Bound states in the continuum (BICs) are typically investigated in terms of distinct formation mechanisms, such as symmetry-protected (SP), Friedrich-Wintgen (FW), or Fabry-Perot (FP) BICs. However, their coexistence and mutual interaction within a single plasmonic architecture have not been systematically examined so far. In this work, we show that these distinct BIC classes can coexist and interact in a metal-insulator-metal waveguide incorporating a double T-shaped cavity. Using an analytically tractable Green's function formalism supported by full-wave finite element simulations, we identify the simultaneous emergence of twin FW-BICs and FP-BICs. Unlike FW-BICs, which are independent of the separation between the two cavities, FP-BICs occur at a discrete set of cavity separations. We show that the interaction between the two types of BICs gives rise to multi-BICs, featuring highly confined, non-radiative modes. We further analyze how breaking the BIC condition leads to Fano-like and plasmon-induced reflection resonances as well as the Dicke effect. The combined analytical-numerical analysis provides physical insight into BIC formation in plasmonic waveguides and underscores the potential of these nanostructures for sensing and integrated optical filtering applications.

Reconstructing scattering spectra from limited spectral measurements is a recurring need for plasmonic nanostructures, both in numerical workflows where full-wave simulations are computationally expensive and in experimental settings where broadband scans can be slow, bandwidth-limited, or sparsely sampled. We address this problem with a supervised deep learning framework targeted at gold nanoridge dimer-on-mirror structures. Two complementary masking schemes are studied: a contiguous spectral window, which forces the model to extrapolate beyond the observed region, and a random sparse mask, which reduces the task to interpolation between scattered observations. Three architectures with different inductive biases are compared on a dataset of 1307 COMSOL-simulated scattering spectra: a multilayer perceptron (MLP), a one-dimensional convolutional neural network (CNN) with dilated kernels, and a one-dimensional U-Net, a U-shaped encoder–decoder network with skip connections. We find that no architecture is universally optimal. Under contiguous masking, the MLP outperforms both convolutional models, with R2≈0.98 from only one third of the spectral range, because its dense connectivity provides a global receptive field suited to long-range extrapolation. Under random sparse masking, all three architectures reach R2≥0.989, but the CNN matches the larger U-Net while using roughly five times fewer parameters. A bandwidth ablation in the contiguous case shows that one third of the spectral range is sufficient for high-fidelity reconstruction, with diminishing returns beyond about 40% visibility, and a visibility ablation in the random case shows that 25%–30% of the spectral points is sufficient for reliable reconstruction. Classical interpolation, Gaussian-process, and linear baselines confirm that the networks are most valuable at low visibility and for contiguous-window extrapolation, also a noise and disorder analysis shows that the random-sparse reconstruction tolerates realistic fabrication variations The architecture-task coupling identified here gives a concrete design rule for sparse spectral acquisition and neural post-processing pipelines in nanophotonics.

Reconstructing scattering spectra from limited spectral measurements is a recurring need for plasmonic nanostructures, both in numerical workflows where full-wave simulations are computationally expensive and in experimental settings where broadband scans can be slow, bandwidth-limited, or sparsely sampled. We address this problem with a supervised deep learning framework targeted at gold nanoridge dimer-on-mirror structures. Two complementary masking schemes are studied: a contiguous spectral window, which forces the model to extrapolate beyond the observed region, and a random sparse mask, which reduces the task to interpolation between scattered observations. Three architectures with different inductive biases are compared on a dataset of 1307 COMSOL-simulated scattering spectra: a multilayer perceptron (MLP), a one-dimensional convolutional neural network (CNN) with dilated kernels, and a one-dimensional U-Net, a U-shaped encoder-decoder network with skip connections. We find that no architecture is universally optimal. Under contiguous masking, the MLP outperforms both convolutional models, with R 2 ≈ 0.98 from only one third of the spectral range, because its dense connectivity provides a global receptive field suited to long-range extrapolation. Under random sparse masking, all three architectures reach R 2 ≥ 0.989, but the CNN matches the larger U-Net while using roughly five times fewer parameters. A bandwidth ablation in the contiguous case shows that one third of the spectral range is sufficient for high-fidelity reconstruction, with diminishing returns beyond about 40% visibility, and a visibility ablation in the random case shows that 25-30% of the spectral points is sufficient for reliable reconstruction. The architecture-task coupling identified here gives a concrete design rule for sparse spectral acquisition and neural post-processing pipelines in nanophotonics.

