Auger process in CdS/ZnS core-shell quantum dots

Auger processes are investigated for CdS/ZnS core-shell quantum dots. Auger recombination (AR) lifetime and electron relaxation inside the core are computed. Using the effective-mass theory and by solving a three-dimension Schrödinger equation we predict the dependence of Auger relaxation on size of core-shell nanocrystals. We considered in this work different AR processes: the excited electron (EE), excited hole (EH), multiexciton AR type. Likewise, Auger multiexciton recombination rates are predicted for biexciton. Our results show that biexciton AR type is more efficient than the other AR process (excited electron (EE) and excited hole (EH)). We also found that electron Auger relaxation P→S is very efficient in core-shell nanostructures.     

Ratio problem in single carbon nanotube fluorescence spectroscopy

The electronic band gaps measured in fluorescence spectroscopy on individual single wall carbon nanotubes isolated within micelles show significant deviations from the predictions of one electron band theory. We resolve this problem by developing a theory of the electron-hole interaction in the photoexcited states. The one-dimensional character and tubular structure introduce a novel relaxation pathway for carriers photoexcited above the fundamental band edge. Analytic expression for the energies and line shapes of higher subband excitons are derived, and a comparison with experiment is used to extract the value of the screened electron-hole interaction.     

Spin diffusion of optically oriented electrons and photon entrainment in n-gallium arsenide

An experimental and theoretical study of spin transport in the n-GaAs semiconductor is reported. Transport of average electron spin from the photoexcited crystal surface is shown to be determined by the spin diffusion process. At the same time the transport of photoexcited carriers takes place primarily through photon entrainment, which transfers nonequilibrium carriers into the bulk of the semiconductor to distances considerably in excess of the electron spin diffusion length. A comparison of the experimental results with theory permits one to determine the average-spin diffusion length and electron-spin relaxation time. Fiz. Tverd. Tela (St. Petersburg) 39, 1975–1979 (November 1997)     

(110)-GaAs量子阱中光生载流子对电子自旋弛豫的影响

我们实验研究了(110)-GaAs量子阱中光生载流子对电子自旋弛豫的影响。通过测量量子阱的荧光寿命和光学吸收计算,我们能得到不同泵浦光功率下的带间吸收所产生的空穴浓度;相对应地,通过双色磁光科尔旋转技术,我们测量了该GaAs量子阱中电子自旋的动力学过程。结合两者,我们得到了电子自旋弛豫速率与空穴浓度的关系。实验结果表明电子自旋弛豫速率与空穴浓度呈线性依赖关系,验证了BirAronov-Pikus机制主导该体系的电子自旋弛豫。     

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.     

Monitoring laser cleaning of titanium alloys by probe beam reflection and emission spectroscopy

The energy relaxation of electrons in InN epilayers is investigated by excitation- and electric field-dependent photoluminescence (PL). From the high-energy tail of PL, we determine the electron temperature of the hot carriers. It was found that the electron temperature variation can be explained by a model in which the longitudinal optical (LO)-phonon emission is the dominant energy relaxation process. The LO-phonon lifetime is fitted to be 0.89 ps, which is higher than the theoretical phonon lifetime. This deviation is attributed to the presence of the non-equilibrium hot-phonon effects. PACS 78.55.Cr; 78.66.Fd; 61.66.Fn; 78.20.Jq; 63.20.kd     

A second order full coulomb relaxation theory of spectral line broadening in plasmas

A second-order, quantum-mechanical relaxation theory of spectral line broadening in plasmas that uses a full Coulomb potential to describe the radiator-perturbing electron interactions is presented. A Fourier representation of the interaction is used rather than a multipole expansion. The results for the Lyman-alpha transition in hydrogen are compared with experimental data and with a corresponding relaxation theory computation that uses a dipole approximation.     

