed excitation have been measured. It is shown that a considerable modulation of the thermal emission in a semiconductor can be obtained without a temperature change, through changing its emissivity alone. A number of features of the thermal emission in the region under consideration has been revealed: a spectral maximum which position depends on the optical thickness of the crystal, a kinetics delay effect, etc. It has been established that the thermal emission of an excited semiconductor can be used as a means for determining various parameters of a material. Experiments were carried out on Ge, Si plates and Ge diode structures at 310 ⩽ T ⩽ 350 K.     

Photogeneration of electrons and holes in amorphous molecular semiconductors

This paper reports on the results of investigations into the photoconducting properties of amorphous molecular semiconductors based on films of two types: (i) poly(styrene) films doped with epoxypropylcarbazole (EPC) and a cationic polymethine dye (PD1) and (ii) poly(styrene) films doped with tetranitrofluorenone (TNF) and an anionic polymethine dye (PD2). Films of the first type possess p-type conductivity, whereas films of the second type exhibit n-type conductivity. It is found that, for films with n-type conductivity, unlike films with p-type conductivity, the activation energy of photogeneration of mobile charge carriers decreases with a decrease in the optical wavelength in the absorption range of the dyes. The possible mechanisms of the influence of the photoexcitation energy on the initial distance between charge carriers in electron-hole pairs are analyzed. The inference is made that, when the excess thermal energy of excited dye molecules dissipates at a low rate, the distance between the photogenerated electrons and photogeneration centers increases as compared to the distance between the photogenerated holes and photogeneration centers due to the electron-nucleus interaction.     

Fermi-Pasta-Ulam β 格点链系统能量载流子研究

Fermi-Pasta-Ulam (FPU) β格点链中能量输运的载流子是孤子还是声子一直存在较多的争议. 本文通过单脉冲方法, 明确了一个能量波包在该格点链系统中从声子波包转变成为孤子波包的条件, 即波包能量达到一定阈值. 基于纯四次势链的声子真空效应, 构造了由FPU-β链与纯四次势链构成的双段链系统. 通过对比研究双段链系统和单段FPU-β链中的热流, 发现低温下声子是FPU-β链中能量的主要载流子, 而随着温度的升高孤子逐步取代声子成为能量的主要载流子. 关键词： Fermi-Pasta-Ulam格点链 声子 孤子 热传导     

Theory of the hot free carrier intraband-absorption coefficient of n-inversion layers

The absorption coefficient K of a quasi two dimensional (2D) hot free electron gas is calculated for the first time as a function of the lattice temperature T, the photon angular frequency w, the carrier density N_s as well as the electron temperature T_e when the carriers are scattered by ionized impurities, acoustic phonons and polar optical phonons. Analytical expressions are derived in the limiting cases of non-degeneracy and degeneracy of the electron system. In the quantum limit ħw/k_BT_e ≳ 1 where the interaction between the electron and the photon is inelastic K sensitively depends on the limiting scattering mechanism showing that the electron motion is completely controlled by the photon field. In the classical limit ħw/k_BT_e ⪡ 1 the absorption decreases proportional to w¹ independent of the limiting scattering mechanism in agreement with the experimental data deduced from far-infrared absorptivity measurements on GaAs heterolayers.     

Coherent acoustic phonon oscillations in semiconductor multiple quantum wells with piezoelectric fields

Large coherent acoustic phonon oscillations were demonstrated using InGaN/GaN multiple quantum wells with piezoelectric fields. With UV femtosecond pulse excitation, photogenerated carriers screened the piezoelectric field and initiated the displacive coherent phonon oscillations. The specific phonon frequency was selected by the coupling between the periodic carrier distribution and the corresponding acoustic phonon mode. The induced acoustic phonon oscillation resulted in piezoelectric field modulation and then caused absorption variation through the quantum confined Franz-Keldysh effect. The wave vector uncertainty due to the finite sample width was found to determine the observed dephasing time.     

双光子激发ZnSe自由载流子超快动力学研究

采用Z扫描和抽运-探测实验技术, 在波长为532 nm、脉冲宽度为41 fs的条件下测得ZnSe晶体的双光子吸收系数, 并获得了不同激发光强下的自由载流子吸收截面、电子-空穴带间复合时间和电子-声子耦合时间. 研究发现, 随着激发光强的增大, 自由载流子吸收截面减小, 复合时间变短. 当激发光强增大导致载流子浓度大于10¹⁸ cm^-3时, 抽运-探测信号出现明显改变, 原因归结为强光场激发导致样品在短时间内带隙变窄和电子-空穴等离子体的形成.     

