Plasmon-driven chemical transformation has become a promising approach for enhancing sluggish electrocatalytic reactions. Herein, an alternative enhancement strategy employing plasmon-induced hot electrons was developed and found to be competitive with oxygen reduction reaction (ORR) on platinum electrodes. We demonstrated that, by using the intertwined plasmon-catalytic-electrochemical properties of nanoperforated gold thin film electrodes, the ORR under alkaline conditions could be significantly enhanced by careful tuning of the laser wavelength and power density. Irradiation at 980 nm and 2 W cm⁻² displays maximal current densities of j = −6.0 A cm⁻² at 0.95 V vs. RHE, under hydrodynamic conditions, comparable to that of commercial Pt/C (40 wt%) catalysts, with good long-term stability. The wavelength-dependent electrochemical reduction confirmed that the hot carriers formed during plasmon decay are responsible for the improved electrocatalytic performance.

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 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.

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 μm. 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.

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.