Spontaneous luminescence due to high density electron-hole plasma in GaAs under nano- and pico-second pulse excitation

Spontaneous luminescence due to high density electron-hole plasma in GaAs is observed at 4.2 K under nano- and pico-second pulse excitation. From the pico-second time-resolved spectra, it is found that the hot carriers are cooled down rapidly within 150 psec, and the changes of spectra are not appreciable in the later stage. One may consider, together with results of the spectral shape analysis, that the electron-hole liquid formation is improbable at least within the time range observed.     

Light scattering from nonequilibrium electron-hole plasma excited by picosecond laser pulses in GaAs

A picosecond dye laser is used to excite electron-hole plasma of density between 10¹⁸ to 10¹⁹ cm^-3 in GaAs. The plasma density and distribution function within ? 20 psec of excitation is probed by Raman scattering. The lineshape of the single particle excitation spectra of the plasma can be explained only by assuming that the electron distribution function is in nonthermal equilibrium.     

Subnanosecond luminescence spectroscopy of Cd1−xMnxSe under high intensity optical excitation

Transient photoluminescence of Cd_1?xMn_xSe has been investigated for compositions x = 0, .05 and .10, in zero magnetic field, under high excitation intensity. The spectra yield information about the temporal evolution of inelastic scattering processes in a dense exciton gas, with a likely contribution by an electron-hole plasma. The short carrier lifetimes measured, less than 100 psec for x = .10, and a possible plasma expansion, might inhibit the generation of carrier densities sufficient for suggested exchange induced magnetic ordering.     

Pico-second spectroscopy of highly excited CdSe and CdSI high density exciton system

Time-resolved luminescence spectra are measured at 4.2 K most in detail for CdSe under 531 nm pico-second pulse excitation. The M, P, and A-LO lines from high density excitons rise in intensity after pulse excitation, reach maxima, and then fall, the rise time being in a range of 100–600 psec and becoming faster with increasing excitation density. This time dependence is due to the light amplification effect by stimulated emission.     

Optical gains associated with exciton collision processes and high density electron-hole plasma in CdSe

Optical gain spectra for CdSe are measured at 4.2 K changing excitation density. The gain in the 683–689 nm region is concluded to be due to exciton collision processes, in disagreement with the assignment to electron-hole liquid by other investigators. The gain due to high density electron-hole plasma, which has been found recently by the authors to be generated under very high excitation density, is observed in the 688–691 nm region.     

Picosecond time-resolved luminescence of hydrogenated amorphous silicon-carbon

Photoluminescence decay curves and time-resolved luminescence spectra of amorphous Si_0.15C_0.85 : H films are measure. The decay curves are nearly exponential with a time-constant of about 200 psec at room temperature. The peak of the emission spectrum is shifted from 440 nm immediately after the excitation to 490 nm at 400 psec after the excitation.     

Measurements of exciton absorption with pico-second light pulses produced by a self-phase modulation technique

Time-resolved absorption spectra of the A exciton in CdSe are measured by using pico-second white-light pulses produced by a self-phase modulation technique. The absorption peak of the n = 1 A exciton shifts to the high-energy side due to the high-density effect. The magnitude of the shift reaches a maximum at 20 psec after the pulse excitation, which is considered to indicate that the time constant for the formation of the n = 1 excitons by band-to-band excitation is about 20 psec.     

Nonequilibrium properties of electron-hole plasma in direct-gap semiconductors

The gain spectra of the electron-hole plasma recombination in CdS are investigated as a function of the excitation conditions and of the lattice temperature. From a lineshape analysis which includes such many-body effects as collision broadening, single-particle energy renormalization and excitonic enhancement, average plasma parameters are obtained. In contrast to the predictions of quasi-equilibrium theory, one finds that the electron-hole plasma does not reach a full thermal quasi-equilibrium in direct-gap materials because of the short lifetimes of the carriers. The nonequilibrium effects are shown to lead to the formation of electron-hole plasma density fluctuations. No well-defined coexistence region exists. The experimental results in the phase transition region can consistently be explained by theoretical treatments of this nonequilibrium phase transition.     

