Generation of tunable broadband deep ultraviolet radiation from gaseous media using ultrashort laser pulses.

Abstract: Most modern ultrafast lasers are based on mode-locked fiber or solid-state lasers and operate around the near infrared part of the electromagnetic spectrum. The widely spread CPA based Titanium Sapphire lasers can emit pulses centred around 800nm with durations down to 20 fs. However, for many ultrafast experiments light at different centre frequencies are required. In order to obtain other centre frequencies, nonlinear optical processes are employed. The conventional way of frequency conversion is to use nonlinear crystals, which generally offers ways of generating ultrashort ranging from the near-infrared to the near-ultraviolet. At shorter wavelengths this technique is limited due to strong dispersion of the existing nonlinear crystals. Ultrashort extreme ultraviolet light can be generated through the process of high harmonic generation. However, a gap in available ultrashort pulses with centre frequencies exists in the intermediate deep ultraviolet (_ < 300 nm) and vacuum ultraviolet ( < 200 nm) regions. This Master Thesis, which was performed in Sweden at Lund High-Power Laser Facility, aimed to close the gap with main focus on the deep ultraviolet regions. The generation of ultrashort deep ultraviolet was studied both experimentally and theoretically. A particular focus of the project was to keep the experimental relatively simple without the use of highly sophisticated vacuum installations, nor complicated pulse re-compressions schemes. To avoid limitations caused by dispersion only gaseous interaction media was considered. Different experimental four-wave mixing schemes for nonlinear frequency conversion of the output from CEP stable 3 mJ, 1 kHz 30 fs Titanium Sapphire laser was studied. Direct third harmonic generation was experimentally investigated in a static gas cell, which was designed and realized in this project. The gas cell experiments were supported by numerical simulations of phase-matching conditions for third harmonic generation. Additionally was third harmonic generation experimentally studied in a pulsed gas jet, as well as in a gas filled hollow capillary. Finally, was third harmonic frequency light experimentally realized by phasematched difference frequency mixing between the fundamental frequency pulses from the laser, and frequency doubled pulses (generated in a KDP crystal), in a hollow capillary. Direct third harmonic generation in the static pressure gas cell provided the most powerful deep ultraviolet pulses generated within this work. The pulsed gas jet provided slightly less output power. However, increased reproducibility and mode quality strongly outweighed the loss of power. Phase-matched di_erence frequency generation in a hollow capillary, although experimentally the most demanding, yielded the most promising results. Deep ultraviolet pulses centred around 260nm with estimated energies exceeding 1 µJ, and a spectral width of 50nm was demonstrated at low pressures in the capillary. The bandwidth principally supports pulse durations down to 2 fs. The scheme also allowed for tunability of the output pulses; by changing the intensity of the fundamental _eld, the centre wavelength of the deep ultraviolet pulse could be tuned over 10 nm. Finally, at high pressures broadband light with spectral components covering the entire spectrum from 200 - 1100nm was demonstrated.