'It's a strange phenomenon, like locking up light in a prison with invisible walls', says Hugo Doeleman of the UvA Institute of Physics and the AMOLF group Resonant Nanophotonics. Doeleman explains that many of his colleagues are fascinated by a phenomenon discovered in 2013. This phenomenon, first considered in 1929 for the case of electrons, by physics giants John van Neumann and Eugene Wigner, turned out to apply to light as well. In physics lingo, the phenomenon is called a BIC: bound state in the continuum. The term BIC is used when light waves are captured somewhere, without anything stopping them. 'The light could go out, but it doesn't', says Doeleman.

Hurricane of light

In 2014, a group of researchers from MIT, Harvard and Yale, predicted that these BICs can be the center of a vortex of light. Together with his colleagues Wouter den Hollander (AMOLF) and Femius Koenderink (UvA/AMOLF), Doeleman has now experimentally shown the existence of such a vortex. They managed to create a hurricane of light, and showed that light is indeed captured inside a  so-called polarization vortex: the light cannot excape the eye. In this research, the Amsterdam physicists collaborated with two visiting scientists from the USA, Francesco Monticone and Andrea Alù.

To make this unexpectedly capured light, that is, a BIC, one needs a sample in which incoming laser light is scattered by two independent light resonators – similar to acoustic resonance boxes. The sample that was made at AMOLF consisted of a row of silicon nitride bars on a thin glass membrane. Interference between the light coming from the two resonators forms a whirlpool-like shape in the direction of the light's polarization. This occurs close to the point where the light is captured – the eye. Together with the visiting scientists, Doeleman developed a model that the researchers can use to show what happens inside the sample. 'In the eye of the whirlpool, the two resonators are balanced in such a way that they cancel each other out using interference, forming the invisible wall that imprisons the light.'

Robust situation

A special feature is that the vortex makes the seemingly captured light robust, says Doeleman. 'As long as the vortex exists, there is an eye. Tiny imperfections can move or deform the vortex, but if that happens, the eye just moves along with the vortex. It's like in a hurricane: if it moves, the eye moves as well. Only when the vortex completely disappears does the captured light escape as well.' This will be useful for applications in photonics: it means that the hurricane of light has a tolerance for errors, wich will always be present.

In the long run, light hurricanes could be useful for constructing small lasers. Doeleman: 'To make a good laser, one has to capture and focus light for some time.' Such a small laser could be placed on a photonic chip. 'But this is long term thinking. Right now, we are mostly interested in the phenomenon itself. We're only beginning to uncover the possibilities it has.'

Reference

Experimental observation of a polarization vortex at an optical bound state in the continuum, Hugo M. Doeleman, Francesco Monticone, Wouter den Hollander, Andrea Alù and A. Femius Koenderink, Nature Photonics. Doi: 10.1038/s41566-018-0177-5