Quantum emitters in two-dimensional materials are promising candidates for studies of light-matter interaction and next generation, integrated on-chip quantum nanophotonics. However, the realization of integrated nanophotonic systems requires the coupling of emitters to optical cavities and resonators. In this work, we demonstrate hybrid systems in which quantum emitters in 2D hexagonal boron nitride (hBN) are deterministically coupled to high-quality plasmonic nanocavity arrays. The plasmonic nanoparticle arrays offer a high-quality, low-loss cavity in the same spectral range as the quantum emitters in hBN. The coupled emitters exhibit enhanced emission rates and reduced fluorescence lifetimes, consistent with Purcell enhancement in the weak coupling regime. Our results provide the foundation for a versatile approach for achieving scalable, integrated hybrid systems based on low-loss plasmonic nanoparticle arrays and 2D materials.

CONSPECTUS:Plasmonic surface lattice resonances (SLRs) are mixed light−matter states emergent in a system of periodically arranged metallic nanoparticles (NPs) under the constraint that the array spacing is able to support a standing wave of optical-frequency light. The properties of SLRs derive from two separate physical effects; the electromagnetic (plasmonic) response of metal NPs and the electromagnetic states (photonic cavity modes) associated with the array of NPs. Metal NPs, especially free-electron metals such as silver, gold, aluminum, and alkali metals, support optical-frequency electron density oscillations known as localized surface plasmons (LSPs). The high density of conduction-band electrons in these metals gives rise to plasmon excitations that strongly couple to light even for particles that are several orders of magnitude smaller than the wavelength of the excitation source. In this sense, LSPs have the remarkable ability to squeeze far-field light into intensely localized electric near-fields that can enhance the intensity of light by factors of ∼10 3 or more. Moreover, as a result of advances in the synthesis and fabrication of NPs, the intrinsic dependence of LSPs on the NP geometry, composition, and size can readily be exploited to design NPs with a wide range of optical properties. One drawback in using LSPs to enhance optical, electronic, or chemical processes is the losses introduced into the system by dephasing and Ohmic dampingan effect that must either be tolerated or mitigated. Plasmonic SLRs enable the mitigation of loss effects through the coupling of LSPs to diffractive states that arise from arrays satisfying Bragg scattering conditions, also known as Rayleigh anomalies. Bragg modes are well-known for arrays of dielectric NPs, where they funnel and trap incoming light into the plane of the lattice, defining a photonic cavity. The low losses and narrow linewidths associated with dielectric NPs produce Bragg modes that oscillate for ∼10 3 −10 4 cycles before decaying. These modes are of great interest to the metamaterials community but have relatively weak electric fields associated with dielectric NPs and therefore are not used for applications where local field enhancements are needed. Plasmonic lattices, i.e., photonic crystals composed of metallic NPs, combine the characteristics of both LSPs and diffractive states, enabling both enhanced local fields and narrow-linewidth excitations, in many respects providing the best advantages of both materials. Thus, by control of the periodicity and global symmetry of the lattice in addition to the material composition and shape of the constituent NPs, SLRs can be designed to simultaneously survive for up to 10 3 cycles while maintaining the electric field enhancements near the NP surface that have made the use of LSPs ubiquitous in nanoscience. Modern fabrication methods allow for square-centimeter-scale patches of two-dimensional arrays that are composed of approximately one trillion NPs, making them effectively infinite at the nanoscal...