Localized surface plasmons (LSPs) in nanoparticles-on-mirror (NPsoM) structures enable extreme field confinement for sensing, optomechanics, and quantum photonics. However, their inverse design remains challenging due to complex geometry-spectrum relationships and the absence of analytical models. This study introduces a data-efficient deep learning framework for bidirectional LSP resonance design in gold nanoridge dimer-on-mirror structures. The forward model, enhanced with physics-guided regularization, achieves a test mean squared error of 2.2 & times; 10-a while reducing the generalization gap by over 80 %. For inverse design, a dual-objective loss jointly optimizes spectral fidelity and geometric accuracy, addressing the non-uniqueness limitations of standard tandem networks in low-data regimes. A systematic grid search identifies optimal weighting parameters, yielding a relative improvement of 30 % in spectral consistency and 15.4 % in geometric accuracy compared with uniform weighting. The resulting model predicts geometry within fabrication tolerances, providing a physically consistent and fabrication-ready approach for scalable nanophotonic device design.

Nanocomposites assembled from polymer-grafted plasmonic nanoparticles (PGNs) can combine strong light-matter interactions with soft-matter functionalities and a high degree of translational symmetry. This work explored the potential of gold nanoparticles (16 nm diameter) grafted with polystyrene chains (degree of polymerization, N ≈ 63) as building blocks for acoustoplasmonic metasurfaces. We have decorated inorganic surfaces─crystalline silicon and SiO2 glass─with PGN monolayers and explored their surface acoustic waves with micro-Brillouin Light Scattering (μ-BLS) at various photon energies. Aided by finite-element-method calculations of acoustic phonons, plasmons and optomechanics, we show that PGNs sustain coupled sphere modes with rattling, torsional, and quadrupolar features. The coupled sphere modes exhibit plasmon-enhanced BLS and form a wide acoustic band gap below the line of sound. In the long wavelength limit, the coupling to the substrate leads to the emergence of shear-horizontal and Sezawa waves, whose dispersion relationships yield the local scale elasticity of ultrathin PGN monolayers.

The study of localized surface plasmons (LSPs) in nanoscale structures is an essential step towards identifying optimal plasmonic modes that can facilitate robust optomechanical coupling and deepen our understanding of light–matter interactions at the nanoscale. This paper investigates, numerically, using the finite element method, LSP modes in a design comprising two coupled nano-ridges deposited on a gold layer with an interposing polymer spacer layer. Such a structure, usually referred to as a particle-on-mirror structure, shows exquisite optical properties at the nanoscale. We first examine the LSP modes of a single nano-ridge through the analysis of its scattering cross-section in the visible and infrared ranges. To enhance the plasmonic response, a thin polymer layer is placed at the middle of the ridge, which introduces additional LSP modes confined within the former. Then, we extend the analysis to the dimer configuration, which exhibits more complex and enhanced plasmonic behavior compared to a single nano-ridge. In particular, the dimer configuration yields LSP resonances with a quality factor enhancement of approximately threefold relative to a single nano-ridge. Furthermore, the presence of the polymer layer within the ridges significantly improves plasmon field localization and the quality factor. These findings underscore the potential of nano-ridge-based structures in advancing optomechanical coupling and offering valuable insights for the development of high-performance acousto-plasmonic devices. In particular, the proposed device could help significantly improve the design of nano-acousto-optic modulators, operating in the visible or in the near-infrared ranges, that require an enhanced light–phonon coupling rate.

We report an experimental study of surface acoustic wave (SAW) localization and propagation in random metasurfaces composed of Al scatters using pump–probe spectroscopy. Thanks to this technique, wideband high frequency acoustic modes are generated, and their dynamical propagation directly from inside of the media with a high (micrometric) spatial resolution is enabled. During SAW propagation, part of the acoustic wavefront energy is trapped within free areas between the scatterers, acting as cavities. The spectral content of the localized modes of a few GHz is found to depend on the shape and size of the cavities but also on the landscape seen by the wave during its propagation before arriving inside them. The experimental results are supported by numerical simulations using the finite element method. This study is the phononic part of a more global research on the co-localization of elastic and optical waves on random metasurfaces, with the main objective of enhancing the photon–phonon interaction. Applications could range from the design of acousto-optic modulators to ultrasensitive sensors.

