Tech Briefs Magazine - February 2022 - 30

Optics
Optical Coating Exhibits the Same Color in Reflection and
Transmission
Fano Resonance Optical Coatings can both transmit and reflect the same color
simultaneously.
University of Rochester, New York and Case Western Reserve University, Cleveland, OH
O
ptical coatings have been used to
better reflect certain wavelengths of
light from lenses and other devices or
conversely, to better transmit certain
wavelengths through them. The coatings
on tinted eyeglasses, for example,
reflect, or block out harmful blue light
and ultraviolet rays. But until now, no
optical coating had ever been developed
that could simultaneously reflect and
transmit the same wavelength, or color.
Researchers developed a new class of
optical coatings - Fano Resonance
Optical Coatings (FROCs) - that can
be used on filters to reflect and transmit
colors of remarkable purity. In addition,
the coating can be made to fully reflect
only a very narrow wavelength range.
Previously, the only coating that could
do this was a multilayered dielectric mirror
that is much thicker, suffers from a
strong angular dependence, and is far
more expensive to make.
The researchers envision several ap -
plications for the new technology. FROCs
could be used to separate thermal and
photovoltaic bands of the solar spectrum.
Such capability could im prove the effectiveness
of devices that use hybrid thermalelectric
power generation as a solar energy
option. Directing only the useful band of
the solar spectrum to a photovoltaic cell
Researchers developed an optical coating that exhibits the same color in reflection (pictured) and
transmission. (Photo: University of Rochester photo/J. Adam Fenster)
prevents its overheating. The technology
could also lead to a six-fold increase in the
life of a photovoltaic cell. And the rest of
the spectrum is absorbed as thermal energy,
which could be used in other ways
including energy storage at night, electricity
generation, solar-driven water sanitation,
or heating a supply of water.
Fano resonance, named after the
physicist Ugo Fano, is a widespread wavescattering
phenomenon first observed as
a fundamental principle of atomic phys -
ics involving electrons. Later, research ers
discovered that the same phenomenon
can also be observed in optical systems.
The team applied a thin, 15-nanometer-thick
film of germanium to a metal
surface, creating a surface capable of
absorbing a broad band of wavelengths.
They combined that with a cavity that
supports a narrowband resonance. The
coupled cavities exhibit Fano resonance
that is capable of reflecting a very narrow
band of light.
For more information, contact Bob
Marcotte at bmarcotte@ur.rochester.edu.
Light Helps Autonomous Vehicles Better Scan for Nearby
Fast-Moving Objects
Mechanical control and modulation of light on a silicon chip could enhance LiDAR.
Purdue University, West Lafayette, IN
self-driving car has a hard time recognizing
the difference between a
toddler and a brown bag that suddenly
appears because of limitations in how it
senses objects using Light Detection and
Ranging (LiDAR). The autonomous
vehicle industry is exploring frequency
modulated continuous wave (FMCW)
LiDAR to solve this problem.
Researchers have built a way that this
type of LiDAR could achieve higher-resA
30
Cov
olution
detection of nearby fast-moving
objects through mechanical control and
modulation of light on a silicon chip.
FMCW LiDAR detects objects by scanning
laser light from the top of an
autonomous vehicle. A single laser beam
splits into a comb of other wavelengths,
called a microcomb, to scan an area.
Light bounces off of an object and goes
to the detector through an optical isolator
or circulator, which ensures all
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ToC
reflected light ends up at the detector
array. The new method uses acoustic
waves to enable faster tuning of these
components, which could bring higherresolution
FMCW LiDAR detection of
nearby objects.
The technology integrates microelec -
tromechanical systems (MEMS) transducers
made of aluminum nitride to
modulate the microcomb at high frequencies
ranging from megahertz to
Tech Briefs, February 2022
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