Relaxation time for a charge carrier due to its scattering from other charge carriers in superlattices

A calculation of relaxation time for (i) electron–electron scattering in a modulation-doped superlattice of type-I and (ii) electron–electron, hole–hole and electron–hole scattering processes in a compositional superlattice of type-II has been performed, using Fermi's golden rule. As compared to a two-dimensional electron gas system, both intralayer and interlayer interactions, between charge carriers in a superlattice, contribute to relaxation time. It is found that scattering processes at all possible value of momentum transfer contribute to relaxation time, for a given value of temperature and carrier density. We further find interlayer interactions in a superlattice make a significant contribution to relaxation time. Relaxation time is found to decrease on increasing temperature, carrier density and single particle energy, in a superlattice. The computed relaxation time for an electron (hole) in a superlattice enhances on increasing the width of layer consisting of electrons (holes). The electron–hole (hole–electron) scattering process in a type-II superlattice yields maximum contribution to the relaxation time when a hole layer lies exactly in between two consecutive electron layers.     

Gauge theory approach for diffusive and precessional spin dynamics in a two-dimensional electron gas

We develop a gauge theory for diffusive and precessional spin dynamics in a two-dimensional electron gas. Our approach reveals a direct connection between the absence of the equilibrium spin current and a strong anisotropy in the spin relaxation: both effects arise if spin-orbit coupling is reduced to a pure gauge SU(2) field. In this case, the spin-orbit coupling can be removed by a gauge transformation in the form of a local SU(2) spin rotation. The resulting spin dynamics is exactly described in terms of two kinetic coefficients: the spin diffusion and electron mobility. After the inverse transformation, full diffusive and precessional spin density dynamics, including the anisotropic spin relaxation, formation of stable spin structures, and spin precession induced by a macroscopic current are restored. Explicit solutions of the spin evolution equations are found for the initially uniform spin density and for stable, nonuniform structures. Our analysis demonstrates a universal relation between the spin relaxation rate and spin-diffusion coefficient.     

Energy relaxation by hot carriers in wurtzite GaN epilayers

In this paper, we report hot carrier energy relaxation processes studied by acoustic phonon emission in wurtzite GaN epilayers, using the heat pulse technique. In this method, the carriers were heated up by means of short (≈ 10 ns) voltage pulses and emitted phonons were detected by Al bolometers biased at their superconducting transition. Obtained phonon signals indicate that the optical phonon emission threshold has not been reached and longitudinal acoustic and transverse acoustic modes can be clearly resolved. This paper specifically concentrates on the electron temperature dependence of the energy relaxation rates and compares the experimental results with the existing theory.     

Dopant size dependent variable range hopping conduction in polyaniline nanorods

The present work investigates the electrical transport and dielectric relaxation of polyaniline (PAni) nanorods doped with organic camphorsulfonic acid (CSA) and inorganic hydrochloric acid (HCl) synthesized by interfacial polymerization technique. High resolution transmission electron micrographs (HRTEM) depict that initially spherical nuclei directionally grow into nanorods and CSA doped PAni produces more uniform and aligned structures. The electrical transport studies reveal that the CSA doped nanorods follow 1D Mott variable-range hopping (VRH), whereas the HCl doped nanorods exhibit 2D VRH conduction mechanism. The value of interchain charge transfer integral is found to be higher for smaller size HCl doped PAni than that for larger size CSA doped PAni. The resistivity measurements exhibit semiconducting behavior for both organic and inorganic dopants and the resistivity of the CSA doped nanorods is found to be smaller than that of the HCl doped nanorods. The dielectric relaxation studies suggest Debye type relaxation with a single relaxation peak for both the dopants and the relaxation time of the carriers of the CSA doped PAni nanorods is smaller than that of the HCl doped nanorods.     

Spin-polarization dependent carrier recombination dynamics and spin relaxation mechanism in asymmetrically doped (110) n-GaAs quantum wells

Carrier recombination and electron spin relaxation dynamics in asymmetric n-doped (110) GaAs/AlGaAs quantum wells are investigated with time-resolved pump-probe spectroscopy. The experiment results reveal that the measured carrier recombination time depends strongly on the polarization of pump pulse. With the same pump photon flux densities, the recombination time of spin-polarized carriers is always longer than that of the spin-balanced carriers except at low pump photon flux densities, this anomaly originates from the polarization-sensitive nonlinear absorption effect. Differing from the traditional views, in the low carrier density regime, the D'yakonov–Perel' (DP) mechanism can be more important than the Bir–Aronov–Pikus (BAP) mechanism, since the DP mechanism takes effect, the spin relaxation time in (110) GaAs QWs is shortened obviously via asymmetric doping.     

