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.     

Built-in electric field effect on the linear and nonlinear intersubband optical absorptions in InGaN strained single quantum wells

Considering the strong built-in electric field (BEF) induced by the spontaneous and piezoelectric polarizations and the intrasubband relaxation, we investigate the linear and nonlinear intersubband optical absorptions in In_xGa_1-xN/Al_yGa_1-yN strained single quantum wells (QWs) by means of the density matrix formalism. Our numerical results show that the strong BEF is on the order of MV/cm, which can be modulated effectively by the In composition in the QW. This electric field greatly increases the electron energy difference between the ground and the first excited states. The electron wave functions are also significantly localized in the QW due to the BEF. The intersubband optical absorption peak sensitively depends on the compositions of In in the well layer and Al in the barrier layers. The intersubband absorption coefficient can be remarkably modified by the electron concentration and the incident optical intensity. The group-III nitride semiconductor QWs are suitable candidate for infrared photodetectors and near-infrared laser amplifiers.     

Linear and nonlinear optical absorption coefficients in inverse parabolic quantum wells under static external electric field

In the present theoretical study, the linear and third-order nonlinear optical absorption coefficients have been calculated in GaAs/Ga_1−x Al _x As inverse parabolic quantum wells (single and double) subjected to an external electric field. Our calculations are based on the potential morphing method in the effective mass approximation. The systematic theoretical investigation contains results with all possible combinations of the involved parameters, as quantum well width, quantum barrier width, Al concentration at each well center and magnitude of the external electric field. Our results indicate that in most cases investigated, the increase of the electric field blue-shifts the peak positions of the total absorption coefficient. In all cases studied it became apparent that the incident optical intensity considerably affects the total absorption coefficient.     

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.     

Phonon-assisted intersubband transitions in wurtzite GaN/InxGa1-xN quantum wells

A detailed numerical calculation on the phonon-assisted intersubband transition rates of electrons in wurtzite GaN/In_xGa_1-xN quantum wells is presented. The quantum-confined Stark effect induced by the built-in electric field and the ternary mixed crystal effect are considered. The electron states are obtained by iteratively solving the coupled Schrödinger and Poisson equations and the dispersion property of each type of phonon modes is considered in the derivation of Fermi's golden rule to evaluate the transition rates. It is indicated that the interface and half-space phonon scattering play an important role in the process of 1-2 radiative transition. The transition rate is also greatly reduced by the built-in electric field. The present work can be helpful for the structural design and simulation of new semiconductor lasers.     

Intersubband transitions and refractive index changes in coupled double quantum well with different well shapes

In this study, both the linear intersubband transitions and the refractive index changes in coupled double quantum well (DQW) with different well shapes for different electric fields are theoretically calculated within framework of the effective mass approximation. Results obtained show that intersubband transitions and the energy levels in coupled DQW can importantly be modified and controlled by the electric field strength and direction. By considering the variation of the energy differences, it should point out that by varying electric field we can obtain a blue or red shift in the intersubband optical transitions. The modulation of the absorption coefficients and the refractive index changes which can be suitable for good performance optical modulators and various infrared optical device applications can be easy obtained by tuning applied electric field strength and direction.     

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.     

Linear and nonlinear intersubband optical absorption coefficients and refractive index changes in a quantum box with finite confining potential

In this work, both the intersubband optical absorption coefficients and the refractive index changes are calculated exactly in a quantum box. Analytical expressions for the linear and nonlinear intersubband absorption coefficients and refractive index changes are obtained by using the compact-density matrix approach. Numerical results are presented for typical GaAs/Al_xGa_1−x As quantum box system. The linear, third-order nonlinear and total absorption and refractive index changes are investigated as a function of the incident optical intensity and structure parameters such as box-edge length and stoichiometric ratio. Our results show that both the incident optical intensity and the structure parameters have a great effect on the total absorption and refractive index changes.     

