Short Pulse Laser Absorption and Energy Partition at Relativistic Laser Intensities

We present the first absorption measurements at laser intensity between $10^{17}$ to $10^{20}$ W/cm$^{2}$ using an intergrating sphere and a suite of diagnostics that measures scale length, hot electrons and laser harmonics. A much-enhanced absorption in the regime of relativestic electron heating was observed. Furthermore, we present measurements on the partitioning of absorbed laser energy into thermal and non-thermal electrons when illuminating solid targets from $10^{17}$ to $10^{19}$ W/cm$^{2}$. This was measured using a sub-picosecond x-ray streak camera interfaced to a dual crystal von H\'{a}mos crystal spectrograph, a spherical crystal x-ray imaging spectrometer, an electron spectrometer and optical spectrometer. Our data suggests an intensity dependent energy-coupling transition with greater energy portion into non-thermal electrons that rapidly transition to thermal electrons. The details of these experimental results and modeling simulations will be presented.