O. A. C. Nunes

Unexpected transition from single to double quantum well potential induced by intense laser fields in a semiconductor quantum well

When an electronic system is irradiated by an intense laser field, the potential “seen” by electrons is modified, which affects significantly the bound-state energy levels, a feature that has been observed in transition energy experiments. For lasers for which the dipole approximation applies, a nonperturbative approach based upon the Kramers–Henneberger translation transformation, followed by Floquet series expansions, yields, for sufficiently high frequencies, the so-called “laser-dressed” potential, which is taken for composing a time-independent Schrodinger equation whose solutions are the desired quasistationary states. This approach, developed originally for atoms, has been verified to be useful also for carriers in semiconductor nanostructures under intense laser fields. In quantum wells, analytical expressions for the dressed potential have been proposed in literature for a nonresonant, intense laser field polarized perpendicularly to the interfaces. By noting that they apply only for α0≤L/2, wher...

Intense field effects on hydrogen impurities in quantum dots

Calculations of the binding energy of an on-center donor hydrogenic impurity in a quasizero-dimensional quantum-well system [quantum dot (QD)] placed in an intense, high-frequency laser field are presented. A nonperturbative theory and a variational approach are used as the framework for this calculation. The effect of the intense laser field is to “dress” the impurity potential making it dependent upon the laser field amplitude. A rapid decrease of the binding energy, for different values of the QD radius and for both infinite and finite potential barriers, with increasing field intensity is predicted. An application is made for a spherical QD made of GaAs/Ga1−xAlxAs heterostructures.

Parametric distortion of the optical absorption edge of a magnetic semiconductor by a strong laser field

The influence of a strong laser field on the optical absorption edge of a direct‐gap magnetic semiconductor is considered. It is shown that as the strong laser intensity increases the absorption coefficient is modified so as to give rise to an absorption tail below the free‐field forbidden gap. An application is made for the case of the EuO.

LO-phonon instability due to indirect interband absorption of a laser field in semiconductors

Abstract We investigate here the possibility of excitation and amplification of longitudinal optical (LO) lattice vibrations by electrons due to interband absorption of a laser field in semiconductors. We show that under certain conditions the phonon excitation rate may become greater than the rate of relaxation and the LO-phonon system may reach instability.

Electric field effects on electron mobility in n-AlGaAs/GaAs/AlGaAs single asymmetric quantum wells

We calculated low-temperature electron mobility in n-AlGaAs/GaAs/AlGaAs single asymmetric quantum wells in the presence of a uniform electric field directed perpendicularly to the interfaces. The quantum well asymmetry is due to the doping profile (one-side modulation doping). Following a variational scheme, we solved both Schrodinger and Poisson equations simultaneously and the results were used to calculate the low-temperature (quasielastic) scattering rates. Only relevant scattering mechanisms were taken into account, namely ionized impurity, interface roughness, alloy disorder, and acoustic phonons (deformation potential and piezoelectric coupling). Our results show that both interface roughness and alloy disorder scattering rates are strongly dependent upon the electric field strength. We also show that there are interesting changes in the dominance of the mobility among different scattering processes, which leads to the formation of a maximum in the mobility dependence on the electric field strength...

Amplification of optical phonons in the multiphoton absorption in semiconductors

Abstract The optical phonon scattering by electrons in the presence of a strong radiation field is discussed. It is shown that in the strong-field limit the optical phonon population may become unstable.

LO-phonon instability in semiconductors in the presence of two laser radiation fields

Abstract The rate of change of the LO-phonon population due to scattering by free carriers in the presence of two laser fields is calculated. It is found that LO-phonon propagating parallel to the direction of polarization of the radiation fields may become unstable over a relatively narrow range of phonon wavenumbers.

The influence of intense electromagnetic field on the energy-gap between spin branches in ferromagnetic semiconductors

Abstract The influence of an intense laser field on the intraband motion of a charge carrier in the localized moment potential of a ferromagnetic semiconductor is discussed. The quasienergy spectrum of a free carrier is calculated and it is shown that for very large fields the energy-gap between the spin branches decreases with increasing laser amplitude.

Dichotomy of the exciton wave function in semiconductors under intense laser fields

We study the behavior of excitons in a semiconductor irradiated by a monochromatic, linearly polarized, intense laser field. By taking the finiteness of the hole effective mass into account and including the radiation field in a semiclassical manner, we solved the two-body quantum problem in the framework of a nonperturbative theory based upon the Kramers-Henneberger translation transformation for the Schrodinger equation. In the Kramers frame, the rapidly oscillating potential is expanded in a Fourier-Floquet series and, for laser frequencies high enough, only the zeroth-order term survives, the so-called “laser-dressed” potential. By applying the Ehlotzky’s approximation, this potential simplifies to a two-center potential that resembles that for the electronic motion in the H2+ molecule ion. The binding energy for an exciton in bulk GaAs under a nonresonant laser field is then computed by following a variational scheme we recently adapted from the linear combination of atomic orbitals-molecular orbital...

Amplification of acoustic lattice vibrations by electrons in semiconductors under intense laser radiation

The rate of change in the acoustic phonon population due to scattering by conduction electrons in the presence of an intense laser is obtained from which the phonon generation rate is calculated. It is found that under certain conditions the generation rate may exceed the phonon relaxation rate and the acoustic phonon system may reach instability. An application is made for the case of an InSb sample.