Ballistic electron transport in non-equilibrium warm dense gold

We have measured the time evolution of the phase shift at the front and back surfaces of gold nano-foils that have been excited with a 150 fs (λ = 400 nm) laser pulse. The thickness of the foils (d∼30 nm) is roughly one-third of the ballistic electron transport range at ambient conditions (∼100 nm). At lower pump fluences, the front and back sides of the foil behave similarly, indicating uniform heating by ballistic electrons. As the pump fluence is increased, the phase shift on the front side is higher than that on the back, indicating a significant reduction in the ballistic electron transport range.