Ultrafast carrier dynamics of resonantly excited InAs/GaAs self-assembled quantum dots

We study theoretically the time development of electronic relaxation in quantum dots. We consider the process of relaxation of the state with an electron prepared at the beginning of relaxation in the electronic ground state. We obtain a fast (in picoseconds) increase of electronic population in the excited state. Also, we consider the process of relaxation of an electron from an excited state in the dot. Here we obtain an incomplete depopulation of the electron from the excited state. We compare these results to experiments in which a fast decrease of luminescence is reported during the first period of relaxation after resonant excitation of the ground state. We estimate numerically the role of electron–LO–phonon (Fröhlich's coupling) mechanism in these processes. We show that this effect may be attributed to the influence of multiple scattering of quantum dot electrons on LO phonons. A single-electron two-energy-level quantum dot model is used to demonstrate this effect in an isolated semiconductor quantum dot.     

Time resolved scattering relaxation mechanisms of microcavity polaritons

We study the polariton relaxation dynamics for different scattering mechanisms as: Phonon and electron scattering procesess. The relaxation polariton is obtained at very short times by solving the Boltzman equation. Instead of the well-known relaxation process by phonons, we show that the bottleneck effect relaxes to the ground state more efficiently at low pump power intensity when the electron relaxation process is included. In this way, we clearly demonstrate that different relaxation times exist, for which any of these two mechanism is more efficient to relax the polariton population to the ground state.     

NO2分子内能传递和弛豫过程的光声和荧光光谱探测

将荧光光谱和光声光谱两种互补的探测技术结合起来,从辐射和无辐射跃迁两个方面,分析了 532nm激光作用下,NO2分子的激发和弛豫过程.发现NO2分子在激光作用下,将跃迁至第一激发电子态.当样品气压较低时,受激NO2分子除辐射荧光外,可通过快速的内能转移过程实现在几个振转能级的再布居;随样品气压的升高,分子间碰撞加剧,受激NO2分子通过分子间的碰撞,实现在多个振转能级的再布居.激光布居能级的荧光辐射效率随样品气压的升高逐渐降低,而长波区域的荧光辐射及光声信号强度逐渐增强,说明在高样品气压条件下,受激NO2分子的弛豫过程除辐射荧光外,还存在很强的碰撞弛豫过程,在碰撞弛豫过程中受激NO2分子将振动能转化为热运动的平动能,引起温度升高而产生很强的声信号.     

Measurement of the argon-gas-induced broadening and line shifting of the barium Rydberg level 6s24d ¹D₂ by two-photon resonant nondegenerate four-wave mixing

We apply two-photon resonant nondegenerate four-wave mixing with a resonant intermediate state to the observation of the broadening and shifting of the barium Rydberg level 6s24d 1D2 by collision with argon.The collision broadening and shifting cross sections are measured.This technique is purely optical,and can investigate the pressure dependence of the transverse relaxation rate Γ21 between the Rydberg state and an intermediate state,as well as the transverse relaxation rate Γ20 between the Rydberg state and the ground state.     

相移法测量寿命的理论分析

本文从理论上分析了利用调制的单模激光场共振激发来测量原子或分子的寿命.结果表明:感生荧光或共振荧光信号与调制信号间的相移主要来自三个过程:(1)由原子的横向弛豫时间及激光线宽决定的激发过程;(2)由激发能级及其他能级上粒子数衰减过程;(3)与系统达到平衡态粒子数分布所需的弛豫时间有关的碰撞激发过程.讨论了在不同的情况下它们的影响,并在分析中考虑了多普勒加宽的影响.     

Inelastic collisions of ultracold heteronuclear molecules in an optical trap

Ultracold RbCs molecules in high-lying vibrational levels of the a3Sigma+ ground electronic state are confined in an optical trap. Inelastic collision rates of these molecules with both Rb and Cs atoms are determined for individual vibrational levels, across an order of magnitude of binding energies. The long-range dispersion coefficients for the collision process are calculated and used in a model that accurately reproduce the observed scattering rates.     

