The polaron effect on the binding energy of a shallow donor impurity in quantum-well wires in an electric field

Using a variational technique, the effect of electron-longitudinal optical (LO) phonon interaction on the ground and the first few excited states of a hydrogenic impurity in a semiconductor quantum wire of rectangular cross section under an external electric field is studied theoretically for the impurity atom doped at various positions. The results for the binding energy as well as polaronic correction are obtained as a function of the size of the wire, the applied uniform electric field and the position of the impurity. It is found that the presence of optical phonons changes significantly the values of the impurity binding energies of the system. Taking into account the electron–LO phonon interaction the 1s→2p_y and 1s→2p_z transition energies are calculated as a function of applied electric field for different impurity positions.     

Intense laser effects on nonlinear optical absorption and optical rectification in single quantum wells under applied electric and magnetic field

In this work the effects of intense laser on the electron-related nonlinear optical absorption and nonlinear optical rectification in GaAs-Ga_1−xAl_xAs quantum wells are studied under, applied electric and magnetic field. The electric field is applied along the growth direction of the quantum well whereas the magnetic field has been considered to be in-plane. The calculations were performed within the density matrix formalism with the use of the effective mass and parabolic band approximations. The intense laser effects are included through the Floquet method, by modifying the confining potential associated to the heterostructure. Results are presented for the nonlinear optical absorption, the nonlinear optical rectification and the resonant peak of these two optical processes. Several configurations of the dimensions of the quantum well, the applied electric and magnetic fields, and the incident intense laser radiation have been considered. The outcome of the calculation suggests that the nonlinear optical absorption and optical rectification are non-monotonic functions of the dimensions of the heterostructure and of the external perturbations considered in this work.     

Polaron effect on the optical rectification in spherical quantum dots with electric field

The polaron effect on the optical rectification in spherical quantum dots with a shallow hydrogenic impurity in the presence of electric field is theoretically investigated by taking into account the interactions of the electrons with both confined and surface optical phonons. Besides, the interaction between impurity and phonons is also considered. Numerical calculations are presented for typical Zn_(1-x)Cd_xSe/ZnSe material. It is found that the polaronic effect or electric field leads to the redshifted resonant peaks of the optical rectification coefficients. It is also found that the peak values of the optical rectification coefficients with the polaronic effect are larger than without the polaronic effect, especially for smaller Cd concentrations or stronger electric field.     

Linear and nonlinear optical properties in a semiconductor quantum well under intense laser radiation: Effects of applied electromagnetic fields

In this work we are studying the intense laser effects on the electron-related linear and nonlinear optical properties in GaAs–Ga_1?xAl_xAs quantum wells under applied electric and magnetic fields. The calculated quantities include linear optical absorption coefficient and relative change of the refractive index, as well as their corresponding third-order nonlinear corrections. The nonlinear optical rectification and the second and third harmonic generation coefficients are also reported. The DC applied electric field is oriented along the hererostructure growth direction whereas the magnetic field is taken in-plane. The calculations make use of the density matrix formalism to express the different orders of the dielectric susceptibility. Additionally, the model includes the effective mass and parabolic band approximations. The intense laser effects upon the system enter through the Floquet method that modifies the confinement potential associated to the heterostructure. The results correspond to several configurations of the dimensions of the quantum well, the applied electric and magnetic fields, and the incident intense laser radiation. They suggest that the nonlinear optical absorption and optical rectification are nonmonotone functions of the dimensions of the heterostructure and of the external perturbations considered in this work.     

Effects of an electric field on the confined hydrogen impurity states in a spherical parabolic quantum dot

In this paper, we studied the effects of an electric field on a hydrogenic impurity confined in a spherical parabolic quantum dot using nondegenerate and degenerate perturbation methods. The binding energies of the ground and three low-excited states are calculated as a function of the confinement strength and as a function of the intensity of an applied electric field. Moreover, we computed the oscillator strength and the second-order nonlinear optical rectification coefficient based on the computed energies and wave functions. The results show that the electric and optical properties of hydrogenic impurity states are strongly affected by the confinement strength and the applied electric field.     

