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International Conference on Ultrafast Phenomena

Dynamics of excitons probed by accumulated photon echo

T. Bouma, P. Vledder, J. I. Dijkhuis

Published Jan 1, 1992• 0 citations
Publisher

Abstract

Accumulated photon echo1 (AccPE) is an ultra-sensitive tool for studying optical dephasing of excitations in solids. The essential prerequisite for the succesful application of this technique is the presence of metastable states in the optical pumping cycle, preventing the system from a quick return to the ground state. A frequency grating in the ground state may than be formed and serve to create a stimulated echo. The technique has been applied to study long-lived optical impurity centers and bound excitons in solids. Here we present the first exploratory measurements on short-lived excitons in semi-insulating GaAs, and we will discuss the accumulation mechanisms. These mechanisms hold promise for general application of AccPE to study excitons in semiconductors.

DOI
10.1364/up.1992.fc22

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Accumulated photon echo1 (AccPE) is an ultra-sensitive tool for studying optical dephasing of excitations in solids. The essential prerequisite for the succesful application of this technique is the presence of metastable states in the optical pumping cycle, preventing the system from a quick return to the ground state. A frequency grating in the ground state may than be formed and serve to create a stimulated echo. The technique has been applied to study long-lived optical impurity centers and bound excitons in solids. Here we present the first exploratory measurements on short-lived excitons in semi-insulating GaAs, and we will discuss the accumulation mechanisms. These mechanisms hold promise for general application of AccPE to study excitons in semiconductors.
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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.
Accumulated photon echoes (APE) were used to measure the dephasing rates of Nd3+ in various inorganic glasses as a function of excitation wavelength, sample temperature, and laser power. Although dephasing processes of optical transitions in solids are of considerable interest, they are poorly understood, largely due to the lack of experimental data on femtosecond and picosecond time scales. The APE technique using sub-picosecond laser sources has made it possible to measure in the time domain dephasing rates of inhomogeneously broadened transitions in amorphous systems.1,2
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.
Time-resolved demonstration of picosecond photon echoes due to intrinsic optical excitations of a semiconductor is presented. Phase relaxation times up to several hundred picoseconds are found for excitons localized by chemical disorder in CdSx{\mathrm{CdS}}_{\mathrm{x}}CdSx​${\mathrm{Se}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$ mixed crystals. These long dephasing times are attributed to reduced scattering of the localized excitons as compared to free excitons. Dephasing is identified as due to energy relaxation of the excitons.
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.