High harmonic generation in gas-filled photonic crystal fibers

High harmonic generation (HHG) is a promising tabletop source of coherent short wavelength radiation, with applications spanning science and engineering [1]. However, the low conversion efficiency and low average power of conventional few-kHz near-infrared (NIR) driving lasers limits the photon flux of such sources. Scaling this technique to MHz driving lasers requires strong focusing due to the limited pulse energy, and as a result the interaction volume is greatly reduced. It has been shown that this may be mitigated by restricting HHG to a photonic crystal fiber (PCF) [2, 3]. Here, we explore HHG in the latest generation of negative curvature PCFs [4] and achieve the highest photon energies to date