The optical Stern-Gerlach diversion and Youn

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Image: Young’s experiment for photons in reciprocal space. Spin patterns that correspond to the persistent spin helix.
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Photo: Mateusz Krol, Faculty of Physics UW

Scientists have demonstrated Young’s experiment for photons in reciprocal space for the first time. Spin patterns that correspond to the persistent spin helix and the Stern-Gerlach experiment are realized in an optically anisotropic liquid crystal microcavity. By applying an electrical voltage to the micro-cavity, 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 clockwise and counter-clockwise circularly polarized components.

Young’s experiment almost 220 years ago shows that light waves, when they pass through two slits in a plate, experience diffraction that creates an image composed of many stripes (the so-called interference image). The closer the slots are to each other, the further apart the interference fringes are. In this way, the two slits transform information about the light from the position space into the so-called “reciprocal space” – the space of directions. Changing the distance between the slits changes the angle (and therefore the direction) at which the light is diffracted. From 1801, Young’s experiment was carried out not only on light, but also on electrons, atoms and even large molecules.

It turns out that a similar experiment can be carried out in reciprocal space, where light rays emitted in two directions should also lead to a periodic pattern in spatial space.

In an article published in Physical Review Letters, scientists from the University of Warsaw, the Military Technical University of 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 for the first time. For this purpose, a special optical micro-cavity filled with a liquid crystal was made. The microcavity consists of two perfect mirrors that are so close to each other that a standing electromagnetic wave is formed inside. By applying an electrical voltage to the micro-cavity, 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-handed and left-handed circularly polarized components that deflected inwardly in opposite directions from the original beam path.

It was similar to Young’s experiment – but this time two different directions of light played the role of the slits in “reciprocal space”. On the sample surface – that is, on the “position space” – an interference pattern of the light polarization was observed, which consists of linearly polarized strips. A similar phenomenon was previously observed for electrons – the modulation of the polarization of electron spins in spatial 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 the polarization of light. Scientists interpreted this phenomenon as the classic entanglement of two degrees of freedom – the direction and polarization of light.

The observation that the optical microcavity with a liquid crystal separates the “spin” of the light in a certain way – whereby the circular polarization plays the role of the spin – almost coincided with the 100 Stern and Gerlach in 1922. So one work became an optical one Analogy of two fundamental experiments in quantum mechanics observed. The work was recognized by the editor of the Physical Review Letters and is included in the renowned article group “Physical Review Letters, Editors’ Suggestion”.

The research is carried out in the Polariton group at the Faculty of Physics of the University of Warsaw under the joint direction of 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 authors are Mateusz Krol and Katarzyna Rechcinska from the Faculty of Physics at the University of Warsaw.

This work was supported by National Science Center Grant 2019/35 / B / ST3 / 04147, 2019/33 / B / ST5 / 02658, 2018/31 / N / ST3 / 03046, and 2017/27 / B / ST3 / 00271. This project was funded under grant agreement No. 964770 (TopoLight) by the research and innovation program FET Open of the European Union Horizon 2020. HS and PGL thank the British Engineering and Physical Sciences Research Council (Grant EP / M025330 / 1 on Hybrid Polaritonics), the support of the RFBR project No. 20-52-12026 (together with the DFG) and No. 20 – 02-00919 and the European Union’s Horizon 2020 program through an FET action for open research and innovation under grant agreement No. 899141 (PoLLoC). HS thanks the Icelandic Research Fund, Grant No. 217631-051.

Physics and Astronomy at the University of Warsaw emerged in 1816 as part of the then Philosophical Faculty. In 1825 the Astronomical Observatory was founded. Currently, the Faculty of Physics of the University of Warsaw consists of the following institutes: Experimental Physics, Theoretical Physics, Geophysics, the Department of Mathematical Methods and the Astronomical Observatory. The research covers almost all areas of modern physics, on scales from quantum to cosmology. The faculty’s research and teaching staff consists of over 200 academic lecturers, including 81 professors. Around 1,000 students and over 170 doctoral students study at the Faculty of Physics at the University of Warsaw.

SCIENTIFIC PAPERS:

Mateusz Krol, Katarzyna Rechcinska, Helgi Sigurdsson, Przemyslaw Oliwa, Rafal Mazur, Przemyslaw Morawiak, Wiktor Piecek, Przemyslaw Kula, Pavlos G. Lagoudakis, Michal Matuszewski, Witold Bardyszewski, Barbarak Pietz and Jako Realization of an optically persistent spin helix and a Stern-Gerlach deflection in an anisotropic liquid crystal microcavity, Phys. Rev. Lett. 127, 190401 (2021), DOI: 10.1103 / PhysRevLett.127.190401

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.127.190401

CONTACTS:

Jacek Szczytko
Faculty of Physics University of Warsaw
Email: [email protected]

tel. +48 225532764

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Website of the Polariton group at the Faculty of Physics, University of Warsaw

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https://www.fuw.edu.pl/press-releases.html

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GRAPHIC MATERIALS:

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https://www.fuw.edu.pl/tl_files/press/images/2021/FUW211109b_fot01.jpg

Spin pattern corresponding to the persistent spin helix (Source: M. Krol, Faculty of Physics UW)


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