Phase matching of high-order harmonic generation in helium- and neon-filled gas cells

摘要

10mJ laser pulses are used to study high-order harmonic generation in helium- and neon-filled gas cells of various lengths. Harmonic orders in the range 50-100 are investigated. A semi-infinite cell geometry produces brighter harmonics than subcentimetre cells. In the semi-infinite geometry, the gas occupies the region from the focusing lens to a thin exit foil near the laser focus. Restricting the laser beam with an aperture in front of the focusing lens increases the emission of most harmonic orders observed by as much as an order of magnitude. Counter-propagating light is used to probe the region in the focus where the high harmonics are generated. In neon, the harmonics are generated in the last few millimetres before the exit foil. In helium, the harmonics are produced over a longer distance, indicating good phase-matching conditions.