Scaling of soliton dynamics in gas-filled hollow capillary fibres
Grigorova, Teodora Filipova
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Soliton-effect dynamics during the propagation of an ultrashort pump pulses in gas-filled hollow-core fibres offer unique capabilities for applications in many different fields of science and technology, such as strong-field physics and time-resolved spectroscopy. In recent years many advancements have been made by the use of gas-filled hollow-core micro-structured fibres. However, because of their small core sizes these systems have been limited with respect to the maximum energy that can be used to observe these dynamics. In this thesis a method of scaling the energy of soliton-effect dynamics thought the use of simple fused silica hollow capillary fibres (HCFs) is studied. First, the viability of HCFs as a platform for high-energy soliton dynamics is demonstrated by the compression of mJ-level pump pulses to sub-cycle duration, combined with the emission of tunable ultrafast pulses, called resonant dispersive-wave (RDW) emission, over the ultraviolet spectral region with brightness comparable to that of free-electron lasers for these wavelengths. This is achieved by pumping a 3 - long helium-filled HCF with inner diameter of 250 µm with 10 fs pulses centered at 800 nm and 1 kHz repetition rate. The soliton-effect self-compression was shown to yield pulses with envelope duration of 1.2 f fs and 340 µJ energy when the HCF was filled with 400 mbar of helium. The wavelength of the RDW emission can be varied simply by changing the filling gas pressure, and this tunability was demonstrated for RDW emission from 120 nm to 350 nm by using He-pressures in the range of 230 mbar-4 bar. The energy of the UV-RDW was measured to nearly 1 µJ around 120 nm and up to 16 µJ around 220 nm. These represent two orders of magnitude increase both in the self-compressed pulse energy and the RDW energy in comparison to previous demonstrations in gas-filled hollow-core micro-structured fibres. Further, the variation of the observed dynamics as a function of the dispersion regime, in which the pulses are propagating, is studied using the same optical setup, but using argon as HCF-filling gas. The filling-gas pressure is varied from 7 mbar to 3.344 bar in order to continuously change the dispersion at the pump wavelength of 800 nm from anomalous to normal. This has shown that the wide variety of soliton dynamics, which have so far been observed in gas-filled micro-structured fibres, can also be observed in HCFs. In addition, a new regime of interest is identified when the short, 10 fs pump pulses are propagating near the zero-dispersion wavelength (ZDW) of the gas-filled HCF. In this regime, in addition to spectral broadening, a RDW-like band of radiation is generated at wavelengths shorter than the pump. This is attributed to self-phase-modulation-induced pulse splitting of the pump pulse near the ZDW and the consequent collision and cross-phase modulation of the split pulses. In this study it is also shown that in certain cases, for example when the pulse is experiencing significant self-focusing or ionisation, modelling by simply assuming pure fundamental-mode initial coupling into the HCF is not appropriate. Instead, a full-spatial propagation of the pulse to the input of the fibre is necessary to appropriately calculate the initial modal excitation before the propagation through the HCF. Lastly, the short-wavelength extent of the process of RDW emission in different noble gases is studied. A previous result present in the literature, the existence of a gas-dependent RDW emission wavelength cut-off, which is shifted to shorter wavelengths for lighter noble gases, is experimentally confirmed. Using the same HCF system as the previous studies, it is shown that the shortest RDW than can be generated is 115 nm in helium, 125 nm in neon, 160 nm in argon, and 180 nm in krypton. A possible reason for this cut-off is presented, linking it with the ionisation-influenced dynamics of the pump pulse. Further studies will aim to find a way of extending the RDW emission cut-off to shorter wavelengths.