Charge carrier recombination mechanisms in Sb-containing quantum well laser structures

The dynamics of interband luminescence in structures with InGaAsSb-based quantum wells and barriers of different types was studied at different temperatures and excitation levels. The lifetime of optically injected charge carriers in quantum wells at different temperatures and optical excitation levels was determined. An increased recombination rate in structures with deep electron quantum wells was discovered; it is associated with the occurrence of resonance Auger recombination. It was concluded that the application of quinary solid solutions as barriers in laser structures for a 3—4 fum wavelength range is to be preferred.     

Structure formation in electromagnetically driven granular media

We report structure formation in submonolayers of magnetic microparticles subjected to periodic electrostatic and magnetic excitations. Depending on the excitation parameters, we observe the formation of a rich variety of structures: clusters, rings, chains, and networks. The dynamics and shapes of the structures are strongly dependent on the amplitude and frequency of the external magnetic field. We find that for pure ac magnetic driving the low-frequency magnetic excitation favors compact clusters, whereas high frequency driving favors chains and netlike structures. An abrupt phase transition from chains to a network phase was observed for a high density of particles.     

Probing subcycle spectral structures and dynamics of high-order harmonic generation in crystals

Subcycle spectral structures and dynamics of high-order harmonic generation (HHG) processes of atoms and molecules driven by intense laser fields on the attosecond time scale have been originally studied theoretically and experimentally. However, the time scale of HHG dynamics in crystals is in the order of sub-femtosecond, and the carrier dynamics of HHG in crystals driven by subcycle laser pulses are largely unexplored. Here we perform a theoretical study of subcycle structures, spectra, and dynamics of HHG of crystals in mid-infrared laser fields subject to excitation by a subcycle laser pulse with a time delay. The HHG spectra as a function of time delay between two laser fields are calculated by using a single-band model for the intra-band carrier dynamics in crystal momentum space and by solving the time-dependent Schrödinger equation in velocity gauge for the treatment of multi-band crystal systems. The results exhibit a complex time-delay-dependent oscillatory pattern, and the enhancement and suppression of the HHG related to subcycle pulse are observed at the given time delay in either single-band or multi-band crystal systems. To understand oscillation structures with respect to the dependence for the subcycle laser fields, the time-frequency characteristics of the HHG as well as the probability density distribution of the radiation are analyzed in detail.     

Er-Y共掺杂ZrO2的Raman光谱和Er3+的荧光光谱

采用柠檬酸溶胶凝胶法制备Er-Y共掺杂的具有单斜、四方和立方结构的ZrO2基复合氧化物．用XRD和Raman对复合氧化物结构和不同晶相环境中Er3+的荧光光谱进行了表征．结果表明，掺杂Er3+在三种不同晶相Y-ZrO2中表现出不同的荧光特征．随着基体ZrO2由单斜相逐渐向四方和立方相转变，其荧光谱峰位置对称性提高，峰分裂数减少，谱带逐步向单峰转变．在632.8 nm激发下的Raman图谱有Er3+的荧光干扰，在514.5 nm下则以荧光光谱为主体，不能获得Raman信号，在325 nm激发下无荧光干扰．XRD和紫外Raman二种方法对物相表征结果的差别是由于样品体相和表面不一致，表层容易生成比内层结构对称性更低的物相造成的．     

Polymorphic and regular localized activity structures in a two-dimensional two-component reaction-diffusion lattice with complex threshold excitation

Space-time dynamics of the system modeling collective behaviour of electrically coupled nonlinear units is investigated. The dynamics of a local cell is described by the FitzHugh-Nagumo system with complex threshold excitation. It is shown that such a system supports formation of two distinct kinds of stable two-dimensional spatially localized moving structures without any external stabilizing actions. These are regular and polymorphic structures. The regular structures preserve their shape and velocity under propagation while the shape and velocity as well as other integral characteristics of polymorphic structures show rather complex temporal behaviour. Both kinds of structures represent novel sorts of spatially temporal patterns which have not been observed before in typical two-component reaction-diffusion type systems. It is demonstrated that there exist two types of regular structures: single and bound states and three types of polymorphic structures: periodic, quasiperiodic and even chaotic ones. The partition of the parameter plane into regions corresponding to the existence of these different types of structures is carried out. High multistability of regular structures is indicated. The interaction of regular structures is investigated. The correspondence between the structures and trajectories in multidimensional phase space associated with the system is given. Bifurcation mechanisms leading to the loss of stability of regular structures as well as to a transition from one type of polymorphic structure to another are indicated. The mechanisms of formation of regular and polymorphic structures are discussed.     

Dynamics of photostimulated structure metastability formation in vitreous GeS2.2 by optical electroabsorption

It is shown by optical electroabsorption (EAB) measurements, that photostimulated long time metastability of energy spectrum in vitreous GeS_2.2 arises as result of the formation of the low energy quasi-molecular structures under intensive light excitation. These structures oscillate in time and change the energy spectrum. The dynamics of the phenomena testifies that the formation of atomic superclusters takes place. The results of measurements of dynamics of photostimulated structure formation coincide with earlier EAB measurements of photostimulated structure dissociation.     

