Optical shock-enhanced self-photon acceleration
P. Franke, D. Ramsey, T. T. Simpson, D. Turnbull, D. H. Froula, J. P. Palastro
Abstract
Photon accelerators can spectrally broaden laser pulses with high efficiency in moving electron density gradients driven in a rapidly ionizing plasma. When driven by a conventional laser pulse, the group velocity walk-off experienced by the accelerated photons and deterioration of the gradient from diffraction and plasma refraction limit the extent of spectral broadening. Here we show that a laser pulse with a shaped space-time and transverse intensity profile overcomes these limitations by creating a guiding density profile at a tunable velocity. Self-photon acceleration in this profile leads to dramatic spectral broadening and intensity steepening, forming an optical shock that further enhances the rate of spectral broadening. In this new regime, multi-octave spectra extending from 400 to 60 nm wavelengths, which support near-transform-limited $<400$ as pulses, are generated over $<100 \ensuremath{\mu}\mathrm{m}$ of interaction length.