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Superlattices and Microstructures

Excitonic effects on the two-photon transition rate in quantum wells

Alfredo Pasquarello, A. Quattropani

Published Jan 1, 1991• 8 citations
Publisher
DOI
10.1016/0749-6036(91)90273-t

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NATO ASI series. Series B : Physics•Jan 1, 1995•2 citations
We calculate two-photon transition rates of exciton states in GaAs quantum wells for varying well widths. The exciton states are described in the effective-mass approximation and are obtained variationally assuming wave functions that are separable in the r coordinates. The two-photon transition rate is calculated taking explicitly into account the sum over all the intermediate states, using a variational procedure. We calculate transition rates of excitons of s symmetry as well as of p symmetry, which are allowed for polarization of the radiation beam along the growth axis and in the layer planes, respectively. From our calculations we find that transitions to s-type excitons have sufficiently large rates to be observable, whereas transitions to p-type excitons seem to have rates too small to be observable; these results are in agreement with experiment. The transition rate of s excitons increases significantly for decreasing well width, whereas the transition rate of p excitons is found to be almost independent of the well width. Moreover, our model allows us to calculate transition rates for photons of different energy. We have shown the resonant enhancement of the transition rate of s-type excitons when one of the photon energies approaches the energy of an intermediate state.
We theoretically propose coherent control of the nonradiative decay of excitons in infrared-coupled asymmetric quantum wells. This is done by (i) inducing intraband multiphoton quantum coherences between exciton states associated with three conduction subbands (E1, E2, and E3), and (ii) adjusting the contributions of these coherences using competition between one- and two-photon coupling of these states. The intraband excitonic transitions are caused by one or two infrared laser beams resonant with the intersubband transitions between E1, E2, and E3. We show that by tuning these beams to allow coupling of the E1--HH1, E2--HH1, and E3--HH1 excitons with different configurations one can coherently manipulate the quantum efficiency of exciton emission. In an asymmetric double ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ quantum well this could lead to nearly complete destruction of the emission spectrum, or to the immunity of excitons from enhancement of their nonradiative decay rates via the intraband transitions. We discuss these effects in terms of coherent population trapping of excitons and show how field-coherent destruction effects influence the multiphoton excitonic coherences.
Abstract The three‐level optical Stark effect is due to a dynamical coupling of two excited levels. The coupling is achieved by a high intensity CO2 laser. Results of experiments on multiple quantum wells (MQW) and excitons in Cu2O are presented. In the MQW the coupling between the two lowest conduction band sublevels leads to characteristic changes in the one‐photon spectra of the lowest exciton transitions. In Cu2O the CO2 laser pulse couples 1 S and 2 P excitons. In this case the one‐photon spectrum of the 2 P exciton (dipole allowed) and also of the 1 S spectrum (quadrupole allowed) show drastic changes. Most of the experimental results can be explained quantitatively in terms of a nonlinear susceptibility.