Coherent control of the lattice dynamics of bismuth near the lindemann stability limit

The relative contributions of the anharmonicity of the lattice potential and the nonequilibrium concentration of charge carriers to the time dependence of the coherent A _1g phonon frequency in bismuth excited by high-energy ultrashort laser pulses are studied by the coherent control method. The contributions are separated by the pump-probe method in which excitation is performed by two pulses with a controlled time delay. It is shown that, depending on the relative delay between the pump pulses, both correlation and anticorrelation between the amplitude and the initial frequency of oscillations are observed while the chirp and the initial frequency of the coherent phonon are anticorrelated. In addition, it has been found that the contributions of the lattice and the electronic subsystem are always anticorrelated. Therefore, a certain phase can be assigned to an electronic excitation and it may be suggested that the time dependence of the phonon frequency is determined not only by instantaneous values of the lattice and electronic response but also by the phase relations between them.     

On the nature of coherent phonons generated by ultrashort laser pulses in single-crystal antimony

Reflectivity oscillations generated by A_1g coherent phonons in an antimony single crystal have been studied by a method involving pumping and probing by femtosecond laser pulses, which was complemented by spectral filtration of the signal. An analysis of the spectrally resolved signal showed that not only the integrated intensity but also the spectrum of the probe pulse are functions of the delay time between the pumping and probing and oscillate between the Stokes and anti-Stokes components at the optical-phonon frequency. A comparison of the integrated lattice excitation relaxation dynamics with the spectrally resolved lattice excitation relaxation dynamics revealed new facets in the nature and generation mechanism of coherent phonons.     

亚皮秒脉冲激光辐照硅薄膜热效应的模拟研究

基于Boltzmann方程,采用了Chen J K等人建立的自相关模型,考虑了Si薄膜的热容、热导率、弛豫时间等热力学参量随温度非线性变化的影响.采用有限差分法,数值求解了脉宽为500 fs的激光脉冲辐照2 μm厚硅膜的自相关模型.分析了膜表面载流子浓度、载流子温度、晶格温度等随入射激光功率和脉宽等的变化规律.结果表明：在脉冲辐照初期（t<0.68 ps）,载流子和晶格之间存在着明显的非热平衡性,之后通过相互之间的弛豫碰撞,逐渐达到热平衡,载流子热容是引起载流子温度在早期迅速上升的原因;载流子温度速率方程中单光子吸收、载流子-晶格能量交换和载流子能流变化率对载流子温升影响较大,而多光子吸收、双极能流和带隙能量变化率对载流子温升的影响较小,可以忽略;较高脉冲激光能量（Ф＞0.02 J·cm－2）辐照Si膜,会引起载流子密度方程中的俄歇复合项增大,从而使载流子密度下降率增大,导致载流子温度出现双峰.     

Cd1-xMnxTe混晶的远红外吸收光谱

本文报道4.2—300K和20—400cm^-1范围内不同组份的Cd_1-xMn_xTe混晶远红外声子吸收光谱的研究结果。实验分辨了类CdTe,类MnTe剩余射线带及其TO-LO分裂,首次观察到位于混晶剩余射线吸收区内侧的低频吸收带、低x值情况下的准定域模吸收峰及其两侧的诸双声子吸收带。用缺陷晶格动力学理论估计了准定域模的出现及位置,讨论了其他吸收带的物理起源和判定。 关键词：     

Frequency-tunable terahertz electromagnetic wave emitters based on undoped GaAs/n-type GaAs epitaxial layer structures utilizing sub-picosecond-range carrier-transport processes

We have investigated sub-picosecond-range carrier-transport processes in undoped GaAs/n-type GaAs (i-GaAs/n-GaAs) epitaxial layer structures with various i-GaAs-layer thicknesses d ranging from 200 to 2000 nm, focusing on the relation between carrier-transport processes and terahertz electromagnetic wave frequency. Initially, using numerical simulation and photoreflectance measurement, we confirm that a decrease in d enhances the built-in electric field in the i-GaAs layer. In the time-domain terahertz waveform, it is observed that the intense monocycle oscillation induced by the surge current of photogenerated carriers, the so-called first burst, is followed by the oscillation patterns originating from the coherent GaAs longitudinal optical (LO) phonon. From the Fourier power spectra of the terahertz waveforms, it is clarified that the decrease in d causes a high frequency shift of the band of the first burst. Consequently, we conclude that, in the sub-picosecond time range, the photogenerated carriers are monotonously accelerated by the built-in electric field without being affected by intervalley scattering. The present conclusion signifies that the frequency-tunable terahertz emitters are realized by controlling i-GaAs-layer thickness. We also find the intensity of the coherent LO phonon band is enhanced by a decrease in d.     

Study of the correlation of scintillation decay and emission wavelength

In photoluminescence which directly excites the emission center of phosphor material is known to have a correlation between the emission wavelength and the decay time based on quantum mechanics. In scintillation phenomenon, host lattice of the material is first excited by ionizing radiation and then the excitation energy is transferred to emission centers. For the first time, we investigated the correlation between the scintillation decay and the emission wavelength by using pulse X-ray equipped streak camera system which could observe time and wavelength resolved scintillation phenomenon. Investigated materials were Ce³⁺, Pr³⁺ and Nd³⁺ doped oxides and fluorides which all showed 5d-4f transition based emission. As a result, we obtained the relation that τ (scintillation decay time) was proportional to the λ^2.15 (emission wavelength).