Linear Optical Absorption Coefficients Research Articles

Simultaneous Effects of External Electric Field and Conduction Band Nonparabolicity on Optical Properties of a GaAs Quantum Dot Embedded at the Center of a GaAlAs Nano-Wire

An investigation of the optical properties of a GaAs spherical quantum dot which is located at the center of a Ga1−xAlxAs cylindrical nano-wire has been performed in the presence of an external electric held. The band nonparabolicity effect is also considered using the energy dependent effective mass approximation. The energy eigenvalues and corresponding wave functions are calculated by finite difference approximation and the reliability of calculated wave functions is checked by computing orthogonality. Using computed energy eigenvalues and wave functions, the linear, third-order nonlinear and total optical absorption coefficients and refractive index changes are examined in detail. It is found that (i) Presence of electric field causes both blue and red shifts in absorption spectrum; (ii) The absorption coefficients shift toward lower energies by taking into account the conduction band nonparabolicity; (iii) For large values of electric field the effect of conduction band nonparabolicity is less dominant and parabolic band is estimated correctly; (iv) In the presence of electric field and conduction band nonparabolicity the nonlinear term of absorption coefficient rapidly increases by increasing incident optical intensity. In other words, the saturation in optical spectrum occurs at lower incident optical intensities.

Electronic and optical properties of InGaAs/GaAs quantum dots with tunable aspect-ratio

In this paper, a combined approach of finite element method (FEM) and quadratic programming optimization method is proposed to investigate the nonuniform equilibrium composition profile of InGaAs / GaAs quantum dots (QDs) in the framework of Gibbs energy optimization (GEO). The proposed QDs are varied with aspect ratio from 0.3 to 0.5. The wave functions of electron and heavy hole are predicted by using the k ⋅ p method. The changes of wave functions before and after optimization can be observed by using composition optimization. Both the eigenvalues and transition energy change obviously with the increasing aspect ratio. The linear optical absorption coefficients corresponding to the interband ground state transition are obtained via the density matrix approach and perturbation expansion method. The numerical results reveal that the aspect ratio and composition profile play significant roles in determining the electronic and optical properties.

Polaron effects on the linear and nonlinear intersubband optical absorption coefficients in quantum wells with asymmetrical semi-exponential potential

Linear and nonlinear intersubband optical absorption coefficients in quantum wells with asymmetrical semi-exponential potential are studied theoretically. We obtain unperturbed eigenfunctions and the energy eigenvalues by means of solving Schrödinger equation within the framework of effective mass approximation. The wave functions and energies with consideration of polaron are acquired through perturbation method. Besides, the analytic expressions of optical absorption coefficients are obtained using compact-density-matrix approach and iterative method. The results of calculation show that optical absorption is enhanced when effects of polaron are considered. Additionally, the parameters of confinement potential have great influences on optical absorption.

Nonlinear intersubband optical absorption in semiparabolic quantum wells

The linear and third-order nonlinear optical absorptions in semiparabolic quantum wells are studied in detail. Analytic formulas for the linear and third-order nonlinear optical absorption coefficients are obtained using the compact density matrix approach. Based on this model, numerical results are presented for typical GaAs/AlGaAs semiparabolic quantum wells. The results show that the factors of the incident optical intensity and the semiparabolic confinement frequency have great influences on the total optical absorption coefficients.

Magnetic field control of the intraband optical absorption in two-dimensional quantum rings

Linear and nonlinear optical absorption coefficients of the two-dimensional semiconductor ring in the perpendicular magnetic field B are calculated within independent electron approximation. Characteristic feature of the energy spectrum are crossings of the levels with adjacent nonpositive magnetic quantum numbers as the intensity B changes. It is shown that the absorption coefficient of the associated optical transition is drastically decreased at the fields corresponding to the crossing. Proposed model of the Volcano disc allows to get simple mathematical analytical results, which provide clear physical interpretation. An interplay between positive linear and intensity-dependent negative cubic absorption coefficients is discussed; in particular, critical light intensity at which additional resonances appear in the total absorption dependence on the light frequency is calculated as a function of the magnetic field and levels' broadening.

Effects of applied lateral electric field and hydrostatic pressure on the intraband optical transitions in a [formula omitted] quantum ring

In this paper the simultaneous effects of applied lateral electric field and hydrostatic pressure on the intraband linear optical absorption coefficient have been investigated in a two-dimensional GaAs/Ga1−xAlxAs single quantum ring. The lateral confinement potential of the ring is approximated by square finite potential well model. The one electron energy spectrum has been found using the effective mass approximation and an exact diagonalization technique. We find that for fixed geometric dimensions, the hydrostatic pressure can lead to both blue- and red-shift of the intraband optical absorption spectrum, while only a blue-shift is observed as a result of an electric field. The effect of hydrostatic pressure on the absorption coefficient׳s peaks is negligibly small. With the increase of electric field strength the absorption coefficient׳s peaks decrease.

