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.
Highlights
We report high aspect ratio nanochannel and void formation in the bulk of the material, with direct observations by Scanning Electron Microscopy (SEM) after Focused Ion Beam (FIB) milling of the laser-processed areas
We report different behavior with picosecond illumination: high aspect ratio voids can be formed with dimensions close to those obtained by femtosecond illumination, the voids are surrounded by a relatively thick shell of resolidified material, with debris apparent within the void
We have demonstrated the generation of high aspect ratio voids in the bulk of sapphire with single femtosecond and picosecond laser pulses with high-angle Bessel beams
Summary
We report high aspect ratio nanochannel and void formation in the bulk of the material, with direct observations by Scanning Electron Microscopy (SEM) after Focused Ion Beam (FIB) milling of the laser-processed areas. We report different behavior with picosecond illumination: high aspect ratio voids can be formed with dimensions close to those obtained by femtosecond illumination, the voids are surrounded by a relatively thick shell of resolidified material, with debris apparent within the void. Specific care was taken to avoid any supplementary damage created by FIB milling (see Methods section).
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