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Physical review. B, Condensed matter

Coherent transient in photoluminescence of excitonic molecules in GaAs quantum wells

A. L. Ivanov, Hailin Wang, Jagdeep Shah, Theo C. Damen, Leonid V. Keldysh, H. Haug, Loren N. Pfeiffer

Published Aug 15, 1997• 23 citations
Publisher

Abstract

Time-resolved photoluminescence (PL) of excitonic molecules in GaAs quantum wells (QW's) reveals an initial transient characterized by a finite rise and an extremely fast nonexponential decay [H. Wang, J. Shah, and T. C. Damen et al., Solid State Commun. 98, 807 (1996)]. The transient is attributed to coherent quantum evolution towards the molecule ground state of two optically correlated excitons, ${\ensuremath{\sigma}}^{+}$ and ${\ensuremath{\sigma}}^{\ensuremath{-}},$ which undergo the Coulombic attraction. In the present paper, we develop a theory of the transient PL and fit the experimental data. The radiative decay of a quasi-two-dimensional (2D) excitonic molecule is analyzed with the giant oscillator strength model adapted to QW's and with the bipolariton model. Within the 2D bipolariton model, the main ``hidden'' channel of the radiative decay of an excitonic molecule is the resonant dissociation into two outgoing interface (surface) polaritons rather than the observable decay into the bulk radiative modes. We conclude that quasi-2D molecules dissociate already within the initial transient of PL, while the following exponential decay of the molecule-mediated PL is due to the escape of secondary interface polaritons into the bulk modes. According to the 2D bipolariton model, the sequence ``two incoming $\mathrm{photons}\ensuremath{\rightarrow}\mathrm{two}$ virtual $\mathrm{excitons}\ensuremath{\rightarrow}2\mathrm{D}$ $\mathrm{molecule}\ensuremath{\rightarrow}\mathrm{two}$ outgoing interface polaritons'' is a completely coherent process. The fit of the time-resolved molecule-mediated PL provides us with numerical estimates of the exciton-photon coupling in GaAs QW's.

DOI
10.1103/physrevb.56.3941

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Time-resolved photoluminescence (PL) of excitonic molecules in GaAs quantum wells (QW's) reveals an initial transient characterized by a finite rise and an extremely fast nonexponential decay [H. Wang, J. Shah, and T. C. Damen et al., Solid State Commun. 98, 807 (1996)]. The transient is attributed to coherent quantum evolution towards the molecule ground state of two optically correlated excitons, \ensuremathσ+{\ensuremath{\sigma}}^{+}\ensuremathσ+ and \ensuremathσ\ensuremath−,{\ensuremath{\sigma}}^{\ensuremath{-}},\ensuremathσ\ensuremath−, which undergo the Coulombic attraction. In the present paper, we develop a theory of the transient PL and fit the experimental data. The radiative decay of a quasi-two-dimensional (2D) excitonic molecule is analyzed with the giant oscillator strength model adapted to QW's and with the bipolariton model. Within the 2D bipolariton model, the main hidden'' channel of the radiative decay of an excitonic molecule is the resonant dissociation into two outgoing interface (surface) polaritons rather than the observable decay into the bulk radiative modes. We conclude that quasi-2D molecules dissociate already within the initial transient of PL, while the following exponential decay of the molecule-mediated PL is due to the escape of secondary interface polaritons into the bulk modes. According to the 2D bipolariton model, the sequence two incoming photons\ensuremath→two\mathrm{photons}\ensuremath{\rightarrow}\mathrm{two}photons\ensuremath→two virtual excitons\ensuremath→2D\mathrm{excitons}\ensuremath{\rightarrow}2\mathrm{D}excitons\ensuremath→2D molecule\ensuremath→two\mathrm{molecule}\ensuremath{\rightarrow}\mathrm{two}molecule\ensuremath→two outgoing interface polaritons'' is a completely coherent process. The fit of the time-resolved molecule-mediated PL provides us with numerical estimates of the exciton-photon coupling in GaAs QW's.
Summary form only given. In this paper we present time-resolved photoluminescence (PL) studies of biexcitons in GaAs quantum wells (QW) with femtosecond resolution. Using polarization selection rules for biexcitons, we separate the biexciton PL from the exciton PL while still retaining adequate time resolution. A surprising initial transient characterized by a finite rise and an extremely fast decay is revealed in the biexciton PL. We show that this initial transient stems from coherent quantum evolution of relative momentum of two excitons in the molecule, in sharp contrast to the initial dynamics of exciton radiative recombination. The initial transient also reflects a type of quantum beat that differs from the usual quantum beats in transient optical measurements.
Resonant photoexcitation of a predominantly homogeneously broadened exciton line in a GaAs quantum well results in a nonexponential time-resolved photoluminescence decay transient due to the decrease in the exciton-exciton scattering contribution to the homogeneous linewidth, ${\mathrm{\ensuremath{\Gamma}}}_{\mathit{h}}$, as the exciton population decays. This result confirms the theoretically predicted relationship between the exciton radiative recombination lifetime and the homogenous linewidth (and coherence area) of the exciton state.
The time evolution and kinetics of photoluminescence (PL) spectra of interwell excitons in double GaAs/AlGaAs quantum wells ( n-i-n structures) have been investigated under the pulse resonance excitation of intrawell 1sHH excitons using a pulsed tunable laser. It is found that the collective exciton phase arises with a time delay relative to the exciting pulse (several nanoseconds), which is due to density and temperature relaxation to the equilibrium values. The origination of the collective phase of interwell excitons is accompanied by a strong narrowing of the corresponding photoluminescence line (the line width is about 1.1 meV), a superlinear rise in its intensity, a long time in the change of the degree of circular polarization, a displacement of the PL spectrum toward lower energies (about 1.5 meV) in accordance with the filling of the lowest state with the exciton Bose condensate, and a significant increase in the radiative decay rate of the condensed phase. The collective exciton phase arises at temperatures T <6 K and interwell exciton densities n =3×10 10 cm −2 . Coherent properties of the collective phase of interwell excitons and experimental manifestations of this coherence are discussed.
Secondary emission from GaAs quantum wells following resonant pulsed excitation is studied in the time domain. Selective excitation of heavy- and light-hole excitons is accomplished in a two-pulse excitation experiment which preserves sub-ps time resolution. After the initial rise the emission decays exponentially at a fast rate and then with another much slower exponential decay at longer times. The variations in the initial decay times with excitation density and sample temperature are similar to the variation in dephasing rates measured in four-wave mixing experiments. The decay in the visibility of heavy- and light-hole exciton beating is consistent with the decay rates measured for both species separately.
The dynamics of biexciton radiative decay in semiconductor quantum wells is treated within the excitonic-molecule mode including super-radiance, exciton recoil, and nonzero photon momentum. For parameters describing GaAs/Alx{\mathrm{Al}}_{\mathit{x}}Alx​${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum wells, we find that the rate for the decay of the excitonic molecule (in the ground state of the relative motion between excitons) into an exciton and a photon is less than the decay rate characteristic of radiative free one-excitons.
Rapid superradiant decay of coherently excited excitons in thin quantum wells (QWs) on a time scale of approximately 10 ps has been theoretically predicted by several groups [1], however, the experimental observation of this ultrafast radiative recombination is still the subject of controversial discussions. A recent theoretical paper [2] concludes, that this radiative decay rate should be strongly enhanced in multiple QW (MQW) Bragg structures due to radiative coupling of excitons excited in different wells via the exchange of transverse photons.