Direct Laser Printing of Infrared Antireflective Laser-Induced Periodic Surface Structures on the Surface of LiInS2 Nonlinear Optical Single Crystals

The development of advanced infrared-operating optical devices is facing the problem of Fresnel reflection losses at the surface of optical components, caused by mismatch between their refractive index and that of the surrounding environment, limiting in its turn the devices’ practical efficiency. This task becomes especially challenging when designing optical systems that rely on the utilization of nonlinear optical single-crystals, typically exhibiting a rather high refractive index (n > 2) in the near-IR spectral range with a corresponding strong index jump at the air-crystal interface and increased Fresnel losses. Unreliable methods, based on deposition of antireflection thin-film coatings onto the crystal surface, are being rapidly replaced by high-resolution, but low-performance, lithographic techniques for fabrication anti-reflective nanostructures (ARNs) directly on the surface of nonlinear optical crystals. However, direct lithography-free technologies are undoubtedly demanded to convert proof-of-concept demonstrations of broadband high-transmittance properties of ARNs into real practical applications. Here, anti-reflection relief representing laser-induced periodic surface structures (LIPSS) was fabricated on a novel and promising IR-transparent LiInS2 (LIS) single crystals by direct femtosecond laser nanopatterning. The effects of applied laser pulse energy and scanning velocity on the morphological features (nanotrenches periodicity and height modulation amplitude) and the structural-phase composition of produced LIPSS were systematically investigated and characterized by means of scanning electron microscopy and Raman spectroscopy. Fourier-transform infrared spectroscopy revealed a 10%-increased transmittance within NIR spectral range of one-sided LIPSS-patterned LIS crystal, compared to the untreated one. This study confirms the prospects of direct laser printing as a high-performance and high-resolution technology for fabrication of anti-reflective nanostructures on the surface of functional nonlinear optical crystals, applicable in noninvasive medical diagnostics, broadband laser spectrometers, nonlinear bio- and chemosensing and so on.