Novel Ultrafast Light Sources Based on Third-Order Parametric Nonlinear Effects

Parametric third-order nonlinear effects offer a promising approach for the generation of ultrashort pulses within the ultraviolet (UV) spectral range through the conversion of near infrared laser radiation. Third-harmonic generation (THG) combines three identical photons into a single high-energy photon, thereby enabling direct generation of ultrashort UV pulses without intermediate conversion steps across the visible spectrum. In addition, four-wave mixing (FWM), which involves the combination of three independent photons by summation and subtraction, offers broad tunability while achieving the high photon energies required for UV pulse generation. However, the practical realisation of these processes is often hindered by weak nonlinearities, absorption losses in nonlinear crystals and difficulties in phase matching. This study proposes innovative approaches to exploit third-order nonlinear effects for efficient ultrashort pulse generation, compression and frequency conversion. The first approach is based on THG in dielectric coatings. A study of THG in a single dielectric layer is conducted to reveal the influence of interference effects, Kerr nonlinearity and higher order effects. Based on these findings, multilayer structures are utilised to enhance the THG efficiency by several orders of magnitude in comparison to the single layer. An alternative is the use of gaseous media, which have the advantage of lower absorption and greater control over dispersion and nonlinearity through pressure tuning. In combination with hollow-core fibres, gases offer an extremely versatile platform for high power nonlinear optics. In this work, the generation of tunable UV pulses by dispersive wave generation in Kagome fibres is investigated. Dispersive wave generation using four-wave mixing offers spectral tunability, making it a powerful technique for nonlinear pulse generation. The first experimental demonstration of dispersive waves driven by few-cycle pulses is presented, with numerical simulations showing the potential for sub-femtosecond pulses in the extreme ultraviolet region. Instead of focusing solely on enhancing the efficiency of UV generation via nonlinear processes, an alternative approach involves enhancing the pulse energy in the UV range by increasing the pulse energy of the pump laser systems. Self-phase modulation, a third-order parametric process, can be employed for pulse compression in multipass cells (MPCs). This technique is explored in this work to achieve the compression of pulses in the near infrared spectral range with a duration of several hundred femtoseconds down to the range of a few-cycles while maintaining high transmission efficiency. For this purpose, a two-stage MPC system is designed based on numerical simulations, with the first stage successfully implemented in experiments. The results of this study highlight the potential of third-order nonlinear effects for the development of more efficient and versatile ultrafast optical systems, which offer new opportunities for applications such as spectroscopy and strong-field physics.