AbstractWe present a comprehensive study using finite element numerical analysis of the acoustic localized phonons supported by a gold nanoridges dimer-based multilayer design. The latter consists in a SiO2-substrate over which a gold film covered with a thin polymer is deposited. We investigate first the mechanical eigen-modes analysis of a single monomer ridge, where we find flexural and compressional type modes in the sub-GHz frequency range. This is realized by either setting the ridge in a periodic structure, which enables to get the dispersion curve of the modes, or by considering an isolated system bounded by perfect matched layers, where we use the equivalent of the local density of states to track the modes. A good agreement is obtained between the two methods. Similarly, we find in case of the coupled dimer ridges hybridized modes, namely the flexural and compressional modes of a monomer split-up into in- and -out-of-phase type modes. We demonstrate efficient coupling between the monomer/dimer localized phonons with surface acoustic waves (SAWs) as the simulated transmission spectra show dips at the frequencies of the monomer/dimer eigenmodes. For symmetry reasons, some of the dimer modes are expected to be optomechanically active. The proposed SAW-based device is meant to help design acousto-optic modulators or ultrasensitive sensors.

A Friedrich–Wintgen bound state in the continuum (FW-BIC) is of particular interest in the field of wave physics phenomena. It is induced via the destructive interference of two modes that belong to the same cavity. In this work, we analytically and numerically show the existence of FW-BIC in a T-shaped cavity composed of a stub of length d0 and two lateral branches of lengths d1 and d2, attached to an infinite waveguide. The whole system consists of metal–insulator–metal (MIM) plasmonic waveguides that operate in the telecommunication range. Theoretically, when d1 and d2 are commensurated, BIC is induced by these two branches. This latter is independent of d0 and the infinite waveguide, where the T structure is grafted. By breaking the BIC condition, we obtain a plasmon-induced transparency (PIT) resonance. The PIT resonance’s sensitivity to the dielectric material of the waveguide may be exploited to design a sensitive nanosensor suitable for sensing platforms, thanks to its very small footprint. A sensitivity of 1400 nm/RIU and a resolution of 1.86×10−2 RIU showed a high level of performance that the designed structure achieved. Moreover, this structure could also be used as a biosensor, in which we have studied the detection of the concentration in the human body, such as Na+, K+, and glucose solutions, and these sensitivities can reach 0.21, 0.28, and 1.74 nm dL/mg, respectively. Our designed structure advances with technology and has good application prospects, working as a biosensor to detect the blood’s hemoglobin level. The analytical results, obtained via Green’s function method, are validated via numerical simulations using Comsol Multiphysics software based on the finite element method.

The interaction between phonons and localized plasmons in film-coupled nanoparticles designs can be exploited both for modulating the scattered electromagnetic field and the understanding of the mechanical vibrations at nanoscale. In this paper, we show by finite element numerical analysis an enhanced optomechanical interaction in a film-coupled gold nanoridges or pillars mediated by surface acoustic waves. The metallic nanoparticles are placed atop a multilayer structure consisting of a thin dielectric spacer covering a gold film layer on a silicon dioxide/or silicon substrate. Optical simulations reveal the existence of surface localized plasmons in the infrared range confined under the nanoparticles in the dielectric spacer and/or in between such particles. Optomechanical coupling between the plasmonic modes and localized phonons is evaluated from the shift in the plasmon eigenfrequency. It is found that the compressional, the in-phase compressional and the out-of-phase flexural modes, yield the highest coupling rates. Such phonons are excited by means of SAW launched from the system inlet in front of the particles. The findings in this paper could help design new generation of acousto-optic modulators monitored by fast coherent surface acoustics.

Graphene plasmonic devices have been demonstrated to show great potential for reconfigurable metasurfaces due to the tuneable electronic charge transport properties of graphene in response to electrostatic gating. Iron‐doped lithium niobate is proposed as a platform for patterning‐free optically reconfigurable graphene metasurfaces in the THz spectral region. Under structured illumination, the lithium niobate undergoes charge migration in the bulk, where carriers migrate away from illuminated regions, forming spatially patterned charge distributions capable of electrostatic tuning of graphene. These charge distributions are stable in the dark, however, can be redefined by subsequent illumination. Through the use of numerical simulations, it is demonstrated that optically defined charge distributions in lithium niobate can tune locally the graphene Fermi level allowing for plasmonic resonances at THz frequencies.