Study of optical properties in a cubic quantum dot

In this paper, the effect of hydrostatic pressure on both the intersubband optical absorption coefficients and the refractive index changes is studied for typical GaAs/Al _x  Ga_1?x As cubic quantum dot. We use analytical expressions for the linear and third-order nonlinear intersubband absorption coefficients and refractive index changes obtained by the compact-density matrix formalism. The linear, third-order nonlinear, and total intersubband absorption coefficients and refractive index changes are calculated at different pressures as a function of the photon energy with known values of box length (L), the incident optical intensity (I), and Al concentration (x). According to the results obtained from the present work, we have found that the pressure plays an important role in the intersubband optical absorption coefficient and refractive index changes in a cubic quantum dot.     

Linear and nonlinear intersubband optical absorption in symmetric double semi-parabolic quantum wells

The linear and the nonlinear intersubband optical absorption in the symmetric double semi-parabolic quantum wells are investigated for typical GaAs/AlxGa1−xAs. Energy eigenvalues and eigenfunctions of an electron confined in finite potential double quantum wells are calculated by numerical methods from Schrödinger equation. Optical properties are obtained using the compact density matrix approach. In this work, the effects of the barrier width, the well width and the incident optical intensity on the optical properties of the symmetric double semi-parabolic quantum wells are investigated. Our results show that not only optical incident intensity but also structure parameters such as the barrier and the well width really affect the optical characteristics of these structures.     

Linear and nonlinear intersubband optical absorption in double triangular quantum wells

The linear and the third-order nonlinear optical absorptions in the asymmetric double triangular quantum wells (DTQWs) are investigated theoretically. The dependence of the optical absorption on the right-well width of the DTQWs is studied, and the influence of the applied electric field on the optical absorption is also taken into account. The analytical expressions of the linear and the nonlinear optical absorption coefficients are obtained by using the compact density-matrix approach and the iterative method. The numerical calculations are presented for the typical GaAs/Al_xGa_1?xAs asymmetric DTQWs. The results show that the linear as well as the nonlinear optical absorption coefficients are not a monotonous function of the right-well width, but have complex relationships with it. Moreover, the calculated results also reveal that applying an electric field to the DTQWs with a thinner right-well can enhance the linear optical absorption but has no prominent influence on the nonlinear optical absorption. In addition, the total optical absorption is strongly dependent on the incident optical intensity.     

Impurity-related polarizability and photoionization-cross section in double quantum wells under electric fields and hydrostatic pressure

Double quantum well heterostructures are quite important for the exploration of correlated electron states in two-dimensional systems. By using the variational procedure, within the effective-mass and parabolic-band approximations, the effects of both electric field and hydrostatic pressure on the shallow-donor-impurity related polarizability and photoionization cross-section in GaAs–Ga_1−xAl_xAs double asymmetric quantum wells are presented. The electric field is considered to be applied along the growth direction. It is found that the impurity binding energy and polarizability can be tuned by means of an applied external electric field or hydrostatic pressure in asymmetric double quantum wells, a behavior which could be used in the design and construction of semiconductor devices. The photoionization cross-section magnitude increases as the pressure and applied electric field are increased, except beyond the Γ–X crossover in the barrier material, where a decrease of the photoionization cross-section is expected due the smaller confinement of the impurity wave function.     

Laser effects on the donor states in V-shaped and inverse V-shaped quantum wells

Laser effects on the electronic states in GaAs/ Ga_1−xAl_xAs V-shaped and inverse V-shaped quantum wells under a static electric field are studied using the transfer matrix method. The dependence of the donor binding energy on the laser field strength and the density of states associated with the impurity is also calculated. It is demonstrated that in inverse V-shaped quantum wells under electric fields, with an asymmetric distribution of the electron density, the position of the binding energy maximum versus the impurity location in the structure can be adjusted by the intensity of the laser field. This effect could be used to tune the electronic levels in quantum wells operating under electric and laser fields without modifying the physical size of the structures.     

