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Annual Meeting Optical Society of America

Nonlinear optics near the band edge in multiple-quantum-well structures

J.P. Heritage, A. Von Lehmen, Jane E. Zucker, D. S. Chemla

Published Jan 1, 1987• 0 citations
Publisher

Abstract

We have investigated the optical properties of GaAs quantum wells in the spectral region below the band edge using a two-wavelength picosecond optical pulse technique. Optical pumping in the transparent region produces modulation of the excitonic absorption which is probed with an optical pulse tuned to the excitonic absorption. We identify two distinct physical mechanisms that lead to the absorption modulation. First, a real population of carriers is produced by phonon-assisted absorption. These carriers bleach the excitonic transition by the action of the exclusion principle. The second mechanism involves virtual populations induced by the off-resonant optical excitation.

DOI
10.1364/oam.1987.mr2

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We have identified several physical processes by which below-gap photons can modify excitonic absorption in GaAs quantum wells.1-3 These different phenomena can be distinguished by their temporal response times, which range from tens of nanoseconds to less than one picosecond. Our experiments also show that the relative contributions of these mechanisms to below-gap optical modulation of exciton absorption depend strongly on temperature as well as on the wavelength of the below-gap modulating beam. The size of the modulation indicates that a new class of absorptive and dispersive devices for optical signal processing is possible in which signals propagate in the transparent region of the crystal and experience negligible attenuation. In order to successfully interpret and calculate the magnitude of these exciting new effects, we show that it is necessary to take into account the special properties of two-dimensional excitons in semiconductor quantum wells.
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Excitonic resonances in semiconductors are very sensitive to applied optical fields. We present here a comparison between the effects of resonant and non resonant optical excitation, creating respectively real or virtual excitons. A blue shift of the exciton line and a reduction of the oscillator strength are simultaneously observed in multiple-quantum-well-structures for the two following cases : presence of a population of real excitons in very thin wells (two-dimensional systems), or presence of an intense offresonance non absorbed optical field (virtual excitons) independently of the well thickness. The latter configuration (optical Stark effect) has been studied as a function of pump intensity, determining in particular how the saturation of the blue shift is related to the exciton bleaching.
I report on the effect of a moderate excess electron population on nonlinearities in modulation-doped CdTe quantum wells. I show that the electron population does not qualitatively affect the nature of correlations between excitons. In this respect, I bring strong evidence of the existence of unbound and bound (stable) two-exciton states in the presence of electrons and charged excitons (trions). In time-resolved pump and probe experiment, they lead to the observation of electromagnetically induced transparency and optical Stark shift of the exciton resonance. Rabi flopping of excitons within a sea of electrons is also clearly evidenced through ac Stark splitting and gain without inversion. The quantum coherence is more robust to electron induced dephasing than what would have been expected. I demonstrate third and higher-order exciton correlations in the presence of electrons, which manifest, for increasing exciton densities, through the blue-shift of counter-polarized exciton resonance, the red-shift of the biexciton resonance and the exciton to biexciton crossover. I also evidence correlated behavior of excitons and trions under excitation which manifests itself by crossed trion-exciton effects. I observe a wealth of phenomena encompassing bleaching, crossed bleaching, induced-absorption and energy shifts of the resonances. Significant differences are found between the nonlinear optical effects induced by an exciton and a trion population. Electron scattering with electron and exciton is shown to strongly broaden the high energy tail of exciton and trion resonance lineshapes. Variations of electron density result in a clear modification of both resonance lineshapes. The dynamics of the formation of trions is also studied. I propose a two-channel mechanism for their formation; they are formed through bi- and tri-molecular processes. This implies that both negatively and positively charged excitons coexist in a quantum well, even in the absence of excess carriers. The model is applied to a time-resolved photoluminescence experiment performed on a very high quality InGaAs quantum well sample, in which the photoluminescence contributions at the energy of the trion, exciton and at the band edge can be clearly separated and traced over a broad range of times and densities. The unresolved discrepancy between the theoretical and experimental radiative decay time of the exciton in a doped semiconductor is explained.