Scientists from the University of Warsaw, the Military University of Technology and the University of Southampton presented a new type of tunable micro­laser emitting two beams. These beams are polarized circularly and directed at different angles. This achievement was obtained by creating a persistent-spin helix on the surface of the micro­cavity. 

To achieve this effect, scientists around Jacek Szczytko from the faculty of physics filled the optical micro­cavity with a 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, turning also other molecules filling the cavity.

The light in the cavity interacts with the molecules in a different way, when the electric field of the propa­gating wave oscillates along the molecules and differently, when oscillation are perpen­dicular to them. 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, i.e. 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 i.e. 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 of the emitted light depends. 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 stimu­lating optically the organic dye placed between the molecules of the liquid crystal, lasing effect was obtained. The gradual rotation of the liquid crystal molecules leaded to unexpected properties of this lasing. The lasing was achieved for this tunable 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 nanometer. “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, 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”,  says Jacek Szczytko.

The phase coherence of the laser has been confirmed in an interesting way. “The persistent-spin helix –pattern of stripes with different polari­zation of light, spaced 3 microns apart – appeared on the surface of the sample. Theoretical calculations show that such a 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”, says Marcin Muszynski.

So far, the laser works in pulses, because the organic dye that was used slowly photo­degrades under the influence of intensive light. Scientists hope that replacing the organic emitter with more durable polymers or inorganic materials (e.g. perovskites) will allow for longer lifetime. “The obtained precisely tunable 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”, says Barbara Pietka from the faculty of physics. (Source: U Warsaw)

Reference: M. Muszyński et al.: Realizing Persistent-Spin-Helix Lasing in the Regime of Rashba-Dresselhaus Spin-Orbit Coupling in a Dye-Filled Liquid-Crystal Optical Microcavity, Phys. Rev. Applied 17, 014041 (2022); DOI: 10.1103/PhysRevApplied.17.014041

Link: Institute of Experimental Physics (J. Szczytko), Faculty of Physics, University of Warsaw, Warsaw, Poland