  Effect of the femtosecond and nanosecond UV laser irradiation (below the
ablation threshold) of graphite-like hexagonal boron nitride (hBN) has been
studied. Experiments were carried out with the compacted powder under high
vacuum at room temperature using excimer KrF laser (248 nm). In the nanosecond
operation mode, the laser-induced fluorescence spectra are found strongly
modified depending on the integrated doze, which is attributed to a progressive
enrichment of the surface layer by elemental boron. A slow sample recovery
after the laser irradiation has been observed. On the other hand, in the
femtosecond mode the fluorescence spectra depend on the laser fluence, and the
changes are reversible: low energy fluorescence spectra are restored
immediately when the laser energy decreases. This effect can be explained by a
material bleaching, which favors a bulk centers emission. The ablation
threshold has been determined as 78 mJ/cm2 in the femtosecond laser operational
mode.