Electric field effects on the linear and nonlinear intersubband optical properties of double semi-parabolic quantum wells

In this work, the effects of the electric field on the optical properties of the symmetric and asymmetric double semi-parabolic quantum wells (DSPQWs) are investigated numerically for typical GaAs/Al_xGa_1−xAs. Optical properties are obtained using the compact density matrix approach. Our calculations for the asymmetric DSPQW show that the resonant peak values of the total refractive index change and total optical absorption coefficient are maximum for a certain value of the applied electric field, due to the anti-crossing effect. However, for the symmetric DSPQW, the resonant peak values of these optical properties decrease monotonically with increasing the applied electric field. Also, our results indicate that a larger value of the optical rectification coefficient of the symmetric DSPQW can be induced by applying a small electric field.     

Off-centre impurity-related nonlinear optical absorption,second and third harmonic generation in a two-dimensional quantum ring under magnetic field

Abstract

The effects of an off-centre donor impurity on the non-linear optical absorption, second and third harmonic generation in a GaAs two-dimensional disc-shaped quantum ring under magnetic field are investigated within the compact density-matrix formalism and the effective mass approximation. The results reveal that: (i) the absorption spectra extend on larger energy intervals at the increment of the magnetic field; (ii) the possibility of generating second harmonic response from the system is demonstrated for an impurity placed on the repulsive part of the confining potential; (iii) both second harmonic and third harmonic coefficients are one order of magnitude larger for an impurity placed on the repulsive part of the potential and are blue-shifted by the increment of the magnetic field regardless the impurity position.     

Depending on the electric and magnetic field of the linear optical absorption and rectification coefficient in triple quantum well

In this study, the alteration of the potential profile, the energy levels, the dipole matrix element and the resonant peaks of the linear optical absorption (OA) and optical rectification (OR) coefficients in GaAs/GaAlAs triple quantum well (TQW) are calculated as dependent on the applied electric field and the magnetic field. The results show that the shape of confined potential profile, the energy levels and the dipole moment matrix elements are changed as dependent on the external fields. Also, the resonant peaks of the OA and OR coefficients depend on the applied external field effects. Therefore, I hope that these results will provide important improvement in semiconductor device applications, for suitable choice of electric and magnetic field values. It may particularly be useful in technological applications that the structure of TQW changes with the strength and direction of the external electric field.     

Oscillatory behaviour in the energy and nonlinear optical rectification spectra of elliptic quantum rings under electric field: influence of impurity and eccentricity

We have theoretically investigated the electronic properties and nonlinear optical rectification spectra of GaAs/AlGaAs anisotropic quantum ring, modelled by an outer ellipsis and an inner circle, in connection to the presence of a donor off-centre impurity, structural distortions and in-plane electric field. The one-electron energy spectrum and wave functions are found using the adiabatic approximation and the finite element method within the effective-mass model. The energy spectrum of concentric ring reveals an anomalous oscillatory behaviour in the region of relatively small values of the electric field (< 12?kV/cm) followed by linear Stark effect at higher field values. We showed that this unusual behaviour is strongly affected by the ratio of the outer/inner ring radii, the displacement of the inner circle (eccentricity) along the x or y axis and the impurity presence. The related nonlinear optical rectification spectra present maxima whose positions mirror this oscillatory behaviour and consequently can be used as an excellent tool to distinguish the presence of an impurity or the direction of the eccentricity.     

Ag-BaO薄膜在电场作用下的可见——近红外波段光学吸收特性

对超额Ba激活的Ag-BaO复合薄膜在外加垂直表面电场作用下的光学吸收特性进行了测量.结果显示,薄膜在可见—近红外光波段存在两个吸收峰,其中近红外光区的吸收峰强度随垂直表面电场的作用而降低.理论分析表明,可见光区的主吸收峰源于埋藏在BaO半导体中的Ag超微粒子的表面等离激元共振吸收;近红外光区的次吸收峰则由BaO半导体基质中杂质能级的光吸收引起,杂质能级的产生与超额Ba在BaO晶体中造成的氧缺位有关.在外加垂直表面电场作用下,BaO基质中的杂质发生电离,并导致杂质能级上束缚电子浓度减小,表现为薄膜在与杂质 关键词： 光吸收 金属超微粒子半导体复合薄膜 表面等离激元 杂质能级     

Intense laser effects on the optical properties of asymmetric GaAs double quantum dots under applied electric field

We investigated the combined effects of a non-resonant intense laser field and a static electric field on the electronic structure and the nonlinear optical properties (absorption, optical rectification) of a GaAs asymmetric double quantum dot under a strong probe field excitation. The calculations were performed within the compact density-matrix formalism under steady state conditions using the effective mass approximation. Our results show that: (i) the electronic structure and optical properties are sensitive to the dressed potential; (ii) under applied electric fields, an increase of the laser intensity induces a redshift of the optical absorption and rectification spectra; (iii) the augment of the electric field strength leads to a blueshift of the spectra; (iv) for high electric fields the optical spectra show a shoulder-like feature, related with the occurrence of an anti-crossing between the two first excited levels.     