特丁基对苯二酚的光谱及密度泛函研究

特丁基对苯二酚是重要的食品抗氧化剂.理论上,基于密度泛函理论,采用B3LYP泛函及6-311G(d,p)基组在气相环境下优化分子的结构并进行频率计算.在此基础上,基于含时密度泛函理论,选用SMD(solvation model based on density)溶剂模型,利用B3LYP泛函并结合def2-TZVP基组计...     

Time-dependent density-functional molecular-dynamics study of the isotope effect in chemicurrents

The energy dissipation into electron-hole pairs has been simulated ab initio within time-dependent density-functional theory for spin-unpolarized hydrogen atoms interacting with the Al on-top site at the Al(1 1 1) surface. The electron-hole pair excitation spectra are characterized by an approximately exponentially decaying tail of the electron energy distribution. It is shown that both the energy dissipated into electron-hole pairs and the excitation spectra, and hence the chemicurrent yield, show an isotope dependence identical to what expected from the linear friction ansatz and the forced oscillator model.     

Luminescence due to metallic electron-hole liquid in GaAs

Emission spectra of high-purity GaAs have been studied at 4.2K under very intense optical excitation. The results give the first experimental evidence for an important contribution of the electron-hole liquid phase in the luminescence of a direct-allowed semiconductor. Theoretical fit of the luminescence band-shape is satisfactory. The effects of applied electric field on the emission spectra are explained well using a concept of the metallic electron—hole plasma state.     

Ground state energy of the electron-hole liquid in type-II (GaAs)m/(AlAs)m quantum wells

The ground state energy of quasi-two-dimensional electron-hole liquid (EHL) at zero temperature is calculated for type-II (GaAs)_m/(AlAs)_m (5≤m≤10) quantum wells (QWs). The correlation effects of Coulomb interaction are taken into account by a random phase approximation of Hubbard. Our EHL ground state energy per electron-hole pair is lower than the exciton energy calculated recently for superlattices, so we expected that EHL is more stable state than excitons at high excitation density. It is also demonstrated that the equilibrium density of EHL in type-II GaAs/AlAs QWs is of one order of magnitude larger than that in type-I GaAs/AlAs QWs.     

Luminescence and gain spectroscopy of disordered CdS1−x Se x under high excitation

The absorption-, reflection-, and luminescence spectra of CdS_1–x Se _x are measured under low excitation. The luminescence under high excitation is also observed. For the first time, the gain spectra of disordered crystals are investigated with the excite- and probebeam technique to get a better understanding of the high density electron-hole pair system in this type of materials. We compare the results with those obtained in pure CdS and CdSe.     

Dynamics of the phase transitions in the system of nonequilibrium charge carriers in quantum-dimensional Si1 − x Ge x /Si structures

The dynamics of the phase transition from an electron-hole plasma to an exciton gas is studied during pulsed excitation of heterostructures with Si_{1 ? x} Ge _x /Si quantum wells. The scenario of the phase transition is shown to depend radically on the germanium content in the Si_{1 ? x} Ge _x layer. The electron-hole system decomposes into a rarefied exciton and a dense plasma phases for quantum wells with a germanium content x = 3.5% in the time range 100–500 ns after an excitation pulse. In this case, the electron-hole plasma existing in quantum wells has all signs of an electron-hole liquid. A qualitatively different picture of the phase transition is observed for quantum wells with x = 9.5%, where no separation into phases with different electronic spectra is detected. The carrier recombination in the electron-hole plasma leads a gradual weakening of screening and the appearance of exciton states. For a germanium content of 5–7%, the scenario of the phase transition is complex: 20–250 ns after an excitation pulse, the properties of the electron-hole system are described in terms of a homogeneous electron-hole plasma, whereas its separation into an electron-hole liquid and an exciton gas is detected after 350 ns. It is shown that, for the electron-hole liquid to exist in quantum wells with x = 5–7% Ge, the exciton gas should have a substantially higher density than in quantum wells with x = 3.5% Ge. This finding agrees with a decrease in the depth of the local minimum of the electron-hole plasma energy with increasing germanium concentration in the SiGe layer. An increase in the density of the exciton gas coexisting with the electron-hole liquid is shown to enhance the role of multiparticle states, which are likely to be represented by trions T ⁺ and biexcitons, in the exciton gas.     

