Direct and stand-off fabrication of black silicon with enhanced absorbance in the short-wavelength infrared region using femtosecond laser filament

Abstract

We report on the experimental demonstration of a technique for rapid fabrication of black silicon with enhanced absorption in the short-wavelength infrared (SWIR) region, directly in the ambient air, and at a standoff distance of about 1 m. Our approach is based on the irradiation of a surface of the crystalline silicon, coated by a 100-nm-thick aluminum film, by femtosecond laser pulses that are propagating in the air in the self-channeling regime known as laser filamentation. By adjusting the processing parameters, we achieve up to 50% absorptivity enhancement in the SWIR spectral range between 1.5 and 2.5 μm wavelength, compared to the case when no aluminum coating is applied prior to the laser processing under the otherwise same conditions. Microscopic and elemental analyses of the processed samples suggest that the absorptivity enhancement is due to both the changes in the morphology of the microstructures formed on the sample surface by the laser irradiation and the modification of the energy-band structure of silicon, as a result of aluminum implantation. Our technique could enable rapid and cost-effective fabrication of silicon-based opto-electronic devices for applications in the infrared spectral range.