Dynamics of ultrafast laser nanostructuring in bulk sapphire in non-diffractive mode: a micro-explosion in slow motion

Recently, the ultrafast Bessel beam has emerged as an efficient tool to structure nano-volumes in sapphire crystals with a high aspect ratio of more than 100. To increase the controllability of the process, there is a quest to understand better the interaction of ultrafast Bessel beams with sapphire in intensity ranges pertinent for nanostructuring. In the present study, we investigate the ultrafast interaction of a short-pulsed Bessel beam with a single-crystal sapphire sample using time-resolved phase contrast and quantitative phase contrast microscopy techniques. The time-resolved relaxation dynamics reveal that the plasma phase in sapphire persisted for the duration of a few ns followed by the tens of ns long hot phase that triggers amorphisation. Around 100 ns the manifestation of nano-volume expansion becomes visible. This study contributes to attaining precise control in the laser processing of sapphire for scientific and industrial applications.