Photonic spin-orbit coupling and topology in organic microcavities

Organic semiconductors have received considerable attention as the active medium in microcavity devices that exploit the regime of strong exciton–photon coupling. The eigenstates of these systems are microcavity polaritons, whose properties are an admixture of the uncoupled exciton and photon. Organic microcavities are particularly interesting due to their large exciton binding energy which permits the electrical excitation of polaritons at room temperature. Measurements of electroluminescence are often facilitated through the use of metallic reflectors that form the optical microcavity and also serve as device electrodes. Here, we demonstrate that such structures exhibit a significant polarization splitting under both optical and electrical excitation. The size of the polarization splitting rivals those observed in strongly coupled microcavities based on distributed Bragg reflectors having a long optical penetration depth.

Observation of strong exciton–photon coupling in an organic microcavity

Observation of strong exciton–photon coupling in an organic microcavity in transmission and photoluminescence

Using an extended Tavis-Cummings model, we study the effect of the spin-orbit coupling between the singlet and the triplet molecular excitons in organic microcavities in the strong coupling regime. The model is solved in the single excitation space for polaritons, which contains the bright (permutation symmetric) singlet and triplet excitons, as well as the dark bands consisting of the nonsymmetric excitons of either type. We find that the spin-orbit coupling splits the lower polariton into two branches, and also creates a triplet polariton when the cavity mode is in resonance with the triplet excitons. The optical absorption spectrum of the system that can reveal this splitting in experiments is presented and the effect of disorder in exciton energies and couplings is explored. An important consequence of the disorder in the spin-orbit coupling -- a weak coupling between the otherwise decoupled bright and dark sectors -- is explored and detailed calculations of the squared transition matrix elements between the dark bands and polaritons are presented along with derivation of some approximate yet quite accurate analytical expressions. This relaxation channel for the dark states contains an interference between two transition paths that, for a given polariton state, suppresses the relaxation of one dark band and enhances it for the other.

We observe hybrid states of cavity photons and Tamm plasmons in an organic microcavity with an incorporated thin silver layer of increasing thickness up to 40 nm. Via μ-photoluminescence spectroscopy, we investigate their angular dependence. At oblique angles, we observe a TE-TM polarization splitting of more than 40 meV for each mode. An analytical model is developed to describe the coupling of Tamm plasmons and cavity photons and to account for the splitting of the orthogonally polarized resonances.

The polarization control of micro- and nanolasers is an important topic in nanophotonics. Up to now, the simultaneous generation of two distinguishable orthogonally polarized lasing modes from a single organic microlaser remains a critical challenge. Here, we demonstrate simultaneously orthogonally polarized dual lasing from a microcavity filled with an organic single crystal exhibiting selective strong coupling. We show that the non-Hermiticity due to polarization-dependent losses leads to the formation of real and imaginary Fermi arcs with exceptional points. Simultaneous orthogonally polarized lasing becomes possible thanks to the eigenstate mixing by the photonic spin-orbit coupling at the imaginary Fermi arcs. Our work provides a novel way to develop linearly polarized lasers and paves the way for the future fundamental research in topological photonics, non-Hermitian optics, and other fields.

The optical spin Hall effect (OSHE) bridges photonics and spintronics by enabling spin-dependent manipulation of light-where the "spin" of light refers to its polarization state-which is critical for on-chip photonic technologies. While OSHE with distinct topological textures has been demonstrated separately using transverse electric-transverse magnetic (TE-TM) splitting or birefringent crystals, achieving multiple spin textures within a single system remains elusive. Here, we report the first observation of OSHE driven by the interplay between TE-TM splitting and Rashba-Dresselhaus spin-orbit coupling (RDSOC) in an organic microcavity at room temperature. Polarization-resolved measurements reveal hybrid spin textures: quadrupole patterns at high momenta from TE-TM splitting and mirror-symmetric textures at low momenta from RDSOC. This interplay generates a persistent spin bias with a polarization lifetime of ∼300ps, indicating robust spin coherence for stable spin-photonic and polarization-preserving devices. Our findings establish organic microcavities as versatile platforms for engineering hybrid spin-orbit coupling, advancing topological photonics and integrated spin-based information processing.

Exciton‐polaritons in organic microcavities are applied in devices including lasers, light‐emitting devices, and photodetectors, as well as in structures capable of tuning exciton kinetics and energy transfer. To enable a broader tailoring of polariton properties, it is important to develop means to better control molecular orientation and tune the intensity of the exciton–photon interaction. Vapor‐processed, glassy organic thin films are previously shown to have tunable molecular orientation as evidenced by phenomena including birefringence and transition dipole moment (TDM) alignment. Here, this tunability in TDM orientation with thin film processing conditions is exploited to continuously vary the interaction between the exciton and confined cavity photon mode. By embedding a thin film of 4,4′‐bis[(N‐carbazole)styryl]biphenyl (BSB‐Cz) in a metal‐reflector microcavity, ultrastrong coupling and hybridization of multiple electronic transitions of BSB‐Cz are demonstrated with a common cavity mode. Increasing the temperature during BSB‐Cz deposition tunes the TDM orientation from predominantly in‐plane to random to slightly vertical. This leads to a corresponding ≈30% variation in the associated Rabi splitting, consistent with theoretical predictions. This work demonstrates a means to continuously tune coupling strength from a materials perspective while also providing a handle to tune orientation disorder in thin film.

