Coherent Ultra-Broadband Laser-Assisted Injection Radiation from a Laser Plasma Accelerator

The injection of electrons into a laser wakefield accelerator (LWFA) is observed to generate an intense coherent ultra-broadband and ultrashort pulse radiation flash, consistent with the acceleration of electrons from rest to nearly the speed of light in a distance <∼1μm. Under certain conditions, the radiation occurs at harmonics of the local plasma frequency. The flash is sufficiently bright to induce large nonlinear refractive index shifts in optical materials; we estimate a source brightness temperature of ∼1018K. We present measurements of the flash spectra, coherence, pulse duration, polarization, and angular distribution, providing a detailed picture of electron injection dynamics in LWFA. These are characteristic of laser-assisted injection of off-axis electrons, which preserves wake coherence.