Ultrahigh Transmittance Biomimetic Fused Quartz Windows Enabled by Frequency-Doubling Femtosecond Laser Processing

Abstract

Owing to the intricate parameter design of antireflective subwavelength structures (ASS) and the stringent fabrication precision constraints associated with Gaussian beams, directly fabricating ultrahigh-transmittance ASS on the surfaces of fused silica (SiO₂) via femtosecond laser processing remains a considerable challenge. In this study, an analysis was first conducted on the characteristics of two representative types of ASS, through which hole-type microstructures were identified as exhibiting superior capability in enhancing the transmittance of infrared windows. Based on this observation, a dual-pulse femtosecond laser setup operating at a wavelength of 1030 nm was constructed and successfully employed to fabricate hole-type microstructures. To enable the fabrication of finer microstructures, a frequency-doubling crystal was further introduced for beam reshaping, through which a dual-pulse femtosecond laser optical path at 515 nm was successfully constructed. Concurrently, the performance differences of ASS fabricated using femtosecond lasers at different wavelengths were investigated to comprehensively evaluate the effects of laser wavelength on processing outcomes. Comparative analysis revealed that the ASS produced with the 515 nm femtosecond laser exhibited significantly enhanced antireflective performance, achieving a transmittance as high as 99% at a wavelength of 2.5 μm. In addition, to rigorously assess the stability and durability of the antireflective windows under various environmental conditions, the water contact angle and surface roughness with different processing methods were characterized, along with abrasion testing. The results demonstrated that the ASS produced by the 515 nm femtosecond laser maintained high transmittance after multiple abrasion cycles and exhibited superior performance under normal conditions as well as when covered with a black coating.