Demonstration of tailorable and durable substrate-engraved antireflective metasurfaces for high power laser applications

A critical component for all high-power laser systems that is particularly susceptible to laser damage are the antireflective coatings that maximize energy transmission and minimize scattered and stray light. We demonstrate the ability to generate substrate-engraved metasurfaces (MS) 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, metasurface damage, has been observed and is likely the result of thermally driven material reflow accompanied by plasma initiation on the metasurface. At 1053 nm, metasurface damage onsets at 49 J/cm² with sample cleaning and 88 J/cm² after laser conditioning – very close to the reference substrate at 93 J/cm². At 351 nm we show damage onset of 30 J/cm², with reference substrate material damage onset of 47 J/cm². Therefore, damage is close to the bulk material and represents an improvement with respect to other methods. The metasurfaces 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 metasurfaces were minimally impacted at pressures orders of magnitude higher than an average fingerprint pressure – indicating that incidental handling contact will not affect MS structures. Mechanical damage is attributed to plastic compression, not fracturing of the MS features. We demonstrate for the first time that MS AR coatings, despite being rich in etched surface features, can tolerate laser fluences comparable to unprocessed optical surfaces. Furthermore, laser damage features of MS 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.