Optical Shock-Enhanced Self-Photon Acceleration

Broadband sources of coherent radiation find diverse utility across many scientific disciplines as experimental drivers and diagnostic tools. State-of-the-art supercontinuum sources, which primarily achieve spectral broadening through Kerr-induced self-phase modulation of ultrashort laser pulses, have been limited to wavelengths >100 nm. Extending broadband sources into the extreme ultraviolet (EUV, \lambda< 100 nm) would benefit several key applications. In principle, large spectral shifts can be obtained over short interaction lengths in a rapidly ionizing medium, but so far, photon acceleration from optical to EUV frequencies has met with limited success. Combining spatiotemporal and transverse intensity profile shaping of a short pulse laser can largely eliminate the adverse effects of diffraction, refraction, and dephasing in a photon accelerator. This structured flying focus drives a guiding plasma density profile that moves at a tunable focal velocity in a homogenous, partially ionized plasma, opening a new self-shocked photon acceleration regime. This regime is characterized by extreme self-steepening and spectral broadening of the laser light, culminating in the formation and collapse of an optical shock. We demonstrate this optical shock-enhanced self-photon acceleration in a structured flying-focus driven plasma, and show that multi-octave spectra extending well into the EUV (400 nm to 60 nm) can be generated over <100 \mum of interaction length. A simple short-pass filter can isolate coherent, high intensity (>1 x 10¹⁵ W/cm²), sub-fs pulses (<400 as) from the accelerator output, without the need for post-compression.