Young's experiment for photons in the reciprocal space. Spin patterns corresponding to the persistent spin Helix. Credit: Mateusz Krol, Faculty of Physics UW
Young's experiment for photons in reciprocal space has been demonstrated for the first time by scientists. In an optically anisotropic liquid crystal microcavity, spin patterns matching to the persistent spin helix and the Stern-Gerlach experiment are achieved. The liquid crystal molecules inside the microcavity may be rotated by putting electric voltage across it, forcing the light traveling through it to alter its internal state into right- and left-handed circular polarized components.
When light waves travel through two slits in a plate, they suffer diffraction, resulting in a picture formed of multiple fringes, as demonstrated by Young's experiment about 220 years ago (the so-called interference image). The interference fringes are spread further apart the closer the slits are to each other. The two slits convert information about the light from position space to "reciprocal space," or the space of directions, in this fashion. The angle (and consequently direction) at which light is diffracted changes as the distance between the slits changes. Young's experiment has been carried out on electrons, atoms, and even huge molecules since 1801, but not simply on light.
It turns out that a similar experiment may be carried out in reciprocal space, with light beams emitted in opposite directions producing a periodic pattern in position space.
Scientists from the University of Warsaw, the Military University of Technology in Warsaw, the Institute of Physics of the Polish Academy of Sciences, and the University of Southampton demonstrated Young's experiment for photons in reciprocal space in a paper published in Physical Review Letters. A specific optical microcavity filled with a liquid crystal was created for this purpose. The microcavity is made up of two perfect mirrors that are so close together that they produce a standing electromagnetic wave within.The liquid crystal molecules inside the microcavity could be rotated by applying electric voltage across it, forcing linearly polarized plane wave light passing through it to change its internal state into right- and left-handed circular polarized components that deflected in opposite directions from the original beam path.
It was identical to Young's experiment, except that instead of slits, two distinct directions of light in the "reciprocal space" fulfilled the function of slits. A linearly polarized interference pattern of light polarization was detected on the sample surface—that is, on the "position space." A similar phenomena was previously reported for electrons, when the polarization of electron spins in position space was modulated, resulting in the development of the so-called persistent spin helix. The liquid crystal microcavity led to the same mathematical description of such a helix for the electron spin as well as for light polarization. This phenomena was regarded by scientists as a conventional entanglement of two degrees of freedom—light direction and polarization.
The finding that the optical microcavity with a liquid crystal separates the "spin" of light—with circular polarization acting as the spin—almost coincided with the 100th anniversary of the discovery of spin in Stern and Gerlach's famous experiment in 1922. Thus, an optical analogue of two fundamental quantum physics experiments was discovered in one effort. Physical Review Letters published the research.
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