We study analytically and numerically the design of plasmonic demultiplexers based on Fano and plasmonic induced transparency (PIT) resonances. The demultiplexers consist of T-shaped structures with an input waveguide and two output waveguides. Each output contains two waveguide stubs grafted either at the same position or at two different positions far from the input waveguide. We derive closed form analytical expressions of the geometrical parameters allowing a selective transfer of a single mode in one waveguide without affecting the other one. This is performed by implementing the Fano and PIT resonances which are characterized by a resonance placed near an antiresonance or placed between two antiresonances respectively. In particular, we show the possibility of trapped modes, also called bound in continuum (BIC) modes. These modes appear as resonances with zero width in the transmission spectra for appropriate lengths of the stubs. Then, by detuning slightly the stubs, BICs transform to PIT or Fano resonances. The existence of a full transmission besides a transmission zero, enables to filter a given wavelength on one output waveguide, by vanishing both the transmission on the second waveguide as well as the reflection in the input waveguide. The demultiplexer is capable to separate two fundamental optical windows (i.e. 1310 and 1550 nm). The performance of the demultiplexer platform is measured using the crosstalk of the two outputs and quality factor. The lowest value of the crosstalk −96.8 dB with an average of −84.7 dB is achieved and a maximum quality factor 45 is obtained. The maximum transmission reaches a high value of 85% despite the large metallic losses. These values are suitable for integrated photonic circuits in the optical communication. The analytical results are obtained by means of the Green’s function method which enables us to deduce the transmission and reflection coefficients, as well as the delay times and density of states. These results are confirmed by numerical simulations using a 2D finite element method. The analytical analysis developed in this work represent a predictive method to understand deeply different physical phenomena in more complex plasmonic devices.

Plasmonic coupling between adjacent metallic nanoparticles can be exploited for acousto-plasmonics, single-molecule sensing, and photochemistry. Light absorption or electron probes can be used to study plasmons and their interactions, but their use is challenging for disordered systems and colloids dispersed in insulating matrices. Here, we investigate the effect of plasmonic coupling on optomechanics with Brillouin light spectroscopy (BLS) in a prototypical metal-polymer nanocomposite, gold nanorods (Au NRs) in polyvinyl alcohol. The intensity of the light inelastically scattered on thermal phonons captured by BLS is strongly affected by the wavelength of the probing light. When light is resonant with the transverse plasmons, BLS reveals mostly the normal vibrational modes of single NRs. For lower energy off-resonant light, BLS is dominated by coupled bending modes of NR dimers. The experimental results, supported by optomechanical calculations, document plasmonically enhanced BLS and reveal energy-dependent confinement of coupled plasmons close to the tips of NR dimers, generating BLS hot-spots. Our work establishes BLS as an optomechanical probe of plasmons and promotes nanorod- soft matter nanocomposites for acousto-plasmonic applications.

We theoretically investigate, with the help of the finite element method, the interaction between aluminum pillars erected on top of a semi-infinite substrate of silicon in the low frequency range. Our interest is to control and manipulate the propagation of the surface acoustic waves through a linear chain of pillars for frequencies in the [0, 2] GHz range. We show that two pillars can interact together in near field through the excitation of their resonant compressional eigen mode. We will then investigate the resonant modes of a finite linear chain of N pillars, in order to analyze the possibility of propagation along the chain. To reach this objective and to relax the numerical stresses, we have considered a periodicity in the direction perpendicular to the propagation where the lattice parameter is significantly higher to avoid any interaction between two adjacent lines of pillars.

