This paper reports how the spectral linewidths of plasmon resonances can be narrowed down to a few nanometers by optimizing the morphology, surface roughness, and crystallinity of metal nanoparticles (NPs) in two-dimensional (2D) lattices. We developed thermal annealing procedures to achieve ultranarrow surface lattice resonances (SLRs) with full-width at half-maxima linewidths as narrow as 4 nm from arrays of Au, Ag, Al, and Cu NPs. Besides annealing, we developed a chemical vapor deposition process to use Cu NPs as catalytic substrates for graphene growth. Graphene-encapsulated Cu NPs showed the narrowest SLR linewidths (2 nm) and were stable for months. These ultranarrow SLR nanocavity modes supported even narrower lasing emission spectra and high nonlinearity in the input–output light–light curves.

This Letter describes strong coupling of densely packed molecular emitters in metal−organic frameworks (MOFs) and plasmonic nanoparticle (NP) lattices. Porphyrin-derived ligands with small transition dipole moments in an ordered MOF film were grown on Ag NP arrays. Angle-resolved optical measurements of the MOF-NP lattice system showed the formation of a polariton that is lower in energy and does not cross the uncoupled MOF Q 1 band. Modeling predicted the upper polariton energy and a calculated Rabi splitting of 110 meV. The coupling strength was systematically controlled by detuning the plasmon energy by changing the refractive index of the solvents infiltrating the MOF pores. Through transient absorption spectroscopy, we found that the lower polariton decays quickly at shorter time scales (<500 ps) and slowly at longer times because of energy transfer from the upper polariton. This hybrid system demonstrates how MOFs can function as an accessible excitonic material for polariton chemistry.

This Review focuses on the integration of plasmonic and dielectric metasurfaces with emissive or stimuli-responsive materials for manipulating light–matter interactions at the nanoscale. Metasurfaces, engineered planar structures with rationally designed building blocks, can change the local phase and intensity of electromagnetic waves at the subwavelength unit level and offers more degrees of freedom to control the flow of light. A combination of metasurfaces and nanoscale emitters facilitates access to weak and strong coupling regimes for enhanced photoluminescence, nanoscale lasing, controlled quantum emission, and formation of exciton–polaritons. In addition to emissive materials, functional materials that respond to external stimuli can be combined with metasurfaces to engineer tunable nanophotonic devices. Emerging metasurface designs including surface-functionalized, chemically tunable, and multilayer hybrid metasurfaces open prospects for diverse applications, including photocatalysis, sensing, displays, and quantum information.

Strong coupling between light and matter can produce hybrid eigenstates known as exciton–polaritons. Although polariton dynamics are important photophysical properties, the relaxation pathways of polaritons in different coupling regimes have seen limited attention. This paper reports the dynamics of hybridized states from 2D Ruddlesden–Popper perovskites coupled to plasmonic nanoparticle lattices. The open cavity architecture of Al lattices enables the coupling strength to be modulated by varying either the lead halide perovskite film thickness or the superstrate refractive index. Both experiments and finite-difference time-domain simulations of the optical dispersion diagrams showed avoided crossings that are a signature of strong coupling. Our analytical model also elucidated the correlation between the exciton/plasmon mixing ratio and polariton coupling strength. Using fs-transient absorption spectroscopy, we found that both the upper and lower polaritons have shorter lifetimes than the excitons and that polaritons can show faster excited-state dynamics when they have access to additional energy transfer channels.

Significance Molecules interacting with metallic nanostructures can show tunable exciton-plasmon coupling, ranging from weak to strong. One factor that influences the interactions is the spatial organization of the molecules relative to the localized plasmon-enhanced electromagnetic fields. In this work, we show that the arrangement of aromatic dye molecules can be tuned within plasmonic hotspots by interfacial engineering of nanoparticle surfaces. By controlling the local chemical and physical interactions, we could modulate lasing thresholds. Surface-functionalized plasmonic metasurfaces open prospects for programmable light-matter interactions at the nanoscale.

