Physicists have created a laser that generates spiralling light, emitted by a surface with thickness a mere fraction of a human hair.
Light with a spiral – i.e. rotating polarisation – has been created before, but only in large bulky apparatuses. However, this new flat-optics device is so tiny that it could be incorporated into silicon chips and used for biosensors of proteins, opto-magnetic devices, or even to provide torque to micro-machines.
“Physicists have had a dream to separate light into left-handed and right-handed components in this way, but it was hard to work out how to do it,” said lead scientist Professor Yuri Kivshar, from the Nonlinear Physics Centre in RSPhys.
“We came up with a theory in 2020, but it took 2 years to devise a way to bring it to reality.”
Such a device hinges on a material that responds differently to light with left- or right-rotating polarisation. This property occurs in nature, for example in some crystals, but the response to the two polarisations differs only slightly; to separate the two parts needs a thick crystal to enable a long enough interaction to become significant.
In a publication in Science, Professor Kivshar and team relate how they drew on his work with a phenomenon known as Bound States in the Continuum, which enables light to be trapped in flat, precisely engineered dielectric structures. Clever manipulation of interference effects means light can be contained for surprisingly long periods of time - many hundreds of cycles of the light’s frequency.
Trapping light in this way increases interaction time, thereby removing the need for a thick material, but finding a thin material with rotationally asymmetric properties – known as chiral – proved tricky. A promising option was metamaterials – a class of thin materials that can be engineered to have a wide range of strong interactions with light, due to regular patterns of surface structures smaller than the wavelength of light.
Most metamaterials have surfaces patterned with symmetric cylinders or rectangles, like identical mini skyscrapers in a city grid – the proportions of these structures can be tweaked to precisely control how it interacts with light. But the puzzle that faced Professor Kivshar and the team was that they needed an asymmetric pattern, to interact with only one of the polarisation rotations.
Eventually Professor Kivshar and close collaborator Professor Maxim Gorkunov, from the Russian Academy of Sciences, came up with the idea to break the symmetry in the vertical direction. Modelling showed such a metamaterial would create the chiral bound states required to generate a spiral laser.
To fabricate the structures Professor Kivshar collaborated with the group of Professor Qinghai Song from China who used tilted structures of nanopillars, like a city of many leaning towers of Pisa. To their delight the tilted structures behaved as predicted, taking linearly polarised laser light and turning it to almost 100 percent rotationally polarised – with a quality factor of over 2500.
“The new chiral metasurfaces could help photonics to become more broadly applicable”, said Professor Kivshar. “They provide an exciting prospect for future chiral optoelectronics and devices, which is crucial to moving photonics closer in functionality to that of electronics”