Novel ultrafast pulse propagation dynamics in hollow-core fibres

Abstract

In this thesis, I describe experimental and numerical work on understanding the complex nonlinear dynamics of the propagation of high-intensity laser pulses in gas-filled hollow-core fibres (HCFs). The long interaction length and the ability to control the dispersion and nonlinearity make HCF a great platform for exploring a wide variety of nonlinear optical phenomena. By employing high-order soliton dynamics, I experimentally demonstrated compression of µJ-level pulses directly from a 220 fs commercial pump laser to ∼ 13 fs in a single stage without the need for external elements such as chirped mirrors. Moreover, I demonstrated the generation of wavelength-tunable sub-15 fs pulses through soliton-plasma interactions using the same commercial pump source. I temporally characterized the output pulses using sum-frequency generation (SFG) cross-correlation frequency-resolved optical gating (XFROG). Using extreme modulation instability (MI) dynamics, I demonstrated the generation of a linearly flat supercontinuum (SC) extending from 350 nm up to 2 µm in argon-filled broadband-guiding HCF. Moreover, I investigated the role of the Raman response on such dynamics by using nitrogen-filled HCF. I found that due to the close rotational lines in N2, gain suppression in the fundamental mode causes the pulse to be coupled into higher-order modes (HOMs), which reduces the energy density of the SC. Molecules can dissociate due to the high optical intensity of the propagating pulse, as has been previously observed in filamentation experiments. By using molecular gases, I was able to observe, for the first time, evidence indicating the dissociation of molecular gases inside HCF. In particular, I observed the formation of ozone molecules inside the fibre which is a result of the chemical reactions between the dissociated oxygen molecules due to the high intensity of the propagating pulse. I studied the effect of such chemical reactions on pulse propagation dynamics assisted by numerical modeling. In addition, by using a gas mixture of molecular gases, I observed a novel phenomenon caused by the chemical reaction between the different gas constituents.