Surface energy transport following relativistic laser-solid interaction

A planar Al target is excited by a 25 fs laser pulse focused to intensity up to 3x10(18) W/cm(2) in a similar to 1 mu m radius spot; subsequent heat propagation along the target surface, imaged by a delayed probe pulse, appears as a roughly circular area of reduced reflectivity centered on the pump spot, that expands to as much as 12 +/- 3 mu m in radius within 500 fs. We present a semiempirical model in which the pump laser pulse drives hot electrons into the target via collisionless interactions. A return current heats the target and, above a critical temperature, includes runaway electrons that return to the surface before dissipating their energy. Ultrafast radial expansion of the heated surface layer is explained by lateral diffusive motion of returning runaway electrons oscillating across the target surface layer confined by space charge. Isotropy of the observed expansion is consistent with dominance of resonance absorption over jxB heating, indicating prepulse heating is important.