Skip to main content
European Commission logo print header

SOFT MATTER PLATFORM FOR OPTICAL DEVICES VIA ENGINEERING OF NON-LINEAR TOPOLOGICAL STATES OF LIGHT

Periodic Reporting for period 2 - TopoLight (SOFT MATTER PLATFORM FOR OPTICAL DEVICES VIA ENGINEERING OF NON-LINEAR TOPOLOGICAL STATES OF LIGHT)

Reporting period: 2022-09-01 to 2023-08-31

Liquid crystals are advanced materials known for their anisotropic optical properties allowing to control the polarization of light and are used in various optical devices. Now the time has come to push the liquid crystals applications further by implementing them into novel polariton devices to control topological properties of light. TopoLight deals for the first time with non-linear effects in room temperature Bose-Einstein condensate and topological states of light uncovering astonishing possibilities of external electrical control over spin-orbit interaction due to artificially engineered fields acting on photons. Our platform will combine a strong emissivity with the ease of fabrication, low costs, and scalability and room temperature operation.

The results of TopoLight will have an impact on all various scientific fields. TopoLight will embed control of topological light where a control of polarization. Another important topic, which will be addressed by TopoLight, concerns the tunability of photonic structures, which is a difficult issue. TopoLight will open possibilities for the realization of practical, versatile chiral lasers which offer new opportunities for information encoding, as well as novel spatial light modulation devices active both on the polarization and spectral bandwidth of light.

For instance, in 2022 scientists from the TopoLight consortium 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 the so-called a persistent-spin helix on the surface of the microcavity. The results have been published in "Physical Review Applied". (Physical Review Applied, 17, 014041 (2022)).

To achieve this effect, scientists 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 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, i.e. this interaction depends on the electromagnetic wave polarization. The precise arrangement of molecules inside the microcavity resulted in the appearance of two linearly polarized light modes in the cavity - i.e. two standing waves of light with opposite linear polarizations. One mode 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 - coherent light radiation with a strictly defined energy. 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. However, when the liquid crystal molecules were rotated 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 (they 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.

The phase coherence of the laser has been confirmed in an interesting way. The so-called 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 (Physical Review Letters, 127, 190401 (2021)).

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.
Novel liquid crystal microcavity structures were proposed and tested to demonstrate spin-orbit coupling of light, tuneability of liquid crystal microcavities and controlled light emission.
Optical persistent spin helix and Stern-Gerlach experiment was demonstrated.
We achieved classically entangled emission in dye-doped liquid crystal microcavity and chiral lasing.
We achieved electrical control over topological exceptional points of light in a liquid crystal microcavity.
Condensation can occur at the Dirac points in staggered polariton graphene and the symmetry breaking occurring during the condensate build-up leads to the formation of valley-polarized domains.
Giant effective Zeeman splitting in a monolayer semiconductor realized by spin-selective strong light–matter coupling was demonstrated.
Optical isolators are required to implement any photonic circuits. These isolators are so far macroscopic devices based on the Faraday effect. In the Rashba-Dresselhaus regime our device already separates circularly- polarized light into two counter-propagating planar modes of the microcavity with transverse angular momentum. TopoLight will further apply this phenomenon to demonstrate miniaturized topological optical isolator integrated with an emitting device. Another promise of topological photonics technology is guiding light around sharp, subwavelength corners. TopoLight will demonstrate the feasibility of integrable photonic devices with topological elements. They will be working at optical wavelength, using scalable micro-devices, easy and cheap to fabricate, with excellent basic optical properties.

A perspective developed technology of generation of topological light with photon entanglement has a natural application in quantum cryptography. Using a structural light source, by shaping higher optical angular momentum (OAM) modes, one can achieve multi-dimensional Quantum Key Distribution tools consistent with experimentally confirmed technique developed for static and dynamic optical vortices, which promise preservation of entanglement with fiber OAM states . The advantage of the proposed technique will be the ability to modulate "on demand" a portion of light with the use of the source itself. This promises a new level of the data transfer safety for the society.

The participation of four academic institutions will create opportunities for training early career researchers (MSc, PhD and post-doctoral) who will work with us in interdisciplinary international environment and become the next generation of leaders in Europe.
Tunable microlaser emitting two circularly polarized beams (illustration)
TopoLight logo