Generation of deep ultra-violet pulses in hollow capillary fibres

Abstract

Femtosecond pulses in the deep ultraviolet region (DUV, 200-400 nm) are in high demand for time-resolved studies in several fields of research such as quantum chemistry (UV photoelectron spectroscopy), material sciences (lithography in semiconductors, polymers) and biology (protein analysis, DNA sequencing, cell imaging). When combined with external polarization control, they can provide excellent tools for chiral molecule analysis and ultrafast magnetism studies. A well-established route for the generation of linearly polarized pulses in the DUV has been via frequency up-conversion in bulk materials such as nonlinear crystals or gases. However, the former suffer from limited transmission range, narrow phase-matching bandwidths and low intensity damage thresholds while the latter require high-peak power and provide low frequency up-conversion efficiencies. For circularly polarized pulses in the DUV, an additional stage for polarization conversion is required, commonly implemented by using phase retarders in the ultraviolet. This induces unwanted dispersion that compromises ultra-short duration and achromatic phase retardation. This thesis focuses on the experimental generation of high-energy, ultrashort DUV pulses with controlled polarization using two alternative frequency up-conversion techniques in hollow capillary fibres; four-wave mixing and resonant dispersive wave emission. Using the first technique, record-breaking energy conversion efficiency up to 50% and large spectral bandwidths in the DUV with high pulse energy are demonstrated with linear polarization. This scheme is then extended to achieve direct generation of DUV pulses with circular polarization without the need for dispersive optics in the UV. Using resonant dispersive wave emission, high energy, circularly polarized pulses tunable across the DUV are generated when driven by circularly polarized 800 nm pulses. These studies allow to experimentally verify the polarization-induced dynamics with respect to fundamental science such as the angular momentum conservation and the energy scaling of nonlinearity. Although this work is focused on DUV generation, the same techniques can be applied to other spectral regions such as the vacuum-ultraviolet (VUV, 100-200 nm) and the mid-infrared (mid-IR, 3-8 µm) by proper selection of the driving frequencies, and can be scaled up in output energy using larger fibre systems.