Low-temperature time-resolved vacuum ultraviolet spectroscopy of self-trapped excitons in KH<Subscript>2</Subscript> PO<Subscript>4</Subscript> crystals

A complex investigation of the dynamics of electronic excitations in potassium dihydrophosphate (KDP) crystals is performed by low-temperature time-resolved vacuum ultraviolet optical luminescence spectroscopy with subnanosecond time resolution and with selective photoexcitation by synchrotron radiation. For KDP crystals, data on the kinetics of the photoluminescence (PL) decay, time-resolved PL spectra (2–6.2 eV), and time-resolved excitation PL spectra (4–24 eV) at 10 K were obtained for the first time. The intrinsic character of the PL of KDP in the vicinity of 5.2 eV, which is caused by the radiative annihilation of self-trapped excitons (STEs), is ascertained; σ and π bands in the luminescence spectra of the STEs, which are due to singlet and triplet radiative transitions, are resolved; and the shift of the σ band with respect to the π band in the spectra of the STEs is explained.     

Ultrafast optical response in polydiacetylenes and polythiophenes

Ultrafast optical response in the films of poly(3-dodecylthiophene) (P3DT) and blue-and red-phase polydiacetylenes (PDA-4BCMU) has been investigated by femtosecond absorption and picosecond luminescence spectroscopies. Several nonlinear optical processes, i.e., hole burning, Raman gain, inverse Raman scattering, and induced-frequency shift, have been observed. The relaxation processes from photoexcited free excitons to self-trapped excitons (STEs) has been observed. The time constant is estimated as 140±40 fs in the blue-phase PDA-4BCMU and 100±50 fs in P3DT. The generated unthermalized STEs thermalize with the time constant of about 1 ps. The STEs in the blue-phase PDA-4BCMU decay exponentially with lifetime of 1.6±0.1 ps at 290 K and 2.1±0.2 ps at 10 K. The decay curves in the red-phase PDA-4BCMU and P3DT are not single exponential but can be fitted to biexponential functions with time constants of slightly shorter than 1 ps and about 5 ps. These two decay time constants correspond to relaxations to the ground state, respectively, from the free exciton and unthermalized STE and from the thermalized STE.     

Relaxation Dynamics In Photo-Excited Halogen-Bridged Pt-Complexes Studied By Femtosecond Luminescence Spectroscopy

The ultrafast dynamics of excited electronic states in a series of quasi-one-dimensional halogen-bridged platinum-complexes was investigated by using femtosecond luminescence spectroscopy. In Pt-Br system, the wave-packet oscillation of the local vibration of self-trapped excitons (STEs) was observed. The time evolutions of luminescence intensity in Pt-Br and Pt-Cl were successfully described by using a model based on a wave-packet motion on the interaction mode coordinate. In Pt-I, very fast decay was observed, suggesting the existence of efficient non-radiative decay channel directing to structural change.     

共轭高分子中激子演化过程的非绝热动力学研究

有机共轭高分子受光激发或被电荷掺杂后可能会产生各种激发状态的激子,激子的演化过程对有机发光光谱有着至关重要的影响.通过非绝热动力学演化的方法模拟了受光激发后有机高分子中激子驰豫的动力学过程,结果表明高激发态激子不稳定,由于电声耦合作用,高激发态激子会持续向低激发态激子演化,同时,低激发态激子的复合发光会发生红移.稳定的激子复合发光光谱中,基态激子发光强度最大,可高达70-80%;第一激发态激子及其它激发态激子发光强度的总和不超过20%.     

The role of self-trapped excitons and defects in the formation of nanogratings in fused silica

We investigate the role of self-trapped excitons (STEs) and defects in the formation of femtosecond laser pulse induced nanogratings (NGs) in fused silica. Our experiments reveal strongly enhanced NG formation for pulse separations up to the STE lifetime. In addition, the absorption spectra show that the weaker cumulative action of laser pulses for longer temporal separations is predominantly mediated by dangling-bond-type lattice defects that emerge from decaying STEs.     

Spatial and orientational ordering of charge-transfer excitons at intense pumping. Absorption and fluorescence spectra

The non-linear properties of charge-transfer exciton dynamics at intense pumping are investigated. The shift of an absorption band of an arbitrary cell, induced by the electric field of the available excitons, results in spatial and orientational ordering of the excitons. The side bands appear in the absorption and fluorescence spectra.     

