Exciton-polaritons (polaritons), hybrid quasiparticles formed by the strong coupling between semiconductor excitons and microcavity photons, are promising for developing low-threshold coherent light sources crucial for addressing the escalating energy demands of optical communications. The pursuit of efficient polariton condensation, a macroscopic quantum state analogous to Bose-Einstein condensation, is a central focus in this field. Bound states in the continuum (BICs), characterized by theoretically infinite quality factors (Q factors), offer a compelling pathway towards achieving this goal. Their non-radiative nature and prolonged lifetimes are exceptionally advantageous for accumulating polaritons and enhancing exciton-photon coupling. However, previous demonstrations of BIC polariton condensation have been limited to cryogenic temperatures, largely due to the small exciton binding energies of conventional materials like GaAs. Lead halide perovskites, known for their large exciton binding energies, present a promising alternative for achieving room-temperature operation. This research aims to overcome the limitations of cryogenic temperatures and explore the potential of perovskite-based BIC polariton condensation, offering a path to practical integrated photonic and topological circuits. The high optical gain, easily tunable bandgap, high defect tolerance, and good processability of perovskite single crystals make them ideal candidates for this application. Previous work has demonstrated BIC photonic lasing in perovskite single crystals, further solidifying their suitability for this pursuit. This study specifically investigates room-temperature BIC polariton condensation in perovskite-based photonic crystal (PhC) lattices, combining the advantages of high-quality BIC modes with the robust excitons of perovskite materials.

Extensive research has explored exciton-polariton condensation in various systems. Early studies demonstrated Bose-Einstein condensation of exciton polaritons at low temperatures, opening up possibilities for low-threshold coherent emitters, all-optical logic circuits, and quantum simulators. The quest for improved exciton-photon coupling has driven the exploration of microcavities with higher Q factors and longer coherence times. BICs, with their theoretically infinite Q factors, have emerged as a particularly attractive option. While BICs have shown promise in vortex beam generation, topological modulation, and low-threshold nanolasers in the linear regime, their potential in strong interaction scenarios for generating unrivaled nonlinearities and topological characteristics remained largely unexplored until recently. Recent advances demonstrated BIC polariton condensation in a patterned GaAs quantum well waveguide at cryogenic temperatures (-4 K). However, achieving room-temperature operation remains a significant challenge. Lead halide perovskites, with their large exciton binding energies, have shown potential for room-temperature polariton condensation, but demonstrations have been largely limited to vertical Fabry-Pérot microcavities, hindering their integration into compact, designable structures. This study builds upon the existing literature by exploring perovskite single crystal-based structures, aiming to exploit the strong coupling between excitons and BIC cavity photons to generate low-threshold, highly nonlinear, and topologically rich polariton condensates at room temperature.

This study employed all-inorganic cesium lead bromide (CsPbBr3) perovskite, chosen for its large exciton binding energy and superior optical properties. Large-area, single-crystalline CsPbBr3 microplatelets were synthesized on silicon substrates via chemical vapor deposition, leveraging the lattice match between the perovskite and silicon to achieve vertically grown microplatelets with exceptional crystal quality and smooth surfaces. Focused ion beam (FIB) milling was used to create a periodic air-hole PhC lattice on the CsPbBr3 microplatelets. The PhC lattice parameters (hole radius, thickness, and periodicity) were carefully controlled to optimize the BIC mode properties. A protective polymethyl methacrylate layer was added to protect the structure and match refractive indices. The transverse magnetic (TM) polarized energy-angle dispersion of Bloch resonances in the PhC lattice was calculated using the Lumerical FDTD software. Angle-resolved reflectance spectroscopy, using a custom-built Fourier imaging system, was employed to experimentally characterize the BIC modes. Nonlinear polariton interactions were investigated using femtosecond-pulsed laser excitation with a controlled spot size. The transition from the linear to superlinear regime was assessed through analysis of integrated intensity, linewidth, and energy blueshift as functions of pump density. Time-resolved and angle-resolved photoluminescence (PL) emission were obtained using a time-correlated single photon counting (TCSPC) system and an ultrafast optical Kerr gating (OKG) system to study temporal dynamics. Spatial and temporal coherence of the polariton condensates were characterized using a Michelson interferometer, enabling the measurement of first-order correlation functions (g(1)) and determination of coherence time. Far-field emission characteristics, including vortex beam profile and polarization singularity, were analyzed using Fourier space imaging and polarization-resolved measurements. Finally, switching between multiple orders of miniaturized BIC polariton modes was achieved using two excitation beams with varying time delays, examining the resulting angle-resolved PL spectra and far-field momentum-space images and polarization.

