Fig. 1

Experimental results. a Generated output spectra for wavelengths from 180 nm to 5 μm, when 7 cm of fibre was filled with 4 bar of argon and pumped with pulses of 4, 6, 8 and 10 μJ (the spectra are offset by 25 dB each for clarity). The power spectral density (PSD) is normalized to the peak of the spectra. The PSD from −100 dB to 0 is mapped to a colour gradient from green to blue. At wavelengths longer than 900 nm, the spectra are corrected for the theoretical relative responsivity of the detectors (“Methods” section), but are otherwise intensity-uncalibrated. This leads to a wavelength-dependent noise level (indicated with black lines between 3 and 5 μm) that follows the correction curve for the detector response. Below 200 nm, the spectrum is intensity-uncalibrated. In the vicinity of 4.25 μm, absorption by carbon dioxide in the 2-m-long detection path in air reduces the signal. The relative humidity was ~30%, so that water absorption was negligible in the spectral region of interest. The numerically simulated spectrum (“Methods” section) at 9.7 µJ is shown by the curve under-shaded in grey. Dispersive waves (DWs) are emitted in the deep ultraviolet (DUV) and mid-infrared (MIR). b Same as in a, but for 30 bar of neon and 15.7 μJ for the simulation. c, d Spectra versus frequency corresponding to a, b, scaled by λ ²/c (λ is wavelength and c the speed of light in vacuum) to represent the correct frequency-scaled PSD. e Output energy (blue) and overall transmission (red) of the fibre (including coupling efficiency into the fibre) for the two cases in a (circles) and b (squares). f Near-field optical micrograph of the output mode for wavelengths longer than 3.1 μm, when the fibre was filled with 4 bar of argon and pumped with 6.6 μJ pulses, with the microstructure of the kagomé-photonic crystal fibre superimposed. The intensity is normalized to the maximum value. g Normalized intensity profiles in orthogonal directions through the centre of the mode in f. The dots are measured points and the lines Gaussian fits