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

View Article: PubMed Central - PubMed

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.

No MeSH data available.


Scanning electron microscope images of the void channels after HF etching.(a–d) Femtosecond regime (channel presented in Figs 1 and 3), (e–h) picosecond regime (channel presented in Fig. 4).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC5037470&req=5

f5: Scanning electron microscope images of the void channels after HF etching.(a–d) Femtosecond regime (channel presented in Figs 1 and 3), (e–h) picosecond regime (channel presented in Fig. 4).

Mentions: We repeated the high-aspect ratio microexplosion experiments using longer pulses of 3 ps duration obtained by stretching the pulse in the compressor of our chirped pulse amplification laser source. Under these conditions, the threshold for sapphire index modification was 0.6 μJ and the threshold for crack formation was 2.3 μJ. Even under bulk focusing conditions, it is possible to generate void structures with ps single pulses. To enable comparison with the femtosecond regime, we show in Fig. 4 the void created by a 2 μJ pulse (the same energy as for the structure shown in Fig. 1). The diameter for the picosecond illumination is slightly higher than in the femtosecond regime, i.e. 350 nm. Also in contrast with the femtosecond regime results, the void cross-section remains circular irrespective of the polarization. However, we note that the channel morphology is very different from the case of femtosecond illumination. In particular, the picosecond regime results show sidewalls that are more irregular, and the presence of a shell of resolidified material can be seen surrounding the void. This shell has a diameter of approximately 700 nm and shows a granular structure as is apparent in Fig. 5(b–d). Finally, we note that the bottom of the void shows more particles than the top part of the void. A possible qualitative interpretation for this could be that the vapor phase has been condensed within the void. In the case of picosecond illumination regime, we understand the absence of asymmetry as originating from the much thicker melted shell around the void channel. In this case, ellipticity cannot be maintained in the liquid phase.


High aspect ratio micro-explosions in the bulk of sapphire generated by femtosecond Bessel beams
Scanning electron microscope images of the void channels after HF etching.(a–d) Femtosecond regime (channel presented in Figs 1 and 3), (e–h) picosecond regime (channel presented in Fig. 4).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5037470&req=5

f5: Scanning electron microscope images of the void channels after HF etching.(a–d) Femtosecond regime (channel presented in Figs 1 and 3), (e–h) picosecond regime (channel presented in Fig. 4).
Mentions: We repeated the high-aspect ratio microexplosion experiments using longer pulses of 3 ps duration obtained by stretching the pulse in the compressor of our chirped pulse amplification laser source. Under these conditions, the threshold for sapphire index modification was 0.6 μJ and the threshold for crack formation was 2.3 μJ. Even under bulk focusing conditions, it is possible to generate void structures with ps single pulses. To enable comparison with the femtosecond regime, we show in Fig. 4 the void created by a 2 μJ pulse (the same energy as for the structure shown in Fig. 1). The diameter for the picosecond illumination is slightly higher than in the femtosecond regime, i.e. 350 nm. Also in contrast with the femtosecond regime results, the void cross-section remains circular irrespective of the polarization. However, we note that the channel morphology is very different from the case of femtosecond illumination. In particular, the picosecond regime results show sidewalls that are more irregular, and the presence of a shell of resolidified material can be seen surrounding the void. This shell has a diameter of approximately 700 nm and shows a granular structure as is apparent in Fig. 5(b–d). Finally, we note that the bottom of the void shows more particles than the top part of the void. A possible qualitative interpretation for this could be that the vapor phase has been condensed within the void. In the case of picosecond illumination regime, we understand the absence of asymmetry as originating from the much thicker melted shell around the void channel. In this case, ellipticity cannot be maintained in the liquid phase.

View Article: PubMed Central - PubMed

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.

No MeSH data available.