M. A. Amato

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...

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...

Effect of a terahertz laser field on the electron-DOS in a GaAs/AlGaAs cylindrical quantum wire: finite well model

The influence of a linearly polarized terahertz laser field on the density of states (DOS) for carriers confined in a cylindrical semiconductor quantum wire is investigated here within a nonperturbative scheme, based on the Greens function approach introduced by Xu (1997 Semicond. Sci. Technol. 12 1559). In our finite potential well model, the functional dependence of the DOS on energy is significantly modified by a monochromatic, nonresonant, intense laser radiation polarized along the wire axis. Apart from a uniform blueshift with respect to the laser-free DOS, which is proportional to the laser intensity and to the inverse of the square of the frequency, a remarkable reduction of the DOS, followed by pronounced Franz–Keldysh-like oscillations above the bottom of each 1D sub-band, is obtained. The period of these oscillations is seen to depend more on the laser frequency than on the intensity. Interestingly, with an increase of the laser intensity and/or a reduction of the laser frequency, the DOS profile changes from the usual shape found in 1D electronic systems to a series of discrete peaks similar to that found in quasi-0D systems, such as quantum dots.

Dielectric screening, exciton binding energy and the insulator-metal transition in molecular hydrogen

Abstract Using an appropriate model for dielectric screening in molecular hydrogen we calculate the static dielectric constant ϵ()) in the proximity of the insulator-metal transition. The dielectric constant ϵ(0) is constructed in terms of the energy gap, the exciton binding energy and the plasmon frequency. Considering the energy gap from the band-structure calculation of Friedli and Ashcroft [1] and making an estimate of the lower exciton binding energy in the instability region, ϵ(0) is shown to diverge at a critical density given by r s ∗ = 1.57 a.u.

Increase of the positronium lifetime under high-frequency, intense laser fields

In this work, a nonlinear variational linear combination of atomic orbitals scheme we introduced recently (Lima et al 2007 Phys. Rev. B 75 073201) for studying hydrogenic systems in semiconductors is adapted for the computation of the effect of a high-frequency, intense laser field on the positronium (Ps) quantum states. Our nonperturbative approach, based upon the Kramers–Henneberger translation transformation, allows us to show that, for sufficiently high intensities, the radiation field induces a wavefunction dichotomy, which causes a reduction on the positronium decay probability (i.e., enhances its lifetime). Our results point out that an enhancement of up to four orders of magnitude in the Ps lifetime can be attained with the current laser technology. Our careful quantitative analysis also corrects some mistakes in the approximate formulae proposed by Ehlotzky in a previous work (Ehlotzky 1988 Phys. Lett. A 126 524).