Enhanced absorptance of gold following multipulse femtosecond laser ablation

In contrast to the common belief for femtosecond laser ablation that the thermal energy remaining in the ablated sample should be negligible, we recently found that a significant amount of residual thermal energy is deposited in metal samples following multishot femtosecond laser ablation. This suggests that there might be a significant enhancement in laser light absorption following ablation. To understand the physical mechanisms of laser energy absorption, we perform a direct measurement of the change in absorptance of gold due to structural modification following multishot femtosecond laser ablation. We show that besides the known mechanisms of absorption enhancement via microstructuring and macrostructuring, there is also a significant absorption enhancement due to nanostructuring. It is found that nanostructuring alone can enhance the absorptance by a factor of about three. The physical mechanism of the total enhanced absorption is due to a combined effect of nanostructural, microstructural, and macrostructural surface modifications induced by femtosecond laser ablation. Virtually, at a sufficiently high fluence and with a large number of applied pulses, the absorptance of gold surface can reach a value close to 100%.