Superfluid Light

The flow of polaritons encounters an obstacle in the superfluid regime. (Source: Polytec. Montreal)

Scien­tists have known for centuries that light is composed of waves. The fact that light can also behave as a liquid, rippling and spiraling around obstacles like the current of a river, is a much more recent finding that is still a subject of active research. The liquid properties of light emerge under special circum­stances, when the photons that form the light wave are able to interact with each other. Researchers from CNR Nanotec of Lecce in Italy, in colla­boration with Poly­technique Montreal in Canada have shown that for light dressed with electrons, an even more dramatic effect occurs. Light become superfluid, showing friction­less flow when flowing across an obstacle and recon­necting behind it without any ripples.

Daniele Sanvitto, leading the experimental research group that observed this pheno­menon, states that “Super­fluidity is an impressive effect, normally observed only at temperatures close to absolute zero (-273 degrees Celsius), such as in liquid Helium and ultracold atomic gasses. The extra­ordinary observation in our work is that we have demon­strated that super­fluidity can also occur at room-tempera­ture, under ambient conditions, using light-matter polaritons.”

“Super­fluidity, which allows a fluid in the absence of visco­sity to literally leak out of its container”, adds Sanvitto, “is linked to the ability of all the particles to condense in a state called a Bose-Einstein conden­sate, also known as the fifth state of matter, in which particles behave like a single macro­scopic wave, oscillating all at the same frequency. Something similar happens, for example, in super­conductors: electrons, in pairs, condense, giving rise to super­fluids or super-currents able to conduct elec­tricity without losses.”

These experi­ments have shown that it is possible to obtain super­fluidity at room-tempera­ture, whereas until now this property was achievable only at tempera­tures close to absolute zero. This could allow for its use in future photonic devices. Stéphane Kéna-Cohen, the coor­dinator of the Montreal team, states: “To achieve super­fluidity at room tempera­ture, we sandwiched an ultrathin film of organic molecules between two highly reflec­tive mirrors. Light interacts very strongly with the molecules as it bounces back and forth between the mirrors and this allowed us to form the hybrid light-matter fluid. In this way, we can combine the pr­operties of photons such as their light effective mass and fast velocity, with strong interactions due to the electrons within the molecules. Under normal conditions, a fluid ripples and whirls around anything that interferes with its flow. In a superfluid, this turbu­lence is suppressed around obstacles, causing the flow to continue on its way un­altered”.

“The fact that such an effect is observed under ambient condi­tions”, says the research team, “can spark an enormous amount of future work, not only to study funda­mental phenomena related to Bose-Einstein conden­sates with table-top experiments, but also to conceive and design future photonic super­fluid-based devices where losses are completely suppressed and new unex­pected phenomena can be exploited”. (Source: Polytec. Montreal)

Reference: R. Lerario et al.: Room-temperature superfluidity in a polariton condensate, Nat. Phys., online 05 June 2017; DOI: 10.1038/nphys4147

Links : CNR NANOTEC Institute of Nanotechnology, Lecce, Italy • Dept. of Engineering Physics, École Polytechnique de Montréal, Montréal, Canada

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