Femtosecond laser-induced selective etching (FLISE) is a promising technology for fabrication of a wide range of optical, mechanical and microfluidic devices. Various etching conditions, together with significant process optimisations, have already been demonstrated. However, the FLISE technology still faces severe limitations for a wide range of applications due to limited processing speed and polarization-dependent etching. In this article, we report our novel results on the double-pulse processing approach on the improvement of chemical etching anisotropy and >30% faster processing speed in fused silica. The effects of pulse delay and pulse duration were investigated for further understanding of the relations between nanograting formation and etching. The internal sub-surface modifications were recorded with double cross-polarised pulses of a femtosecond laser, and a new nanograting morphology (grid-like) was demonstrated by precisely adjusting the processing parameters in a narrow processing window. It was suggested that this grid-like morphology impacts the etching anisotropy, which could be improved by varying the delay between two orthogonally polarized laser pulses.
In this study, we demonstrate the elongated Type I modifications in fused silica with an axial length > 50 µm. Such extended longitudinal dimensions were obtained by deep focusing radiation of a femtosecond laser inside fused silica at a depth of 2 mm. The transition from the Type II modification (nanogratings) to the Type I modification (refraction index change) was observed with increasing focusing depth at the constant pulse energy. The refractive index changes of ∼ 1.5×10−3 for a single pass and 2.4×10−3 for multiple passes were demonstrated. The radial dimensions of the deep-focused modifications were confined to 0.5-1.5 µm size. By overlapping the modifications in radial and axial directions, 1D phase grating in the depth range from 2 to 5 mm was recorded, allowing to split of the beam with a diffraction efficiency of > 96%. We demonstrate that the aberration-based recording with a Gaussian beam in fused silica is a simple tool for fabricating complex phase diffractive optical elements.
This manuscript presents a simple approach to the manufacturing and optimization of a multilevel phase-only diffractive conical lens (Fresnel axicon or “fraxicon”). The method for recording deep type I modifications in fused silica was established and its ability proven. We showed the prospects and limitations of elements processed using this method. The fine and advanced parameters optimization allowed us to get a compensation mechanism for almost uniform refractive index change for each separate layer. The maximum diffraction efficiency of the fraxicon for a wavelength of 515 nm was ∼80%. The measured Bessel beam depth of field was compared with commercially available conical lens axicons and showed good agreement.
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