The interaction between phonons and localized plasmons in coupled nanoparticles can be exploited both for modulating the scattered electromagnetic field and the understanding of the mechanical vibrations at nanoscale. In this paper, we demonstrate by numerical analysis an enhanced optomechanical interaction in a film-coupled gold dimer nanoparticles mediated by surface acoustic waves. Two gold nanoridges are placed atop a multilayer structure consisting of a thin dielectric spacer covering a gold film layer on a silicon dioxide substrate. Numerical simulations of the optical properties reveal the existence of three surface localized plasmons in the infrared range with enhanced scattering and narrower linewidths than with a single nanoridge. The physical origin of such modes as well as their tunability as function of key geometrical parameters are successfully captured with a simple model based on effective Metal-Insulator-Metal (MIM)-like plasmonic cavity. We calculate the optomechanic coupling rates between the GHz localized mechanical modes and plasmonic modes of the dimer, finding that the strongest coupling is observed for the in-phase compressional mode followed by the out-of-phase flexural mode. Both such modes can be excited by launching a surface acoustic wave (Sezawa wave) at the inlet in front of the dimer structure. It is also found that the flexural mode which is inactive optomechanically in case of a monomer becomes active due to dimer coupling, with a significant phonon-plasmon coupling rate. The findings in this work may facilitate design of new optomechanical components monitored with fast coherent acoustics, leading to new generation of light acousto-optic modulators where strong optomechanical interactions are required.

Particle vibrational spectroscopy has emerged as a new tool for the measurement of elasticity, glass transition, and interactions at a nanoscale. For colloid-based materials, however, the weakly localized particle resonances in a fluid or solid medium renders their detection difficult. The strong amplification of the inelastic light scattering near surface plasmon resonance of metallic nanoparticles (NPs) allowed not only the detection of single NP eigenvibrations but also the interparticle interaction effects on the acoustic vibrations of NPs mediated by strong optomechanical coupling. The "rattling" and quadrupolar modes of Ag/polymer and polymer-grafted Ag NPs with different diameters in their assemblies are probed by Brillouin light spectroscopy (BLS). We present thorough theoretical 3D calculations for anisotropic Ag elasticity to quantify the frequency and intensity of the "rattling" mode and hence its BLS activity for different interparticle separations and matrix rigidity. Theoretically, a liquidlike environment, e.g., poly(isobutylene) (PIB) does not support rattling vibration of Ag dimers but unexpectedly hardening of the extremely confined graft melt renders both activation of the former and a frequency blue shift of the fundamental quadrupolar mode in the grafted nanoparticle Ag@PIB film.

We study numerically the interaction of surface acoustic waves propagating at the surface of a multilayer structure (the so-called Sezawa waves) with localized surface plasmons at the bottom of metallic pillars deposited on the substrate. The pillars are made of gold and the multilayer structure is constituted by a layer of gold on a silicon substrate and covered by a thin spacer of dielectric material. We are interested in localized plasmons, which are the analog of metal-dielectric-metal surface plasmons. The physical characteristics of these plasmons, such as their eigenvalues and absorption or reflection spectra, are modulated by surface acoustic waves due to deformation of the structure in the vicinity of the pillars. The surface waves are generated in the area in front of the pillars deposited on the substrate. Two types of acoustic modes are envisioned to interact with the localized plasmons, namely, confined modes associated with the local resonances of the pillars and the propagating Sezawa modes that deform the structure in the vicinity of the surface. We discuss the efficiency of phonon-plasmon coupling for both types of modes and select those that can be useful for an experimental realization based on this platform. Also, we show that, besides the plasmon frequency, the phonon-plasmon coupling magnitude strength is sensitive to the distance between the pillars, which reveals the role of interactions between the pillars on the coupling. The geometrical parameters are chosen such that the acoustic waves are in the sub-GHz range and the plasmons are around the telecommunication wavelength of 1.55 mu m. This work may help with the design of optomechanical devices that can be controlled by fast coherent acoustics and facilitate applications involving enhanced sound-light interactions, such as light modulation.

We study both analytically and numerically the possibility to realize a simple plasmonic Y-shaped demultiplexer made of an input line and two output lines. Each line consisting of a metal-insulator-metal (MIM) waveguide contains a specific resonator made of two stubs grafted at a given position from the input line. The two stubs on each line induce a plasmonic induced transparency (PIT) resonance in the transmission spectra characterized by a resonance squeezed between two zeros. The idea consists in coinciding at a given wavelength, a resonance on one line with a transmission zero on the other line. We give closed-form expressions of the geometrical parameters allowing the selective transfer of a single mode in one line without affecting the other line. The analytical results, obtained by means of the Green’s function method, are confirmed by numerical simulation using finite element method via Comsol Multiphysics software.