Multiphoton transitions in direct-gap semiconductors

Multiphoton transitions in direct-gap semiconductors are studied considering a “non-perturbative” approach in which the final state is represented by a Volkov wavefunction. Analytical solutions are obtained for multiphoton transition rates. The relation between multiphoton absorption and tunneling is discussed.     

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.     

Electron-hole plasma generation and evolution in semiconductors

We discuss various physical problems related to dense electron-hole (e-h) plasmas in silicon-like semiconductors such as: the e-h plasma energy in the quantum and classical limits including the actual band structure; the dielectric constant using the appropriate plasma frequency mass and the variation of the plasma relaxation time due to e-h collisions; the plasma generation including its nonlinear aspect when the e-h density passes the plasma frequency density of the pump beam; the e-h plasma evolution due to Auger recombination and impact ionization as well as the e-h plasma hydrodynamics; finally, we will discuss also the possibility of a very fascinating melting at T=0.     

The dispersion of excitons,polaritons and biexcitons in direct-gap semiconductors

Excitons are the energetically lowest excitations of the electronic system in an ideal semiconductor at zero temperature. If the excitons couple to the electromagnetic field, a mixed state is formed, the quanta of which are called excitonic polaritons. Associates of two excitons, so-called biexcitons, have been observed in many semiconductors. Excitons are known for about forty years. During the first three decades, they have been investigated mainly by the classical spectroscopic methods, i.e., reflection, transmission and luminescence spectroscopy. In the last decade, several new techniques have been developed, which allow for a direct spectroscopy in momentum space. In this contribution, we review these novel techniques, both linear and nonlinear ones, and present results obtained for excitons, polaritons and biexcitons. The review is restricted to semiconductors which have their conduction band minimum and their valence band maximum at the same point of the Brillouin zone (direct-gap materials) and which have a band to band transition which is dipole allowed.     

Defects in semiconductors observed by positron annihilation

The origin of the effect of defects on positron annihilation in semiconductors has been studied. The electron-positron momentum densities in elemental semiconductors (Si and Ge), III-V compound semiconductors (GaAs, InP and GaSb), diamond and the proton irradiated Si were investigated by a full-scale use of the two-dimensional angular correlation of positron annihilation radiations (2D-ACAR). The obtained results showed, as a whole, good agreement with the electron momentum distribution of the fully occupied Jones zone with a small exception for the fact that the low density channels are running along the three principal axes. This anisotropy was strong in elemental semiconductors, while it was weakened in compound semiconductors. This anisotropy and its dependence on the material were found to be generally understood by the incorporation of crystal symmetry. The anisotropy will be discussed by group theory in conjunction to the effect of defects on positron annihilation.     

Electron-hole droplet condensation — A phase transition in an open system

We derive from hydrodynamical equations a generalized Ginzburg-Landau potential for the amplitude of the u stable mode, which describes a first order phase transition in the electron-hole plasma.     

Spin-independent and spin-dependent re-excitation in CdZnTe studied by pump-probe spectroscopy

Time-resolved pump-probe spectroscopy is used to study the re-excitation processes of spin-balanced and spin-polarized electrons trapped by the defect levels in Cd_0.86Zn_0.14Te. In CdTe, Cd_0.96Zn_0.04Te, and Cd_0.86Zn_0.14Te, the electron trapping time and lifetime decrease with the increasing defect density of states that depends on the Zn composition. The abnormal linearly and circularly polarized pump-probe spectroscopies, respectively, demonstrate the presence of spin-independent and spin-dependent re-excitation processes in Cd_0.86Zn_0.14Te which have high defect density of states.     

基于中红外光谱检测牛奶中掺杂的尿素

采用中红外光谱技术对牛奶中掺杂尿素目标物进行检测.配置含有尿素浓度范围为1～18 g·L-1之间18个牛奶样品,分别研究了纯牛奶和掺杂尿素牛奶的红外光谱特性,并进行了对比分析.利用尿素1 562 cm-1处酰胺Ⅲ带C=O伸缩振动吸收峰面积A1 562与1 464 cm-1处C-N伸缩振动吸收峰面积A1464的比值A1 ...     

Two-dimensional Fourier transform spectroscopy in the pump-probe geometry

Two-dimensional (2D) Fourier transform (FT) infrared spectroscopy is performed by using a collinear pulse-pair pump and probe geometry with conventional optics. Simultaneous collection of the third-order response and pulse-pair timing permit automated phasing and rapid acquisition of 2D absorptive spectra. To demonstrate the ability of this method to capture molecular dynamics, couplings and structure found in the conventional boxcar 2D FT spectroscopy, a series of 2D spectra of a metal carbonyl, and a beta-sheet protein are acquired.     

