Half Light, Half Matter

This image shows how researchers launched and studied half-light, half-matter quasiparticles, exciton-polaritons. A laser from the top left shines on the sharp tip of a nano-imaging system aimed at a flat semiconductor. The red circles inside the semiconductor are the waves associated with the quasiparticles. (Source: Z. Fei)

Zhe Fei pointed to the bright and dark vertical lines running across his computer screen. This nano-image, he explained, shows the waves asso­ciated with a half-light, half-matter quasi­particle moving inside a semi­conductor. “These are waves just like water waves,” said Fei, an Iowa State Uni­versity assistant professor of physics and astro­nomy and an asso­ciate of the U.S. Department of Energy’s Ames Labora­tory. “It’s like dropping a rock on the surface of water and seeing waves. But these waves are exciton-polari­tons.”

Exciton-pola­ritons are a combination of light and matter. Like all quasi­particles, they’re created within a solid and have physical properties such as energy and momentum. Now, they were launched by shining a laser on the sharp tip of a nano-imaging system aimed at a thin flake of molyb­denum dise­lenide, a layered semi­conductor that supports excitons. Excitons can form when light is absorbed by a semiconductor. When excitons couple strongly with photons, they create exciton-pola­ritons.

It’s the first time researchers have made real-space images of exciton-pola­ritons. Fei said past research projects have used spectro­scopic studies to record exciton-pola­ritons as resonance peaks or dips in optical spectra. Until recent years, most studies have only observed the quasi­particles at extremely cold temperatures – down to about -450 degrees Fahren­heit. But Fei and his research group worked at room tempera­ture with the scanning near-field optical micro­scope in his campus lab to take nano-optical images of the quasi­particles.

“We are the first to show a picture of these quasi­particles and how they propa­gate, interfere and emit,” Fei said. The researchers, for example, measured a propa­gation length of more than 12 microns for the exciton-polaritons at room tempera­ture. Fei said the creation of exciton-pola­ritons at room tempera­ture and their propa­gation charac­teristics are significant for developing future appli­cations for the quasi­particles. One day they could even be used to build nano­photonic circuits to replace elec­tronic circuits for nano­scale energy or infor­mation transfer. Fei said nano­photonic circuits with their large band­width could be up to 1 million times faster than current electrical circuits.

The researchers also learned that by changing the thickness of the flat semi­conductor, they could mani­pulate the properties of the exciton-pola­ritons. Fei, who has been studying quasi­particles in graphene and other 2-D materials since his graduate school days at University of Cali­fornia San Diego, said his earlier work opened the doors for studies of exciton-pola­ritons. “We need to explore further the physics of exciton-pola­ritons and how these quasi­particles can be mani­pulated,” he said. That could lead to new devices such as pola­riton tran­sistors, Fei said. And that could one day lead to break­throughs in photonic and quantum techno­logies. (Source: Iowa State U.)

Reference: F. Hu et al.: Imaging exciton–polariton transport in MoSe2 waveguides, Nat. Phot. 11, 356 (2017). DOI: 10.1038/nphoton.2017.65

Link: Div. of Materials Sciences and Engineering, Ames Laboratory, Iowa State University, Ames, USA

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