The rectangular diffraction grating on single crystal diamond was fabricated with the wavelength of 10.6 μm. A novel method called bi-layer lift-off technology was used to form the hard mask. This approach simplified the patterning process of the thick Al film and made the deep etching on single crystal diamond achievable according to our requirement. The fabrication steps and the bi-layer lift-off technology are demonstrated in detail. We characterized the diamond grating and found that the angles of its sidewalls were almost vertical (within 3°), with a mean roughness of Ra = 3.01 nm on the bottom and 12.4 nm on the top.
We studied the structural and elemental evolutions during the femtosecond laser percussion drilling of high-aspect-ratio diamond microholes. Microholes 225-μm-deep having an aspect ratio of 15 were drilled with an exposure time of 100 s and a laser power of 60 mW. It is found that when the specimen was machined by femtosecond laser at low power and short exposure time, the laser-affected zone (LAZ) may be still solid in the interior rather being void, even it reached the bottom of the diamond. A clear crack appeared between the solid portion and pristine diamond. Elemental analysis revealed that oxygen was incorporated into the solid portion of the LAZs, and its atomic percentage reached 6.5% for a laser power of 10 mW at initial position and decreased as the depth increased in the solid portion. The wall of the void contained nearly no oxygen. Furthermore, nanoripples were observed on the sidewall surface of the hole.
Grating is an important optical element widely used in high power laser system which is required for higher diffraction efficiency and high laser damage threshold. Compared with the traditional grating materials, diamond has excellent optical transmission properties, high anti-damage threshold, high thermal conductivity etc. Diamond grating has wide application prospect in high power laser system. Based on rigorous couple wave analysis method (RCWA) diffraction characteristics of diamond gratings with different structures were simulated and optimized; Also the diffraction characteristics of between diamond gratings and fused silica gratings were compared and analyzed. The studies showed that the optimized diamond grating could obtain high diffraction efficiency more than 94% in wider bandwidth and the bulk of diamond grating would be much smaller than that of fused silica gratings.
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