High aspect ratio micro-explosions in the bulk of sapphire generated by femtosecond Bessel beams

Abstract

Femtosecond pulses provide an extreme degree of confinement of light matter-interactions in high-bandgap materials because of the nonlinear nature of ionization. It was recognized very early on that a highly focused single pulse of only nanojoule energy could generate spherical voids in fused silica and sapphire crystal as the nanometric scale plasma generated has energy sufficient to compress the material around it and to generate new material phases. But the volumes of the nanometric void and of the compressed material are extremely small. Here we use single femtosecond pulses shaped into high-angle Bessel beams at microjoule energy, allowing for the creation of very high 100:1 aspect ratio voids in sapphire crystal, which is one of the hardest materials, twice as dense as glass. The void volume is 2 orders of magnitude higher than those created with Gaussian beams. Femtosecond and picosecond illumination regimes yield qualitatively different damage morphologies. These results open novel perspectives for laser processing and new materials synthesis by laser-induced compression.

Single-shot ultrafast laser processing of high-aspect-ratio nanochannels using elliptical Bessel beams

“…This shows the elliptical Bessel beam preserves the main properties of propagation invariance of regular Bessel beams even in the nonlinear ionization regime [18,19]. A careful FIB milling procedure allowed us to minimize the damage produced to the laser-produced channels [13]. High resolution FIB images show a variation in diameter of about ∼50 % along the length of the channel, understood from inhomogeneity of the on-axis beam intensity, as can be seen in Fig 2(f).…”

Extreme Energy Density Confined inside a Transparent Crystal: Status and Perspectives of Solid-Plasma-Solid Transformations

“…It was demonstrated recently that the BB (150 fs, 2 µJ) focused inside sapphire produced the cylindrical void of 30 µm length and 300 nm diameter [8]. The void volume, V = 30 µm×πr 2 = 2.12 × 10 −12 cm −3 , appears to be two orders of magnitude larger than that generated by the GB.…”

“…Resonant absorption of the E r component (along the radial direction k r ) allows a higher energy density deposition for the cylindrical micro-explosion. With the central void diameter in sapphire comparable for the GB [1] and BB [8] pulses the volume was 180 times larger in the case of BB.…”

High-resolution material structuring using ultrafast laser non-diffractive beams

“…The temperatures estimated for type II changes of about 4000 K [46,92] indicates processes at the limit between liquid cavitation and gas-phase nucleation. Rapp et al [103] have created embedded voids in sapphire using single pulse Bessel illumination of fs and ps durations. The results are shown in Figure 6.…”