Inherent spectral distortions in pump-probe Brillouin scattering spectroscopy

It is shown that the pump-probe technique in Brillouin scattering spectroscopy is accompanied by two specific distortions to a registered spectrum. The first appears due to an acoustic wave that unavoidably accompanies in a continuous medium the density wave driven by the pump and probe signals. The registered spectrum then splits to two spectral components, one corresponding to the propagation velocity of the driven density wave, and the other to the acoustic velocity. The other distortion is asymmetry of the spectrum which is shown to occur when the velocity of the driven density wave is greater than the acoustic velocity in the medium.     

Electron-hole interaction of the d core levels in III–V semiconductors

Recently Aspnes and Olson reported the first electroreflectance spectra obtained on GaP in the vacuum ultraviolet. They identified the structures above 20 eV as originating from critical points occuring between the flat Ga 3d corelevels and the sp³ conduction bands. We show that photoemission (E.D.C.) and optical spectroscopy (absorption, electroreflectance) give different results. The one-electron calculation fails to explain the experimental data. Electron-hole interaction and relaxation effects have to be included.     

On the choice of initial approximation in the problem of parameter identification of direct-gap semiconductors by cathodoluminescence microscopy

The problem of the choice of initial approximation has been considered to identify the parameters of direct-gap semiconductors by cathodoluminescence microscopy using the dependence of monochromatic cathodoluminescence intensity on electron beam energy for the case of a low level of signal excitation. It has been shown that the realization of the confluent analysis method can lead to the problem of multidimensional numerical minimization of a quadratic functional, the solution of which depends significantly on the choice of initial approximation. On this basis, some conditions of correct processing of the experimental data have been determined to identify the required parameters for gallium arsenide.     

Transient interband light absorption by quantum dots: Degenerate pump-probe spectroscopy

The optical pump-probe method, which makes it possible to determine the energy relaxation rate for excited electron-hole pairs and excitons in semiconductor quantum dots (QDs), is theoretically described. A scheme in which the carrier frequencies of optical pump and probe pulses are close to resonance with the same interband transition in the QD electron subsystem (degenerate case) is considered. The pump-induced probe energy absorption is analyzed as a function of the delay time between the pump and probe pulses. It is shown that under certain conditions this dependence is reduced to monoexponential, whose exponent is proportional to the energy relaxation rate for the considered state of electron-hole pairs and excitons. The size dependence of the energy relaxation rate of the electron-hole pair states is modeled by the example of PbSe-based QDs, whose electron subsystem is in the strong-confinement regime.     

Ultrafast electron spin dynamics in bulk CdTe investigated by femtosecond pump-probe reflection spectroscopy

The transient spin polarization dynamics in bulk cadmium telluride (CdTe) at 70 K is investigated by the circularly polarized pump-probe reflection technique. A general expression is derived from the rate equations of a two-level system with small signal approximation to describe the light-helicity-dependent reflection spectrum. The initial degree of electron spin polarization in the excited state and the electron spin relaxation time in bulk CdTe at low temperature with different carrier density are analyzed according to this model. Our experimental results reveal that the D?yakonov–Perel? mechanism based on a fully microscopic kinetic spin Bloch equation (microscopic KSBE) approach dominates in the electron spin relaxation process in bulk CdTe crystal.     

Frequency-comb-referenced molecular spectroscopy in the mid-infrared region

A simple method for absolute-frequency measurements of molecular transitions in the mid-IR region is reported. The method is based on a cw singly resonant optical parametric oscillator (SRO), which is tunable from 3.2 to 3.45?μm. The mid-IR frequency of the SRO is referenced to an optical frequency comb through its pump and signal beams. Sub-Doppler spectroscopy and absolute-frequency measurement of the P(7) transition of the ν3 band of CH4 are demonstrated.     

Rapid detection of illicit fipronil in phoxim granules by mid-infrared spectroscopy

In this paper, we use mid-infrared spectroscopy to detect illicit fipronil in phoxim granules by partial least discrimination analysis (PLS-DA). The classification accuracy (CA) of the calibration set and prediction set are both 100.0% with the additive level of fipronil ranges from 0.1% to 5.0%. The results indicate that MIR coupled with chemometric method has the potential in detecting illicit pesticide addition and the proposed method provides a new idea on pesticide quality control.     

Ion implantation in semiconductors investigated by nuclear spectroscopy methods

Nuclear methods have proved to be very useful for structural investigations of implanted layers. Through the hyperfine interaction of the impurity atoms, valuable information can be obtained on the charge state, location and the vibrational state of implanted atoms. Changes in the atomic locations after various annealing processes were successfully understood by applying the M?ssbauer effect and perturbed angular correlation. This review pays special attention to the physical interactions which determine the location of implanted atoms, the ballistics, the size and the chemical effects. The structure of disordered layers formed after implantation is also reviewed and discussed.