Photon Acceleration in the Ionization Front of a Flying Focus

A high-intensity laser pulse propagating through a medium triggers an ionization front that can frequency upshift and accelerate the photons of a secondary pulse. Dramatic frequency shifts, for instance from the optical to extreme UV, require that the photons remain in the ionization front over an extended distance. Traditionally, however, several effects have limited the interaction distance: the accelerated photons quickly outpace the ionization front, and the ionizing pulse diffracts or refracts from the plasma. The “flying focus”—a moving focal point resulting from a chirped laser pulse focused by a chromatic lens—overcomes these limitations. A flying focus pulse can drive a counter-propagating ionization front that travels at the speed of light in vacuum over a distance much greater than the Rayleigh range. Here we present photon kinetics simulations demonstrating photon acceleration in such a front.