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Physical Review Letters

Two-photon absorption in GaAs/AlGaAs multiple quantum wells

M. Nithisoontorn, K. Unterrainer, Steffen Michaelis, Nobuhiko Sawaki, E. Gornik, H. Kano

Published Jun 26, 1989• 35 citations
Publisher

Abstract

Photoconductivity measurements revealing two-photon absorption in GaAs/AlGaAs multiple quantum wells are presented. It is shown that both the forbidden 1S and the allowed 2P excitons can be observed. The energy separation between 1S and 2P excitons is determined directly from the absorption spectra to high accuracy. The data give evidence for two different absorption mechanisms: One is the two-photon absorption and the other one is the absorption of the second-harmonic wave which is generated inside the sample by nonlinear optical processes. By means of this additional absorption mechanism, the observation of the 1S excitions in two-phonon absorption spectra can be explained qualitatively.

DOI
10.1103/physrevlett.62.3078

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Photoconductivity measurements revealing two-photon absorption in GaAs/AlGaAs multiple quantum wells are presented. It is shown that both the forbidden 1S and the allowed 2P excitons can be observed. The energy separation between 1S and 2P excitons is determined directly from the absorption spectra to high accuracy. The data give evidence for two different absorption mechanisms: One is the two-photon absorption and the other one is the absorption of the second-harmonic wave which is generated inside the sample by nonlinear optical processes. By means of this additional absorption mechanism, the observation of the 1S excitions in two-phonon absorption spectra can be explained qualitatively.
Two-photon absorption spectroscopy was performed in GaAs quantum wells with photon energy \ensuremath{\Elzxh}\ensuremath{\omega} near half the band gap Eg{\mathrm{E}}_{\mathrm{g}}Eg​. For the case of incident light polarized perpendicular to the confinement direction \ensuremath{\epsilon}^\ensuremath{\perp}z^, the onset of two-photon absorption occurs at the 2P state of the lowest-energy heavy-hole exciton and the absorption increases linearly with 2\ensuremath{\Elzxh}\ensuremath{\omega}-Eg{\mathrm{E}}_{\mathrm{g}}Eg​. For \ensuremath{\epsilon}^\ensuremath{\parallel}z^, the two-photon spectrum is stepped with superpositions of sharp 1S exciton features at odd \ensuremath{\Delta}N light-hole--conduction-band transitions. These results are used to accurately determine exciton binding energy, successive subband energy splittings, and band offsets.
An examination of the contribution of excitons, compared to the continuum states, to the optical properties of GaAs/AlGaAs multiple quantum wells illuminates the mechanisms underlying nonlinear absorption and nonlinear refraction and yields information which is useful for optimizing devices which incorporate these materials. We have measured the absorption as a function of intensity at three different wavelengths—above the band edge, on the excitonic resonance, and far below the band edge—to separate the effects. We find that different well thicknesses have different saturation intensities both for excitonic and continuum absorption.
Dipole-forbidden transitions have been investigated by means of coherent control techniques. The 2p heavy-hole excitonic transition of a GaAs/AlGaAs multiple quantum well system was excited by a pair of phase-locked pulses with their energy at half the transition energy. The temporal evolution of the amplitude of coherent control, monitored in the secondary emission at the 1s heavy-hole exciton energy, showed two different contributions: one at the frequency of the optical field, and a second one at twice that frequency, from the interference of the excitonic polarisation. The latter component contains, besides the energy of the dark transition, information about the dynamics of the state.
We have investigated the polarization-dependent two-photon absorption in GaAs/AlGaAs quantum wires. The anisotropic selection rules of the multiphoton absorption process are exploited to study the one-dimensional 2p exciton states and the transitions between quantum wire subbands of different quantum numbers (\ensuremath{\Delta}ny{\mathit{n}}_{\mathit{y}}ny​\ensuremath{\ne}0 selection rule). The deviations from the selection rules derived for the strict one-dimensional case are discussed, and depend on the actual quasi-one-dimensional character of the excitonic wave functions.
We have observed two-wave mixing in semi-insulating AlGaAs/GaAs multiple quantum well structures at wavelengths near the exciton absorption. The photorefractive index changes are caused by the Franz–Keldysh effect on quantum-confined excitons. Photorefractive gains larger than 200 cm−1 are obtained for the first time at wavelengths near 836 nm using stationary fringes and dc applied fields up to 5 kV/cm. The direction of energy transfer between the two beams is determined by the direction of applied electric field.
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.