BUFFALO, N.Y. – University at Buffalo engineers have
created a more efficient way to catch rainbows, an advancement in
photonics that could lead to technological breakthroughs in solar
energy, stealth technology and other areas of research.
Qiaoqiang Gan, PhD, an assistant professor of electrical
engineering at UB, and a team of graduate students described their
work in a paper called “Rainbow Trapping in Hyperbolic
Metamaterial Waveguide,” published Feb. 13 in the online
journal Scientific Reports.
They developed a “hyperbolic metamaterial
waveguide,” which is essentially an advanced microchip made
of alternate ultra-thin films of metal and semiconductors and/or
insulators. The waveguide halts and ultimately absorbs each
frequency of light, at slightly different places in a vertical
direction (see the above figure), to catch a “rainbow”
Gan is a researcher within UB’s new Center of Excellence
in Materials Informatics.
“Electromagnetic absorbers have been studied for many
years, especially for military radar systems,” Gan said.
“Right now, researchers are developing compact light
absorbers based on optically thick semiconductors or carbon
nanotubes. However, it is still challenging to realize the perfect
absorber in ultra-thin films with tunable absorption band.
“We are developing ultra-thin films that will slow the
light and therefore allow much more efficient absorption, which
will address the long existing challenge.”
Light is made of photons that, because they move extremely fast
(i.e., at the speed of light), are difficult to tame. In their
initial attempts to slow light, researchers relied upon cryogenic
gases. But because cryogenic gases are very cold – roughly
240 degrees below zero Fahrenheit – they are difficult to
work with outside a laboratory.
Before joining UB, Gan helped pioneer a way to slow light
without cryogenic gases. He and other researchers at Lehigh
University made nano-scale-sized grooves in metallic surfaces at
different depths, a process that altered the optical properties of
the metal. While the grooves worked, they had limitations. For
example, the energy of the incident light cannot be transferred
onto the metal surface efficiently, which hampered its use for
practical applications, Gan said.
The hyperbolic metamaterial waveguide solves that problem
because it is a large area of patterned film that can collect the
incident light efficiently. It is referred to as an artificial
medium with subwavelength features whose frequency surface is
hyperboloid, which allows it to capture a wide range of wavelengths
in different frequencies including visible, near-infrared,
mid-infrared, terahertz and microwaves.
It could lead to advancements in an array of fields.
For example, in electronics there is a phenomenon known as
crosstalk, in which a signal transmitted on one circuit or channel
creates an undesired effect in another circuit or channel. The
on-chip absorber could potentially prevent this.
The on-chip absorber may also be applied to solar panels and
other energy-harvesting devices. It could be especially useful in
mid-infrared spectral regions as thermal absorber for devices that
recycle heat after sundown, Gan said.
Technology such as the Stealth bomber involves materials that
make planes, ships and other devices invisible to radar, infrared,
sonar and other detection methods. Because the on-chip absorber has
the potential to absorb different wavelengths at a multitude of
frequencies, it could be useful as a stealth coating material.
Additional authors of the paper include Haifeng Hu, Dengxin Ji,
Xie Zeng and Kai Liu, all PhD candidates in UB’s Department
of Electrical Engineering. The work was sponsored by the National
Science Foundation and UB’s electrical engineering