Light-exciton coupling in semiconductor micro- and nano-structures

Abstract

The optical properties of planar semiconductor microstructures and three-dimensional nanostructures containing narrow linewidth In₀.₀₄Ga₀.₉₆As quantum wells are studied in this dissertation. The interaction of quantum-well excitons with light in environments different from free space gives a pronounced effect on the optical response. N periodically arranged quantum wells are coupled to each other by light leading to N exciton-polariton eigenmodes. Each eigenmode is characterized by a distinct energy and radiative lifetime depending on the periodicity of the quantum wells. For a period of about half the excitonic transition wavelength, linear measurements of reflection, transmission, and absorption show significant features of the light-coupled eigenmodes. At Bragg periodicity, the oscillator strengths of all quantum well excitons are concentrated into one superradiant mode resulting in an N times increased radiative decay rate. The slope of the reflectivity linewidths versus N gives the radiative linewidth of the quantum well exciton. For off-Bragg periodicity, however, other eigenmodes become optically active and show their features in reflection and absorption spectra. Oxide-aperture three-dimensional nanocavities containing a single quantum well are investigated. The discrete transverse modes due to the lateral confinement of the optical field are observed in empty cavities with various aperture sizes. The linewidth measurements of the cavity modes show quality-factor values around 2000 for aperture diameters down to 2 μm. This is high enough to give a strong light-coupling effect in the nonperturbative regime, named normal mode coupling or vacuum Rabi splitting. The anti-crossing behavior of exciton and cavity modes for a 2 μm diameter aperture cavity is measured in transmission by temperature tuning of the exciton resonance through the lowest transverse cavity mode. A minimum splitting value of 3.9 meV and a splitting-to-linewidth ratio of 4.9 are obtained. Then, nonlinear pump-probe measurements on nanocavities with several aperture sizes are performed. The transition from the nonperturbative regime to the weak coupling regime is observed as the pump power increases. From the measured saturation powers for various aperture diameters, a photon density of 90 photons/μm² is found necessary to saturate the normal-mode peaks. The effect of quantum fluctuations of the light field in the nonperturbative regime of planar semiconductor microcavities containing quantum wells is studied. A pronounced third transmission peak lying spectrally between the two normal modes is observed in resonant single-beam-transmission and pump-probe measurements. Measurements on three-dimensional nanocavities confirm the important role of guided modes for this intriguing effect.