Intraband transitions in magnetoexcitons in coupled double quantum wells

A theory of far-infrared (FIR) magneto-optical intraband s→p ^± transitions of direct and indirect excitons in semiconductor coupled double quantum wells has been developed. The case of symmetric strained In_xGa_1−x As/GaAs quantum wells with nondegenerate valence band in the regime of both narrow and wide barriers has been analyzed. The energies and dipole matrix elements of transitions between the ground s and excited p ^± states in a quantizing magnetic field B>2 T and electric field ℰ perpendicular to the quantum well plane have been studied. The regimes of direct (in a weak electric field) and indirect (in a strong electric field) transitions, and the transition between the direct and indirect regimes, have been investigated. Zh. éksp. Teor. Fiz. 113, 1446–1459 (April 1998)     

Effect of position-dependent effective mass on linear and nonlinear optical properties of a cubic quantum dot

In this paper, we first obtain an analytic relation for studying the position-dependent effective mass in a GaAs/Al_xGa_1−xAs cubic quantum dot. Then, the effect of position-dependent effective mass on the intersubband optical absorption coefficient and the refractive index change in the quantum dot are studied. Our numerical calculations are performed using both a constant effective mass and the position-dependent effective mass. We calculate the linear, nonlinear and total intersubband absorption coefficient and refractive index change as a function of the incident optical intensity and structural parameters such as dot length. The results obtained from the present work show that spatially varying electron effective mass plays an important role in the intersubband optical absorption coefficient and refractive index change in a cubic quantum dot.     

Influence of Polarization-Induced Electric Field on Subband Structure in AlxGa1-xN/GaN Double Quantum Wells

The influence of the polarization-induced electric field and other parameters on the subband structure in Al_xGa_1-xN /GaN coupled double quantum wells (DQWs) has been studied by solving the Schrödinger and Poisson equations self-consistently. It is found that the polarization effect leads to an asymmetric potential profile of Al_xGa_1-xN/GaN DQWs although the two wells have the same width and depth. The polarization effect also leads to a very large Stark shift between the odd and the even order subband levels that can reach 0.54eV. Due to the polarization-induced Stark shift, the wavelength of the intersubband transition between the first odd order and the second even order subband levels becomes smaller, which is useful for realization of optoelectronic devices operating within the telecommunication window region.     

Optical absorption of a quantum well with an adjustable asymmetry

The effects of asymmetry and the electric field on the electronic subbands and the nonlinear intersubband optical absorption of GaAs quantum wells represented by a P?schl-Teller confining potential are studied. The potential itself can be made asymmetric through a correct choice of its parameter set and this adjustable asymmetry is important for optimizing the absorption. In that way optimal cases can be created. We calculate the modified wave functions and electronic subbands variationally. The linear and the nonlinear optical intersubband absorption coefficients are calculated. Numerical results for a typical GaAs quantum well are presented. The nonlinear part of the absorption coefficient is strongly modified by the asymmetry parameters while the electric field affects it at smaller values of the parameters. - 78.67.De Quantum wells     

Intensity-dependent nonlinear optical properties in a modulation-doped single quantum well

In the present work, the changes in the intersubband optical absorption coefficients and the refractive index in a modulation-doped quantum well have been investigated theoretically. Within the envelope function approach and the effective mass approximation, the electronic structure of the quantum well is calculated from the self-consistent numerical solution of the coupled Schrödinger-Poisson equations. The analytical expressions of optical properties are obtained by using the compact density-matrix approach. The numerical results GaAs/Al_xGa_1−xAs are presented for typical modulation-doped quantum well system. The linear, third-order nonlinear and total absorption and refractive index changes depending on the doping concentration are investigated as a function of the incident optical intensity and structure parameters, such as quantum well width and stoichiometric ratio. The results show that the doping concentration, the structure parameters and the incident optical intensity have a great effect on the optical characteristics of these structures.     

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.