Development of a tuneable few-femtosecond ultraviolet source up to 50 kHz repetition rate

Abstract

Ultraviolet (UV) light plays a crucial role across various scientific fields such as biochemistry, material science and nanotechnology. Producing and using UV pulses with sub-5 fs durations is extremely demanding, but offers new possibilities for studying electron dynamics in light-induced biochemical re- actions. This thesis presents a tuneable ultrafast UV source operating at high repetition rates (up to 50 kHz), designed to be the key component of a table-top few-femtosecond UV-pump and IR-probe beamline to perform time-resolved spectroscopy measurements. Firstly, pulse post-compression of 150 fs pulses with a pulse energy of 250 μJ, provided by an ytterbium (Yb)-based fibre laser system, down to 20 fs with 90% throughput is achieved via a conventional single-stage multi-pass cell (MPC) filled with 1.4 bar krypton gas. Limited by the narrow bandwidth of the cell mirrors, dispersion-engineered cell mirrors are utilised for the direct control of dispersion inside the MPC to enhance nonlinear spectral broadening based on self-phase modulation (SPM). Through this approach, pulse compression down to 15 fs within a compact single-stage MPC is demonstrated, maintaining an overall throughput of > 90% . Additionally, the average power and repetition rate scalability up to 50 W and 200 kHz, respectively, of this method is proven while preserving the spectral broadening characteristics of the output beam. These short pulses are subsequently used to drive resonant dispersive wave (RDW) emission via a 1.7 meter long hollow-core fibre, filled with argon gas in a negative pressure gradient. A tuneability range of the RDW from 270 nm to 530 nm at 1 and 10 kHz repetition rate is demonstrated, with a transform limited UV pulse of about 3 fs and microjoule level pulse energy at the short- est wavelength. It is also shown that at 50 kHz repetition rate the tuneability range is limited to 370 nm due to the onset of filamentation at higher gas pressures. Moreover, the overall design of the UV-pump and IR-probe beamline is presented, which will benefit statistically demanding experiments.