We show the first experimental demonstration of room-temperature exciton–polariton (EP) condensation from a bound state in the continuum (BIC). This demonstration is achieved by strongly coupling stable excitons in an organic perylene dye with the extremely long-lived BIC in a dielectric metasurface of silicon nanoparticles. The long lifetime of the BIC, mainly due to the suppression of radiation leakage, allows for EP thermalization to the ground state before decaying. This property results in a condensation threshold of less than 5 μJ cm–2, 1 order of magnitude lower than the lasing threshold reported in similar systems in the weak coupling limit.

https://pubs.acs.org/doi/pdf/10.1021/acs.nanolett.3c01102

Room Temperature Exciton–Polariton Condensation in Silicon Metasurfaces Emerging from Bound States in the Continuum

Abstract Exciton–polaritons (polaritons) resulting from the strong exciton–photon interaction stimulates the development of novel low-threshold coherent light sources to circumvent the ever-increasing energy demands of optical communications 1–3 . Polaritons from bound states in the continuum (BICs) are promising for Bose–Einstein condensation owing to their theoretically infinite quality factors, which provide prolonged lifetimes and benefit the polariton accumulations 4–7 . However, BIC polariton condensation remains limited to cryogenic temperatures ascribed to the small exciton binding energies of conventional material platforms. Herein, we demonstrated room-temperature BIC polariton condensation in perovskite photonic crystal lattices. BIC polariton condensation was demonstrated at the vicinity of the saddle point of polariton dispersion that generates directional vortex beam emission with long-range coherence. We also explore the peculiar switching effect among the miniaturized BIC polariton modes through effective polariton−polariton scattering. Our work paves the way for the practical implementation of BIC polariton condensates for integrated photonic and topological circuits. 

Exciton polariton condensation from bound states in the continuum at room temperature

Multicolor light sources can be used in applications such as lighting and multiplexing signals. In photonic and plasmonic systems, one way to achieve multicolor light is via multimode lasing. To achieve this, plasmonic nanoparticle arrays are typically arranged in superlattices that lead to multiple dispersions of the single arrays coupled via the Bragg superlattice Bragg modes. Here, we show an alternative way to enable multimode lasing in plasmonic nanoparticle arrays. We design a supercell in a square lattice by leaving part of the lattice sites empty. This results in multiple dispersive branches caused by the supercell period and hence creates additional band edges that can support lasing. We experimentally demonstrate multimode lasing in such a supercell array. Furthermore, we identify the lasing modes by calculating the dispersion by combining the structure factor of the array design with an empty lattice approximation. We conclude that the lasing modes are the 74th Γ- and 106th X-point of the supercell. By tuning the square lattice period with respect to the gain emission, we can control the modes that lase. Finally, we show that the lasing modes exhibit a combination of transverse electric and transverse magnetic mode characteristics in polarization-resolved measurements. 

Multimode Lasing in Supercell Plasmonic Nanoparticle Arrays

Most of the nanophotonic devices such as metasurfaces control light by coupling external waves to the eigenmodes of the nanostructure. Notably, nonlocal modes have garnered considerable attention owing to their exceptional high-quality factor characteristics. Specifically, employing multiple nonlocal modes provides the unique benefit of broadening the detection range for a variety of sensing applications. However, the simultaneous alteration of these modes by altering the geometric parameters impedes the ability to execute distinct functions across each spectrum. Thus, the necessity of disentangling the interdependence of these modes and regulating them individually is underscored. In this work, we demonstrated the independent regulation of multiple nonlocal modes by introducing an eigenmode termed a quasi-trapped guided mode (QTGM). We explore the characteristics of the QTGM within an integrated structure of a metasurface manifesting a quasi-trapped mode (QTM) and a waveguide capable of exhibiting a leaky guided mode (LGM). QTGM inherits the characteristics of both QTM and LGM, rendering it especially responsive to variations in the in-plane symmetry of the metasurface notwithstanding its confinement within the waveguide slab. Conversely, LGM is confined within the waveguide slab and remains unresponsive to such perturbations, thereby enabling the exclusive alteration of QTGM. Additionally, we demonstrated that this methodology permits the tuning of the Rabi-splitting and quality factor of Friedrich–Wintgen bound states in the continuum. The proposed approach offers significant potential for a wide array of applications that leverage multiple nonlocal modes, including biomolecular sensing, multispectral filtering, and multiphoton nonlinear processes. 

Quasi-Trapped Guided Mode in a Metasurface Waveguide for Independent Control of Multiple Nonlocal Modes

This paper reports how a hybrid system composed of transparent dielectric lattices over a metal mirror can produce high-quality lattice resonances for unidirectional lasing. The enhanced electromagnetic fields are concentrated in the cladding of the periodic dielectric structures and away from the metal. Based on a mirror-image model, we reveal that such high-quality lattice resonances are governed by bound states in the continuum resulting from destructive interference. Using hexagonal arrays of titanium dioxide nanoparticles on a silica-coated silver mirror, we observed lattice resonances with quality factors of up to 2750 in the visible regime. With the lattice resonances as optical feedback and dye solution as the gain medium, we demonstrated unidirectional lasing under optical pumping, where the array size was down to 100 μm × 100 μm. Our scheme can be extended to other spectral regimes to simultaneously achieve strongly enhanced surface fields and high quality factors.

Unidirectional Lasing from Mirror-Coupled Dielectric Lattices.

Spatially extended bound states in the continuum (BICs) can provide high-quality optical feedback for vortex lasing. This work reports that three out of the four supported BIC modes in 2D square lattices of dye-covered TiO2 nanoparticles can lase simultaneously under pulsed pumping, while the remaining one can be tuned into the gain bandwidth by adjusting the nanoparticle size. All the BIC modes in the in-plane symmetric lattices exhibited doughnut-shaped lasing patterns, regardless of the pump polarization, but with a polarization-dependent threshold. Multiple vortex beams can be generated by illuminating with multiple pump spots. When changing the nanoparticle shape from square to trapezoidal, the symmetry breaking resulted in predominantly single-lobed lasing at symmetry-breaking-induced band edges and dramatically suppressed two-lobe lasing at BIC modes. Such on-chip multimode lasing with different polarization patterns thus enables multiplexing in not only wavelength, but also polarization. 

Multimode Vortex Lasing from Dye–TiO2 Lattices via Bound States in the Continuum

Perovskite nanocrystals (PNCs) are attractive emissive materials for developing compact lasers. However, manipulation of PNC laser directionality has been difficult, which limits their usage in photonic devices that require on-demand tunability. Here we demonstrate PNC metasurface lasers with engineered emission angles. We fabricated millimeter-scale CsPbBr3 PNC metasurfaces using an all-solution-processing technique based on soft nanoimprinting lithography. By designing band-edge photonic modes at the high-symmetry X point of the reciprocal lattice, we achieved four linearly polarized lasing beams along a polar angle of ∼30° under optical pumping. The device architecture further allows tuning of the lasing emission angles to 0° and ∼50°, respectively, by adjusting the PNC thickness to shift other high-symmetry points (Γ and M) to the PNC emission wavelength range. Our laser design strategies offer prospects for applications in directional optical antennas and detectors, 3D laser projection displays, and multichannel visible light communication.

Large-Area Perovskite Nanocrystal Metasurfaces for Direction-Tunable Lasing.

High-Quality Plasmonic Lasing with Topologically Trivial or Nontrivial Polarization

Symmetry-Related Large-Area Corner Mode with a Tunable Mode Area and Stable Frequency

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