Rensselaer Researchers Create World’s First Ideal Anti-Reflection Coating
From left to right, light reflecting off
surfaces made from aluminum, silicon, and aluminum
nitride. At right is a piece of aluminum nitride coated
with the new anti-reflection material.
Credit: Rensselaer/Fred Schubert
A team of Rensselaer researchers has created the world’s
first material that reflects virtually no light. Reporting in
the March issue of Nature Photonics, they describe an
optical coating made from the material that enables vastly
improved control over the basic properties of light. The
research could open the door to much brighter LEDs, more
efficient solar cells, and a new class of “smart” light sources
that adjust to specific environments, among many other
Most surfaces reflect some light — from a puddle of water
all the way to a mirror. The new material has almost the same
refractive index as air, making it an ideal building block for
anti-reflection coatings. It sets a world record by decreasing
the reflectivity compared to conventional anti-reflection
coatings by an order of magnitude.
A fundamental property called the refractive index governs
the amount of light a material reflects, as well as other
optical properties such as diffraction, refraction, and the
speed of light inside the material. “The refractive index is
the most fundamental quantity in optics and photonics. It goes
all the way back to Isaac Newton, who called it the ‘optical
density,’” said E. Fred Schubert, the Wellfleet Senior
Constellation Professor of the Future Chips Constellation at
Rensselaer and senior author of the paper.
Schubert and his coworkers have created a material with a
refractive index of 1.05, which is extremely close to the
refractive index of air and the lowest ever reported. Window
glass, for comparison, has a refractive index of about
Scientists have attempted for years to create materials that
can eliminate unwanted reflections, which can degrade the
performance of various optical components and devices. “We
started thinking, there is no viable material available in the
refractive index range 1.0-1.4,” Schubert said. “If we had such
a material, we could do incredible new things in optics and
To achieve a very low refractive index,
silica nanorods are deposited at an angle of precisely 45
degrees on top of a thin film of aluminum nitride.
Credit: Rensselaer/Fred Schubert
So the team created one. Using a technique called oblique
angle deposition, the researchers deposited silica nanorods at
an angle of precisely 45 degrees on top of a thin film of
aluminum nitride, which is a semiconducting material used in
advanced light-emitting diodes (LEDs). From the side, the films
look much like the cross section of a piece of lawn turf with
the blades slightly flattened.
The technique allows the researchers to strongly reduce or
even eliminate reflection at all wavelengths and incoming
angles of light, Schubert said. Conventional anti-reflection
coatings, although widely used, work only at a single
wavelength and when the light source is positioned directly
perpendicular to the material.
The new optical coating could find use in just about any
application where light travels into or out of a material,
including more efficient solar cells, brighter LEDs, optical
interconnects, high-reflectance mirrors, and “smart” light
sources that offer the potential for totally new
The development could also advance fundamental science. A
material that reflects no light is known as an ideal “black
body.” No such material has been available to scientists, until
now. Researchers could use an ideal black body to shed light on
quantum mechanics, the much-touted theory from physics that
explains the inherent “weirdness” of the atomic realm.
Schubert and his coworkers have only made several samples of
the new material to prove it can be done, but the oblique angle
evaporation technique is already widely used in industry, and
the design can be applied to any type of substrate — not just
an expensive semiconductor such as aluminum nitride.
Several other Rensselaer researchers also were involved with
the project: Professors Shawn-Yu Lin and Jong Kyu Kim; and
graduate students J.-Q. Xi, Martin F. Schubert, and Minfeng
See campus interview
with Fred Schubert.
Listen to Schubert
interview on NPR Morning Edition.
Read the press