Polarization dependence of quantum beats in quantum wells: Raman coherence and two‐exciton correlations

Abstract

Time-integrated differential detection techniques are used to study quantum beats involving the light-hole and heavy-hole excitons in an optically thin InGaAs multiple quantum well. The contributions of the exciton-exciton correlations are separated from those of the intervalence-band Raman coherence and the interband (population) coherence by monitoring the signal in the probe direction and by taking advantage of the polarization selection rules for the excitonic transitions. Quantum beats are observed for pump and probe pulses with the same circular polarization, providing evidence for two-exciton correlations. A theoretical analysis based on the dynamics-controlled truncation formalism suggests that the beats for the same linear polarization arise from Raman, population, and two-exciton contributions. 1 Introduction Interest in semiconductor analogues to quantum coherent processes in three-level systems has increased recently, at least in part, because of the prediction and observation of lasing without inversion, electromagnetically induced transparency and related effects in molecular and atomic systems [1][2][3][4]. One of the most direct manifestations of quantum coherence in a three-level system is the observation of quantum beats. In semiconductors, such beats have been observed in quasi-two-dimensional multiple quantum wells (MQWs) between light-hole (lh) and heavy-hole (hh) excitons [5][6][7][8][9][10][11][12][13]. In order for quantum beats (as opposed to polarization interference) to be produced, the hh and lh excitons must be coupled in some way. When both hh and lh excitons of the same spin are excited, this coupling can be provided by the common conduction band energy level shared by the hh and lh oscillators, as illustrated phenomenologically in Fig. 1. Recent experimental and theoretical work, however, has demonstrated that the lh-hh excitonic coupling necessary for lh-hh quantum beating also can be produced by the manybody Coulomb interactions between excitons [14-22] (schematically represented by the coupling arrow in Fig. 1). This hh-lh coupling, in turn, can produce two kinds of coherence: (i) the radiative coherence between the conduction and valence bands that is associated with the interband density matrix elements ρ c-hh and ρ c-lh and (ii) the non-radiative coherence between the hh and lh valence bands that is associated with the intervalence-band matrix element ρ hh-lh . The latter has also been referred to as Raman coherence [23,24], and its existence in MQWs has been established by the observation of coherently coupled hh