The ability to control the spin‐orbit interaction (SOI) of light in optical microresonators is of fundamental importance for future photonics. Organic microcrystals, due to their giant optical anisotropy, play a crucial role in spin‐optics and topological photonics. Here, the controllable and wavelength‐dependent Rashba–Dresselhaus (RD) SOI is realized that is attributed to the anisotropic excitonic response in an optical microcavity filled with an organic microcrystalline. This work investigates the transition of the spin‐splitting from twice winding caused by the splitting of the transverse‐electric and transverse‐magnetic modes to once winding caused by the RD effect. The interplay of the two allows engineer the SOI of light in organic microcavities, which besides its fundamental interest promises applications in spin‐controlled on‐chip integrated nanophotonic elements, toward exploiting nonmagnetic and low‐cost spin‐photonic devices.

Effective manipulation of photonic spin–orbit coupling (SOC) in microcavities is of fundamental importance within topological photonics and applications. Anisotropic organic single‐crystalline materials can induce abundant SOC phenomenon due to their flexible tunability of molecular geometries, however, the intrinsic relationship between molecular geometries/orientations in 3D space and photonic SOC is lacking. In this study, we design two kinds of 2D organic polymorphs for the construction of organic microcavities to investigate the structure‐performance relationships. In two polymorphic microcavities, two distinctive photonic SOC phenomena are observed regardless of the in‐plane anisotropy of organic polymorphs. Theoretical analysis indicates that the photonic SOC strength is strongly influenced by the synergies between the crystal anisotropy and the tilted collective molecular transition dipole moment. Our results uncover the correlation mechanism between the structure of molecules and photonic SOC and open an avenue to engineer complex photonic SOC by use of organic microstructures towards the development of diverse integrated photonic devices.

Exciton-polaritons are half-light, half-matter bosonic quasiparticles formed by strong exciton-photon coupling in semiconductor microcavities. These hybrid particles possess the strong nonlinear interactions of excitons and keep most of the characteristics of the underlying photons. As bosons, above a threshold density they can undergo Bose-Einstein condensation to a polariton condensate phase and exhibit a rich variety of exotic macroscopic quantum phenomena in solids. Recently, organic semiconductors have been considered as a promising material platform for these studies due to their room-temperature stability, good processability, and abundant photophysics and photochemistry. Herein, recent advances of exciton-polaritons and their Bose-Einstein condensates in organic semiconductor microcavities are summarized. First, the basic physics is introduced, and then their emerging applications are highlighted. The remaining questions are also discussed and a personal viewpoint about the potential directions for future research is given.

The modification and control of exciton–photon interactions in semiconductors is of both fundamental1,2,3,4 and practical interest, being of direct relevance to the design of improved light-emitting diodes, photodetectors and lasers5,6,7. In a semiconductor microcavity, the confined electromagnetic field modifies the optical transitions of the material. Two distinct types of interaction are possible: weak and strong coupling1,2,3,4. In the former perturbative regime, the spectral and spatial distribution of the emission is modified but exciton dynamics are little altered. In the latter case, however, mixing of exciton and photon states occurs leading to strongly modified dynamics. Both types of effect have been observed in planar microcavity structures in inorganic semiconductor quantum wells and bulk layers1,2,3,4,5,6,7,8. But organic semiconductor microcavities have been studied only in the weak-coupling regime9,10,11,12,13,14,15,16,17,18. Here we report an organic semiconductor microcavity that operates in the strong-coupling regime. We see characteristic mixing of the exciton and photon modes (anti-crossing), and a room-temperature vacuum Rabi splitting (an indicator of interaction strength) that is an order of magnitude larger than the previously reported highest values for inorganic semiconductors. Our results may lead to new structures and device concepts incorporating hybrid states of organic and inorganic excitons19, and suggest that polariton lasing20,21,22 may be possible.

The large exciton binding energies and oscillator strengths of organic semiconductors have allowed the realization of strong exciton–photon coupling at room temperature in microcavities containing a wide variety of materials. The first part of this chapter reviews the physics of organic semiconductors and initial observations of cavity polaritons in this class of materials. In the second part, the linear optical properties of crystalline organic microcavities are discussed and contrasted to previous organic and inorganic microcavity results. The chapter concludes with a discussion of recent organic polariton lasing results and future prospects for realizing nonlinear behavior using organic polaritons.

Optical strong coupling in microcavities containing J-aggregates absorbing in near-infrared spectral range

Small-footprint, low-power, and reprogrammable arrays of coupled coherent emitters are highly sought in modern nanophotonics. Among existing solutions, only inorganic semiconductor microcavities operating in a strong light-matter coupling regime exhibit controlled on-chip interaction between individual coherent states, predominantly at cryogenic temperatures. Here, we demonstrate electrically controlled in-plane interaction between optically reconfigurable spatially separated lasing states, operating at room temperature in the weak light-matter coupling regime. An organic liquid crystal-filled microcavity is introduced as a new material platform where a spatially extended coherent lasing state, or “supermode", appears due to the blueshift-induced near-field transverse coupling between distinct spatially pumped states. We demonstrate a wide-range microscale control of supermode near- and far-field with on-chip phase-locking tuning functionality. We realize electrical control over the interaction strength between lasing states and corresponding mutual coherence going beyond nearest neighbours, and a spin-selective directional coupling regime by using a photonic analogue of the Rashba-Dresselhaus spin-orbit interaction.