Theoretical investigations of material modification using temporally shaped femtosecond laser pulses

We present a two-dimensional model, based on a drift–diffusion approach, developed to describe the dynamics of electronic excitation and lattice heating in several dielectric materials with different electron–phonon coupling properties (e.g. fused silica and sapphire) under the action of femtosecond near-infrared laser pulse trains with variable separation time between pulses. The modeling approach was aimed to describe the mechanisms that enable the spatial modulation of the structures induced by temporally modulated laser excitation and ablation of wide-band-gap dielectric materials. The possible geometric contours of the laser-induced craters on the target surfaces are discussed on the basis of the lattice-temperature profiles obtained by modeling. It was found that the observed difference in the crater shapes generated in fused silica and sapphire is conditioned by the difference in dynamics of electron excitation and recombination channels characteristic of these two materials. This effect can be used to convert a given temporal pulse modulation into spatial modulation, opening up new perspectives for material processing in order to obtain desired structure profiles. PACS 79.20.Ds; 42.62.-b     

Nonresonant two-photon generation of excitonic matter in a CuCl pellet

A pressed CuCl pellet is optically excited at 2 K using an excitation energy in the range from 1892 to 2843 meV, which is far below the bandgap. The steady-state population dynamics unambiguously indicates an unusual two-photon generation of ground-state excitons. At high-excitation levels, the observed spectra exhibit rich spectral features arising from electron-hole plasma and electron-hole droplets formation. This nonresonant two-photon excitation is presumably assisted by impurity bands due to grain boundaries and surfaces in this random semiconductor.     

甲基自由基在212.5 nm的光分解:母体内能态的激发对产物态分布的影响

利用里德堡氢原子飞渡时间谱的方法,进行了甲基自由基在212.5nm 的光解动力学的实验研究.甲基自由基来自于碘甲烷分子在266nm 的光分解.实验测量了不同振动态的甲基自由基通过3s 电子激发态解离的产物 CH_2自由基的平动能分布和角分布,并从平动能分布得到了 CH_2自由基的转动布居.实验同时详细地研究了母体分子甲基自由基的振动和转动激发对光解动力学的影响.实验结果表明母体分乙甲基自由基的伞形振动对于产物的转动激发和角分布均有较大的影响,而母体分子的转动激发只对产物的角分布有较明显的影响.     

Excitation-density-dependent generation of broadband terahertz radiation in an asymmetrically excited photoconductive antenna

The generation of terahertz (THz) transients in photoconductive emitters has been studied by varying the spatial extent and density of the optically excited photocarriers in asymmetrically excited, biased low-temperature-grown GaAs antenna structures. We find a pronounced dependence of the THz pulse intensity and broadband (>6.0 THz) spectral distribution on the pump excitation density and simulate this with a three-dimensional carrier dynamics model. We attribute the observed variation in THz emission to changes in the strength of the screening field.     

Ultrafast carrier dynamics of Cu2O thin film induced by two-photon excitation

Cuprous oxide (Cu₂O) has attracted plenty of attention for potential nonlinear photonic applications due to its superior third-order nonlinear optical property such as two-photon absorption. In this paper, we investigated the two-photon excitation induced carrier dynamics of a Cu₂O thin film prepared by radio-frequency magnetron sputtering, using the femtosecond transient absorption experiments. Biexponential dynamics including an ultrafast carrier scattering (< 1 ps) followed by a carrier recombination (> 50 ps) were observed. The time constant of carrier scattering under two-photon excitation is larger than that under one-photon excitation, due to the different transition selection rules and smaller absorption coefficient of the two-photon excitation.     

Optical properties of InGaN quantum dots

We present time-resolved and spatially-resolved photoluminescence (PL) measurements of InGaN inclusions in a GaN matrix. The structures were grown by metal-organic chemical vapor deposition on sapphire and Si(111) substrates. Nonresonant pulsed excitation yields a broad PL peak, while resonant excitation into the nonresonant PL intensity maximum results in an evolution of a sharp resonant PL peak, having a spectral shape defined by the excitation laser pulse and a radiative decay time close to that revealed for PL under nonresonant excitation. Observation of a resonantly excited narrow PL line gives clear proof of the quantum dot (QD) nature of luminescence in InGaN–GaN samples. Cathodoluminescence (CL) and micro-PL measurements demonstrate sharp emission lines from single QD states. The recombination dynamics of single QD’s and the whole QD ensemble were investigated. Monoexponential decay was observed for the PL of single QD’s. For similar transition energies different time constants were obtained. Therefore the nonexponential decay observed for the whole ensemble is attributed to the coexistence of QD’s having similar ground-state transition energies, but significantly different electron–hole overlap.     