磁控溅射制备ZnO薄膜的受激发射特性的研究

用射频磁控反应溅射法在二氧化硅衬底上制备ZnO薄膜。得到了在不同温度下ZnO薄膜的吸收与光致发光。观测到了纵光学波 (LO)声子吸收峰与自由激子吸收峰 ;室温 (30 0K)下 ,PL谱中仅有自由激子发光峰。这些结果证实了ZnO薄膜具有较高的质量。探讨了变温ZnO薄膜的发光特性。研究了ZnO薄膜的受激发射特性。     

Electron-hole liquid in GaS

Photoluminescence spectra of “pure” GaS are recorded under high excitation levels and low temperatures between 1.9 and 10K. This work reports a new and broad band which dominates the spectrum at high excitation intensities and temperatures near 1.9K. Its dependence on the excitation level is highly superlinear. When temperature is raised from 1.9 to 10K the spectrum observed of low excitation levels is recovered. We interpret the band here observed as due to radiative recombination from electron-hole droplets. From best fitting between experimental spectrum and calculated theoretical curves the critical density and the condensation energy are estimated as, n_c = 4.5 × 10²⁰ cm^?3 and φ = 9.0 meV.     

Macroscopic Quantum State in Optically Driven Semiconductors: Crossover from Electron-Hole BCS to Exciton Bose Condensation

Luminescence spectra from high-density electron-hole ( e-h ) systems at zero temperature are studied. The calculated spectra clearly show the crossover between the e-h BCS state and the exciton Bose-Einstein condensate; this behavior analyzed neither with the BCS-like mean-field theory nor with the interacting Boson model. The strong phase fluctuation associated with the center-of-mass motion of e-h pairs is considered with the generalized random phase approximation combined with the Bethe-Salpeter equation for e-h pairs. The calculated spectra show that the broad spectral components from the e-h BCS state splits into P- and P 2 -lines with decreasing e-h density. The calculated density dependence of the band-gap renormalization excellently agrees with experiments for CuCl and ZnO.     

4氢-吡喃-4-硫酮1(n-π*),1Ag-(π→π*)和1Bu+(π→π*)等的激发态特征

理论上4氢-吡喃-4-硫酮的激发态特征用量子化学方法以及二维实空间分析法和三维立体实空间分析法加以研究.理论上的结果揭示1(n→π*)和1Ag-(π→π*)激发态是分子内电荷转移(ICT)激发态,而1Bu (π→π*)激发态是离域激发态.它们的跃迁偶极矩的强度和方向用三维密度跃迁(TD)加以解释,用三维电荷密度差(CDD)观察分子间电荷转移(ICT)的激发态特征或局部激发态特征.用二维实空间分析法研究电子.空穴相关性,离域化和激子的尺寸.     

Pico-second transient luminescence of resonantly excited excitonic molecules in CuCl

Time-resolved luminescence spectra of excitonic molecules in CuCl created by the two-photon resonance excitation are studied at 4.2 K with a time resolution of ～ 60 psec. Two sharp emission lines observed are ascribed to the radiative decay of “cold” excitonic molecules with k ～ 2K₀. Stimulated emission process gives a considerable influence on luminescence properties.     

CdS/CdTe多晶薄膜的时间光谱特性

采用超短皮秒脉冲激光激发制备了SnO₂:F 玻璃衬底和石英衬底的CdTe多晶薄膜,在室温下测量其稳态和瞬态荧光光谱。结果表明,随着激发功率的增大,其荧光光谱峰逐渐展宽,并发生蓝移。对于退火处理的样品,其荧光光谱伴随主峰旁出现一个肩峰,是Cl 原子与富Cd区存在的空位形成复合体引起的发射。其瞬态荧光光谱寿命呈双指数衰减,即表面效应所引起的慢过程和带边激子态发射的快过程组成。样品退火处理后荧光寿命变长,有利于电子-空穴对在复合发光前分离,提高电子空穴的迁移率,改善其光伏性能。