Miniaturized lasers are an emerging platform for generating coherent light for quantum photonics, in-vivo cellular imaging, solidstate lighting, and fast 3D sensing in smartphones 1-3. Continuouswave (CW) lasing at room temperature is critical for integration with opto-electronic devices and optimal modulation of optical interactions 4,5. Plasmonic nanocavities integrated with gain can generate coherent light at sub-wavelength scales 6-9 , beyond the diffraction limit that constrains mode volumes in dielectric cavities such as semiconducting nanowires 10,11. However, insufficient gain 1 with respect to losses and thermal instabilities in nanocavities has limited all nanoscale lasers to pulsed pump sources and/or lowtemperature operation 6-9,12-15. Here we show CW upconverting lasing at room temperature with record-low thresholds and high photostability from sub-wavelength plasmons. We achieve selective, single-mode lasing from Yb 3+ /Er 3+-co-doped upconverting nanoparticles (UCNPs) conformally coated on Ag nanopillar arrays that support a single, sharp lattice plasmon cavity mode and < /20 field confinement in the vertical dimension. The intense electromagnetic near-fields localized in the vicinity of the nanopillars result in a threshold of 70 W/cm 2 , orders of magnitude lower than other small lasers. Our plasmon-nanoarray upconverting lasers provide directional, ultra-stable output at visible frequencies under near-infrared pumping, even after six hours of constant operation, which offers prospects in previously unrealizable applications of coherent nanoscale light. Lanthanide-based UCNPs are photostable solid-state nonlinear emitters that are efficient at sequentially absorbing multiple near-infrared (NIR) photons and emitting at visible and shorter-NIR wavelengths 16-19. Recently, UCNPs have been used as gain media in small lasers, and their integration with dielectric microcavities and hyperbolic metamaterials has resulted in multi-wavelength upconverted lasing 20-22. UCNPs also exhibit long radiative lifetimes (typically 100s of µs) compared to other gain materials 18,23,24 , which

Conspectus Rationally assembled nanostructures exhibit distinct physical and chemical properties beyond their individual units. Developments in nanofabrication techniques have enabled the patterning of a wide range of nanomaterial designs over macroscale (>in.2) areas. Periodic metal nanostructures show long-range diffractive interactions when the lattice spacing is close to the wavelength of the incident light. The collective coupling between metal nanoparticles in a lattice introduces sharp and intense plasmonic surface lattice resonances, in contrast to the broad localized resonances from single nanoparticles. Plasmonic nanoparticle lattices exhibit strongly enhanced optical fields within the subwavelength vicinity of the nanoparticle unit cells that are 2 orders of magnitude higher than that of individual units. These intense electromagnetic fields can manipulate nanoscale processes such as photocatalysis, optical spectroscopy, nonlinear optics, and light harvesting. This Account focuses on advances in exciton–plasmon coupling and light–matter interactions with plasmonic nanoparticle lattices. First, we introduce the fundamentals of ultrasharp surface lattice resonances; these resonances arise from the coupling of the localized surface plasmons of a nanoparticle to the diffraction mode from the lattice. Second, we discuss how integrating dye molecules with plasmonic nanoparticle lattices can result in an architecture for nanoscale lasing at room temperature. The lasing emission wavelength can be tuned in real time by adjusting the refractive index environment or varying the lattice spacing. Third, we describe how manipulating either the shape of the unit cell or the lattice geometry can control the lasing emission properties. Low-symmetry plasmonic nanoparticle lattices can show polarization-dependent lasing responses, and multiscale plasmonic superlatticesfinite patches of nanoparticles grouped into microscale arrayscan support multiple plasmon resonances for controlled multimodal nanolasing. Fourth, we discuss how the assembly of photoactive emitters on the nanocavity arrays behaves as a hybrid materials system with enhanced exciton–plasmon coupling. Positioning metal–organic framework materials around nanoparticles produces mixed photon modes with strongly enhanced photoluminescence at wavelengths determined by the lattice. Deterministic coupling of quantum emitters in two-dimensional materials to plasmonic lattices leads to preserved single-photon emission and reduced decay lifetimes. Finally, we highlight emerging applications of nanoparticle lattices from compact, fully reconfigurable imaging devices to solid-state emitter structures. Plasmonic nanoparticle lattices are a versatile, scalable platform for tunable flat optics, nontrivial topological photonics, and modified chemical reactivities.

Periodic metal nanoparticle (NP) arrays support narrow lattice plasmon resonances that can be tuned by changing the localized surface plasmons of the individual NPs in the array, NP periodicity, and dielectric environment. In this paper, we report superlattice plasmons that can be supported by hierarchical Au NP arrays, where finite arrays of NPs (patches) are organized into arrays with larger periodicities. We show that superlattice plasmons can be described by the coupling of single-patch lattice plasmons and Bragg modes defined by the patch periodicity. Superlattice plasmon resonances are often significantly narrower than that of single-patch lattice plasmon resonances and exhibit stronger local peak fields. By varying the periodicity of the patches, we demonstrated that the number and spectral location of superlattice plasmon resonances can be tailored in hierarchical Au NP arrays. These narrow superlattice plasmon resonances open prospects in ultrasensitive sensing and energy transfer and plasmon amplification in plasmonic cavities.