Measurements of Optical Spectra in Short Pulse Laser-Solid Interactions at Moderately Relativistic and Relativistic Laser Intensities

Summary form only given. We present a series of measured spectra, 200 nm-1090 nm with 1 nm resolution, for various target materials including: Al, Ti, Au, Si, Cu and Al coated Ti obtained during laser-solid interaction experiments performed at the ultra-intense short pulse laser facilities JanUSP (LLNL), Comet (LLNL) and Vulcan (RAL). Laser intensities ranged from moderately relativistic, ~10¹⁸ W/cm², to relativistic, ~10²¹ W/cm². Measured spectra were sampled from within the specular reflection cone off of the target and delivered via discreet optics to a fiber coupled diffraction grating spectrometer. In addition to the expected higher integer harmonics, 2omega and 3omega, of the laser frequency, several shots measured high levels of 1/2omega, 3/2omega, and 5/2omega. These are attributed to two plasmon decay in the underdense plasma blow-off caused by the prepulse interacting with the solid before the main pulse. Density profiles for several shots will also be presented, in an attempt to correlate the presence of the 1/2 harmonics with long scale pre-plasmas