Hyperbolic materials that exhibit metal and dielectric responses to orthogonal polarizations offer possibilities for controlling nanophotonic modes beyond those of conventional isotropic materials. Here, we demonstrate the coexistence of metal and dielectric resonance modes in single MoOCl2 nanostructures, a hyperbolic plasmon material with metal-dielectric duality. The two modes are nonhybrid and are controlled independently by the two orthogonal in-plane polarizations, without mode mixing or crosstalk. According to mode analysis, localized plasmon resonance is excited along the metal permittivity axis of MoOCl2, while the dielectric magnetic dipole mode is excited along the orthogonal dielectric axis. Compared to the plasmonic mode, the dielectric mode has a much higher Q factor and a different hotspot distribution. The dielectric mode also exhibits much stronger photoemission enhancement due to the different hotspot distribution along the z-axis, as measured by photoemission electron microscopy. By adjusting the structural parameters, the two modes can be tuned to overlap in spectra while maintaining the polarization control. The coexistence of these modes and their polarization dependence in single nanostructures provide a fundamental strategy for nanophotonic design, particularly for engineering polarizations, enhancing nonlinear processes, and achieving multifunctional metasurfaces.

Achieving precise control over chiral light-matter interactions at the nanometer-femtosecond scale, where both spatial and temporal limits are approached, remains a central challenge in nano- and quantum optics. This control is essential for next-generation ultracompact, ultrafast chiral photonic devices, but no method has realized both the construction and the active regulation of a localized chiroptical source within a single plasmonic nanoantenna. In this study, we report a strategy for creating and dynamically controlling a spatiotemporally localized chiroptical source via plasmonic eigenmode engineering in an achiral rectangular nanoantenna. With time-resolved photoemission electron microscopy, we directly image and analyze eigenmode dynamics and interference-induced near-field chirality across the space, time, and wavelength domains, revealing polarization-dependent hotspots in the nanoantenna. An interferometric pump-probe method further enables the on/off switching and near-field chirality reversal of the chiroptical photon source between nanoantenna corners with a sub-1.37 fs time delay between pump and probe pulses. This approach also yields a tunable superchiral photon source, providing a versatile platform for integrated ultrafast chiral nanophotonic applications.

Plasmon-driven hot-carrier generation (HCG) is entering a new stage: performance is not determined only by local absorption, but can be shaped by coherent electromagnetic design. Recent experiments show that (i) coupling plasmons to optical cavities and (ii) introducing controlled interactions among plasmons produce hybridized collective modes and reorganize optical eigenstates. These modes extend coherence areas beyond isolated near-field hot spots, enable an antenna-like redistribution of electromagnetic energy, and can substantially increase the yieldper absorbed excitation photon(apparent quantum efficiency, AQE), making coherence a practical design principle for plasmon-driven energy conversion. At the same time, this progress brings out a deeper microscopic question:photocurrent generation per incident photon, when normalized byopticalabsorption (often discussed as theinternal quantum efficiency), can show strong mode-dependent structures that are not fully captured by pictures based on local absorption or Landau-damping-based relaxation alone. In this Perspective, we organize these observations by introducing a hierarchy of coherence that follows the energy-conversion pathway from electromagnetic mode formation to microscopic electronic excitation. We show that cavity-mediated coherence and inter-plasmon coherence mainly determine plasmonic mode formation and electromagnetic energy redistribution, and therefore largely control the system-level AQE. By contrast, the mode selectivity seen in optical-absorption-normalized efficiencies points to a different conversion-stage mechanism-plasmon-electron-hole coherence-by which collective plasmonic excitations couple selectively to electronic channels inside metals. In this view, plasmon-driven HCG involves multiple, physically distinct coherence mechanisms that may act independently or interact coherently. This hierarchy-of-coherence perspective summarizes recent experimental and theoretical progress, clarifies both the power and the limitations of absorption-based intuition, and motivates the development of unified quantum approaches beyond Poisson-equation-based models for mode-specific plasmon-electron-hole coupling and for the predictive design of coherently engineered plasmonic energy-conversion systems.