Binding energy and nonlinear optical properties of an on-center hydrogenic impurity in a spherical quantum dot placed at the center of a cylindrical nano-wire: Comparison of hydrogenic donor and acceptor impurities

The binding energies and corresponding wave functions of ground and first excited states (1s, 2p) of on-center hydrogenic donor and acceptor impurities are calculated using finite difference approximation in the effective mass framework. The on-center impurities are assumed to be in an InAs spherical quantum dot which is located at the center of a GaAs cylindrical nano-wire. To test the validity of applied method the eigenvalues of ground and first excited states are compared with reported results which have been calculated by finite element methods and it is shown that the finite difference approximation is more accurate in this particular case. In addition, the oscillator strength, linear, third-order nonlinear and total optical absorption coefficients and refractive index changes have been calculated for 1s–2p transition by means of the compact density matrix approach. The results show that presence of impurity causes a blue shift in optical spectrum which is larger for acceptor impurity. Also, the amplitude of absorption coefficient, refractive index changes and oscillator strength depend on the absence, presence and type of impurity. Moreover, it is found that the saturation condition can be tuned by type of impurity and critical incident optical intensity is enhanced in presence of acceptor impurity.

Strain induced optical properties of exciton in a CdTe/ZnTe quantum dot

The influence of strain on the binding energies of heavy and light hole is obtained in a ZnxCd1−xTe/ZnTe quantum dot taking into account the phonon confinement effect. The band offsets are calculated using model-solid approach. The dielectric mismatch effect is introduced between the dielectric constants of dot and the barrier. The strain induced energies of excitonic transitions in a CdZnTe quantum dot with ZnTe barrier are brought out considering the internal electric field induced by the spontaneous and piezoelectric polarizations. Calculations have been obtained using Bessel function as an orthonormal basis for different confinement potentials of barrier height, strain induced linear and third-order nonlinear optical absorption coefficients and the changes of refractive index with the incident photon energy are observed. Our results show that the exciton binding energy is enhanced with the inclusion of potential taking into account the effects of dielectric mismatch and the geometry of quantum dot with various Zinc alloy content has a great influence on the optical properties of the dot.

Laser radiation effect on the optical properties of a spherical quantum dot confined in a cylindrical nanowire

An investigation is done in order to calculate the linear, third-order nonlinear and total optical absorption coefficient and refractive index changes of an impure GaAs spherical quantum dot placed at the center of a Ga1−xAlxAs cylindrical nano-wire. In this regard, the finite difference approximation is considered to calculate electronic structure as well as compact density-matrix approach is applied to investigate optical properties. In this study the effects of laser-field radiation along with aluminum (Al) concentration on optical properties are examined for three transitions between four first lower-lying electronic levels, i.e. 1→2, 2→3 and 3→4 transitions. Also the effect of relaxation time on saturation is studied and compared for these transitions. It is found that the magnitude of linear and third order nonlinear absorption coefficient and refractive index changes as well as the optical saturation can be modified by both Al concentration and the amplitude of laser-field radiation. Moreover, for 1→2 transition (3→4 transition) the effect of quantum dot (nano-wire) is dominant whereas for 2→3 transition Al concentration is a major determinant factor. Additionally, for all transitions by increasing the value of laser field a red shift appears in optical spectrum where the extent of this red shift depends on the Al concentration and the energy levels between which the transitions occur. Finally, in contrast to QDs, a red shift appears in absorption spectrum as going up the transitions between higher energy levels.

Asymmetric GaAs n-type double δ-doped quantum wells as a source of intersubband-related nonlinear optical response: Effects of an applied electric field

In this work, the conduction band electron states and the associated intersubband-related linear and nonlinear optical absorption coefficient and relative refractive index change are calculated for an asymmetric double n-type δ-doped quantum well in a GaAs-matrix. The effects of an external applied static electric field are included. Values of the two-dimensional impurities density (N2d) of each single δ-doped quantum well are taken to vary within the range of 1.0×1012 to 7.0×1012cm−2, consistent with the experimental data growth regime. The optical responses are reported as a function of the δ-doped impurities density and the applied electric field. It is shown that single electron states and the related optical quantities are significantly affected by the structural asymmetry of the double δ-doped quantum well system. In addition, a brief comparison with the free-carrier-related optical response is presented.

