Scientists for the first time demonstrated Young's experiment for photons in
the reciprocal space. Spin patterns corresponding to the persistent spin
helix and the Stern-Gerlach experiment are realized in an optically
anisotropic liquid crystal microcavity. By applying electric voltage across
the microcavity, the liquid crystal molecules inside could be rotated in
such a way that the light passing through the cavity was forced to change
its internal state into right- and left-handed circular polarized
components.
Young's experiment from almost 220 years ago shows that when light waves
pass through two slits in a plate they undergo diffraction that creates an
image composed of many fringes (the so-called interference image). The
closer the slits are to each other, the further the interference fringes are
separated. In this way, the two slits transform information about the light
from position space into the so-called “reciprocal space” - the space of
directions. Changing the distance between the slits changes the angle (and
thus direction) at which the light is diffracted. From 1801, Young's
experiment has been carried out not only on light, but also on electrons,
atoms and even large molecules.
It turns out that a similar experiment can be performed in reciprocal space
where light beams emitted in two directions should also lead to a periodic
pattern in position space.
In an article published in Physical Review Letters, scientists from the
University of Warsaw, the Military University of Technology in Warsaw,
Institute of Physics Polish Academy of Sciences and the University of
Southampton for the first time demonstrated Young's experiment for photons
in the reciprocal space. For this purpose, a special optical microcavity
filled with a liquid crystal was prepared. The microcavity consists of two
perfect mirrors located so close to each other that a standing
electromagnetic wave is formed inside. By applying electric voltage across
the microcavity, the liquid crystal molecules inside could be rotated in
such a way that linearly polarized plane wave light passing through the
cavity was forced 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 similar to the situation of Young's experiment - this time, however,
the role of slits was played by two distinguished directions of light in the
"reciprocal space". On the sample surface - that is, on the "position space"
- an interference pattern of light polarization was observed, composed of
linearly polarized stripes. Previously, a similar phenomenon was observed
for electrons - modulation of the polarization of spins of electrons in
position space led to the formation of the so-called persistent spin helix.
It turned out that the liquid crystal microcavity led to the same
mathematical description of such a helix for the electron spin and for the
polarization of light. Scientists interpreted this phenomenon as a classic
entanglement of two degrees of freedom - the direction and polarization of
light.
The observation that the optical microcavity with a liquid crystal, in a
way, separates the “spin” of light - with the circular polarization playing
the role of the spin - almost coincided with the 100th anniversary of the
discovery of spin in the famous experiment of Stern and Gerlach in 1922.
Thus, in one work, an optical analogy of two fundamental experiments of
quantum mechanics were observed. The work was awarded by the editor of
Physical Review Letters and is included in the prestigious group of articles
" Physical Review Letters, Editors' Suggestion".
The research is carried out in the Polariton group at the Faculty of
Physics, University of Warsaw, led jointly by prof. Jacek Szczytko and prof.
Barbara Pietka in collaboration with the Military University of Technology,
the Institute of Physics of the Polish Academy of Sciences and the
University of Southampton. The first author is Mateusz Krol and Katarzyna
Rechcinska from the Faculty of Physics, University of Warsaw.
Reference:
Mateusz Krol, Katarzyna Rechcinska, Helgi Sigurdsson, Przemyslaw Oliwa, Rafal
Mazur, Przemyslaw Morawiak, Wiktor Piecek, Przemyslaw Kula, Pavlos G.
Lagoudakis, Michal Matuszewski, Witold Bardyszewski, Barbara Pietka, and Jacek
Szczytko, Realizing Optical Persistent Spin Helix and Stern-Gerlach Deflection
in an Anisotropic Liquid Crystal Microcavity, Phys. Rev. Lett. 127, 190401
(2021),
DOI: 10.1103/PhysRevLett.127.190401
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Physics