Optical absorptions of an exciton in a disc-like quantum dot under an electric field

An exciton in a disc-like quantum dot (QD) with the parabolic confinement, under applied electric field, is studied within the framework of the effective-mass approximation. Both the electric field and the confinement effects on the transition energy and the oscillator strength were investigated. Based on the computed energies and wave functions, the linear, the third-order nonlinear and the total optical absorption coefficients were also calculated. We found that the optical absorption coefficients with considering excitonic effects are stronger than those without considering excitonic effects and the absorption peak will move to the right side induced by the electron-hole interaction, which shows an excitonic effect blue-shift of the resonance in QDs. The applied electric field may affect either the size or the position of absorption peaks of excitons. However, the applied electric field may only affect the size of absorption peaks of an electron-hole pair without considering excitonic effects. It is very important to take excitonic effects into account when we study the optical absorption for disc-like QDs. We may observe the excitonic effect induced by the external electric field.     

Tunneling time correction to the intersubband optical absorption in a THz laser-dressed GaAs/Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1-</Subscript><Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>As quantum well

Tunneling effect on the intersubband optical absorption in a GaAs/Al _x Ga_{1- x} As quantum well under simultaneous presence of intense non-resonant laser and static electric fields is theoretically investigated. Based on the shooting method the quasi-stationary energy levels and their corresponding linewidths are obtained. By considering the joint action of the two external fields the linear absorption coefficient is calculated by means of Fermi’s golden rule and taking into account the intersubband relaxation. We found that: (i) the linewidth broadening due to the electron tunneling has an appreciable effect on the absorption spectrum; (ii) a constant relaxation time adopted in the previous studies could not be justified even for moderate electric fields, especially in the laser dressed wells. Our model predicts that the number of absorption peaks can be controlled by the external applied fields. While in the high-electric fields the excited states become unbounded due to a significant tunneling of the electrons, for high laser intensities and low/moderate electric fields the absorption spectrum has a richer structure due to the laser-generated resonant states. The possibility of tuning the resonant absorption energies by using the combined effects of the static electric field and the THz coherent radiation field can be useful in designing new optoelectronic devices.     

外场作用下蒽分子的激发特性研究

蒽(anthracene)具有良好的热稳定性以及较高的荧光量子产率的优点, 是最早用于研究有机发光器件(organic light-emitting device, OLED)的材料之一. 在本文中, 主要利用量子化学方法研究了不同外电场对蒽分子激发特性的影响规律. 首先采用密度泛函理论(density functional theory, DFT)在6-311G(d, p)基组水平上对蒽分子基态结构进行优化, 基于稳定基态结构, 利用含时密度泛函(time-dependent density functional theory, TDDFT)以及同一基组水平, 计算出蒽分子的前十个激发态的激发能、跃迁偶极矩、振子强度和紫外吸收光谱等数据. 然后以密度泛函B3P86方法优化出的不同外电场下蒽分子基态结构为基础, 使用TDDFT方法研究了不同外电场对蒽分子前线轨道能级和激发特性的影响规律. 结果显示, 无场时蒽分子在紫外区域234.50 nm处有一个较强的吸收峰, 对应基态电子跃迁至第5激发态吸收光子波长; 在外电场作用下, 蒽分子电子由基态跃迁到激发态的各项光谱参数均有显著变化, 加场后蒽分子的吸收光谱发生了红移, 由紫外波段移向了紫外–可见光波段, 与实验值相符合. 分子前线轨道的计算结果也表明蒽分子的最高占据轨道(highest occupied molecular orbital, HOMO)和最低未占据轨道(lowest unoccupied molecular orbital, LUMO)能量差值在不同电场下存在差异. 关键词： 蒽 外电场 激发特性     

