Scientists develop 'exceptional' surface to explore exotic physics
Experimental solution provides first observation of previously
theoretical light absorption

Date:
February 2, 2022
Source:
Penn State
Summary:
By demonstrating exceptional control of an open optical system, an
international research team has provided a path to experimentally
measure and test exotic phenomena and gain insights into new
physics with exquisite sensitivity.



FULL STORY ==========================================================================
By demonstrating exceptional control of an open optical system, an international research team has provided a path to experimentally
measure and test exotic phenomena and gain insights into new physics
with exquisite sensitivity.


========================================================================== Reported in Nature Communications, the Penn State, Michigan Technological University and Vienna University of Technology researchers created
a stable surface of 'exceptional' points -- notoriously finicky
singularities that exhibit peculiar properties -- and used it to
facilitate and observe the perfect absorption of light in a coherent,
chiral system. When light enters the system, which operates as a surface
full of exceptional points, in one direction, the light is completely
absorbed. When entering from the opposite direction, relatively little
light is absorbed.

"Our work points the way towards interesting new physics with exceptional points beyond their traditional habitat in sensing and lasing, where
many more insights can be expected," said corresponding author ?ahin
O"zdemir, associate professor of engineering science and mechanics at
Penn State. Harnessing the unique properties of exceptional surfaces,
O"zdemir explained, could elucidate to a deeper understanding of what
happens when physical processes occur exactly at an exceptional point,
leading to potential applications for better sensors and novel ways of controlling the interaction of light with matter.

Open systems constantly move energy in and out, shifting their
parameters and how they react to moving variables in their surrounding environment. In this dance of change, the value of any spectral point
in the system can coalesce with the value of another spectral point,
drawing them together as a spectral singularity -- an exceptional
point. Incredibly sensitive, exceptional points respond to any
interference with strong, measurable signals. Even the smallest of perturbations can knock the exceptional point enough for it to dissociate
and become unexceptional, according to O"zdemir.

"The downside of this fragility is that it becomes very difficult to
operate a system at an exceptional point without being driven away
from the singularity, which poses a serious challenge for exploiting exceptional points for practical applications," O"zdemir said, explaining
that moving close to an exceptional point introduces noise to the
system. "You can bring your system close to these points in a huge
space, but you may fail to stabilize the system long enough to study
precisely at those exceptional points. So, what if every point in that
space was exceptional?" Following their earlier theoretical proposals, published in Physical Review Letters and Optics Letters, the researchers
placed a mirror at one end of a waveguide, a structure that encourages
light waves to follow a particular path.

The waveguide was outfitted with a microresonator, which supports light propagation in clockwise and counterclockwise directions at the same
frequency.

The researchers coupled light to the resonator, bringing its frequency
to that of the exceptional point and sent it into the system -- either
in a clockwise or counterclockwise direction. Clockwise light traveled
along a tapered fiber guide until it encountered the mirror and was
reflected in the direction from where it came. Counterclockwise light
did not encounter the mirror, so it traveled the system once before
dispersing. Detectors on either end tracked light that escaped the system.



========================================================================== "Theoretically, light absorbed at an exceptional point or on an
exceptional surface will measure with a flat top resonance lineshape,"
O"zdemir said.

The lineshape refers to the shape of the measured spectrum. Light sent clockwise first entered the resonator at the exceptional point frequency
and was partially absorbed, with the rest continuing to the mirror. The reflected light traveled back through the resonator, perfectly overlapping
with the resonance and absorbing completely. The measured spectra revealed
a squared Lorentzian lineshape -- the formerly theoretical flat top. Light
sent in a counterclockwise direction only went through the resonator once, since it was not reflected. The measured results demonstrate the system's chirality, or how different directional inputs cause different behavior.

"We saw chiral perfect absorption in an exceptional surface in an open
system," said first author Sina Soleymani, postdoctoral researcher in engineering science and mechanics. Soleymani completed his doctoral
research on this project prior to graduating from Penn State in
2021. "The mirror allows the exceptional surface to form because
it couples the modes to one another in a chiral manner." Without the
mirror, the system behaves exactly the same when light is sent clockwise
or counterclockwise. The mirror makes the system chiral: a portion of
the light sent clockwise couples to counterclockwise direction after it
is reflected by the mirror, but light sent in the counterclockwise does
not couple to the clockwise direction.

"It's so stable and simple," Solyemani said. "Just a mirror in one end of
the waveguide gives us the beautiful surface to more easily investigate
both the classical and quantum dynamics at exceptional points." O"zdemir
is also affiliated with the Penn State Materials Research Institute.

Other contributors include Mohammad Mokim in Penn State's Department of Engineering Science and Mechanics; Q. Zhong and R. El-Ganainy, Michigan Technological University's Department of Physics and Henes Center for
Quantum Phenomena; and S. Rotter, Vienna University of Technology's
Institute for Theoretical Physics.

The U.S. Department of Defense Air Force Office of Scientific Research
has funded this work, in part, through a Multidisciplinary University
Research Initiative led by O"zdemir.

The U.S. Air Force Office of Scientific Research, the National Science Foundation, the Army Research Office, the Alexander von Humboldt
Foundation, the European Commission and the Austrian Science Fund also supported this research.

========================================================================== Story Source: Materials provided by Penn_State. Original written by
Ashley J. WennersHerron.

Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. S. Soleymani, Q. Zhong, M. Mokim, S. Rotter, R. El-Ganainy,
Ş. K.

O"zdemir. Chiral and degenerate perfect absorption on
exceptional surfaces. Nature Communications, 2022; 13 (1) DOI:
10.1038/s41467-022- 27990-w ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/02/220202153856.htm

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