Kinetics of a local “magnetic” moment and a non-stationary spin-polarized current in the single impurity Anderson model

We perform theoretical investigation of the localized state dynamics in the presence of interaction with the reservoir and Coulomb correlations. We analyze kinetic equations for electron occupation numbers with different spins taking into account high order correlation functions for the localized electrons. We reveal that in the stationary state electron occupation numbers with the opposite spins always have the same value: the stationary state is a “paramagnetic” one. “Magnetic” properties can appear only in the non-stationary characteristics of the single-impurity Anderson model and in the dynamics of the localized electrons second order correlation functions. We found that for deep energy levels and strong Coulomb correlations, relaxation time for initial “magnetic” state can be several orders larger than for “paramagnetic” one. So, long-living “magnetic” moment can exist in the system. We also found non-stationary spin polarized currents flowing in opposite directions for the different spins in the particular time interval.     

Quantum computers? … Coherent computers!

Attention is drawn to the fact that the computer that employs a superposition of states as the basis for its operation can be implemented by not only quantum but also classical elements, whose dynamics obey classical laws of motion. It is shown that the term “coherent computer” better reflects the physical principle of the computational devices based on the superposition of states.     

Theory of indirect exciton photoluminescence in elevated quantum trap

Inspired by an experiment of indirect excitons photoluminescence (PL) in elevated quantum trap (High et al., 2009), we theoretically investigate the energy relaxation and nonlinear interactions of indirect excitons in coupled quantum wells. It is shown that, when increasing the laser power, the intensity reversion of two PL peaks is due to the phonon necklace effect. In addition, we use a nonlinear Schrödinger equation including attractive two-body, repulsive three-body interactions and the excitation power dependence of energy distribution to understand the exciton states. This model gives a natural account for the PL blue shift with the increase of the excitation power. This study thus provides an alternative way to understand the underlying physics of the exciton dynamics in coupled potential wells.     

Mid-gap luminescence and its excitation spectrum in trigonal selenium single crystals

Trigonal selenium shows in addition to the narrow “band-edge” luminescence due to the decay of free excitons a broad photoluminescence band in the i.r. region very similar to the luminescence in amorphous selenium. This mid-gap luminescence arises at low temperature from the excitation of a defect and at higher temperature from the trapping of free excitons, both processes involving considerable relaxation of the defect. The excitation spectra exclude photoluminescence via the excitation of free carriers.     

Self-trapped excitons in silicon dioxide: mechanism and properties

Irradiating silica produces self-trapped excitons (STEs) that spontaneously create atomic-scale distortions on which they localize themselves. Despite enduring interest in STEs and subsequent defects in this key technological material, the trapping mechanism and geometry remain a mystery. Our ab initio study of STEs in alpha-quartz using a many-electron Green's function approach answers both questions. The STE comprises a broken O-Si bond with the hole localized on the defected oxygen and the electron on the defected silicon atom in a planar sp2 conformation. The results further explain quantitatively the measured STE spectra.     

Wave Resonance in Media with Modular,Quadratic and Quadratically-Cubic Nonlinearities Described by Inhomogeneous Burgers-Type Equations

The phenomenon of “wave resonance” which occurs at excitation of traveling waves in dissipative media possessing modular, quadratic and quadratically-cubic nonlinearities is studied. The mathematical model of this phenomenon is the inhomogeneous (or “forced”) equation of Burgers type. Such nonlinearities are of interest because the corresponding equations admit exact linearization and describe real physical objects. The presence of “accompanying sources” (traveling with the wave) on the right-hand side of the inhomogeneous equations ensures the inflow of energy into the wave, which thereafter spreads throughout the wave profile, flows to emerging shock fronts, and then dissipates due to linear and nonlinear losses. As an introduction, the phenomenon of wave resonance in ideal and dissipative media is described and physical examples are given. Exact expressions for nonlinear steady-state wave profiles are derived. Non-stationary processes of wave generation, spatial “beating” of amplitudes with different relationship between the speed of motion of the sources and the natural wave velocity in the medium are studied. Resonance curves are constructed that contain a nonlinear shift of the absolute maxima to the “supersonic” region. The features of the resonance in each of the three types of nonlinearity are discussed.     

