The University of Southampton

Novel take on Young's famous double-slit experiment could enable new opportunities for harnessing light

Published: 18 January 2022
The experiment demonstrated 'spin orbit coupling' of light. Image courtesy of Mateusz Krol/Faculty of Physics, University of Warsaw.

In an article published in Physical Review Letters and featured in Editors' Suggestions, a group of researchers including the University of Southampton's Dr Helgi Sigurdsson and Professor Pavlos Lagoudakis have demonstrated Young's famous double-slit experiment in the reciprocal space (the space of directions) rather than the conventional position space. This could enable new opportunities for controlling the properties of light.

Young's double-slit experiment from almost 220 years ago shows that when light waves pass through two slits in a plate they undergo a phenomenon known as diffraction. This creates an image composed of multiple bright fringes. By changing the distance between the slits, the angle and direction of the diffracted waves is affected which, in turn, affects the fringe separation in the interference image. In this way, the two slits transform information about the light from position space into the so-called reciprocal space - the space of directions or momentum.

Scientists from the Hybrid Photonics group at University of Southampton, the University of Warsaw, the Military University of Technology in Warsaw and the Institute of Physics Polish Academy of Sciences, have now discovered that a similar, inverse, experiment can be performed in the reciprocal space. To do this, they used a microscale optical resonator filled with a liquid crystal. The resonator, consisting of two mirrors facing each other, could trap photons, enabling the liquid crystal inside to twist their internal properties. By applying an electric voltage across the resonator, the liquid crystal molecules inside could be rotated so that linearly polarised light was forced to change into right- and left-handed circular polarised waves that belonged to two different 'slits' in reciprocal space.

Helgi from the University of Southampton's Hybrid Photonics Group says, "This type of an effect where polarisation and propagation direction of photons become intertwined, meaning a change in one causes a change in the other, is known in quantum mechanics as spin orbit coupling. Photonic spin orbit coupling is a relatively young field of research and full of opportunities to control the properties of light, like we show in our study."

The ?slits? in the reciprocal space resulted in a polarisation interference image of parallel fringes in the position space (i.e., the space of x-y-z coordinates). Such polarisation interference images had only been observed before in spins of electrons, dubbed the persistent spin helix. The scientists discovered that the liquid crystal microcavity led to the same pattern for the polarisation of light, referring to it as a photonic persistent spin helix.

The scientists then went even further and demonstrated that their liquid crystal microcavity could also separate the circular polarisations of light in position space. This meant that right hand circular polarised light would be deflected in one direction from the beam path and left hand circular light in the opposite. This observation coincided with the famous experiment of Stern and Gerlach in 1922 where the quantum mechanical nature of spin was discovered, but instead the scientists performed the experiment using light.

Therefore, during the one experiment, an optical analogy of two fundamental experiments of quantum mechanics were observed.

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