Transition metal dichalcogenide monolayers in microcavities

Semiconducting transition metal dichalcogenides (TMDs) such as MoS2, MoSe2, WS2 and WSe2 are a group of materials with much attention in recent years. Their crystal structure resembles those of graphene and consists of strongly covalently-bond two-dimensional sheets with only week van der Waals forces between the layers. Of special interests are single layers of semiconducting TMDs exhibiting direct bandgap and therefore are used as light emitting materials.

We investigate the whole family of TMDS in a specially designed, open microcavity structures. In the so-called strong coupling regime interactions between excitons and cavity photons lead to creation of new mixed light–matter quasiparticles: exciton–polaritons. Our recent achievements demonstrate exciton-polaritons at room temperature, spin selective valley polarization effects and strong coupling regime in multilayer TMDS layers.

Semimagnetic exciton-polariton condensates

Exciton-polaritons are quasi-particles formed by strong coupling of excitons in a quantum well and photons confined in optical cavity. Their bosonic character allows for the formation of a macroscopic quantum state: a non-equilibrium Bose-Einstein condensate. The semi-magnetic polaritons studied in our group form a unique class among polaritons.

The magneto-optical effects in semimagnetic polaritons are especially strong due to magnetic ions such as Mn2+ incorporated into crystal lattice. In external magnetic field magnetic moments of magnetic ions orients and through s,p-d interactions acts on charge carriers as an additional effective magnetic field. We demonstrated in those structures giant Zeeman splitting, spin Meissner effect and the formation of circularly polarized polariton condensates. 

Liquid crystal microcavities

Optical cavities allow for localization of electromagnetic field. Typical structures of microcavities contains alternating dielectric or semiconductor layers of different refractive indices with wedged shape allowing for finding resonance with the emitter, as quantum wells.

In our group we developed new kind of structure for investigation of strong coupling at room temperature using as a cavity material liquid crystals (LCs). Their use allow for ease tuning, important to adapt to the cavity resonance with the excitonic mode in the emitter and reaching strong coupling regime. Tunability is possible due to tilting of the anisotropic LC molecules induced by electric field applied to the whole structure. The big advantage of the LC microcavities is the simplicity of operation and the ability to work at room temperature. We investigate LC cavities with entire class of emitters: transition metal dichalcogenides, quantum dots, dyes, perovskites etc.

Spin-orbit coupling in liquid crystal microcavities

Spin-orbit coupling gives rise to many different interesting phenomena. We are focused on the spin-orbit interactions of light in LC optical cavities. This is extremally interesting system because of the tuning, and thus the ability to smoothly change the parameters describing the structure, as dielectric tensor. The rotation of the dielectric tensor leads to a change of TE-TM splitting. This makes LC cavity an interesting system to study optical spin Hall effect (OSHE).

We conducted optical measurements of reflectivity or transmission spectra in real and reciprocal space, which allowed us to examine the tunable optical spin Hall effect. In addition, we observed the optical counterpart of Rashba-Dresselhaus effect known from solid state physics. In the studied system, the spin-orbital interaction for light is observed in the form of oppositely circularly polarized parabolic cavity photon dispersions. For the appropriate angle of rotation, corresponding to the intersection of modes with opposite parity, the cavity modes lose their original linear polarization and create two not concentric, counter-circularly polarized, parabolic dispersion relations.

Ring-shaped excitation of exciton-polariton condensates

Exciton-polaritons are quasi-particles with bosonic properties. They can form Bose-Einstein condensate. Due to the short lifetime and strong interaction with non-condensed polaritons and excitons, they are considered to be in a nonequilibrium state. The nonequilibrium character can be reduced by spatial separation of the condensate from the non-condensed particles i.e. excitonic reservoir. This can be performed by a ring-shaped potential that confines polaritons in its centre. Exciton-polaritons created beneath the excitation laser flow outside the spot to the inside of the ring and exhibit properties more similar to the equilibrium state.

Our main motivation was to obtain a thermally-equilibrium condensate, which was possible owing to the use of a ring-shaped optical trap. Polaritons can flow into the interior of the ring, where, due to the lack of additional light, condensate could exist in separation from the cloud of non-condensed particles.