J. C. Painter

High harmonic generation in a semi-infinite gas cell

Ten-millijoule 35-femtosecond laser pulses interact with a cell of helium or neon that extends from a focusing lens to an exit foil near the laser focus. High harmonic orders in the range of 50 to 100 are investigated as a function of focal position relative to the exit foil. An aperture placed in front of the focusing lens increases the brightness of observed harmonics by more than an order of magnitude. Counter-propagating light is used to directly probe where the high harmonics are generated within the laser focus. In neon, the harmonics are generated in the last few millimeters before the exit foil, limited by absorption. In helium, the harmonics are produced over a much longer distance.

Direct observation of laser filamentation in high-order harmonic generation

We investigate the spatial evolution of a laser pulse used to generate high-order harmonics (orders ranging from 45 to 91) in a semi-infinite helium-filled gas cell. The 5 mJ, 30 fs laser pulses experience elongated focusing with two distinct waists when focused with f/125 optics in 80 Torr of helium. Extended phase matching for the generation of harmonics occurs in the region between the double foci of the laser, where the laser beam changes from diverging to converging.

Measured laser-beam evolution during high-order harmonic generation in a semi-infinite gas cell.

We report on direct measurements of self-guiding of 800 nm, 30 fs, 5 mJ laser pulses used to generate high-order harmonics in 80 torr helium. We track the spatial evolution of the laser pulses as they propagate several centimeters near the focus under conditions suitable for harmonic generation. The laser is observed to focus, diverge, and refocus. This behavior is accompanied by a flattop beam profile. Both of these features are absent when the laser is focused in vacuum. We also observed a 4 nm spectral blue shift in the center of the laser beam near the focus in contrast with no spectral shift at wider radii.