The research results have been published in Physical Review Applied (https://doi.org/10.1103/PhysRevApplied.17.014041), the Faculty of Physics of the University of Warsaw reports in a press release.

'These beams are polarized circularly and directed at different angles', says Professor Jacek Szczytko from the Faculty of Physics of the University of Warsaw, quoted in the press release. This could be achieved by creating a persistent-spin helix on the surface of a microcavity.

To obtain this effect, scientists filled an optical microcavity with liquid crystal doped with an organic laser dye. The microcavity consists of two perfect mirrors placed close to each other - at a distance of 2-3 microns, so that a standing electromagnetic wave is formed inside. The space between the mirrors was filled with a special optical medium - liquid crystal, which was additionally organized using a special mirror coating.

'The characteristic feature of liquid crystals are their elongated molecules and, figuratively speaking, they were +combed+ on the surface of the mirrors and could stand up under the influence of an external electric field, also turning other molecules filling the cavity' comments first author, Marcin Muszyński from the Faculty of Physics of the University of Warsaw, quoted in the release.

The light in the cavity interacts with the molecules differently when the electric field of the propagating wave oscillates along the molecules, and differently, when oscillation is perpendicular. The liquid crystal is a birefringent medium: it can be characterized by two refractive indexes, which depend on the direction of the electric field oscillations (on the electromagnetic wave polarization). The precise arrangement of molecules inside the laser microcavity, obtained at the Military University of Technology resulted in the appearance of two linearly polarized light modes in the cavity - two standing waves of light with opposite linear polarizations. The electric field changed the orientation of the molecules inside the optical cavity, which changed the effective refractive index of the liquid crystal layers. Thus, it controlled the length of the optical path of light - the product of the width of the cavity and the refractive index, on which the energy (colour) of the emitted light depended. One of the modes did not change its energy as the molecules rotated, while the energy of the other increased as the orientation of the molecules changed.

By optically stimulating the organic dye placed between the molecules of medium, the researchers obtained the lasing effect - coherent light radiation with a strictly defined energy. The gradual rotation of the liquid crystal molecules led to unexpected properties of this lasing. The lasing was achieved for this tuneable mode: the laser emitted one linearly polarized beam perpendicular to the surface of the mirrors. The use of liquid crystals allowed for a smooth tuning of the light wavelength with the electric field by as much as 40 nm.

'However, when we rotated the liquid crystal molecules so that both energy of modes (the one sensitive to the orientation of the molecules and the one that did not change its energy) overlapped and were in resonance, the light emitted from the cavity suddenly changed its polarization from linear to two circular: right and left-handed, with both circular polarities propagating in different directions, at an angle of several degrees'. describes Professor Jacek Szczytko.

The phase coherence of the laser has been confirmed in an interesting way. 'A persistent-spin helix - a pattern of stripes with different polarization of light, spaced 3 microns apart - appeared on the surface of the sample. Theoretical calculations show that this pattern can be formed when two oppositely polarized beams are phase coherent and both modes of light are inseparable - this phenomenon is compared to quantum entanglement', explains Marcin Muszynski.

For now, the laser works in pulses, because the organic dye slowly photodegrades under the influence of intensive light. Scientists hope that replacing the organic emitter with more durable polymers or inorganic materials (such as perovskites) will allow to achieve a longer lifetime.

'The obtained precisely tuneable laser can be used in many fields of physics, chemistry, medicine and communication. We use nonlinear phenomena to create a fully optical neuromorphic network. This new photonic architecture can provide a powerful machine learning tool for solving complex classification and inference problems, and for processing large amounts of information with increasing speed and energy efficiency', adds Professor Barbara Pietka from the Faculty of Physics of the University of Warsaw.

The research is carried out by the Polariton group at the Faculty of Physics of the University of Warsaw, led jointly by Professor Jacek Szczytko and Professor Barbara Pietka in collaboration with the Military University of Technology and the University of Southampton. The first author is Marcin Muszynski from the Faculty of Physics of the University of Warsaw.

The research work was co-financed under the IDUB UW, NCN grants and UE H2020 FET Open TopoLight project.

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