Femtosecond Yb:KYW oscillators based high-speed asynchronous optical sampling and coherent acoustic phonon investigations

Abstract

In this thesis, we have first developed a cost-efficient compact GHz femtosecond Yb:KYW oscillators based asynchronous optical sampling (ASOPS) system. Due to the high detection sensitivity and the high time resolution, this system is well suited for exploring ultrafast dynamics of phonons and charge carriers in various materials. Thus picosecond laser ultrasonics using ASOPS techniques raises our particular interest in terms of coherent acoustic phonon (CAP) excitation, propagation and detection in multilayered semiconductor structures and thin films. We have achieved significant progress on CAP investigations in semiconductor saturable absorber mirrors (SESAMs) where not only the imbedded quantum wells as a saturable absorber but also the distributed Bragg mirror (DBR) contributes to the experimental phonon observations. The relevant SESAM structures and multiple quantum well (MQW) structures are tested by one-color or/and two-color ASOPS systems, enriching our understanding on CAP dynamics in those complex SESAM structures. The probe/pump light-dependent CAPs in SESAM structures are also investigated. Our second main progress has been made on optical coherent control of CAPs. Through two pump pulses with adjustable time delay and pump power, a quasi-arbitrary acoustic waveform synthesis is demonstrated, which is also well explained by a simplified model that we developed. In addition, the influence of a second subsequent pump pulse on light absorption in MQWs is explored by the double-pump-pulse method through large time delays between the pump pulses. In the following, the main work will be summarized in three parts, and the potential improvements and inspirations on future work will be proposed.