Nano‑textured optics for removing reflection

Random nano-texturing for fused silica grades across the ultraviolet and infrared spectrum is the latest innovation for reducing reflection rates within optical and laser grade viewports and quantum cells. This is an up-and-coming alternative for applying a thin‑film anti‑reflective coating!

A closer look at a nano-textured optic. This 3D surface map, captured using white light interferometry, reveals microscopic surface features that are invisible to the naked eye. The scan reveals surface features with a peak‑to‑valley height (Spv) of 36.0 µm and an average roughness (Sa) of 2.91 µm. Note: vertical scale can differ from horizontal.

Nano‑texturing process and design

Nano‑texturing is a plasma‑etched design into the bulk material, which creates a pattern made to minimise reflection. It is effective due to the type of design placed into the optic. This is a cone-like randomised etching, where the individual components get thicker until they disappear into the optic. This gradually implements changes in the overall refractive index through the depth of the optics, as the design moves from mostly air and a small area of the optical material, through to mostly optic and a small area of air and then finally merging fully into the optic.

This is in opposition to standard untextured or uncoated optics where the refractive index changes immediately from the low refractive index of air to the much higher refractive index in the optic. This leads to a high level of reflections, due to the Fresnel coefficient.

There are various designes available to target different wavelength ranges and these texture types differ due to the depth of the etching, as the wavelengths required get longer, the depth increases!

The “random” part of the design was created to oppose the diffraction (light scattering) often found in ordered tessellating patterns for nano‑texturing called “motheye”. This occurs when the incidence angle is high, as at a specific angle (depending on the optical material’s atomic structure) the minimal reflections still caused by the optic create constructive interference. If all the etching is exactly the same, when this angle is reached all of the points will cause constructive interference at the same time and creates an iridescent effect, much like the rainbow effect in an oil spill. Impressively the random nano‑textured design shows stable low reflections across an angular shift of up to 60°.

Nano‑texturing vs anti‑reflective coatings

Anti‑reflective coatings are at risk of damage from high-powered lasers, which can cause melting or evaporation of the material. For our anti‑reflective coatings, we recommend for continuous lasers that power not exceed 500 W cm^-2 at 530 nm and for pulsed lasers we rate coatings to 2 J cm^-2 at 530 nm with a 10 ns pulse width. Damage from exceeding these limits can lead to the coating evaporating or fully being vaporised from the optic!

However, as nano‑textured solutions do not require any additional materials to be applied to the optic, this vastly increases the laser damage threshold. Therefore, unlike traditional anti‑reflective coatings, with nano‑textured optics the only limiting factor is the optical material itself. This means that depending on the optic itself, for a continuous wave (CW) laser, the damage threshold is > 15 MW/cm² and for pulsed lasers, the damage threshold is > 60 J/cm².

Nano-textured viewport