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Abstract A growing number of organic materials have recently been reported to achieve room‐temperature exciton‐polariton (polariton) condensation, which is an essential requirement for practical polaritonic applications. Notably, fluorescent dyes utilizing the small‐molecule, ionic isolation lattice (SMILES) method have solved the long‐standing challenges of conventional organic dyes and have been successfully implemented in cavities to realize condensation. However, almost all demonstrations of molecular polariton condensates have inherently large spectral linewidth and poor temporal coherence arising from intrinsic disorder and low quality (Q) factor of the cavity. Here, exciton‐polaritons are realised using fluorescent dye SMILES in a high Q factor microcavity and we observe polariton condensates with a linewidth of 175 µeV. These polariton condensates exhibit temporal coherence of 30.3 ± 8.0 ps, indicating the highly coherent nature of the narrow linewidth condensates. These results set the stage for realizing highly coherent and robust polaritonic devices operating at room temperature. 

The robust spin and momentum valley locking of electrons in two-dimensional semiconductors makes the valley degree of freedom of great utility for functional optoelectronic devices. Owing to the difference in optical selection rules for the different valleys, these valley electrons can be addressed optically. The electrons and excitons in these materials exhibit the valley Hall effect, where the carriers from specific valleys are directed to different directions under electrical or thermal bias. Here we report the optical analog of valley Hall effect, where the light emission from the valley-polarized excitons in a monolayer ${\mathrm{W}\mathrm{S}}_2$propagates in different directions owing to the preferential coupling of excitonic emission to the high momentum states of the hyperbolic metamaterial. The experimentally observed effects are corroborated with theoretical modeling of excitonic emission in the near field of hyperbolic media. The demonstration of the optical valley Hall effect using a bulk artificial photonic media without the need for nanostructuring opens the possibility of realizing valley-based excitonic circuits operating at room temperature. 

Color centers in hexagonal boron nitride (hBN) are presently attracting broad interest as a novel platform for nanoscale sensing and quantum information processing. Unfortunately, their atomic structures remain largely elusive and only a small percentage of the emitters studied thus far have the properties required to serve as optically addressable spin qubits. Here, we use confocal fluorescence microscopy at variable temperatures to study a new class of point defects produced via cerium ion implantation in thin hBN flakes. We find that, to a significant fraction, emitters show bright room-temperature emission, and good optical stability suggesting the formation of Ce-based point defects. Using density functional theory (DFT) we calculate the emission properties of candidate emitters, and single out the CeV_Bcenter—formed by an interlayer Ce atom adjacent to a boron vacancy—as one possible microscopic model. Our results suggest an intriguing route to defect engineering that simultaneously exploits the singular properties of rare-earth ions and the versatility of two-dimensional material hosts. 

Color centers in wide bandgap semiconductors are attracting broad attention for use as platforms for quantum technologies relying on room-temperature single-photon emission (SPE), and for nanoscale metrology applications building on the centers’ response to electric and magnetic fields. Here, we demonstrate room-temperature SPE from defects in cubic boron nitride (cBN) nanocrystals, which we unambiguously assign to the cubic phase using spectrally resolved Raman imaging. These isolated spots show photoluminescence (PL) spectra with zero-phonon lines (ZPLs) within the visible region (496–700 nm) when subject to sub-bandgap laser excitation. Second-order autocorrelation of the emitted photons reveals antibunching withg²(0) ∼ 0.2, and a decay constant of 2.75 ns that is further confirmed through fluorescence lifetime measurements. The results presented herein prove the existence of optically addressable isolated quantum emitters originating from defects in cBN, making this material an interesting platform for opto-electronic devices and quantum applications. 

Abstract Fluorescent proteins (FPs) have recently emerged as a serious contender for realizing ultralow threshold room temperature exciton–polariton condensation and lasing. This contribution investigates the thermalization of FP microcavity exciton–polaritons upon optical pumping under ambient conditions. Polariton cooling is realized using a new FP molecule, called mScarlet, coupled strongly to the optical modes in a Fabry–Pérot cavity. Interestingly, at the threshold excitation energy (fluence) of ≈9 nJ per pulse (15.6 mJ cm⁻²), an effective temperature is observed,T_eff ≈ 350 ± 35 K close to the lattice temperature indicative of strongly thermalized exciton–polaritons at equilibrium. This efficient thermalization results from the interplay of radiative pumping facilitated by the energetics of the lower polariton branch and the cavityQ‐factor. Direct evidence for dramatic switching from an equilibrium state into a metastable state is observed for the organic cavity polariton device at room temperature via deviation from the Maxwell–Boltzmann statistics atk_‖ = 0 above the threshold. Thermalized polariton gases in organic systems at equilibrium hold substantial promise for designing room temperature polaritonic circuits, switches, and lattices for analog simulation.