Electrical engineers at Duke University have discovered that changing the
physical shape of a class of materials commonly used in electronics and
near- and mid-infrared photonics—chalcogenide glasses— can extend their use
into the visible and ultraviolet parts of electromagnetic spectrum. Already
commercially used in detectors, lenses and optical fibers, chalcogenide
glasses may now find a home in applications such as underwater
communications, environmental monitoring and biological imaging.
The results appear online on October 5 in the journal Nature Communications.
As the name implies, chalcogenide glasses contain one or more
chalcogens—chemical elements such as sulfur, selenium and tellurium. But
there's one member of the family they leave out: oxygen. Their material
properties make them a strong choice for advanced electronic applications
such as optical switching, ultra-small direct laser writing (think tiny
rewritable CDs) and molecular fingerprinting. But because they strongly
absorb wavelengths of light in the visible and ultraviolet parts of
electromagnetic spectrum, chalcogenide glasses have long been constrained to
the near- and mid-infrared with respect to their applications in photonics.
"Chalcogenides have been used in the near- and mid-IR for a long time, but
they've always had this fundamental limitation of being lossy at visible and
UV wavelengths," said Natalia Litchinitser, professor of electrical and
computer engineering at Duke. "But recent research into how nanostructures
affect the way these materials respond to light indicated that there might
be a way around these limitations."
In recent theoretical research into the properties of gallium arsenide
(GaAs), a semiconductor commonly used in electronics, Litchinitser' s
collaborators, Michael Scalora of the US Army CCDC Aviation and Missile
Center and Maria Vincenti of the University of Brescia predicted that
nanostructured GaAs might respond to light differently than its bulk or even
thin film counterparts. Because of the way that high intensity optical
pulses interact with the nanostructured material, very thin wires of the
material lined up next to one another might create higher-order harmonic
frequencies (shorter wavelengths) that could travel through them.
Imagine a guitar string that is tuned to resonate at 256 Hertz—otherwise
known as middle C. The researchers were proposing that if fabricated just
right, this string when plucked might also vibrate at frequencies one or two
octaves higher in small amounts.
Litchinitser and her Ph.D. student Jiannan Gao decided to see if the same
might be true for chalcogenide glasses. To test the theory, colleagues at
the Naval Research Laboratory deposited a 300-nanometer-thin film of arsenic
trisulfide onto a glass substrate that was next nanostructured using
electron beam lithography and reactive ion etching to produce arsenic
trisulfide nanowires of 430 nanometers wide and 625 nanometers apart.
Even though arsenic trisulfide completely absorbs light above 600
THz—roughly the color of cyan—the researchers discovered their nanowires
were transmitting tiny signals at 846 THz, which is squarely in the
ultraviolet spectrum.
"We found that illuminating a metasurface made of judiciously designed
nanowires with near-infrared light resulted in generation and transmission
of both the original frequency and its third harmonic, which was very
unexpected because the third harmonic falls into the range where the
material should be absorbing it," Litchinitser said.
This counterintuitive result is due to the effect of nonlinear third
harmonic generation and its "phase locking" with the original frequency.
"The initial pulse traps the third harmonic and sort of tricks the material
into letting them both pass through without any absorption," Litchinitser
said.
Moving forward, Litchinitser and her colleagues are working to see if they
can engineer different shapes of chalcogenides that can carry these harmonic
signals even better than the initial nanostrips. For example, they believe
that pairs of long, thin, Lego-like blocks spaced certain distances apart
might create a stronger signal at both third and second harmonic
frequencies. They also predict that stacking multiple layers of these
metasurfaces on top of one another might enhance the effect.
If successful, the approach could unlock a wide range of visible and
ultraviolet applications for popular electronic material and mid-infrared
photonic materials that have long been shut out of these higher frequencies.
Reference:
Natalia Litchinitser et al, Near-infrared to ultra-violet frequency
conversion in chalcogenide metasurfaces, Nature Communications (2021).
DOI: 10.1038/s41467-021-26094-1
Tags:
Physics