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Picosecond Nonlinear Optical Studies of Exciton Relaxation in Multiple Quantum Well Structures.

Michael D. Webb

Published Jan 1, 1990• 0 citations
OpenAlex

Abstract

This work presents new results of experimental studies of exciton relaxation in GaAs/AlGaAs multiple quantum well materials, using transient nonlinear laser spectroscopy techniques. In the low excitation regime, effective optical Bloch equations similar to the density matrix formalism of a two-level system describe the response. Solutions of these equations appropriate for three-pulse scattering studies are presented, along with a discussion of new experimental techniques used in this work. Non-ideal conditions present in the molecular beam epitaxy growth process introduce monolayer fluctuations in the quantum well thickness, leading to inhomogeneous broadening and exciton localization at sites of minimum energy. Localized excitons are separated in energy from extended, quasi-delocalized states by the mobility edge. Studies of localized exciton relaxation indicate that these excitons relax on a much slower timescale than delocalized excitons. The temperature dependence of the localized exciton relaxation rate shows that above 15K, the excitons are thermally activated to extended states above the mobility edge. Below 15K the excitons relax by phonon assisted migration between localization sites. For temperatures above 5K, and for exciton energies greater than 1 meV below line center, an extra dephasing mechanism characterizes the exciton relaxation. The inhomogeneous broadening of the exciton leads to the emission of stimulated photon echoes in the experimental geometry used in this work. The time-resolved emission demonstrates the stimulated photon echo nature at low excitation densities. At higher excitation densities, the decay dynamics is more complex. The time-resolved emission shows the presence of a stimulated photon echo and a free polarization decay. This unusual result suggests that both a homogeneous and an inhomogeneous exciton distribution comprise the exciton resonance. Exciton relaxation at low excitation densities represents the interaction of the exciton with the lattice. At higher excitation densities, exciton-exciton and exciton-free carrier interactions modify the relaxation rates. The dependence of the exciton relaxation rate on excitation density shows that many-body effects play an important role in the exciton physics. Free carrier interactions are more efficient at increasing the exciton relaxation rate; both types of interactions are more important in quantum well systems than in bulk materials.

