Antireflective Surfaces

Optical Noise-Cancelling for Light

In a previous post, we looked at surface treatments that transform energy from one form to another and back—namely, metallic reflectors. Today, we explore a surface treatment that controls the direction of energy flow to make surfaces virtually disappear. It is another interesting example of how human skill and will can shape a reality built on waves and frequencies.

In contrast to sound, light can also travel in the "nothingness" of a vacuum, without experiencing time or losing any of its energy. When it faces dense earthly reality and travels from a lighter into a denser medium, like from air to glass, its phase velocity drops—roughly to 65% in this case—only to instantly accelerate back to its full speed upon exiting. Some of its energy can be absorbed, and some appears to bounce off the boundaries, creating an unwanted reflection in this context.

Figure: Many frequencies of light can pass effortlessly through a dense medium like 4mm thick float glass. But not all do; infrared radiation, as an example, is absorbed by Si-O molecular vibrations caused by the electric field of the light wave, converting its energy into heat which is then further transferred via conduction and emission back into electromagnetic energy.

To eliminate that annoying visible glare—whether on your glasses or a computer screen—an antireflective surface treatment can be applied. In the simplest case, the trick lies in applying a microscopic layer with an optical thickness of exactly one-quarter of the target wavelength (λ/4) and a refractive index equal to the square root of the two media's indices n=√(n₁ n₂ ). This creates destructive wave interference between waves reflected from the two interfaces, cancelling the totality. Considering the eye sensitivity, target wavelength for a visible light would be near 550nm.  

In practice, achieving close to this ideal means looking for materials from nature like magnesium fluoride (MgF₂), or engineering a new nanostructured materials that blend air and glass in precise porous ratios directly onto the surface.

Figure: Comparison of visible light transmittance from the air-glass interface between uncoated glass and glass with a λ/4 optical thickness at 550nm solid single-layer AR coating.

Antireflective Multilayers

Because the solid layer solution relies strictly on precise wave interference, a single layer perfectly cancels out only one specific wavelength or narrow band. If you want to guide the entire visible spectrum through an interface without glare, several layers—working together to manipulate wave interference across different frequencies—are needed.

Antireflective Structures

While larger physical structures could work by guiding the reflected waves onto the same surface once again, nanostructures work when physical features are significantly smaller than the wavelength of light. According to physics, reflection occurs whenever light encounters a sudden jump in optical density—such as transitioning directly from air to glass. One could consider a graded index film, and in theory it would work, but in practice it’s hard to find a solid material that would be air on top.

Nature has solved it by creating a structure that gradually transforms from solid to air, and it appears to work even at higher incident angles than solid layers can provide. It has been shown that the height of such a structure should be at least half of the wavelength and the period smaller than the wavelength, with optimal shapes approximating a parabolic or sinusoidal transformation.

Fig: The Night Moth can operate in low light conditions with AR structured eye and avoid predators when its eyes do not glare in the dark.

Summary

Recent experiments in quantum mechanics have shown that we have yet to fully understand the fundamental nature of light, but we can observe and, in many ways, manipulate its interaction with the physical reality that surrounds us. Because various applications require distinct surface properties, a wide range of solutions has been developed.