Optimizing high order harmonic generation in absorbing gases

We study high harmonic generation in absorbing and dispersive atomic gas media. For ultrashort fundamental pulses, the harmonic conversion efficiency is mainly limited by the absorption in the gas. This limit is found independent on the gas density for both the case of a loosely focused Gaussian beam or a beam guided in a capillary waveguide. Under optimum conditions, we have measured the highest conversion efficiency to date (4×10−5) for the 15th harmonic generated in xenon using a 40 fs, 800 nm pulse at a 1 kHz repetition rate corresponding to 50 μW for this harmonic.