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Brazilian Journal of Physics

Laser effects in semiconductor heterostructures within an extended dressed-atom approach

Humberto Siqueira Brandi, Andrea Latge, L. E. Oliveira

Published Jun 1, 2002• 5 citations
Publisher

Abstract

We extend the dressed-atom approach to treat the interaction of a laser field with a semiconductor system. The semiconductor is modeled via a simple Kane band-structure scheme and the interaction with the laser field is incorporate through the renormalization of the semiconductor energy gap and conduction/valence effective masses. Far from resonances, such one-body approach allows the study of the effects of laser fields on a variety of optoelectronic phenomena in semiconductor systems for which the effective-mass approximation provides a good physical description. We calculate the effects originated by the laser-dressing on the donor and exciton peack energies in quantum-well heterostructures, and show that they may be quite considerable and observable.

DOI
10.1590/s0103-97332002000200004

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We extend the dressed-atom approach to treat the interaction of a laser field with a semiconductor system. The semiconductor is modeled via a simple Kane band-structure scheme and the interaction with the laser field is incorporate through the renormalization of the semiconductor energy gap and conduction/valence effective masses. Far from resonances, such one-body approach allows the study of the effects of laser fields on a variety of optoelectronic phenomena in semiconductor systems for which the effective-mass approximation provides a good physical description. We calculate the effects originated by the laser-dressing on the donor and exciton peack energies in quantum-well heterostructures, and show that they may be quite considerable and observable.
In the present work, we perform a comparison of theoretical approaches involving Coulomb-interaction corrections within the Hartree–Fock approximation and the renormalized dressed-band scheme in the large detuning limit. We focus on the well-studied optical Stark shift of the exciton peak in bulk and GaAs–(Ga,Al)As semiconductor quantum wells. It is argued that the Hartree–Fock scheme has severe limitations concerning its application to real experimental situations, even in the simplest laser field–perturbative regime. It is also shown, through a comparison between experiments and a three-band Kane-dressed GaAs–(Ga,Al)As quantum well calculation, that a proper treatment of band structure and confinement effects due to the quantum well is of fundamental importance in a theoretical understanding of Stark shift experimental measurements, and that the renormalized dressed-band approach is a very convenient tool to treat, in the large detuning limit, processes involving the laser–semiconductor interaction in low-dimensional heterostructures.
We study the effects of a laser on the splitting of the lowest electronic states of a pair of two-dimensional (2D) GaAs-(Ga,Al)As coupled quantum dots, in the presence of a homogeneous external magnetic field. The interaction of light with the 2D quantum dots is treated within a dressed-band approach in which a two-band scheme is used to model the GaAs bulk semiconductor whereas the interaction with the laser field is treated through the renormalization of the semiconductor energy gap and conduction/valence effective masses. This approach is valid far from resonances and has been successfully used to treat other confined semiconductor heterostructures. We focus our attention on the splitting of the lowest singlet and triplet states and on their double occupation probability. We analyze the exchange coupling (J) in the effective Heisenberg model as a function of the laser field and its detuning, as well as of the magnetic field (B) within the Heitler-London approximation of molecular physics. We find that due to the electronic confinement the laser may play a role similar to the external magnetic field in the qualitative behavior of the exchange parameter J. Furthermore, the presence of the laser may assure both the sizable exchange coupling and the vanishingly small double occupation probability required for efficient quantum computer operations, even in the absence of an external magnetic field. Besides being of fundamental interest, these features may be used as an efficient two-qubit gate control.
A single GaAs/GaAsSb/GaAs quantum well having a valence band profile was described in this study using the V-shaped potential. An external static electric field (E-field) and a high-frequency intense laser irradiation were utilized to examine the first-order linear and third-order nonlinear optical properties. The theoretical modeling was achieved using a compact-density matrix and iterative process in a two-level system framework. The position-dependent effective mass approximation was applied to solve the Schrödinger equation and evaluating the energy states and their corresponding wavefunctions for the two lowest bound states within the quantum well’s valence band with regards to a heavy hole. We extended the investigation of the single V-shaped potential by incorporating the effects of the E-field, the well’s half-width, and antimony (Sb) content on optical characteristics. The study on the combined effects of the intense laser field and the mentioned factors revealed that the peak positions and amplitudes of the linear and nonlinear optical characteristics were strongly influenced by variation in the laser strength. Specifically, we observed, that under higher laser dressing parameters, the application of the electric field played an important role. This occurred when the impact of the well’s half-width and Sb content became prominent at values of lower laser-dressing parameters. It is significant to note that our findings will have potential real-world applications in photonics, optoelectronics, and related areas.
We theoretically study quantum-optical properties of one- and two-dimensional semiconductor nanostructures, where the electronic band structure is described by a two-band tight-binding model. Since the focus of our work is on analyzing quantum-optical effects of systems with a continuous band structure, many-body processes like electron-electron and electron-phonon interactions are neglected. The quantum-optical interband excitations are fully incorporated and described by a Jaynes-Cummings-type model. The exciting quantum light can be a state with arbitrary photon statistics. For simplicity, the results discussed here are limited to single-photon and two-photon Fock states. Our analytical approach is based on the eigenvalue problem and can be utilized to obtain explicit expressions for the steady-state properties of the system. Numerical simulations are based on solutions of the equations of motion and allow for a more precise analysis of the dynamics and nonresonant excitations of the system. We demonstrate that during the interaction process, a collective excitation of the conduction band is formed. For nonresonant excitations, this collective dynamics results in interesting steady states, in which the resonantly addressed eigenstates are occupied. The presented model provides a framework for microscopic simulations of quantum-optically excited extended semiconductor nanostructures.
Ultrahigh refractive indices without absorption1 and reduced absorption2 have been predicted and observed for atomic systems. We apply these considerations to semiconductor structures with complicated energy bands and strong phonon interactions. Bulk semiconductors offer limited flexibility, while quantum well structures offer the ability to tailor the dielectric response through control of the conduction subband states and the separation between heavy and light hole states. To observe high indices without absorption, Raman pumping from the ground conduction state should result in a coherent state at the bottom of heavy hole and light hole bands. The high index then exists between the first excited conduction subband and the range of separation between the holes. Since this scheme gives a large refractive index change, we can observe results by photo- or electro-reflectance. However, interactions between subbands and the strong phonon scattering may inhibit the coherent state. To observe reduced absorption without changes of refractive index, two conduction subbands should be coupled by a CO2 laser. Reduced absorption should be observable from the bottom of the hole state to the range between the two dressed states of the conduction subbands. This approach is simpler than coherent pumping, and modulated transmission offers the best chance for observation.
Dressed states of an atom in a classical laser field of frequency close to an atomic transition frequency are obtained, which are a considerable improvement over the usual two-level rotating-wave approximation. These wave functions are used in a calculation of the cross section for elastic electron-atom scattering in a laser field. Numerical calculations for electron-hydrogen scattering demonstrate that scattering involving absorption or emission of photons can be strongly influenced by coupling to the nonresonant states, regardless of the field strength. Inclusion of the counter-rotating terms in the calculation is shown to be essential for extending the applicability of the theory to scattering in nonresonant fields, where a first-order perturbation-theoretic treatment of the atom-field interaction would be expected to be quite justified.