Optical harmonic generation at exciton resonances in GaAs, GaN, and Cu 2 O semiconductors

Abstract

Nonlinear optics is an important field of fundamental and applied research. The possibility of changing the wavelength of laser light led to many applications in both areas. The integration of optical frequency conversion techniques into semiconductors is an active field of research. Also the inference from observed optical nonlinearities on material properties was very successful and finally led to its own field of research. The high number of photons interacting in this processes offer more degrees of freedom compared to linear optics, since their relative polarization, wave vector and frequency can be tuned. The main focus of this thesis lies on optical second harmonic generation (SHG) and third harmonic generation (THG) in semiconductors. SHG and THG spectroscopy of several model semiconductors is presented. Especially sharp exciton resonances in the vicinity of the bandgap are investigated. The application of an external magnetic field gives rise to a multitude of exciton resonances, due to the formation of Landau levels of the valence and conduction band in GaAs. An electric field on the other hand gives rise to exciton resonances because of the mixing of states by the Stark effect. The microscopic mechanism responsible for the increase of SHG and THG from exciton resonances in external fields is given and supported further by multi-photon absorption measurements. Exciton resonances from all three exciton series (A,B, and C) are found with THG spectroscopy in GaN. The comparison to other measurements allows to identify strain in the material. In the semiconductor Cu2O SHG from excitons is found although it is not expected in the centrosymmetric structure. Possible microscopic mechanisms responsible for the observed resonances are discussed and investigated. Finally, the possibilities of SHG and THG spectroscopy in indirect band gap semiconductors is explored with Si and SiC.