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We present a strong coupling system realized by coupling the localized surface plasmon mode in individual silver nanogrooves and propagating surface plasmon modes launched by periodic nanogroove arrays with varied periodicities on a continuous silver medium. When the propagating modes are in resonance with the localized mode, we observe a √N scaling of Rabi splitting energy, where N is the number of propagating modes coupled to the localized mode. Here, we confirm a giant Rabi splitting on the order of 450-660 meV (N = 2) in the visible spectral range, and the corresponding coupling strength is 160-235 meV. In some of the strong coupling cases studied by us, the coupling strength is about 10% of the mode energy, reaching the ultrastrong coupling regime.

Surface-enhanced Raman spectroscopy (SERS) is an ultrasensitive technique to identify vibrational fingerprints of trace analytes. However, present SERS techniques suffer from the lack of uniform, reproducible, and stable substrates to control the plasmonic hotspots in a wide spectral range. Here, we report the promising application of epitaxial aluminum films as a scalable plasmonic platform for SERS applications. To assess the uniformity of aluminum substrates, atomically thin transition metal dichalcogenide monolayers are used as the benchmark analyte due to their inherent two-dimensional homogeneity. Besides the distinctive spectral capability of aluminum in the ultraviolet (325 nm), we demonstrate that the aluminum substrates can even perform comparably with the silver counterparts made from single-crystalline colloidal silver crystals using the same SERS substrate design in the visible range (532 nm). This is unexpected from the prediction solely based on optical dielectric functions and illustrate the superior surface and interface properties of epitaxial aluminum SERS substrates.

Non-Hermitian photonic systems with gains and/or losses have recently emerged as a powerful approach for topology-protected optical transport and novel device applications. To date, most of these systems employ coupled optical systems of diffraction-limited dielectric waveguides or microcavities, which exchange energy spatially or temporally. Here, we introduce a diffraction-unlimited approach using a plasmon-exciton coupling (polariton) system with tunable plasmonic resonance (energy and line width) and coupling strength. By designing a chirped silver nanogroove cavity array and coupling a single tungsten disulfide monolayer with a large contrast in resonance line width, we show the tuning capability through energy level anticrossing and plasmon-exciton hybridization (line width crossover), as well as spontaneous symmetry breaking across the exceptional point at zero detuning. This two-dimensional hybrid material system can be applied as a scalable and integratable platform for non-Hermitian photonics, featuring seamless integration of two-dimensional materials, broadband tuning, and operation at room temperature.

Bound states in the continuum (BICs) have been observed in a variety of systems. A plasmonic BIC offers interesting opportunities, since a surface plasmon is known to confine light to the nanometer scale. However, the observation and manipulation of plasmonic BICs is a challenge due to the intrinsic loss of metals. Here, we study plasmonic BICs in the visible range in a one-dimensional all-metallic grating. First, by tuning the resonances of localized and propagating surface plasmon modes to resonance, we successfully observe symmetry-protected plasmonic BICs in an all-metallic system. Next, by continuously tuning the localized mode, we demonstrate topological band inversion characterized by a Zak phase transition. In addition, we engineer off-\ensuremath{\Gamma}-point BICs and confirm their formation mechanism. Finally, we experimentally determine that the quality ($Q$) factor of a 10-groove structure can exceed 60, about one order of magnitude greater than conventional metallic structures. The simulations reveal that, with more grooves, the $Q$ factor can be over 200. The plasmonic BICs in the visible range demonstrated in this paper pave the way to promising applications in lasers, sensors, light-matter interactions, nonlinear optics, and quantum optics.

A giant SHG enhancement is achieved from monolayer-WS2 incorporated onto 2D plasmonic metasurfaces (Ag-nanogroove-grating) in resonance with the excitonic resonance of monolayer-WS 2 . An optical encoding technique is demonstrated based on plasmonic manipulation of SHG polarization.

Low-cost materials with large-area, broadband, and strong photoluminescence are of great interest in many fields. Plasmonic enhancement structures, based on nanoparticle self-assembly, are identified as promising candidates for achieving these requirements. This study fabricates Au nanocube assemblies and demonstrates photoluminescence enhancement of quantum dots (QDs) deposited on their surfaces. The enhancement factor of this large-area ensemble is ≈3.5 with the use of a 532 nm pulse laser to excite red-emissive QDs. Furthermore, the broadband response of this assembly is studied and enhanced broadband white-light emission is found. Finally, a new optical information storage method based on this photoluminescence enhancement is demonstrated. The results have significance for applications, such as energy-saving lighting and displays, solar energy harvesting, optical data storage, and anticounterfeiting measures.