Fig. 1. GVD (top) and instantaneous nonlinear coefficient (bottom) at 800 nm of the fundamental mode of a 100 μm core radius HCF filled with Ne or N₂ as a function of gas pressure. Labels indicate the zero-dispersion pressure (p_ZD) in each case.

Fig. 2. FWHM duration (top row) and ratio of output to input peak power (bottom row) of the self-compressed pulses as a function of the input energy and the equivalent constant pressure (see text) in both a statically filled or a negatively pumped 3 m long, 100 μm core radius HCF filled with Ne (left) or N₂ (right). The solid black lines represent the contour lines where L = L_av, which run along the optimal region for self-compression in a decreasing pressure gradient. Note the one order of magnitude change in the pressure range from Ne to N₂ owing to their different dispersion and nonlinearity.

Fig. 3. Temporal intensity profile (top row) and spectrum (bottom row) of the self-compressed sub-cycle pulses obtained after propagation through a HCF filled with Ne (left) or N₂ (right), at both constant or decreasing pressure, for two different pairs of input pulse energy and equivalent gas pressure which lie towards the same area of the optimal self-compression regions in Figure 2.