Near-field enhancement in nanocavities governs the efficiency of nonlinear optical processes and ultrafast light-matter interactions. However, maximizing the cavity quality factor does not necessarily maximize the response under femtosecond excitation. Here, we experimentally demonstrate that hybrid metal-dielectric metasurfaces provide a practical platform for optimizing this trade-off. By coupling a low-Q localized surface plasmon resonance of Au nanodisks to a high-Q dielectric mode of a TiO 2 metasurface, we continuously tune the Q factors of the hybrid modes over a broad range while preserving the plasmonic hotspot geometry. Using four-photon photoemission electron microscopy under 100-fs excitation, we map the nonlinear near-field response and correlate it with spectrally extracted Q factors and ultrafast dynamics measured by time-resolved photoemission electron microscopy. The response varies nonmonotonically with Q and reaches a maximum at Q ≈ 2 0 , where the photoemission yield is enhanced approximately 15-fold relative to the uncoupled metasurface. These results identify pulse-cavity Q-factor matching between the cavity and the driving pulse as a key design principle for pulsed-laser nanophotonics.

Exciton–plasmon coupled enhanced absorption and emission rates, and photoluminescence intensity from halide perovskite quantum dots for high-performance solar cells, light-emitting diodes, lasers, and photocatalysts.

Two-dimensional (2D) van der Waals materials with strong in-plane anisotropy are emerging as fertile platforms for nanophotonics beyond conventional isotropic noble metals and dielectrics. Here, we demonstrate hyperbolic localized plasmon resonances (H-LPRs) in MoOCl2, a representative anisotropic 2D crystal. Unlike conventional plasmons, H-LPRs arise directly from the crystal anisotropy and show unprecedented properties: (i) one-dimensional resonances in circularly symmetric nanodisks, (ii) Z-gap independence in metal-insulator-metal heterostructures, and (iii) twist-induced chirality with circular dichroism values exceeding 0.65. The H-LPRs are characterized by both far-field spectra and near-field imaging. By stacking twisted MoOCl2 flakes, we bridge H-LPRs with concepts of moiré photonics and twistronics, introducing a new degree of freedom in plasmonic design. These findings establish H-LPRs in anisotropic 2D materials as a generalizable and versatile platform for polarization engineering, ultrasensitive chiral sensing, and integration into compact on-chip and quantum nanophotonic devices.

Highly sensitive chirality-dependent Raman analysis capable of probing protein structural information is important for bioanalytical applications. Although chiral plasmonic nanostructures can generate chirality-dependent electromagnetic fields, efficient localization of proteins within these hotspots under hydrated conditions remains challenging, limiting their application to chirality-dependent Raman analysis. Here, peptide-induced chiral plasmonic hotspots are introduced in gold nanotriangle-hydrogel platforms (c-AuTAG) for ultrasensitive chirality-dependent Raman analysis of proteins. Enantiomeric oligo-proline peptides introduce well-defined chirality into plasmonic nanogaps while minimizing protein adsorption and charge-dependent interactions that would otherwise hinder efficient analyte localization within plasmonic hotspots. The peptide-induced chiral hotspots are integrated with the previously developed GFT delivery strategy. Efficient analyte delivery through antifouling interfaces enables protein accumulation within dynamically tunable plasmonic hotspots. The system exhibits exceptionally large SERS chiral anisotropy factors (gSERS-ChA > 1.8) despite a small far-field optical dissymmetry factor (goptical ≈ 0.001), consistent with strong chirality-dependent plasmonic enhancement. The observed response reflects interactions between localized plasmonic near fields and vibrational-mode-dependent molecular polarizability. Ultrasensitive detection down to fg/mL is achieved within seconds using a conventional Raman spectrometer with linearly polarized excitation, enabling differential vibrational spectral analyses analogous to those used in Raman optical activity (ROA) without circular polarization optics. This platform enables ultrasensitive chirality-dependent Raman analysis of biomolecules in hydrated environments while preserving native structures, providing a practical approach for probing protein secondary structures, conformational changes, and biologically relevant molecular modifications.

Surface-enhanced Raman scattering (SERS) is a powerful tool for ultrasensitive molecular detection, yet its performance is critically dependent on the control of light-matter interactions. Here, we demonstrate that self-assembled gold nanoparticle films, composed of anisotropic gold nanotriangles (AuNTs) or isotropic gold nanospheres (AuNSs), exhibit shape-dependent SERS enhancement in a tailorable manner when coupled with a Fabry-Pérot (FP) cavity structure. The nanoparticle films were transferred onto semi-opened FP cavities consisting of a TiO2 dielectric layer on a gold film. Systematic tuning of the TiO2 thickness revealed modal ultrastrong coupling between the cavity and localized surface plasmon modes, yielding a peak splitting energy of ∼650 meV, as confirmed by reflection spectroscopy and finite-difference time-domain (FDTD) simulations. This modal ultrastrong coupling altered the extinction characteristics of the gold nanoparticle films, thereby significantly modulating the local electromagnetic fields and SERS signals. Shape-dependent SERS behaviors highlight the dominant influence of extinction-governed near-field intensity and hotspot distribution with additional contributions from surface chemistry factors associated with molecular accessibility. Under analyte-rich conditions, AuNT films exhibited strong SERS signals that were relatively insensitive to variations in the underlying TiO2 thickness, owing to the broad hybrid band arising from multiple modal coupling. In contrast, AuNS films displayed a greater SERS enhancement but were more susceptible to shifts in the position of the sharp hybrid peaks. In analyte-limited conditions─where SERS is especially valuable due to the need for high sensitivity─AuNT films provided superior SERS signal quality, benefiting from their shape anisotropy, which promotes stronger hotspots and more efficient access to them. These insights contribute to the rational design of practical SERS platforms and highlight the potential of integrating self-assembled plasmonic films with FP cavities in advanced photonic and chemical sensing applications.

