Abstract
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.
Highlights
We recently reported on self-guiding of a laser pulse in a helium-filled cell under conditions well suited for high-order harmonic generation [1]
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
Summary
We recently reported on self-guiding (or filamentation) of a laser pulse in a helium-filled cell under conditions well suited for high-order harmonic generation [1]. We document the evolution of the brightness of the associated highharmonic emission as the distance from the focusing mirror to the exit surface of the gas cell is varied These observations indicate that the self-guiding behavior of the laser is favorable to phase matching. This conclusion is supported by our previous work [2], where we probed the focal region of a helium-filled semi-infinite cell (i.e., a cell where the gas extends from the focusing optic to an exit foil at the focus) with counter-propagating pulses. The magnification of the image was approximately 3x, depending on the exact location of the foil relative to the imaging lens
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