  We report on picosecond UV-laser processing of hexagonal boron nitride (hBN)
at moderately high pressures above 500 bar. The main effect is specific to the
ambient gas and laser pulse duration in the ablation regime: when samples are
irradiated by 5 ps or 0.45 ps laser pulses in nitrogen gas environment,
multiple nucleation of a new crystalline product - BN nanorods - takes place.
This process is triggered on structural defects, which number density strongly
decreases upon recrystallization. Non-linear photon absorption by adsorbed
nitrogen molecules is suggested to mediate the nucleation-growth. High pressure
is responsible for the confinement and strong backscattering of ablation
products. A strong surface structuring also appears at longer 150-ps laser
irradiation in similar experimental conditions. However, the transformed
product in this case is amorphous strongly contaminated by boron suboxides
BxOy.
