Spectral measurements of mid-infrared radiation from a laser wakefield accelerator*

The formation of a plasma “bubble” during Laser Wakefield Acceleration (LWFA) results in a co-moving refractive index gradient that produces time dependent frequency shifts in the driving laser pulse. The positive density gradient at the leading edge of the plasma bubble causes red-shifting of the front of the pulse, generating wavelengths extending into the mid-infrared. The mid-infrared spectral region contains the frequency range of molecular vibrations, and thus is of significant interest for a diversity of scientific and technological applications. High-resolution spectral measurements of mid-infrared radiation extending to 2.5 microns during LWFA in the bubble regime were obtained using the HERCULES laser system at the University of Michigan. The enhancement of this radiation using tailored density targets, as well as the conversion efficiency of this process as a function of plasma density, length and charge generation are presented.