RbH(X1∑+,v=O～2)能级与H2碰撞振动能量转移

利用积分时间分辨荧光光谱方法,研究了RbH(X1∑+,v=0～2)与H2间的振动碰撞能量转移.在Rb-H2混合样品池中,泵浦激光双光子激发Rb原子至6D态,Rb(6D)与H2反应生成RbH(x1∑+)分子,探测激光延迟泵浦激光20 ns,通过激光感应荧光光谱(LIF)的测量,确定了X1∑+(v=0～2,J)原生态的转动...     

Scattering of electrons in the GaAs/AlAs transistor structure

This paper reports on the results of a self-consistent calculation of the rates of electron scattering from surface roughnesses, acoustic phonons, and polar optical phonons in a transistor structure based on a GaAs quantum wire in an AlAs matrix at temperatures T = 77 and 300 K. The rates of electron scattering are calculated in the electric-quantum limit approximation with due regard for both the collisional broadening of the electron energy spectrum and the Pauli principle. The influence of the gate voltage on these rates is investigated. The wave function of electrons and the energy level of their quantum ground state are determined by the self-consistent solution of the Poisson and Schrödinger equations.     

Density dependence of solar emission lines of carbon-like ions

Steady state level population of 15 levels of carbon-like ions: NeV, MgVII, SiIX, and SXI have been computed as a function of electron density and temperature taking into account collisional processes and spontaneous radiative ones. Photo-excitation among the ground term levels has also been considered. Knowing the level density, line intensities have been computed as a function of electron density and temperature. This study indicates that line intensity ratios for carbon-like ions can be used as a diagnostic in the determination of these two parameters of the solar plasma. The resulting line fluxes from these ions at earth distance are compared with observation. Presented at the 5th Annual Radio and Space Sciences Symposium, National Physical Laboratory New Delhi, January 1979.     

Nuclear relaxation due to a paramagnetic impurity

It is shown that the contributions of two-stage resonance processes in which the state of an electron does not vary as a result of nuclear relaxation are not taken into account in the usual equations for the rate of nuclear relaxation by paramagnetic impurities. In particular, these transitions yield a contribution precisely equal to the contribution of cross-over processes. Nuclear relaxation due to a magnetic ion with singlet ground level in which these processes are particularly great is investigated.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 83–87, June, 1976.The author wishes to express his appreciation to B. Io Kochelaev for discussion of the present results.     

Photoluminescence of charged magneto-excitons in InAs single quantum dots

We calculated the photoluminescence spectra of charged magneto-excitons in single two-dimensional parabolic quantum dots, using an unrestricted Hartree–Fock method. The calculated luminescence spectra explain well the observed red shifts of transition energies of InAs/GaAs single quantum dot by additional electron capture in a dot. The magnetic-field-induced transition of the ground state configuration of trapped electrons causes drastic change in the photoluminescence spectra. The dependence of photoluminescence intensities of charged excitons on the excess energies of photogenerated carriers above the bulk GaAs energy gap is studied phenomenologically, by calculating the steady state electron population probability in a dot.     

Effects of field broadening and pump line selection on multi-level absorption in molecular systems

The problem of multi-level absorption of i.r. laser radiation in polyatomic gases is examined using the density matrix formalism. The general condition under consideration is that field broadening effects are significant, but the detailed and discrete features of the absorption spectra remain very important. Collisional effects are taken into account through macroscopic relaxation terms, but the cumulative effects of collisional relaxation are assumed to cause only minor perturbations to the overall vibrational-rotational populations over the optical pumping time scale of interest. Various approximate formulae are then derived for locating the optimum frequency for optical pumping of three consecutive anharmonic vibrational levels within a given normal mode and for predicting the intensity- and frequency-dependence of the resultant uppermost level population density during a quasi-steady state balance among the absorption, stimulated emission, dephasing collision and rotational relaxation rates. Numerical examples are illustrated for pumping of the three lowest vibrational levels within the non-degenerate asymmetric stretching mode of the linear triatomic molecule CO₂ for which the absorption spectra are relatively simple and well known. Some possible applications of these results to the problem of i.r. laser frequency conversion and to the interpretation of wavelength-selective i.r. photodissociation recently observed in many symmetric top molecules, such as BCl₃, SiF₄ and SF₆ for which the absorption spectra are much more complex and the spectroscopic constants much less certain, are also discussed.     

