Whirlpools of light-matter particles
Physicists from The Australian National University (ANU) have developed a spiral laser beam and used it to create a whirlpool of polaritons - hybrid particles that have properties of both matter and light. Their work has been published in the journal Physical Review Letters.
Polaritons form in semiconductors when laser light interacts with electrons and holes (positively charged vacancies) so strongly that it is no longer possible to distinguish light from matter. The ability to control polariton flows could aid the development of novel technology to link conventional electronics with new laser and fibre-based technologies.
Team leader Dr Elena Ostrovskaya, from the ANU Research School of Physics and Engineering, said the creation and control of circulating currents of polaritons (vortices) has been “a long-standing challenge”. Experimental leader Dr Robert Dall added, “The vortices have previously only appeared randomly, and always in pairs that swirl in opposite directions.”
The team created a spiral beam by putting a laser through a piece of brass with a spiral pattern of holes in it. This was directed into a semiconductor microcavity - a tiny wafer of aluminium gallium arsenide, which is used in LEDs - sandwiched between two reflectors. Dr Dall explained, “By using a spiral mask to structure our laser, we create a chiral system that prefers one flow direction. Therefore, we can create a single, stable vortex at will.”
The vortices are an example of quantum fluid behaviour in which the polaritons coalesce into a rare state of matter known as a Bose-Einstein condensate. Dr Ostrovskaya said the vortices provide “a window into the quantum world … [and] could be used to construct extremely sensitive detectors of electromagnetic fields … [or] employed as quantum information carriers”.
The team has pioneered the study of microcavity polaritons in Australia and hope their success will inspire other research groups around the country.
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