Light is composed of waves, but it can also behave like a liquid. In certain circumstances it can ripple and spiral around obstacles. These liquid properties of light emerge when the photons that form the light wave interact with each other. The flow of polaritons encounters an obstacle in the supersonic (top) and superfluid (bottom) regime. Courtesy of Polytechnique Montreal. Researchers from Polytechnique Montreal and CNR Nanotec in Lecce, Italy have shown that combining light with electrons emits an even more dramatic effect. Researcher Daniele Sanvitto said the light can become a superfluid with frictionless flow across and around an obstacle without any ripples. “Superfluidity is an impressive effect, normally observed only at temperatures close to absolute zero, such as in liquid helium and ultra-cold atomic gasses,” said Sanvitto. “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 particles called polaritons." Superfluidity is linked to the ability of 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 at the same frequency. This is a schematic of the organic microcavity used to observe superfluid flow. Courtesy of Polytechnique Montreal. “To achieve superfluidity at room temperature, we sandwiched an ultrathin film of organic molecules between two highly reflective mirrors,” said researcher Stéphane Kéna-Cohen. “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 around anything that interferes with its flow. In a superfluid, the turbulence is suppressed around obstacles, causing the flow to continue unaltered. The teams experiments have shown that it is possible to obtain superfluidity at room temperature. This could allow for its use in future photonic superfluid-based devices where losses are completely suppressed and new unexpected phenomena can be exploited. The research has been published in the journal Nature Physics (doi:10.1038/nphys4147).