Spatial Density Distributions and Correlations in a Quasi-one-Dimensional Polydisperse Granular Gas

By Monte Carlo simulations, the effect of the dispersion of particle size distribution on the spatial density distributions and correlations of a quasi one-dimensional polydisperse granular gas with fractal size distribution is investigated in the same inelasticity. The dispersive degree of the particle size distribution can be measured by a fractal dimension dr, and the smooth particles are constrained to move along a circle of length L, colliding inelastically with each other and thermalized by a viscosity heat bath. When the typical relaxation time τ of the driving Brownian process is longer than the mean collision time To, the system can reach a nonequilibrium steady state. The average energy of the system decays exponentially with time towards a stable asymptotic value, and the energy relaxation time τB to the steady state becomes shorter with increasing values of df. In the steady state, the spatial density distribution becomes more clusterized as df increases, which can be quantitatively characterized by statistical entropy of the system. Furthermore, the spatial correlation functions of density and velocities are found to be a power-law form for small separation distance of particles, and both of the correlations become stronger with the increase of df. Also, tile density clusterization is explained from the correlations.     

Relaxation Processes at High Levels of Vibrational Excitation of Polyatomic Molecules in the Ground Electronic State

By the spectral and kinetic characteristics of the luminescence of vapors of polyatomic molecules (anthracene, anthraquinone, fluorenone) initiated by selective IR multiphoton excitation (IR MPE) of molecules in the ground electronic state S ₀ the relaxation processes proceeding under vibrational excitation of molecules to energies exceeding the energies of the lower excited electronic states have been investigated. The changes in the spectral and kinetic characteristics with increasing CO₂ laser energy density and vapor P _v and foreign gas pressure P _FG are analyzed. They are similar to the characteristics obtained for normal fluorescence of these molecules with changing vibrational energy E _vib content. On the basis of experimental data and model calculations it has been concluded that at the laser radiation densities used in the case of IR MPE the molecules reach energies considerably exceeding the energies of the electronic levels. It is shown that a nonadiabatic connection between the electronic states leads to the population of mixed electronic states isoenergetic to the vibrational levels of the ground electronic state and to emission of delayed luminescence spectrally identical to the normal luminescence of these molecules. It has been found that when high vibrational levels are populated, new relaxation channels, such as reverse electron relaxation, emission from high vibrational levels of the ground electronic state, and multiquantum vibrational energy transfer at collisions leading to a rapid establishment of vibrational equilibrium become important.     

Measurement of the argon-gas-induced broadening and line shifting of the barium Rydberg level 6s24d 1D2 by two-photon resonant nondegenerate four-wave mixing

We apply two-photon resonant nondegenerate four-wave mixing with a resonant intermediate state to the obser-vation of the broadening and shifting of the barium Rydberg level 6s24d 1^D2 by collision with argon. The collision broadening and shifting cross sections are measured. This technique is purely optical, and can investigate the pressure dependence of the transverse relaxation rate-P21 between the Rydberg state and an intermediate state, as well as the transverse relaxation rate F20 between the Rydberg state and the ground state.     

