Nonlinear optics in hollow core PCF filled with gaseous and supercritical xenon

Summary form only given. Gas-filled kagomé-lattice hollow core photonic crystal fibre (HC-PCF) is attractive for nonlinear optical experiments, offering a broad transmission window, a high damage threshold and low loss compared to capillaries [1]. Its dispersion is uniquely low and smooth from the UV to the IR and can be tuned simply by varying the gas pressure. Recently we demonstrated tuning of the zero dispersion wavelength (ZDW) from 300 to 900 nm in an argon-filled kagomé HC-PCF by varying the gas pressure from 1 to 150 bar [2]. The concomitant increase in density brings the nonlinearity to within an order of magnitude of that of fused silica. Nonlinear effects such as the generation of tunable deep-UV light via dispersive wave emission, and the observation of blue-shifting solitons due to ionization effects, can be observed in these fibres. Since both these phenomena rely on soliton pulse compression to achieve high peak intensities (which can be as high as 10¹⁵ W/cm²), the dispersion must be anomalous at the pump laser wavelength (800 nm, pulse duration 150 fs) [3]. At the high gas densities reported in this paper, however, the normal dispersion of the gas dominates over the weaker anomalous dispersion of the kagomé fibre.Interestingly, the supercritical regime can be reached at room temperature in xenon (critical point 16°C and 58 bar) [2,4]. In this thermodynamic regime, liquids and gases are no longer distinguishable. As the gas pressure is increased from 25 to 35 bar (fibre length 28 cm, core diameter 18 μm), spectral broadening is observed up to the energy corresponding to self-focusing in the input cell, at which point the coupling is impaired (Fig. 1). The sharp spectral transitions (shown by red arrows) are indicative of this coupling limitation, the self-focusing threshold dropping with increasing gas pressure and nonlinearity. At 25 bar a UV band at ~340 nm forms by intermodal fourwav- -mixing (iFWM), near-field imaging showing the UV light to be in HE12 mode. Numerical simulations confirm the experimental results of both self-focusing and iFWM. In the supercritical regime at 80 bar, distinct spectral broadening by self-phase modulation was observed as the launched pulse energy was increased from 15 to 60 nJ. Kagomé HC-PCF filled with Xe at high pressure uniquely allows one to study ultrafast dynamics in the absence of Raman scattering at nonlinearity levels as high as silica, while offering pressure-controllable dispersion.