In this study, we fabricated a localized surface plasmon resonance (LSPR)-Fabry-Perot nanocavity strong coupling structure composed of Au nanoparticles (NPs), titanium dioxide (TiO2), and an Au film (ATA) to investigate the activation energy of electrons injected in TiO2 and water oxidation intermediates generation. We performed temperature-dependent transient absorption measurements to determine the activation energy for electron-hole recombination, revealing that electrons injected into TiO2 from Au NPs are primarily trapped in shallow sites within TiO2. In addition, the activation energy of photocurrent generation was calculated based on the temperature-dependent incident photon-to-current conversion efficiency measurements to further evaluate the activation energy of water oxidation intermediates generation in the water oxidation reactions. By analyzing the activation energy for water oxidation intermediate generation, we have discussed the electrochemical potential level of holes trapped at the surface states of the Au/TiO2 interface in both the ATA and Au NPs/TiO2 (AT). Compared with the AT without strong coupling, the frequency factor for the generation of oxidation intermediates in the ATA exhibited a 40-fold enhancement, suggesting that more efficient water oxidation occurred. This enhancement in the strong coupling ATA photoelectrode may be attributed to quantum coherence between the LSPRs of Au NPs through the nanocavity.

This study investigates how structural properties influence the hole migration behavior of nickel oxide (NiOx) by evaluating its photoelectrochemical performance. Two fabrication methods were employed for preparing NiOx layers. One is a sol-gel wet process, which allows wide-area coating without the need for specialized equipment. The other is atomic layer deposition (ALD), which enables uniform and conformal coverage on 3D surfaces with the compact oxide layer by leveraging a self-limiting, layer-by-layer growth mechanism. Our findings reveal that both crystallinity and compactness significantly affect the hole transport capability of NiOx layers. We further integrated the NiOx layer as a hole transport layer in an Au-NPs/NiOx/Pt-film photocathode operating under a modal coupling regime. This photocathode exhibited cathodic photocurrent at wavelengths below 800 nm, with a maximum incident photon-to-current efficiency of 0.16% at 600 nm. These results demonstrate that not only the hole injection process but also hole migration properties of the NiOx layer influence the charge separation under modal coupling conditions.

Modal strong coupling between localized surface plasmon resonance and Fabry-Perot nanocavities offers a promising route to enhance photoreactivity in artificial photosynthesis. In our previous work, we showed that quantum coherence between spatially separated gold nanostructures mediated by a nanocavity enhances hot-carrier injection efficiency. However, how structural dephasing inhomogeneity governs coherence-assisted energy transfer remains unclear. Here, we fabricate mixed arrays of gold nanodisks and gold nanotriangles with varying triangle fractions and investigate their coherence behavior under modal strong coupling. Gold nanotriangles with sharper tips exhibit faster dephasing than gold nanodisks, as confirmed by simulations and photoemission electron microscopy. Transient absorption measurements reveal a triangle-fraction-dependent apparent quantum efficiency showing pronounced branch-dependent asymmetry: under lower-branch excitation, the efficiency first increases, reaches a maximum at 11% nanotriangles, and then decreases before saturating, whereas under upper-branch excitation, it decreases monotonically. This behavior is explained by a coherence-mediated mechanism, where long-dephasing gold nanodisks act as energy-collecting antennas and gold nanotriangles with short dephasing times serve as efficient converters. The branch-dependent near-field localization directs energy either to inefficient linear edges or to efficient sharp tips. We further propose a design principle that the efficiency is maximized when each coherence area contains one fast-dephasing gold nanotriangle surrounded by several long-dephasing gold nanodisks, corresponding to a triangle fraction of about 10-15%, consistent with the observed optimum. These findings demonstrate that structural coherence and dephasing inhomogeneity synergistically control hot-carrier dynamics and provide a pathway toward coherence-assisted design of plasmonic photoelectrodes.