Sidebands in the light absorption of driven metallic nanoparticles

The dynamics of the surface plasmon in laser-driven metallic nanoparticles is described by means of a master-equation formalism. Within the Markov approximation, the dynamics is studied for different regimes ranging from weak excitation in photoabsorption experiments to strong excitation in pump-probe spectroscopy. It is shown that two collective levels are sufficient to describe the dynamics of the surface plasmon. On this basis, we predict the appearance of sidebands in the absorption spectrum of the probe laser field in pump-probe experiments.     

Remodelling of cellular excitation (reaction) and intercellular coupling (diffusion) by chronic atrial fibrillation represented by a reaction-diffusion system

Human atrial tissue is an excitable system, in which myocytes are excitable elements, and cell-to-cell electrotonic interactions are via diffusive interactions of cell membrane potentials. We developed a family of excitable system models for human atrium at cellular, tissue and anatomical levels for both normal and chronic atrial fibrillation (AF) conditions. The effects of AF-induced remodelling of cell membrane ionic channels (reaction kinetics) and intercellular gap junctional coupling (diffusion) on atrial excitability, conduction of excitation waves and dynamics of re-entrant excitation waves are quantified. Both ionic channel and gap junctional coupling remodelling have rate dependent effects on atrial propagation. Membrane channel conductance remodelling allows the propagation of activity at higher rates than those sustained in normal tissue or in tissue with gap junctional remodelling alone. Membrane channel conductance remodelling is essential for the propagation of activity at rates higher than 300/min as seen in AF. Spatially heterogeneous gap junction coupling remodelling increased the risk of conduction block, an essential factor for the genesis of re-entry. In 2D and 3D anatomical models, the dynamical behaviours of re-entrant excitation waves are also altered by membrane channel modelling. This study provides insights to understand the pro-arrhythmic effects of AF-induced reaction and diffusion remodelling in atrial tissue.     

Multiexciton transients in a single quantum dot

The relaxation dynamics of a multiple exciton complex (multiexciton) confined in a semiconductor quantum dot has been investigated. Emission signals from a single self-organized GaAs/Al_0.3Ga_0.7As quantum dot are temporally resolved with picosecond time resolution. The emission spectra consisting of the multiexciton structures are observed to depend on the delay time and the excitation intensity. Quantitative agreement is found between the experimental data and the calculation based on a model describing the successive relaxation of multiexcitons.     

Tunneling in double-barrier ZnSe/ZnTe structures — time-dependent analysis

Wave-packet time-dependent quantum mechanics is used to calculate the tunneling probability through a double-barrier ZnSe/ZnTe structure. The time-dependent transmission characteristics are obtained for several structures, and detailed electron dynamics is presented. The resonant peaks due to the presence of the discrete energy levels in the quantum well as well as in the barrier region are observed.     

Vanishing of the Mott transition in semiconductor nanocrystals

The dynamics of the system of photoexcited electron–hole pairs in semiconductor nanocrystals of different size with increasing excitation intensity was experimentally studied by utilizing the luminescence spectra of semiconductor-doped glasses in order to elucidate the peculiarities of many-body effects in structures approaching the zero-dimensional limit. Vanishing of effects causing the Mott transition in bulk crystals was observed with decreasing nanocrystal radius, and a new type of transformation of excitons to unbound electron–hole pairs was shown to take place in nanocrystals where the energy shift for electrons and holes due to quantum confinement becomes comparable with the exciton binding energy.     

Theory of resonant inelastic X-ray scattering spectrum for Ni impurities in cuprates

We perform the numerically exact diagonalization calculation for small Cu-O clusters with a Ni impurity site, representing the Ni-substituted cuprate, to examine the single-particle excitation spectra as well as the resonant inelastic X-ray scattering (RIXS) spectra. We clarify relations between low-energy electronic structures near the Ni site and excitations seen in the RIXS spectra.     

Time-resolved x-ray microscopy of spin-torque-induced magnetic vortex gyration

Time-resolved x-ray microscopy is used to image the influence of alternating high-density currents on the magnetization dynamics of ferromagnetic vortices. Spin-torque-induced vortex gyration is observed in micrometer-sized permalloy squares. The phases of the gyration in structures with different chirality are compared to an analytical model and micromagnetic simulations, considering both alternating spin-polarized currents and the current's Oersted field. In our case the driving force due to spin-transfer torque is about 70% of the total excitation while the remainder originates from the current's Oersted field. This finding has implications to magnetic storage devices using spin-torque driven magnetization switching and domain-wall motion.     

High-spin multiparticle-hole excitations in 148Eu

Studies by means of 155 MeV ²⁷Al bombardment on a ¹³⁰Te target revealed in ¹⁴⁸Eu high-spin structures up to spin 31?, in addition to a cascade extended to the 11088.1 keV excitation. The observed levels are tentatively assigned as complex multiparticle-hole proton and neutron configurations. Received: 15 December 2000 / Accepted: 29 January 2001