Parametric interaction of space-charge waves in thin-film semiconductor structures

The general theory of parametric coupling between space-charge waves and drifting charge carriers in thin-film semiconductor structures has been worked out. This theory is applicable, in particular, to n-GaAs and n-InP semiconductors with negative differential conductance due to intervalley electron transitions under high electric fields. We started from the electrodynamic theory of waveguide excitation by extraneous currents, which was extended for arbitrary waveguide structures with composite active media. Our theory makes it possible to study parametric interaction between space-charge waves in semiconductor films with regard for boundary conditions, diffusion, the anisotropy and the frequency dispersion of the differential electron mobility, as well as the multifrequency and multimode nature of a wave process in thin-film structures.     

Electron-nuclear dynamics in a quantum dot under nonunitary electron control

We introduce a method for solving the problem of an externally controlled electron spin in a quantum dot interacting with host nuclei via the hyperfine interaction. Our method accounts for generalized (nonunitary) evolution effected by external controls and the environment, such as coherent lasers combined with spontaneous emission. As a concrete example, we develop the microscopic theory of the dynamics of nuclear-induced frequency focusing as first measured in Science 317, 1896 (2007); we find that the nuclear relaxation rates are several orders of magnitude faster than those quoted in that work.     

Spin Localization in Poly(3-Dodecylthiophen)/PCBM Composite

Charge carriers photoinduced in poly(3-dodecylthiophene)/([6,6]-phenyl-C₆₁-butanoic acid methyl ester) (P3DDT/PCBM) by photons with the energy of 1.88–2.75 eV were investigated by X-band light-induced electron paramagnetic resonance (LEPR). LEPR spectra were attributed to non-interacting polarons and methanofullerene anion radicals with different magnetic and relaxation parameters. A part of these charge carriers are trapped in a polymer matrix. Paramagnetic susceptibility and spin–spin relaxation of mobile charge carriers were shown to follow the activation law.     

The conductivity of the spin-polarized two-dimensional electron gas: Exchange/correlation and strong disorder effects

The conductivity of a spin-polarized two-dimensional electron gas is calculated and compared with the conductivity of the unpolarized electron gas. Disorder effects are considered within the self-consistent current relaxation theory, which gives rise to a crossover point from metallic to insulating behavior. Many-body effects due to exchange and correlation are taken into account and are described by a local-field correction. Our calculations are in good agreement with recent experimental results on the magnetoresistance of silicon inversion layers.     

Polarization-dependent carrier dynamics in a passive InAs/InP quantum dot waveguide

We investigate carrier dynamics in a passive InAs/InP quantum dot (QD) waveguide using 255 fs optical pulses at a central wavelength of 1568 nm. We observe strong anisotropy of absorption saturation for different polarizations. Pump-probe measurements indicate the presence of carrier relaxation dynamics on a timescale in the order of tens of picoseconds due to cascaded relaxation of carriers generated by two-photon absorption (TPA) from the bulk region to the QDs via the wetting layer. These relaxation timescales are much longer than in QD amplifiers. Our observations are supported by a rate-equation model which includes TPA, showing good agreement with the pump-probe measurements.     

Modeling of gain and phase dynamics in quantum dot amplifiers

By means of an electron hole rate equation model we explain the phase dynamics of a quantum dot semiconductor optical amplifier and the appearance of different decay times observed in pump and probe experiments. The ultrafast hole relaxation leads to a first ultrafast recovery of the gain, followed by electron relaxation and, in the nanosecond timescale, radiative and non-radiative recombinations. The phase dynamics is slower and is affected by thermal redistribution of carriers within the dot. We explain the ultrafast response of quantum dot amplifiers as an effect of hole escape and recombination without the need to assume Auger processes.