Dirac Cones and Room Temperature Polariton Lasing Evidenced in an Organic Honeycomb Lattice

Artificial one- and two-dimensional lattices have emerged as a powerful platform for the emulation of lattice Hamiltonians, the fundamental study of collective many-body effects, and phenomena arising from non-trivial topology. Exciton-polaritons, bosonic part-light and part-matter quasiparticles, combine pronounced nonlinearities with the possibility of on-chip implementation. In this context, organic semiconductors embedded in microcavities have proven to be versatile candidates to study nonlinear many-body physics and bosonic condensation, and in contrast to most inorganic systems, they allow the use at ambient conditions since they host ultra-stable Frenkel excitons. We implement a well-controlled, high-quality optical lattice that accommodates light-matter quasiparticles. The realized polariton graphene presents with excellent cavity quality factors, showing distinct signatures of Dirac cone and flatband dispersions as well as polariton lasing at room temperature. This is realized by filling coupled dielectric microcavities with the fluorescent protein mCherry. We demonstrate the emergence of a coherent polariton condensate at ambient conditions, taking advantage of coupling conditions as precise and controllable as in state-of-the-art inorganic semiconductor-based systems, without the limitations of e.g. lattice matching in epitaxial growth. This progress allows straightforward extension to more complex systems, such as the study of topological phenomena in two-dimensional lattices including topological lasers and non-Hermitian optics.

💡 Research Summary

The authors present a novel organic polariton platform that realizes a honeycomb (graphene‑like) lattice of exciton‑polaritons operating at room temperature. The active medium is the fluorescent protein mCherry, which provides ultra‑stable Frenkel excitons. The device is fabricated by first milling hemispherical dimples into a borosilicate substrate using focused ion beam (FIB) to create a two‑dimensional honeycomb pattern. Nine pairs of SiO₂/TiO₂ layers are then deposited to form a high‑reflectivity distributed Bragg reflector (DBR). A highly concentrated mCherry solution is spin‑coated onto an identical planar DBR, and the patterned DBR is mechanically pressed onto it, sandwiching the organic layer between the two mirrors. The resulting microcavity exhibits an individual‑site quality factor Q≈4 800 and a lattice‑averaged Q≈1 200–1 400, with a Rabi splitting of 318 meV measured on the planar region.

Angle‑resolved photoluminescence (PL) reveals a well‑defined band structure: the lowest S‑band displays linear Dirac‑cone crossings at the K and K′ points, while higher‑order P‑bands include a nearly flat band arising from destructive interference of local modes. Full‑k‑space tomography and real‑space imaging confirm the expected honeycomb symmetry and allow extraction of the anti‑binding S‑band and flat‑band intensity distributions. By fitting the dispersion with a tight‑binding model that includes nearest‑neighbor (t) and next‑nearest‑neighbor (t′) couplings, the authors obtain t′/t≈0.11, a ratio that matches values reported for inorganic GaAs‑based polariton lattices (see Table 1). This demonstrates that the hemispherical microcavity approach provides coupling strengths comparable to state‑of‑the‑art semiconductor platforms despite the completely different fabrication route.

Nonlinear behavior is probed using a nanosecond‑pulsed optical parametric oscillator tuned to 532 nm (the first Bragg minimum of the top DBR). As the pump fluence is increased from ~0.2 µJ/pulse to >1 µJ/pulse, a pronounced threshold is observed at ≈0.5 µJ/pulse: the integrated emission intensity jumps by three orders of magnitude, the spectral linewidth narrows sharply, and a modest blueshift (~0.6 meV) appears, consistent with phase‑space filling in the exciton‑polariton system. Lasing occurs preferentially in a d‑band mode; the exact lasing mode can be tuned by adjusting exciton‑photon detuning, pump spot size, and position, allowing a shift of up to ~20 meV.

To verify spatial coherence, a Michelson interferometer in a mirror‑prism configuration is employed. Above threshold, clear interference fringes are observed across the entire field of view, and the first‑order correlation function g^(1)(r) remains sizable over distances exceeding 10 µm, i.e., across many unit cells of the lattice. This confirms that the polariton condensate forms a macroscopic coherent state extending throughout the artificial graphene.

The work establishes that organic microcavities incorporating fluorescent proteins can achieve high‑Q, strong‑coupling polariton physics at ambient conditions, with band structures and coupling parameters on par with inorganic semiconductor systems. The demonstrated room‑temperature polariton lasing and long‑range coherence open pathways toward exploring topological polariton phases, non‑Hermitian photonics, and complex many‑body Hamiltonians in a facile, low‑cost platform that avoids lattice‑matching constraints of epitaxial growth. Future extensions could include engineered gauge fields, spin‑orbit coupling, and integration with electrical injection schemes, positioning organic polariton lattices as a versatile testbed for both fundamental quantum optics and emerging photonic technologies.