The research successfully demonstrated room-temperature BIC polariton condensation in perovskite PhC lattices. Strong exciton-photon coupling, with a Rabi splitting exceeding 150 meV, was achieved, evidenced by the clear anti-crossing behavior observed in the angle-resolved reflectance spectrum. The formation of BIC polariton condensates was confirmed through several key observations: a linear-to-superlinear transition in the integrated emission intensity, a narrowing of the emission linewidth, an energy blueshift of the BIC polariton emission, and the emergence of a distinct two-lobe emission pattern at the dispersion saddle point of LP3. Long-range spatial and temporal coherence of the condensate emission was demonstrated, exceeding the coherence time of the pump laser by an order of magnitude. The BIC polariton condensates exhibited directional vortex beam emission with a low divergence angle. Furthermore, this study achieved efficient switching between different orders of miniaturized BIC polariton modes (M11 and M12) through polariton-polariton scattering, demonstrating control over the topological properties of the condensate emission. The switching mechanism is based on the time evolution of the condensed BIC polariton density, offering a new degree of control over the system. The observed polarization singularity and vortex profile in the far-field emission confirm the topological nature of the miniaturized BIC polariton modes. The threshold for BIC polariton condensation was significantly lower than that of photonic lasing from pristine CsPbBr3 microplatelets, highlighting the efficiency of this approach.

The successful demonstration of room-temperature BIC polariton condensation in perovskite PhC lattices addresses a key challenge in the field of polaritonics. The use of perovskite materials with their large exciton binding energies, combined with the unique properties of BIC modes, allows for achieving this macroscopic quantum phenomenon at elevated temperatures. The observed long-range coherence and directional vortex beam emission open up exciting possibilities for applications in integrated photonics and topological devices. The ability to efficiently switch between multiple BIC polariton modes demonstrates a new level of control over the system's behavior, enabling dynamic manipulation of light at the nanoscale. These results are significant because they pave the way for developing room-temperature, compact, and energy-efficient devices based on polariton condensates, potentially leading to breakthroughs in optical communication, quantum information processing, and other areas. The enhanced nonlinear performance observed in the study, which can be tuned by adjusting the detuning energy, showcases the system's versatility and potential for optimization.

This research successfully achieved room-temperature BIC polariton condensation in perovskite air-hole PhCs. The high-quality BIC states suppressed radiative losses and facilitated polariton accumulation, resulting in condensates exhibiting long-range coherence and directional vortex beam emission. Moreover, the demonstration of switching between miniaturized BIC polariton modes with preserved topological properties highlights the potential of this platform for integrated photonic and topological applications. This work provides a critical step towards realizing practical, room-temperature coherent polariton condensates with orbital angular momentum for integrated photonic and topological circuits. Future research could focus on optimizing the fabrication processes to further enhance the Q factor and coherence time, exploring different perovskite materials and PhC designs, and investigating the potential of this platform for realizing more complex topological phenomena.

While the study demonstrates a significant advance in room-temperature BIC polariton condensation, certain limitations exist. The coherence time, although significantly longer than the pump laser, could be further improved through optimized material quality and fabrication techniques. The current switching mechanism relies on pulsed excitation; exploring continuous-wave excitation and alternative switching mechanisms would be beneficial for practical applications. Further investigations into the impact of defects and imperfections within the perovskite crystals on the condensate properties are needed. Finally, the scalability and integration of this platform into larger-scale integrated photonic circuits require further research and development.