Electron-impact ionization cross-sections and ionization rate coefficients for atoms and ions from hydrogen to calcium

Using the empirical formula recently proposed, electron-impact ionization cross-sections for single ionization from the ground state are given for free atoms and for all ionization stages from hydrogen to calcium (Z=20). Ionization rate coefficients are given for these species on the assumption of a Maxwellian distribution of the impacting electrons. Multiple ionization, lowering of ionization potential, or collision limit are not taken into account.     

HeH+体系光缔合反应的动力学研究

采用含时量子波包方法研究基电子态HeH⁺体系优化的光缔合几率,即He+H⁺→HeH⁺随碰撞粒子对初始动量的变化,并研究伴随该过程的多光子跃迁和解离.选取v=6振动能级为目标态,通过优化激光场参数得到碰撞粒子对的最优光缔合几率.结果表明：随着碰撞粒子对初始动量的增加,光缔合所用的最优激光场的持续时间逐渐减少,但光场的强度和失谐逐渐增加.在某些初始碰撞动量附近,由于共振多光子跃迁过程的存在,最优的光场不能严格地将碰撞粒子对缔合到目标态.此外,研究表明随着初始动量的增加,系统的超阈值解离几率近似线性地增加到一个最大值,直至饱和强度.并且,在碰撞粒子对的初始动量较大时,超阈值解离主导解离过程.     

Orientational relaxation of pheophorbide-a molecules in the ground and in the first excited state measured by transient dichroism spectroscopy

We report on transient dichroism spectroscopy carried out on pheophorbide-a in ethanol to measure the response time of the pump-beam induced anisotropy after selective excitation at high excitation levels. From the observed time behaviors of induced absorption, transient amplification, and of the bleaching signal, the orientational relaxation times of pheophorbide-a in the first excited singlet state and in the ground state were obtained separately to be 370 and 250 ps, respectively. It is found that pheophorbide-a behaves like a spherical rotor in the first excited state, while in the ground state randomization of molecules is dominated by the rotation around its quasi-symmetry axis. Because of rotation free measurement of ordinary transient absorption spectra fail in the case of ground state depletion, the observed orientational relaxation time of the ground state have to be taken into account to fit the bleaching kinetics obtained from transient absorption spectra very well to determine the intersystem crossing yield of pheophorbide-a to be 0.46.     

Full configuration interaction studies of phonon and photon transition rates in semiconductor quantum dots

Quantum chemical methods originally developed for studying atomic and molecular systems can be applied successfully to the study of few-body electron-hole systems in semiconductor nanostructures. A new computational approach is presented for studying the energetics and dynamics of interacting electrons and holes in a semiconductor quantum dot. The electron-hole system is described by a two-band effective mass Hamiltonian. The Hamiltonian is diagonalized in a configuration state function basis constructed as antisymmetric products of the electron one-particle functions and antisymmetric products of the hole one-particle functions. The symmetry adapted basis set used for the expansion of the one-particle functions consists of anisotropic Gaussian basis functions. The transition probability between electron-hole states consisting of different numbers of carrier pairs is calculated at the full configuration interaction level. The results show that the electron-hole correlation affects the radiative recombination rates significantly. A method for calculating the phonon relaxation rates between excited states and the ground state of quantum dots is described. The phonon relaxation calculations show that the relaxation rate is strongly dependent on the energy level spacings between the states.     

Population processes in high-pressure inert-gas-mixture lasers

Working-level-population processes are analyzed on the basis of detailed investigations of the amplitude-time structure of the laser and spontaneous emission following a pulsed electric discharge in the mixtures He + R (R = Ar, Kr, Xe), Ar + Xe. Account is taken in the analysis of excitation by electrons (direct and stepwise) and of population as a result of relaxation processes (collisions of second kind with electrons; radiative cascades, recombination processes; collisions with the atoms of the working and buffer gases; excitation transfer from helium molecules). It is concluded that under optimum efficiency conditions inversion is produced in the lasers considered as a result of direct electron collision with the working atoms (Ar, Kr, Xe), which are in the ground state.Translated from Lazernye Sistemy, pp. 15–34, 1982.