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Conference on Lasers and Electro-Optics

Use of four-wave mixing spectroscopy in semiconductor material studies

Duncan G. Steel, J. T. Remillard, H. Wang, Michael D. Webb

Published Apr 25, 1988• 0 citations
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Abstract

Frequency domain laser spectroscopy has been most often used to provide information on the internal Hamiltonian of the resonant system. In recent work, however, we have shown that by analysis of frequency domain four-wave mixing (FWM) line shapes, it is possible to obtain relaxation rates which reflect how the resonant system interacts with its environment. In addition, these experiments are performed with low-intensity cw lasers and give experimental information quite distinct from information obtained with high-intensity pulsed lasers.

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•Jan 1, 1990•0 citations
Frequency domain laser spectroscopy has been most often used to provide information on the internal Hamiltonian of the resonant system. In recent work, however, we have shown that by analysis of frequency domain four-wave mixing (FWM) line shapes, it is possible to obtain relaxation rates which reflect how the resonant system interacts with its environment. In addition, these experiments are performed with low-intensity cw lasers and give experimental information quite distinct from information obtained with high-intensity pulsed lasers.
We report high-resolution nonlinear laser-spectroscopy studies of the n=1 heavy-hole exciton in GaAs multiple quantum wells. Using a new method of cw frequency-domain four-wave mixing, we first show that the excitation relaxation consists of both fast and slow components. We then show that the contribution of well-width fluctuations to inhomogeneous broadening can be elimiated enabling us to obtain a direct measure of the intrinsic exciton line shape. Under certain conditions, this line shape is non-Lorentzian and cannot be explained on the basis of simple excited-state relaxation dynamics.
A study has been made of the effect of relaxation times on the four-wave mixing (FWM) signal response in frequency space for a two-level system where intramolecular coupling is explicitly taken into account within a vibronic model. The model is a generalization of the conventional two-level approach in that it includes a simplified description of the molecular structure and the explicit consideration of permanent dipole moments. Significant changes in both the intensity and shape of the peaks of the FWM spectrum are observed, due mainly to changes in the ratio between the longitudinal and transversal relaxation times, T 1/T 2.
Four-wave mixing processes with double and triple resonances are discussed in detail. Explicit expressions for the resonant nonlinear susceptibilities of various cases are derived. It is shown that in some cases, four-wave mixing can yield a spectrum with reduced inhomogeneous broadening. Possible applications of multiresonant four-wave mixing as a spectroscopic technique are considered. They include high-resolution spectroscopy, deduction of transition matrix elements, study of transitions between excited states, measurement of longitudinal relaxation times, distinction between resonant Raman scattering and resonant fluorescence, etc. Selective polarization arrangement allows us to suppress the nonresonant background, separately measure the real and imaginary parts of the resonant nonlinear susceptibility, minimize the effects of absorption and laser intensity fluctuations, and greatly enhance the signal-to-noise ratio.
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.
The purpose of this research is to study the relaxation of optically generated excitons and carriers in semiconductor microstructures using four wave mixing (FWM) spectroscopy. The systems studied include CdSSe microcrystallite doped glasses and GaAs/AlGaAs multiple quantum well structures (MQWS). Exposing a semiconductor to optical radiation near the fundamental band gap results in the creation of populations of elementary excitations including electrons, holes, and excitons, and also results in the creation of a superposition state between the ground and excited state of the solid. The work in this dissertation examines the relaxation of the populations of elementary excitations and the relaxation of the coherence between the ground and excited state using four wave mixing spectroscopy. First, the nonlinear optical response of simple two level systems is examined in order to provide insight into the types of line shapes expected from semiconductors. It is shown that the line shape is strongly dependent on how the system is coupled to the reservoir and the consequences of coupling to a reservoir are examined in a FWM measurement made in atomic sodium. The first semiconductor system studied is CdSSe microcrystallite doped glass. This system is shown to have a very slow component to the nonlinear response which has a optical intensity dependence and temperature dependence which suggests that the FWM response in these materials is trap mediated. Room temperature FWM measurements in GaAs MQWS enables the measurement of the carrier recombination time and the ambipolar diffusion coefficient. Using the technique of correlated optical fields, a slow component to the nonlinear response was measured showing an interference profile which suggests a possible shift of the exciton resonance due to the optically generated carriers. At low temperatures (2-20 K), measurements of the exciton line shape and relaxation time were made and evidence for exciton spectral diffusion was found. The low temperature line shapes can be qualitatively reproduced using Modified Optical Bloch equations which include the effects of spectral diffusion.