The Confinement and Coherent Generation of Acoustic Plasmons in Photoexcited GaAs.

Abstract

Three-dimensional multi-component plasmas involving charge species with very different masses are expected to show a new branch of charge-density fluctuations with a frequency dispersion that is linear with respect to the wavevector. These bulk excitations are known as acoustic plasmons. Their linear dispersion reflects the quasi-static response of the lighter species as they screen the Coulomb interaction between the heavier carriers. Acoustic plasmons have been previously identified in some gas plasmas and, notably, in photoexcited electron-hole plasmas in GaAs using spontaneous light scattering. It has been theorized that acoustic plasmons may mediate electron-electron interactions in high Tc superconductors, layered, and other low dimensional systems. In this dissertation, I show that when confined to a GaAs layer, the acoustic plasmon exists as a set of discrete standing wave modes. The spontaneous light scattering spectrum from the acoustic plasmon is then the superposition of contributions from each of these modes, and the associated weights can be determined under the framework of the photoelastic model, which strengthens the analogy between acoustic plasmons and LA phonons. Further, I will present the first observation of a coherent acoustic plasmon state in photoexcited GaAs, generated and detected using an ultrafast double pump-probe scheme. This represents a new type of classical sound wave supported by the semiconductor system with velocities much larger than those associated with LA phonons. Based on the plasma density dependence of the oscillatory frequency of this state, it too is a standing wave confined to the GaAs layer, consistent with the spontaneous light scattering experiments. Experimental results indicate a coupling between the acoustic plasmon and coherent lower branch of the phonon-plasmon couple mode, which also serves as a possible driving mechanism for the coherent acoustic plasmon state.