  Femtosecond laser ablation has been demonstrated to be a versatile tool to
produce micro/nanoscale features with high precision and accuracy. However, the
use of high laser fluence to increase the ablation efficiency usually results
in unwanted effects, such as redeposition of debris, formation of recast layer
and heat-affected zone in or around the ablation craters. Here we circumvent
this limitation by exploiting a thin frost layer with a thickness of tens of
microns, which can be directly formed by the condensation of water vapor from
the air onto the exposed surface whose temperature is below the freezing point.
When femtosecond laser beam is focused onto the target surface covered with a
thin frost layer, only the local frost layer around the laser-irradiated spot
melts into water, helping to boost ablation efficiency, suppress the recast
layer and reduce the heat-affect zone, while the remaining frost layer can
prevent ablation debris from adhering to the target surface. By this
frost-assisted strategy, high-quality surface micro/nano-structures are
successfully achieved on both plane and curved surfaces at high laser fluences,
and the mechanism behind the formation of high-spatial-frequency (HSF) laser
induced periodic surface structures (LIPSSs) on silicon is discussed.