Luminescence spectroscopy of excitons and antisite defects in Lu3Al5O12 single crystals and single-crystal films

The nature of the intrinsic luminescence of the lutetium aluminum garnet Lu₃Al₅O₁₂ (LuAG) has been analyzed on the basis of time-resolved spectral kinetic investigations upon excitation of two model objects, LuAG single crystals and single-crystal films, by pulsed X-ray and synchrotron radiations. Due to the differences in the mechanisms and methods of crystallization, these objects are characterized by significantly different concentrations of Lu_Al antisite defects. The energy structure of luminescence centers in LuAG single crystals (self-trapped excitons (STEs), excitons localized near antisite defects, and Lu_Al antisite defects) has been established. For single-crystal LuAG films, grown by liquid-phase epitaxy from a Pb-containing flux, the energy parameters of the following luminescence centers have been determined: STEs in regular (unperturbed by the presence of antisite defects) sites of the garnet lattice and excitons localized near Pb²⁺ ions. The structure of the luminescence centers, related to the background emission of impurity Pb²⁺ ions, has also been established in the UV and visible ranges. It is suggested that, in contrast to the two-halide hole self-trapping, a self-trapped state similar to STEs in simple oxides (Al₂O₃, Y₂O₃) is formed in LuAG; this state is formed by self-trapped holes in the form of singly charged O^? ions and electrons localized at excited levels of Lu³⁺ cations.     

Relaxation of hot excitons in CdZnSe/ZnSe quantum wells and quantum dots

The relaxation dynamics of hot excitons was studied in (Zn,Cd)Se/ZnSe quantum wells and quantum dots. A fast population of the radiative excitonic ground state occurs for an excitation excess energy corresponding to an integer number of optical phonon energies. This is indicated by a spectrally narrow photoluminescence peak observed immediately after the exciting laser pulse. Spatial diffusion of excitons, controlled by the interaction between excitons and acoustic phonons, causes a distinct linewidth broadening with increasing delay time in quantum wells. In contrast, this process is found to be strongly suppressed in quantum dots.     

Evolution of Anion and Cation Excitons in Alkali Halide Crystals

The manifestations of the existence of free anion excitons, the processes of their self-trapping, and the coexistence of mobile and self-trapped excitons (STEs) in wide-gap alkali halide crystals are reviewed. The radiative channel of decay of anion excitons, yielding luminescence, and a particular type of nonradiative channel with the creation of elementary Frenkel defects (FDs) are considered. We analyzed the criteria for the efficiency of this channel for defect formation, possible mechanisms for the decay of self-trapped excitons with the production of neutral and charged anion Frenkel defects, and the processes of multiplication of electronic excitations in alkali halide crystals. Particular attention is paid to the decay of cation excitons, including from the point of view of the possibility of the low-temperature creation of elementary Frenkel defects in the cation sublattice of alkali halide crystals.     

Effect of samarium impurity on the relaxation of the magnetization of a (NdDy)(FeCo)B alloy

Small Sm additions (~1–3 at %) have been found to slow down the relaxation of the magnetization in a magnetic field in the (NdDy)(FeCo)B alloy by several times. The effective “freezing” of the spontaneous relaxation of the magnetic moment is related to the substantial increase in the potential barriers for motion of domain walls when introducing Sm ions that have other symmetry of the single-ion anisotropy than that of Nd and Dy ions. The results can be used to stabilize the properties of hard magnets.     

自旋极化度对GaAs量子阱中吸收饱和效应与载流子复合动力学的影响研究

本文研究了(001) GaAs量子阱薄膜中重空穴激子近共振抽运-探测的载流子自旋弛豫动力学, 发现载流子的自旋极化对传统的线偏振光吸收饱和效应和载流子复合动力学都有影响. 进一步的抽运流依赖的自旋弛豫和复合动力学研究表明, 自旋极化对线偏振光的吸收饱和效应的影响随抽运流降低而变弱. 在低激发流时, 自旋极化对线偏振吸收饱和效应的影响才可忽略. 然而, 又显现出自旋极化对复合动力学的影响. 分析表明复合动力学的自旋极化依赖性起源于重空穴激子形成浓度的自旋极化依赖性. 复合动力学的自旋极化依赖性表明自旋弛豫时间计算中所涉及的复合时间应该使用自旋极化载流子的复合时间. 基于二维质量作用定律的激子浓度计算表明, 库仑屏蔽效应对激子形成的影响在较低激发载流子浓度下可以忽略.     

Tunneling states in ferromagnetic glasses

We study the interaction between structural defects, represented by two level systems, and spin waves in a ferromagnetic glass. The damping and energy shift of the spin wave due to this interaction have a “resonant” and a “relaxation” contribution in analogy with the case of phonons. Ferromagnetic resonance in amorphous ferromagnets can provide useful information on structural relaxation in these materials.