Binding energy and optical properties of an off-center hydrogenic donor impurity in a spherical quantum dot placed at the center of a cylindrical nano-wire

The binding energy as well as the linear, third-order nonlinear and total optical absorption coefficient and refractive index changes of an off-center hydrogenic donor impurity in an InAs spherical quantum dot placed at the center of a GaAs cylindrical nano-wire have been investigated. In this regard, the effective-mass approximation approach is considered and eigenvalues and corresponding eigenfunctions are calculated via the finite element method. The binding energy is plotted as a function of the dot size and impurity position along with optical properties as a function of photon energy. In this study two different directions have been considered for impurity position, along the nano-wire axis and perpendicular to it. It has been found that the binding energy, absorption coefficient and refractive index changes are impressively affected not only by the dot radius but also by the position of the impurity and its direction. Additionally, the optical saturation can be tuned by the direction of the impurity and incident optical intensity.

Linear and nonlinear optical absorption coefficients and refractive index changes in asymmetrical Gaussian potential quantum wells with applied electric field

Linear and nonlinear optical absorption coefficients and refractive index changes in asymmetrical Gaussian potential quantum wells with applied electric field are discussed by the effective mass approximation and the compact density matrix method. The dependence of linear and nonlinear optical absorption coefficients and refractive index changes on the height of the quantum well V0, the range of confinement potential L, and the applied electric field F is studied by the energy eigenvalues and the energy eigenfunctions. Our calculated results show that linear and nonlinear optical absorption coefficients and refractive index changes are affected by the three factors above mentioned. It is also found that the total optical absorption coefficients are greatly affected by the incident optical intensify.

The effects of aluminum concentration and hydrogenic donor impurity on the linear and nonlinear optical properties of a GaAs spherical quantum dot confined at the center of a Ga1−xAlxAs cylindrical nano-wire

The energy eigenvalues and corresponding wave functions of four first lower-lying states of a GaAs spherical quantum dot which is located at the center of a Ga1−xAlxAs cylindrical nano-wire are calculated using finite difference approximation within the effective mass framework. The reliability of calculated wave functions is checked by computing orthogonality and comparing with similar reported results. In addition, the oscillator strength, linear and third-order nonlinear optical absorption coefficients and refractive index changes are calculated based on optical 1–2, 2–3 and 3–4 transitions by means of the compact density-matrix approach, with and without hydrogenic donor impurity. The effect of aluminum concentration on the optical properties is also investigated. The results show that the presence of impurity has great influence on optical properties of the system and shifts optical spectrum towards higher energies. Moreover, through increasing the aluminum concentration both blue and red shifts appear in optical spectrum, which are strongly affected by energy levels between which transition may occur and presence or absence of impurity. Additionally, the calculated results also reveal that the magnitude of absorption coefficient, refractive index changes and oscillator strength as well as the saturation in optical spectrum depend on the aluminum concentration and presence of impurity for all transitions.

Donor impurity-related linear and nonlinear optical absorption coefficients in [formula omitted] concentric double quantum rings: Effects of geometry, hydrostatic pressure, and aluminum concentration

The linear and nonlinear optical absorption associated with the transition between 1s and 2s states corresponding to the electron-donor-impurity complex in GaAs/Ga1−xAlxAs three-dimensional concentric double quantum rings are investigated. Taking into account the combined effects of hydrostatic pressure and the variation of the aluminum concentration, the energies of the ground and first excited s-like states of a donor impurity in such a system have been calculated using the effective mass approximation and a variational method. The energies of these states and the corresponding threshold energy of the optical transitions are examined as functions of hydrostatic pressure, aluminum concentration, radial impurity position, as well as the geometrical dimensions of the structure. The dependencies of the linear, nonlinear and total optical absorption coefficients as functions of the incident photon energy are investigated for different values of those mentioned parameters. It is found that the influences mentioned above lead to either redshifts or blueshifts of the resonant peaks of the optical absorption spectrum. It is particularly discussed the unusual property exhibited by the third-order nonlinear of becoming positive for photon energies below the resonant transition one. It is shown that this phenomenon is associated with the particular features of the system under study, which determine the values of the electric dipole moment matrix elements.