Plasmonic electrochromism, a change in the localized surface plasmon resonance (LSPR) with an applied electric potential, has been attracting increasing attention for the development of spectroscopic tools or optoelectronic systems. There is a consensus on the mechanism of plasmonic electrochromism based on the classical capacitor and the Drude model. However, the electrochromic behaviors of metallic nanoparticles in narrow optical windows have been demonstrated only with small monotonic LSPR shifts, which limits the use of the electrochromism. Here, we observed three distinct electrochromic behaviors of gold nanocubes with a wide potential range through in situ dark-field electrospectroscopy. Interestingly, the nanocubes show a faster frequency shift under the highly negative potential, and this opens the possibility of largely tunable electrochromic LSPR shifts. The reversibility of the electrochemical switching with these cubes are also shown. We attribute this unexpected change beyond classical understandings to the material-specific quantum mechanical electronic structures of the plasmonic materials.

longitudinal beam profiles, [1,2] and because of their increased rotational symmetry, 2D cavities enable multimode lasing, [3,4] vortex polarization, and annular-shaped beams. [2,5] Most lasing work on 2D photonic crystals exploits band edges at high symmetry points (e.g., Γ, X, and M points of a square lattice) in reciprocal space for optical feedback. [6][7][8][9] Since standing waves at these points are biaxially confined, solutions to their wave equation are critically constrained, which limits lasing action to discrete wavelengths and directions. [10] However, 2D photonic crystals can also be considered as lines of 1D arrays, where in-plane scattered waves are decomposed along two orthogonal directions. [2,10,11] In this picture, quasi-propagating photonic modes are slow traveling waves and can be interpreted as uniaxially confined standing waves that propagate along high-symmetry directions (e.g., Γ-M, Γ-X, and M-X). The additional degree of freedom from propagation enables the band edge states to span a continuum of energies and wavevectors. [10,12,13] Although quasi-propagating modes are predicted to support optical feedback, [10,13,14] lasing action via 2D cavities remains primarily focused on that from high symmetry points.Strongly scattering 2D plasmonic nanoparticle (NP) lattices that can trap light in-plane support hybrid photonic-plasmonic modes known as surface lattice resonances (SLRs). [15,16] Feedback from SLRs has enabled nanoscale lasing from NP lattice cavities integrated with index-matched emitter gain materials such as organic dyes in solvents and upconversion NP thin films. [17][18][19][20] NP lattices integrated with high-refractiveindex emissive materials such as colloidal quantum dot films have also demonstrated lasing from transverse electric (TE) and transverse magnetic (TM) waveguide-hybridized SLRs (W TE -SLRs and W TM -SLRs). [21][22][23] Because of the mode structure of the waveguide component, W-SLRs can excite large volumes of active material for lasing. [22,24] For either SLR or W-SLR modes, however, feedback for lasing is from biaxially confined standing waves [21,23,[25][26][27] since their losses are lower than quasipropagating modes. [2,28,29] Lasing from quasi-propagating modes should be possible; we hypothesize that they have been elusive due to insufficient gain coefficients (≈10-200 cm −1 ). [30,31] Although engineered gain materials such as gradient-shell quantum dots [32] or dyes with minimized triplet states [33] may offer higher gain, their syntheses are challenging. In contrast, lead halide perovskite nanocrystals (NCs) can be readily Band edges at the high symmetry points in reciprocal space of periodic structures hold special interest in materials engineering for their high density of states. In optical metamaterials, standing waves found at these points have facilitated lasing, bound-states-in-the-continuum, and Bose-Einstein condensation. However, because high symmetry points by definition are localized, properties associated with them are limited to specific e...

The study of strong coupling between light and matter has gained significant attention in recent years due to its potential applications in diverse fields, including artificial light harvesting, ultraefficient polariton lasing, and quantum information processing. Plasmonic cavities are a compelling alternative of conventional photonic resonators, enabling ultracompact polaritonic systems to operate at room temperature. This review focuses on colloidal metal nanoparticles, highlighting their advantages as plasmonic cavities in terms of their facile synthesis, tunable plasmonic properties, and easy integration with excitonic materials. We explore recent examples of strong coupling in single nanoparticles, dimers, nanoparticle-on-a-mirror configurations, and other types of nanoparticle-based resonators. These systems are coupled with an array of excitonic materials, including atomic emitters, semiconductor quantum dots, two-dimensional materials, and perovskites. In the concluding section, we offer perspectives on the future of strong coupling research in nanoparticle systems, emphasizing the challenges and potentials that lie ahead. By offering a thorough understanding of the current state of research in this field, we aim to inspire further investigations and advances in the study of strongly coupled nanoparticle systems, ultimately unlocking new avenues in nanophotonic applications.