New research by a City College of New York team has uncovered a novel way to
combine two different states of matter. For one of the first times,
topological photons—light—has been combined with lattice vibrations, also
known as phonons, to manipulate their propagation in a robust and
controllable way.

The study utilized topological photonics, an emergent direction in photonics
which leverages fundamental ideas of the mathematical field of topology
about conserved quantities—topological invariants—that remain constant when
altering parts of a geometric object under continuous deformations. One of
the simplest examples of such invariants is number of holes, which, for
instance, makes donut and mug equivalent from the topological point of view.
The topological properties endow photons with helicity, when photons spin as
they propagate, leading to unique and unexpected characteristics, such as
robustness to defects and unidirectional propagation along interfaces
between topologically distinct materials. Thanks to interactions with
vibrations in crystals, these helical photons can then be used to channel
infrared light along with vibrations.

The implications of this work are broad, in particular allowing researchers
to advance Raman spectroscopy, which is used to determine vibrational modes
of molecules. The research also holds promise for vibrational
spectroscopy—also known as infrared spectroscopy—which measures the
interaction of infrared radiation with matter through absorption, emission,
or reflection. This can then be utilized to study and identify and
characterize chemical substances.

"We coupled helical photons with lattice vibrations in hexagonal boron
nitride, creating a new hybrid matter referred to as phonon-polaritons,"
said Alexander Khanikaev, lead author and physicist with affiliation in
CCNY's Grove School of Engineering. "It is half light and half vibrations.
Since infrared light and lattice vibrations are associated with heat, we
created new channels for propagation of light and heat together. Typically,
lattice vibrations are very hard to control, and guiding them around defects
and sharp corners was impossible before."

The new methodology can also implement directional radiative heat transfer,
a form of energy transfer during which heat is dissipated through
electromagnetic waves.

"We can create channels of arbitrary shape for this form of hybrid light and
matter excitations to be guided along within a two-dimensional material we
created," added Dr. Sriram Guddala, postdoctoral researcher in Prof.
Khanikaev's group and the first author of the manuscript. "This method also
allows us to switch the direction of propagation of vibrations along these
channels, forward or backward, simply by switching polarizations handedness
of the incident laser beam. Interestingly, as the phonon-polaritons
propagate, the vibrations also rotate along with the electric field. This is
an entirely novel way of guiding and rotating lattice vibrations, which also
makes them helical."

Entitled "Topological phonon-polariton funneling in midinfrared
metasurfaces," the study appears in the journal Science.

## Reference:

S. Guddala et al, Topological phonon-polariton funneling in mid-infrared
metasurfaces, Science (2021).
DOI: 10.1126/science.abj5488

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