Multi-millijoule femtosecond laser pulse bursts

Abstract

The generation and control of ultrashort laser pulses started a huge field of research and ever since, technology in this field developed further. One of these is the gener- ation of femtosecond (fs) laser pulse bursts: This means the generation of pulse bursts, which consist of pulses with a several magnitudes shorter temporal spacing compared to the burst repetition time. In the past, usual methods for generating fs pulse bursts were developed like picking pulses from the pulse train of a master oscillator, using linear, time-invariant spectral filters or nonlinear methods. However, none of these methods satisfy the needs for fully tunable control of all burst parameters over a wide range, like intraburst frequency, burst energy, number of pulses or individual intraburst pulse parameters such as pulse peak intensities or intraburst CEPs. Methods based on the Vernier effect, which utilize two cavities detuned in round-trip time together with a pulse accumulation mechanism, enable burst generation with a high scalability of the number of pulses in the burst and a tunable pulse spacing down to the sub-picosecond regime, corresponding to THz intraburst frequencies, exceeding the limits given by state-of-the-art methods by several orders of magnitude. Very re- cently, a new approach for the generation of fs pulse bursts, based on the Vernier effect, was invented: It comprises a regenerative amplifier, whose cavity round-trip time is slightly detuned to that of the master oscillator and a Pockels Cell voltage protocol as pulse accumulation mechanism in the regenerative amplifier. However, there is still room for further development of the mentioned approach. Im- plementation of an optical modulator, like an acousto-optical modulator, between the detuned cavities, allows control of individual pulse parameters. The focus in this work is especially on the control and stabilization of individual intraburst CEPs, which enables the generation of reproducible fs pulse bursts and a high amplification of bursts with sub- ps pulse spacing in optical amplifiers when using Chirped Pulse Amplification (CPA), giving access to the multi-millijoule regime. Thus, generation and strong amplification of fs pulse bursts with unprecedented properties and control will be demonstrated. This will be further motivated by proposing several potential applications of the generated bursts. This thesis is structured as following: In chapter 2, a definition of fs pulse bursts together with a presentation of relevant physical properties regarding the technical utilization of bursts is given (Sec. 2.1), followed by a discussion of state-of-the-art methods for the generation of fs pulse bursts (Sec. 2.2), including an introduction to burst genera- tion using the Vernier effect (Sec. 2.2.4). Chapter 3 deals with further development of Vernier methods realized in course of this thesis in the laboratory: The amplifica- tion of fs pulse bursts up to the multi-millijoule regime and its technical requirements (Sec. 3.4) and the stabilization of intraburst CEP drift (Sec. 3.5). Subject of chap- 5 1 Introduction ter 4 are potential applications of the generated bursts, like materials processing (Sec. 4.1), THz-generation (Sec. 4.2), nonlinear spectroscopy (Sec. 4.3) and a technique for quantum control (Sec. 4.4). Finally, a pulse broadening mechanism arising during the generation of continuously-tunable, narrowband THz pulses is investigated in detail in chapter 5, both theoretically and experimentally, leading to a deeper understanding of the THz-generation process and motivating the stabilization of intraburst CEPs