Broadband Transmission Enhancement of B270 Via Random Antireflective Structured Surfaces

Fresnel reflection losses are detrimental to performance of optical systems. Not only do reflections diminish transmission, but rebounding energy can lead to system damage and catastrophic failure in high-energy and highpower laser systems. Suppression of Fresnel reflections by conventional thin-film coatings does not suit all applications, especially high-energy laser (HEL) systems, as foreign material coating can burn off or delaminate. Sub-wavelength structured surfaces minimize reflectivity by gradual change of refractive index from ambient to substrate. Consisting of no foreign material, anti-reflective structured surfaces (ARSS) offer a viable method to suppress reflectivity. ARSS on fused silica (SiO2) windows have been previously shown to exhibit high laser-induced damage thresholds (LIDT), comparable to bulk material LIDT. This study further explores the application of ARSS on low-dispersion crown/soda-lime glass, specifically SCHOTT B270. Random anti-reflective structured surfaces (rARSS) were fabricated by reactive ion etching a surface of 1-inchdiameter, 2-mm-thick, B270 Superwhite optical windows. Transmission results taken via an Agilent CARY 60 spectrometer on single-side processed windows demonstrate greater than 95.0% and 94.5% transmission across 700800 and 530- 1000 nm wavelength bands, respectively. A 100% enhancement of transmission from single facet in this band would theoretically result in 95.6% overall transmission. Therefore, we demonstrate transmission enhancement factor of 85% for one facet etched, as compared to untreated B270. Scanning electron microscopy (SEM) was used to analyze sub-micron surface morphology of the rARSS B270 windows. Randomly orientated "sponge-like" surface features 20-200 nm in width were observed. Successful high broadband transmission enhancement of B270 has been demonstrated using novel rARSS treatment.