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physica status solidi (a)

Coherence versus Coulomb Scattering in Resonantly Excited Quantum Wells

Cristiano Ciuti, Carlo Piermarocchi, Vincenzo Savona, Peter E. Selbmann, P. Schwendimann, A. Quattropani

Published Mar 1, 2000• 0 citations
Publisher

Abstract

The role of Coulomb scattering is studied in the coherent transients of strongly driven exciton resonances in semiconductor quantum wells. Coulomb scattering is treated within the second Born approximation and the light-induced dressing of the electron–hole gas is consistently included. The light dressing is found to decrease the Coulomb collision efficiency and therefore the field-driven dynamics is close to the Hartree-Fock case. Indeed, Coulomb relaxation and dephasing are not strong enough to prevent a dynamical Stark splitting in the pump–probe absorption spectra.

DOI
10.1002/1521-396x(200003)178:1<417::aid-pssa417>3.0.co;2-q

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The role of Coulomb scattering is studied in the coherent transients of strongly driven exciton resonances in semiconductor quantum wells. Coulomb scattering is treated within the second Born approximation and the light-induced dressing of the electron–hole gas is consistently included. The light dressing is found to decrease the Coulomb collision efficiency and therefore the field-driven dynamics is close to the Hartree-Fock case. Indeed, Coulomb relaxation and dephasing are not strong enough to prevent a dynamical Stark splitting in the pump–probe absorption spectra.
The nonequilibrium dynamics of a two-dimensional electron-hole gas is studied in the regime of strong and resonant pumping of the exciton resonance. The Coulomb collision rates are consistently determined by taking into account the light-induced coherence of the two-band system that leads to a dressing of the carrier spectral functions. The light dressing dramatically reduces the Coulomb scattering efficiency. Results are presented for Rabi oscillations in the time domain and dynamical Stark splitting in the pump-probe absorption spectra.
Optical interband excitonic absorption of semiconductor quantum wells (QW’s) driven by a coherent pump field is investigated on the basis of semiconductor Bloch equations. The pump field has a photon energy close to the intersubband spacing between the first two conduction subbands in the QW’s. An external weak optical field probes the interband transition. The excitonic effects and pump-induced population redistribution within the conduction subbands in the QW system are included. When the density of the electron–hole pairs in the QW structure is low, the pump field induces an Autler–Townes splitting of the exciton absorption spectrum. The split size and the peak positions of the absorption doublet depend not only on the pump frequency and intensity but also on the carrier density. As the density of the electron–hole pairs is increased, the split contrast (the ratio between the maximum and the minimum values) is decreased, because the exciton effect is suppressed at higher densities owing to the many-body screening.
Ultrafast pump-probe measurements evidence the dynamical Stark splitting in the exciton absorption spectrum of a single semiconductor quantum well. The intensity of the Stark sidebands in the spectrum is comparable to the linear absorption and their separation scales as the pump field amplitude. In the time domain the Rabi oscillations are observed. The semiconductor Bloch equations solved in the Hartree-Fock approximation demonstrate that the coherent exciton-light coupling dominates over the many-body dephasing.
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.
Differential absorption spectra of high quality InGaAs quantum wells were measured by means of pump–probe spectroscopy as a function of pump–probe delay, for different polarizations of the pulses, and different spectral detunings of the pump beam with respect to the 1s heavy-hole exciton. A clear redshift of the heavy-hole resonance is observed for large detunings and opposite circularly polarized pulses. Microscopic calculations that include not only the Hartree-Fock contribution but also higher-order Coulomb correlations reproduce the experimental findings very well. It is shown that the redshift is caused by Coulomb memory effects and is further enhanced due to the presence of the bound biexciton.
Summary form only given. In the excitonic ac Stark shift in quantum wells the excitonic absorption line is blueshifted by shining on the sample an intense laser pulse, whose frequency is lower than the exciton transition frequency. The effect is bigger when the laser spectrum approaches the resonance with the excitonic absorption. But if the laser and excitonic absorption spectra sensibly overlap, a great amount of electronic population is excited and consequently the Coulomb nonlinearities due to the interaction between excited carries or excitons dramatically increase. One prominent effect under these conditions is the excitation- induced dephasing which can destroy the coherence of the electronic system and therefore the coupling between excitons and light, which is inherently coherent. Some evidence for the resonant Stark effect was already reported, but the direct observation of the spectral splitting was still missing. This observation demonstrates that the coherent exciton light coupling in semiconductors can be strong enough to survive the fast dephasing, and that therefore an efficient ultrafast manipulation of the exciton absorption in the coherent transient is possible. In order to magnify the ac Stark effect and its visibility a special sample was designed: a single In/sub 0.04/Ga/sub 0.96/As quantum well is grown on the top of an AlAs/AlGaAs distributed Bragg reflector; two /spl lambda//2 spacers separate the quantum well from the mirror and from the air. In this configuration the light field incident on the sample and the one reflected by the mirror constructively interfere at the quantum well position, giving a high effective absorption.