Engineers create helical topological exciton-polaritons

Date:
October 13, 2020
Source:
University of Pennsylvania
Summary:
Researchers have created an even more exotic form of the exciton-
polariton, one which has a defined quantum spin that is locked to
its direction of motion. Depending on the direction of their spin,
these helical topological exciton-polaritons move in opposite
directions along the surface of an equally specialized type of
topological insulator.



FULL STORY ==========================================================================
Our understanding of quantum physics has involved the creation of a wide
range of "quasiparticles." These notional constructs describe emergent phenomena that appear to have the properties of multiple other particles
mixed together.


==========================================================================
An exciton, for example, is a quasiparticle that acts like an electron
bound to an electron hole, or the empty space in a semiconducting material where an electron could be. A step further, an exciton-polariton combines
the properties of an exciton with that of a photon, making it behave like
a combination of matter and light. Achieving and actively controlling
the right mixture of these properties -- such as their mass, speed,
direction of motion, and capability to strongly interact with one another
-- is the key to applying quantum phenomena to technology, like computers.

Now, researchers at the University of Pennsylvania's School of Engineering
and Applied Science are the first to create an even more exotic form of
the exciton-polariton, one which has a defined quantum spin that is locked
to its direction of motion. Depending on the direction of their spin,
these helical topological exciton-polaritons move in opposite directions
along the surface of an equally specialized type of topological insulator.

In a study published in the journal Science, they have demonstrated
this phenomenon at temperatures much warmer than the near-absolute-zero
usually required to maintain this sort of quantum phenomenon. The ability
to route these quasiparticles based on their spin in more user-friendly conditions, and an environment where they do not back-scatter, opens
up the possibility of using them to transmit information or perform computations at unprecedented speeds.

The study was led by Ritesh Agarwal, professor in the Department of
Materials Science and Engineering, and Wenjing Liu, a postdoctoral
researcher in his lab.

They collaborated with researchers from Hunan University and George
Washington University.

The study also demonstrates a new type of topological insulators, a class
of material developed at Penn by Charles Kane and Eugene Mele that has
a conductive surface and an insulating core. Topological insulators are
prized for their ability to propagate electrons at their surface without scattering them, and the same idea can be extended to quasiparticles
such as photons or polaritons.



========================================================================== "Replacing electrons with photons would make for even faster computers
and other technologies, but photons are very hard to modulate, route
or switch.

They cannot be transported around sharp turns and leak out of the
waveguide," Agarwal says. "This is where topological exciton-polaritons
can be useful, but that means we need to make new types of topological insulators that can work with polaritons. If we could make this type
of quantum material, we could route exciton-polaritons along certain
channels without any scattering, as well as modulate or switch them via externally applied electric fields or by slight changes in temperature." Agarwal's group has created several types of photonic topological
insulators in the past. While the first "chiral" polariton topological insulator was reported by a group in Europe, it worked at extremely low temperatures while requiring strong magnetic fields The missing piece,
and distinction between "chiral" and "helical" in this case, was the
ability to control the direction of flow via the quasiparticles' spin.

"To create this phase, we used an atomically thin semiconductor,
tungsten disulfide, which forms very tightly bound excitons, and
coupled it strongly to a properly designed photonic crystal via
symmetry engineering. This induced nontrivial topology to the resulting polaritons," Agarwal says. "At the interface between photonic crystals
with different topology, we demonstrated the generation of helical
topological polaritons that did not scatter at sharp corners or defects,
as well as spin-dependent transport." Agarwal and his colleagues
conducted the study at 200K, or roughly -100F without the need for
applying any magnetic fields. While that seems cold, it is considerably
warmer -- and easier to achieve -- than similar systems that operate at
4K, or roughly -450F.

They are confident that further research and improved fabrication
techniques for their semiconductor material will easily allow their
design to operate at room temperature.

"From an academic point of view, 200K is already almost room temperature,
so small advances in material purity could easily push it to working in
ambient conditions," says Agarwal. "Atomically thin, '2D' materials form
very strong excitons that survive room temperature and beyond, so we think
we need only small modifications to how our materials are assembled."
Agarwal's group is now working on studying how topological polaritons
interact with one another, which would bring them a step closer to using
them in practical photonic devices.


========================================================================== Story Source: Materials provided by University_of_Pennsylvania. Original written by Evan Lerner. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Wenjing Liu, Zhurun Ji, Yuhui Wang, Gaurav Modi, Minsoo Hwang,
Biyuan
Zheng, Volker J. Sorger, Anlian Pan, Ritesh Agarwal. Generation
of helical topological exciton-polaritons. Science, 2020; eabc4975
DOI: 10.1126/science.abc4975 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/10/201013124155.htm

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