Substrate-engraved antireflective nanostructured surfaces for high-power laser applications

Abstract

A critical component for all high-power laser systems that is particularly susceptible to laser damage is the antireflective coating, which maximizes energy transmission and minimizes scattered and stray light. We demonstrate the ability to generate substrate-engraved nanostructured surfaces (NS) for scalable and designable antireflective (AR) coatings that are monolithic to the substrate and can handle peak power levels comparable to the bulk material. Experimentally measured reflectance from these fabricated structures has validated our effective index theory-based transmission matrix model, demonstrating the designability of the AR properties. Upon exposure to sufficiently high fluences, a new mode of damage, nanostructured surface damage, has been observed and is likely the result of thermally driven material reflow accompanied by plasma initiation on the nanostructured surface. At 1053 nm, nanostructured surface damage onsets at 39 J / c m 2 with sample cleaning and 74 J / c m 2 after laser conditioning—very close to the reference substrate at 81 J / c m 2 . At 351 nm we show damage onset of 30 J / c m 2 , with reference substrate material damage onset of 47 J / c m 2 . Therefore, damage is close to the bulk material and represents an improvement with respect to other methods. The nanostructured surfaces were found to be mechanically durable and able to withstand cleaning procedures with sonication. Under normal incidence mechanical testing with a 200 µm radius indenter tip, the AR performance of these nanostructured surfaces was minimally impacted at pressures orders of magnitude higher than an average fingerprint pressure—indicating that incidental handling contact will not affect NS structures. Mechanical damage is attributed to plastic compression, not fracturing of the NS features. We demonstrate for the first time, to the best of our knowledge, that NS AR coatings, despite being rich in etched surface features, can tolerate laser fluences comparable to unprocessed optical surfaces. Furthermore, laser-damage features of NS indicate a unique non-growing failure mode whereby following absorption the featureless damage site does not precipitate future damage growth, reducing considerably the burdens for managing optics processing in high-power laser systems.

Designer Metasurfaces for Antireflective Applications Enabled by Advanced Nanoparticle Technology

“…These approaches use dry-etching to take advantage of the creation of in situ nucleation centers [15] or use an etching mask to guide the etching process. [14][15][16] One strength of using an etching mask is the deterministic control the mask gives over feature size, spacing, and sidewall sloping during fabrication. Unlike Optic processing advances have shifted system limitations, with current constraints consisting of antireflective (AR) coatings applied to optics and the inability to reduce size, weight, and price.…”

“…The model has been extensively discussed elsewhere, [19,14] so the discussion here will be focused primarily on the differences in reflectance as the sidewall sloping angle of individual MS features varies-necessitating a discussion of an expression defined as the sloping function (SF). The SF is a quantity to describe the degree to which the MS nanofeature sidewalls are sloped.…”

“…For all feature geometries, the feature-to-feature spacing, or period Λ, should remain below ≈ λ/2 to avoid scattering. [19] Prior work utilized gold nanoparticle ensembles, generated via dewetting, as etching masks, [14][15][16] where dewetting is the self-assembly of a thin film into an ensemble of particles based on surface energy minimization. The particle size and spacing correlate to both the initial film thickness and the dewetting temperature.…”

“…Gold benefits from an inherent resistance to oxidation and possesses a large Hamaker coefficient-and consequently small characteristic inter-particle spacing-which enables nanoparticle ensembles to be fabricated with particle centerto-center spacing, Λ mask , < λ/2. [14][15][16] However, one shortcoming of gold is that the etching selectivity is low compared to other metals [20] ; for the dry etching process implemented here, it was determined that gold is removed at a rate approximately five times slower than the underlying fused silica substrate. In other words, to obtain MS features with height "d", the mean gold nanoparticle height "h particle " should be at least ≈ d/5.…”

“…For substrate‐engraved metasurfaces in which the metasurface (MS) is monolithic to the underlying substrate through a material removal processing step, there are no additional materials or material interfaces and consequently the LIDT has been shown to be near the bulk damage threshold. [ 14 ] Owing to the features being monolithic to the substrate, environmental stability of the AR‐MS is similar to the substrate material itself. Furthermore, it has also been demonstrated that through the ability to control the MS feature geometry, mechanical durability can be obtained.…”