Effect of intense high-frequency laser field on the linear and nonlinear intersubband optical absorption coefficients and refractive index changes in a parabolic quantum well under the applied electric field

The effects of the intense high-frequency laser field on the optical absorption coefficients and the refractive index changes in a GaAs/GaAlAs parabolic quantum well under the applied electric field have been investigated theoretically. The electron energy levels and the envelope wave functions of the parabolic quantum well are calculated within the effective mass approximation. Analytical expressions for optical properties are obtained using the compact density-matrix approach. The numerical results show that the intense high-frequency laser field has a large effect on the optical characteristics of these structures. Also we can observe that the refractive index and absorption coefficient changes are very sensitive to the electric field in large dimension wells. Thus, this result gives a new degree of freedom in the optoelectronic device applications.

Optical absorptions of an exciton in a quantum ring: Effect of the repulsive core

Abstract We study the optical absorptions of an exciton in a quantum ring. The quantum ring is described as a circular quantum dot with a repulsive core. The advantage of our methodology is that one can investigate the influence of the repulsive core by varying two parameters in the confinement potential. The linear, third-order nonlinear and total optical absorption coefficients have been examined with the change of the confinement potential. The results show that the optical absorptions are strongly affected by the repulsive core. Moreover, the repulsive core can influence the oscillation in the resonant peak of the absorption coefficients.

Linear and nonlinear optical absorption coefficients and refractive index changes in modulation-doped quantum wells: Effects of the magnetic field and hydrostatic pressure

In this study, the linear, the third-order nonlinear and total optical absorption coefficients and refractive index changes of a modulation-doped GaAs/AlxGa1−xAs quantum well are investigated numerically. In the effective-mass approximation, the electronic structure of modulation-doped quantum well is calculated by solving the Schrödinger and Poisson equations self-consistently. Optical properties are obtained using the compact density matrix approach. The effects of structure parameters, the applied magnetic field and the hydrostatic pressure on the optical properties of the modulation-doped quantum well are studied. Results show that the resonant peaks shift toward the higher (lower) energies with the increase in the magnetic field (pressure). The magnitude of the resonant peaks of the optical properties decreases with the increasing magnetic field or pressure.

Polaron effect on linear and nonlinear intersubband optical absorption of a wurtzite GaN-based nanowire

Based on the density matrix approach and the perturbation treatment, the polaronic effect on the linear and nonlinear intersubband optical absorption coefficients in quasi-one-dimensional wurtzite nanowires (NWs) is investigated. The simple analytical formulas for polaronic states and the linear and nonlinear optical absorption coefficients in the NW systems are also deduced. Numerical calculation on a freestanding wurtzite GaN NW is performed. The polaronic effect and quantum size effect as well as the influence of incident optical intensity on the optical absorption properties are analyzed and discussed. The results show that, the polaronic effect results in significant increase of the absorption coefficients, and obvious decrease of saturation absorption intensity. In order to observe strong optical absorption, a nitride NW with relative small radius should be chosen. Moreover, a comparison with the situation of GaN-based quantum well systems is also done, and the profound physics is analyzed reasonably. • Polaronic states including the IO and QC phonon modes in wurtzite nitride NWs are obtained. • Polaronic effect results in significant increase of the linear and nonlinear absorption coefficients. • Taking the polaronic effect into account, saturation absorption intensity decreases obviously. • GaN-based NWs with smaller radius should be chosen for observing stronger optical absorption.

Linear and nonlinear optical absorption coefficients and refractive index changes associated with intersubband transitions in a quantum disk with flat cylindrical geometry

The linear and nonlinear optical absorption coefficients and changes in the refractive index in GaAs/AlGaAs quantum disk in the form of a flat cylinder are investigated theoretically in the presence of a static magnetic and a laser field within the framework of the compact-density matrix approach. It is found that the absorption coefficients and the refractive index changes depend not only on the optical wave intensity but also on the strength of the static magnetic field. The intersubband relaxation time, also, has an important influence on the linear and nonlinear optical properties of a quantum disk.

Linear and nonlinear optical properties of anisotropic quantum dots in a magnetic field

We have investigated the linear and nonlinear optical properties of a two-dimensional anisotropic quantum dot in a magnetic field. Based on the computed energies and wave functions, the linear, third-order nonlinear and total optical absorption coefficients as well as the refractive index changes have been examined. The results are presented as a function of the incident photon energy for the different cases of anisotropy, dot size and external magnetic field. The results show that the linear and nonlinear optical properties of anisotropic quantum dots are strongly affected by the degree of anisotropy, the dot size, the external magnetic field and the polarized direction of the incident electromagnetic wave. The result also shows that the size effect of anisotropy quantum dots on the optical absorptions is different from that of isotropic quantum dots.