Fabrication of sapphire optical windows with infrared transmittance enhancement and visible transmittance reduction by femtosecond laser direct writing

Abstract

Enhancing the infrared transmittance of the sapphire windows is an effective approach to improving the infrared detection quality. Herein, the maskless fabrication of anti-reflection subwavelength structures through femtosecond laser direct writing was reported. The sapphire windows with micro-column arrays with different key parameters were simulated through the finite-difference time-domain (FDTD) theory to study the relationship between the structural parameters and optical properties. The calculation and simulation results provide the theoretical basis for devising anti-reflection subwavelength structures. The effects of different polarization states on the morphology and optical properties of micro-column arrays were investigated. The anti-reflection subwavelength structures with a period of 2 μm fabricated by circularly polarized femtosecond laser reached a maximum transmittance of 90.63% at the wavelength of 3.5 μm, while the visible light can be partly filtered. In addition, the contact angle of the sapphire window also increases from 77.48° to 126.42° after the fabrication of anti-reflection subwavelength structures, which is very fit for infrared detection as infrared optical windows. Our study demonstrates the possibility of achieving infrared anti-reflection and visible transmittance reduction by femtosecond laser direct writing, which can also be extended to other superhard windows.