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This work presents new results of experimental studies of exciton relaxation in GaAs/AlGaAs multiple quantum well materials, using transient nonlinear laser spectroscopy techniques. In the low excitation regime, effective optical Bloch equations similar to the density matrix formalism of a two-level system describe the response. Solutions of these equations appropriate for three-pulse scattering studies are presented, along with a discussion of new experimental techniques used in this work. Non-ideal conditions present in the molecular beam epitaxy growth process introduce monolayer fluctuations in the quantum well thickness, leading to inhomogeneous broadening and exciton localization at sites of minimum energy. Localized excitons are separated in energy from extended, quasi-delocalized states by the mobility edge. Studies of localized exciton relaxation indicate that these excitons relax on a much slower timescale than delocalized excitons. The temperature dependence of the localized exciton relaxation rate shows that above 15K, the excitons are thermally activated to extended states above the mobility edge. Below 15K the excitons relax by phonon assisted migration between localization sites. For temperatures above 5K, and for exciton energies greater than 1 meV below line center, an extra dephasing mechanism characterizes the exciton relaxation. The inhomogeneous broadening of the exciton leads to the emission of stimulated photon echoes in the experimental geometry used in this work. The time-resolved emission demonstrates the stimulated photon echo nature at low excitation densities. At higher excitation densities, the decay dynamics is more complex. The time-resolved emission shows the presence of a stimulated photon echo and a free polarization decay. This unusual result suggests that both a homogeneous and an inhomogeneous exciton distribution comprise the exciton resonance. Exciton relaxation at low excitation densities represents the interaction of the exciton with the lattice. At higher excitation densities, exciton-exciton and exciton-free carrier interactions modify the relaxation rates. The dependence of the exciton relaxation rate on excitation density shows that many-body effects play an important role in the exciton physics. Free carrier interactions are more efficient at increasing the exciton relaxation rate; both types of interactions are more important in quantum well systems than in bulk materials.
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Coherent transient optical spectroscopy techniques provide a means of studying the relaxation of electronic excitations in solids. Using these techniques the authors examine the relaxation of excitons in GaAs-AlGaAs multiple quantum wells at low temperature. Localization of excitons due to interface disorder results in inhomogeneous broadening of the absorption spectrum and relaxation due to migration between localization sites. The temperature dependence and energy dependence of the relaxation indicate the presence of complex scattering mechanisms. The dependence of the optical response on the polarization of the incident excitation fields is shown to result in a dramatic change of the dephasing and broadening characteristics of the optical response. This response is interpreted in terms of the presence of both localized and delocalized excitons. Mechanisms for the polarization dependence are discussed.>
Photon echo spectroscopy provides an important tool for studying excitonic relaxation in disor- dered GaAs/AlGaAs quantum wells. In this paper, we review the application of this methodology to the study of excitons localized due to interface fluctuations, which result in inhomogeneous broadening of the spectrum and relaxation due to migration among localization sites. The temperature and energy dependence of the relax- ation rates indicate complex scattering mechanisms. The dependence of the photon echo signal on polarization of the incident pulses shows a dramatic change in the broadening and dephasing characteristics. This response is interpreted in terms of polarization selection of localized versus delocalized excitons. Mechanisms for the polarization dependence are discussed in terms of coupling between opposite spin excitons.
We report high resolution nonlinear laser spectroscopy studies of excitation relaxation associated with the excitonic optical nonlinearity at room temperature and low temperature in GaAs/AlGaAs multiple quantum wells. Using a new method of cw frequency domain four wave mixing, we show that relaxation of the room temperature nonlinear optical response is characterized by free carrier recombination and ambipolar diffusion. At low temperature, the excitation relaxation for localized excitons is dominated by phonon assisted tunneling. In addition, we use four wave mixing methods to eliminate contributions to the excitation line shape from inhomogeneous broadening. The observed line shape is highly asymmetric and shows the presence of spectral diffusion due to the phonon assisted tunneling associated with the excitation relaxation.
Excitons, the bound states of an electron and a hole, in semiconductors are interesting objects and they continue to attract attention of researchers from all over the world. Despite intense research on exciton physics over last three-four decades, many interesting phenomena related to the dynamics and the nonlinear optical properties of excitons in semiconductors and their nanostructures in the coherent and incoherent regimes are still to be explored. This thesis presents our effort to study the nonlinear optical properties as well as the exciton dynamics in coherent and incoherent regime in semiconductor nanostructures including GaAs quantum wells (QWs) and InAs quantum dots (QDs). We have set up several time-resolved and steady-state optical spectroscopic measurement techniques in our laboratory to carry out our study on excitons dynamics in semiconductor QWs and QDs. We work out a clear and detailed theory of the pump-probe signal taking account of all aspect of the measurement, namely, the lock-in detection, spectral selection by the monochromator and time-integration at the slow photodetector. We have used the formalism of optical Bloch equations conventionally used to analyse wave mixing experiments. Our analysis puts the pump-probe technique on the same footing as the wave mixing experiments. We show that pumpprobe experiments can be used to study both coherent and incoherent dynamics. We also present a clear physical picture of the measured signal using the Lorentz harmonic oscillator model. We report the investigation of the coherent exciton dephasing dynamics using pumpprobe reflectivity measurement in GaAs QWs for resonant excitation. Two GaAs/AlGaAs QWs with well thicknesses 8 nm and 17.5 nm are used. We found a strong dependence of the dephasing rate of the excitons with increasing exciton density due to exciton-exciton scattering. We found that the exciton dephasing rate of 8 nm QWs at low excitation density and at low lattice temperature is larger than the corresponding dephasing rate of the 17.5 nm QWs, possibly due to larger effect of inhomogeneous broadening. We also report that the dephasing rates for both QWs are linear with the sample temperature below the 30 K. We report the study of ionization equilibrium between the exciton and the electronhole plasma population in the temperature and pressure (excitation density) phase space. Non-resonant excitation of semiconductor creates a nonequilibrium state of electron-hole plasma. We investigate this ionization equilibrium using steady state photoluminescence and time-resolved excitation correlation PL measurements. We present a clear phase diagram (in temperature and carrier density phase space) of this ionization process.A clear density-dependent increase in the ionization ratio is observed and its connection with the Mott transition is discussed. Semiconductor nanocrystals and QDs are another important area of research, mainly due to their huge potential for device application. We present a simple model which combines carrier loss kinetics, detailed balance principle in thermodynamic equilibrium and Pauli exclusion principle to quantitatively explain the observed behaviour of the experimental data.