Effect of conduction band nonparabolicity on the optical properties in a single quantum well under hydrostatic pressure and electric field

The effect of conduction band nonparabolicity on the linear and nonlinear optical properties such as absorption coefficients, and changes in the refractive index are calculated in the Al_0.3Ga_0.7As/GaAs heterostructure-based symmetric rectangular quantum well under applied hydrostatic pressure and electric field. The electron envelope functions and energies are calculated in the effective mass equation including the conduction band nonparabolicity. The linear and nonlinear optical properties have been calculated in the density matrix formalism with two-level approximation. The conduction band nonparabolicity shifts the positions of the optical properties and decreases their strength compared to those without this correction. Both the optical properties are enhanced with the applied hydrostatic pressure. While the absorption coefficients are bleached under the combined effect of high pressure and electric field, the bleaching effect is reduced when nonparabolicity is included.     

Impurity-related optical response in a 2D and 3D quantum dot with Gaussian confinement under intense laser field

ABSTRACT

Using the two-dimensional (2D) diagonalisation method, the impurity-related electronic states and optical response in a 2D quantum dot with Gaussian confinement potential under nonresonant intense laser field are investigated. The effects of a hydrogenic impurity on the energy spectrum and binding energy of the electron and also intersubband optical absorption are calculated. The obtained numerical results show that the degeneracies of the excited electron states are broken and the absorption spectrum exhibits a redshift with the values of the laser field. The findings indicate a new degree of freedom to tune the performance of novel optoelectronic devices, based on the quantum dots and to control their specific properties by means of intense laser field and hydrogenic donor impurity. Using the same Gaussian confinement model, the electronic properties of a confined electron in the region of a spherical quantum dot are studied under the combined effects of on-centre donor impurity and a linearly polarised intense laser radiation. The three-dimensional problem is used to theoretically model, with very good agreement, some experimental findings reported in the literature related to the photoluminescence peak energy transition.     

Excited state absorptions of an exciton bound to an ionized donor impurity in quantum dots

An investigation of an exciton bound in a parabolic two dimensional quantum dot by a donor impurity has been carried out by using the matrix diagonalization method and the compact density-matrix approach. The linear, third-order nonlinear, total optical absorption coefficients and refractive index changes have been calculated for the s-p, p-d, and d-f transitions. The results show that the parabolic potential has a great effect on the optical absorptions. The calculated results also reveal that as the angular momentum quantum numbers of transitions increase, the optical absorption and refractive index peaks shift towards lower energies and the absorption and refractive index intensities increase.     

Optical detection of an electric field in a GaAs/AlGaAs <Emphasis Type="Italic">n</Emphasis>-<Emphasis Type="Italic">i</Emphasis>-<Emphasis Type="Italic">n</Emphasis> heterostructure with double quantum wells

Processes occurring when a static transverse electric field is applied to a GaAs/AlGaAs n-i-n heterostructure with single quantum wells and asymmetric tunnel-coupled double quantum wells have been investigated by optical methods. The difference between the energies of exciton transitions for quantum wells of different widths makes it possible to attribute the observed photoluminescence peaks to particular pairs of wells or particular single quantum wells. The local electric field for each quantum well has been determined in terms of the Stark shift and splitting of exciton lines in a wide range of external voltage. A qualitative model has been proposed to explain the nonmonotonic distribution of the electric field over the depth of the heterostructure.     

Nonlinear optical absorption and optical rectification in near-surface double quantum wells: combined effects of electric, magnetic fields and hydrostatic pressure

The theoretical study of the combined effects of electric and magnetic fields and hydrostatic pressure on the nonlinear optical absorption and rectification is presented for electrons confined within an asymmetrical GaAs?Ga_1-x Alx As double quantum well. The effective mass, parabolic band, and envelope function approaches are used as tools for the investigation. The electric field is taken to be oriented along the growth direction of the heterostructure and the magnetic field is applied parallel to the interfaces of the quantum wells. The pressure-induced mixing between the two lowest conduction bands is considered both in the low and high pressure regimes. According to the results obtained it can be concluded that the nonlinear optical absorption and rectification coefficients depend in a non-trivial way on some internal and external parameters such as the size of the quantum wells, the direction of applied electric field, the magnitude of hydrostatic pressure, the stoichiometry of the wells and barriers, and the intensity of the applied magnetic field.