Femtosecond imaging of surface heat transport in solid target excited at relativistic intensity

We present an extension of recent fs microscopy experiments [1] in which a planar Al target is excited by a 24 fs pump focused to intensity up to $3 \times 10^{18} W/cm^2$ in a 1 micron radius spot; subsequent heat propagation along the target surface appears as a region of reduced reflectivity that is imaged by a delayed probe pulse. Isotropic expansion of the surface heated region out to 15 microns within 500 fs is observed for pump intensities above $10^{18} W/cm^2$. We present a theoretical model in which the pump drives hot electrons into the target via a mixture of $j \times B$ heating and resonance absorption (RA), and a return current heats the target. Ultrafast expansion of the heated surface layer is explained by two-dimensional diffusive motion of returning electrons that undergo damped oscillations between vacuum and a sub-surface layer confined by positive surface charges. Isotropy of the observed expansion is consistent with dominance of RA over $j \times B$ heating, indicating prepulse heating is important. [1] B. Bowes et al., Opt. Lett. 31, 116 (2006).