A titanium dioxide thin film deposited on a gold reflective layer (TiO₂/Au, referred to as the TA structure) serves as a semiconductor electrode incorporating a nanocavity that confines incident light within the TiO₂ layer. When gold nanoparticles (AuNPs), which exhibit localized surface plasmon resonance (LSPR) modes close to the cavity’s resonant wavelength, are deposited onto the TiO₂ surface (forming the ATA structure), plasmon–nanocavity modal strong coupling is induced, resulting in the formation of two new hybrid modes. Compared to the AT structure, which lacks the nanocavity, the ATA structure exhibits enhanced absorption intensity and a broader absorption bandwidth. Furthermore, when employed as a photoanode for visible-light-driven water splitting, the ATA structure shows a significant improvement in reaction efficiency relative to the AT structure. This enhanced photooxidation performance is attributed to quantum coherence between the LSPRs of multiple AuNPs, mediated by the nanocavity. In this presentation, we report new insights into the kinetics of electron transfer reactions in the ATA structure, obtained from our recent investigations. The activation energy for electron–hole recombination (Ea(r)) between electrons injected into TiO₂ and holes was evaluated based on the temperature dependence of transient absorption decay curves. The Ea(r) values were found to be very small, indicating that the injected electrons are predominantly trapped in shallow trap states within TiO₂. In addition, from temperature-dependent incident photon-to-current conversion efficiency (IPCE) measurements, we derived the activation energy associated with the generation of water oxidation intermediates (Ea(1)). When the lower polaritonic branch (LB) of the ATA structure was excited, Ea(1) was significantly higher than that obtained for excitation in the AT structure. In contrast, excitation of the upper branch (UB) in ATA yielded Ea(1) values comparable to those of AT. These results suggest that holes generated via LB excitation possess more negative electrochemical potentials than those in AT. Furthermore, in the case of UB excitation, the considerable spectral overlap between the UB and LB enables transitions from UB to LB, thereby influencing the energy distribution of the generated holes. Moreover, the frequency factor for water oxidation intermediate generation in the ATA structure was found to be approximately 40 times greater than that in the AT structure. This result suggests that quantum coherence induced by the nanocavity in ATA enhances the reaction cross-section, thereby promoting hole-driven water oxidation reactions.

Semiconductor quantum dots (QDs) and plasmonic metal nanoparticles (PMNPs) have interesting but different physical and optical properties. The combination of QDs and PMNPs modifies QD's excitonic and charge carrier properties, introducing inimitable optical properties and opening many plasmon-enhanced photonic applications. The modal strong coupling between the localized surface plasmon resonance of Au NPs and the optical mode of the Fabry-Perot cavity enhances energy transfer between QDs and Au NPs. By combining helicon sputter coating and atomic layer deposition, we fabricate an Au NPs/TiO2/Au film (ATA) Fabry-Perot cavity that abstracts the excitation energy of CdSe/ZnS QDs placed at 3 to 80 nm from the cavity, where atomistically precise alumina films control the cavity-QD distance. Steady-state and time-resolved absorption and photoluminescence measurements help us to reveal long-range (>80 nm), modal-strong-coupled nonradiative deactivation of photoexcited QDs.

We developed a substrate that enables highly sensitive and spatially uniform surface-enhanced Raman scattering (SERS). This substrate comprises densely packed gold nanoparticles (d-AuNPs)/titanium dioxide/Au film (d-ATA). The d-ATA substrate demonstrates modal ultrastrong coupling between localized surface plasmon resonances (LSPRs) of AuNPs and Fabry-Perot nanocavities. d-ATA exhibits a significant enhancement of the near-field intensity, resulting in a 78-fold increase in the SERS signal for crystal violet (CV) compared to that of d-AuNP/TiO2 substrates. Importantly, high sensitivity and a spatially uniform signal intensity can be obtained without precise control of the shape and arrangement of the nanoscale AuNPs, enabling quantitative SERS measurements. Additionally, SERS measurements of rhodamine 6G (R6G) on this substrate under ultralow adsorption conditions (0.6 R6G molecules/AuNP) show a spatial variation in the signal intensity within 3%. These findings suggest that the SERS signal under modal ultrastrong coupling originates from multiple plasmonic particles with quantum coherence.

