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This paper describes room‐temperature polariton lasing with high circular polarization from plasmonic Kagome lattice cavities strongly coupled to colloidal semiconducting quantum wells. By shrinking and expanding the trimer unit cells in Al nanoparticle lattices, the inversion symmetry is broken, resulting in the spin‐momentum locking of cavity photons. Circularly polarized lattice resonances emerged from crossings of different diffraction orders, denoted as T points, along the Γ‐K direction in momentum space and whose degree of circular polarization is higher than those of high‐symmetry K‐points. Deformed Kagome lattice cavities combined with CdSe nanoplatelets enabled the formation of exciton‐polaritons. Spin‐selectivity from cavity modes resulted in control over the handedness of circular polarization as well as direction of photoluminescence from lower polaritons. Six lasing beams from T points has alternating handedness, low thresholds (8 µJ cm⁻²), and high degrees of circular polarization (≈0.7). The anticipation that spin‐momentum locking via plasmonic Kagome lattices can be extended to other non‐Bravais metasurfaces with hexagonal symmetries and opens prospects for exploring optical analogues of spin and valley physics. 

The ability of nanophotonic cavities to confine and store light to nanoscale dimensions has important implications for enhancing molecular, excitonic, phononic, and plasmonic optical responses. Spectroscopic signatures of processes that are ordinarily exceedingly weak such as pure absorption and Raman scattering have been brought to the single-particle limit of detection, while new emergent polaritonic states of optical matter have been realized through coupling material and photonic cavity degrees of freedom across a wide range of experimentally accessible interaction strengths. In this review, we discuss both optical and electron beam spectroscopies of cavity-coupled material systems in weak, strong, and ultrastrong coupling regimes, providing a theoretical basis for understanding the physics inherent to each while highlighting recent experimental advances and exciting future directions. 

Abstract Nanophotonics research has focused recently on the ability of nonlinear optical processes to mediate and transform optical signals in a myriad of novel devices, including optical modulators, transducers, color filters, photodetectors, photon sources, and ultrafast optical switches. The inherent weakness of optical nonlinearities at smaller scales has, however, hindered the realization of efficient miniaturized devices, and strategies for enhancing both device efficiencies and synthesis throughput via nanoengineering remain limited. Here, we demonstrate a novel mechanism by which second harmonic generation, a prototypical nonlinear optical phenomenon, from individual lithium niobate particles can be significantly enhanced through nonradiative coupling to the localized surface plasmon resonances of embedded gold nanoparticles. A joint experimental and theoretical investigation of single mesoporous lithium niobate particles coated with a dispersed layer of ~10 nm diameter gold nanoparticles shows that a ~32-fold enhancement of second harmonic generation can be achieved without introducing finely tailored radiative nanoantennas to mediate photon transfer to or from the nonlinear material. This work highlights the limitations of current strategies for enhancing nonlinear optical phenomena and proposes a route through which a new class of subwavelength nonlinear optical platforms can be designed to maximize nonlinear efficiencies through near-field energy exchange.