Aspects of stimulated two-photon transitions in bulk semiconductors

Abstract

This thesis aims to further the understanding of the two-photon interaction dynamic in bulk direct semiconductors in an electronically excited state. The central goal is to assess the practicability of controlling the two-photon transition strength using an ultrafast all-optical scheme and ultimately identify the key parameters and requirements necessary to observe degenerate two-photon gain in these media. Here, a transfer of stimulated two-photon emission to the semiconductor medium is very attractive because of its inherently large and fast optical nonlinearities and the capability to inject such materials with very high carrier densities, making it possible to macroscopically utilize this comparatively weak higher order nonlinear optical processes in the potential realization of a semiconductor-based two-photon laser and practical quantum information processing. Utilizing various (ultrafast) spectroscopic techniques, it is found that the application of a strong optical excitation on the order of Iexc>10 mJ/cm2 (yielding carrier densities approaching ne≈1019 cm-3) indeed produces a notable modification to the two-photon coupling for target sum photon energies within a typical bandwidth of ΔE≤80 meV above the optical band gap energy of the sample. Here, in obeying central experimental parameters regarding the excitation process, gain medium and two-photon configuration, a full amplitude inversion of the nonlinear coefficient β(ω), indicating a two-photon emission regime, is observed. Furthermore, the impact of accompanying effects related to the optical excitation on the semiconductor medium, e.g. free-carrier absorption, in competition to the nonlinear interaction have been investigated in this work.