Hot-carrier generation (HCG) through plasmons in nanoscale metals is a crucial element for arguing artificial photosynthesis and photocurrent. The strategic design of nanoscale metals can enhance HCG. A recent study introduced the Au nanoparticle (NP) array/TiO2/Au film (ATA) structure, generating Fabry-Perot "cavity" (FP cavity) modes. Because of the coherent coupling between the plasmons in the NPs via the FP cavity modes and the electromagnetic interactions strengthened by the cavity effect, energy has been found to concentrate at hot sites, which is characterized by a relatively high conversion efficiency. This phenomenon is an antenna effect. Consequently, the overall efficiency of HCG was significantly improved. In this study, we theoretically explored symmetry-breaking NP arrays such as finite clusters and infinite arrays with artificial defects within the ATA structure. The FP cavity effect in the ATA structure strengthened the plasmon hybridization within these arrays, forming a localized mode of the hybridized plasmon. This localization amplified the above-mentioned antenna effect. Our findings demonstrated that under specific conditions, symmetry-breaking NP arrays surpassed perfectly homogeneous arrays in terms of HCG efficiency. These results underlined the significance of optimally exciting the localized modes of hybridized plasmons within symmetry-breaking NP arrays to achieve enhanced photoelectric conversion efficiency.

Label-free sensing techniques, designed for the observation of dynamic cell activities, have the potential to advance studies of cell biology, immunotherapy, and drug discovery. In this context, we introduce the concept of Fano resonances featuring dual evanescent wavelengths in aluminum-coated nanoridge arrays for real-time and label-free analysis of cellular adhesion. The distinctive optical characteristics of dual evanescent lengths were confirmed through finite-difference time-domain (FDTD) calculations and experimental assessments, including tests of the refractive index and surface (thickness) sensitivity. The findings reveal the evanescent lengths measured at 874 and 316 nm for the peak and dip of the Fano resonance at the air/aluminum interface, aligning with FDTD simulations (891 and 338 nm). Wavelength shifts and intensity variations of the Fano resonance correspond to changes in the effective refractive index and absorption/scattering of surface plasmon waves, respectively. Employing four-parameter plots & horbar;time constants of Delta lambda(peak), Delta lambda(dip), Delta I-max, and Delta I-min & horbar;we exemplify the investigation of long-range and short-range adhesion behaviors of lung cancer cells. In contrast to conventional gold-based SPR sensors with evanescent lengths ranging from 100 to 300 nm, the aluminum-coated nanoridge array exhibits an extended sensing depth and dual evanescent lengths. This dual capability enables the simultaneous exploration of cell behaviors in the proximity of and at a distance from the metal surface.

Plasmon-induced carrier transfer at metal/semiconductor Schottky junctions is a novel approach for photo energy conversion. Here, we propose a strategy for improving electron transfer at the Au/TiO2 heterojunction by modifying the Au/TiO2 structures under modal strong coupling conditions to demonstrate the efficacy of our proposal. Via transient absorption measurements, we found that the apparent quantum efficiency (AQE) of electron injection into TiO2 increased by similar to 1.8-fold as the thickness of the Ti layer increased from 0.5 to 5.0 nm under the modal strong coupling conditions. This AQE enhancement was attributed to the energy transfer from AuNDs to the Ti layer through the near-field, resulting in a production of high-energy electrons in the Ti layer to inject into the conduction band of TiO2, thereby improving the electron transfer efficiency. Our observations offer valuable insight into the future design of plasmonic devices aimed at efficiently utilizing plasmon-induced hot carriers.

Surface-enhanced Raman scattering (SERS) is an attractive technique in molecular detection with high sensitivity and label-free characteristics. However, its use in protein detection is limited by the large volume of proteins, hindering its approach to the narrow spaces of hotspots. In this study, we fabricated a Au nanoTriangle plate Array on Gel (AuTAG) as an SERS substrate by attaching a Au nanoTriangle plate (AuNT) arrangement on a thermoresponsive hydrogel surface. The AuTAG acts as an actively tunable plasmonic device, on which the interparticle distance is altered by controlling temperature via changes in hydrogel volume. Further, we designed a Gel Filter Trapping (GFT) method as an active protein delivery strategy based on the characteristics of hydrogels, which can absorb water and separate biopolymers through their three-dimensional (3D) polymer networks. On the AuTAGs, fabricated with AuNTs modified with charged surface ligands to prevent the nonspecific adsorption of analytes to particles, the GFT method helped the delivery of proteins to hotspot areas on the AuNT arrangement. This combination of a AuTAG substrate and the GFT method enables ultrahigh sensitivity for protein detection by SERS up to a single-molecule level as well as a wide quantification concentration range of 6 orders due to their geometric advantages.