When metal nanoparticles are arranged in an ordered array, they may scatter light to produce diffracted waves. If one of the diffracted waves then propagates in the plane of the array, it may couple the localized plasmon resonances associated with individual nanoparticles together, leading to an exciting phenomenon, the drastic narrowing of plasmon resonances, down to 1–2 nm in spectral width. This presents a dramatic improvement compared to a typical single particle resonance line width of >80 nm. The very high quality factors of these diffractively coupled plasmon resonances, often referred to as plasmonic surface lattice resonances, and related effects have made this topic a very active and exciting field for fundamental research, and increasingly, these resonances have been investigated for their potential in the development of practical devices for communications, optoelectronics, photovoltaics, data storage, biosensing, and other applications. In the present review article, we describe the basic phy...

Plasmonic Surface Lattice Resonances: A Review of Properties and Applications

Infrared absorption spectroscopy enabling direct access to vibrational fingerprints of the molecular structure is a powerful method for functional studies of bio-molecules Although the intrinsic absorption cross-sections of IR active modes of proteins are nearly 10 orders of magnitude larger than the corresponding Raman cross-sections, they are still small compared to that of fluorescence-label based methods Here, we developed a new tool based on collective excitation of plasmonic nanoantenna arrays and demonstrated direct detection of vibrational signatures of single protein monolayers We first tailored the geometry of individual nanoantennas to form resonant structures that match the molecular vibrational modes The tailored nanoantennas are then arranged in such a way that their in-phase dipolar coupling leads to a collective excitation of the ensemble with strongly enhanced near fields The combined collective and individual plasmonic responses of the antenna array play a critical role in attaining signal enhancement factors of 10(4)-10(5) We achieved measurement of the vibrational spectra of proteins at zeptomole levels for the entire array, corresponding to only 145 molecules per antenna The near-field nature of the plasmonic enhancement of the absorption signals is demonstrated with progressive loading of the nanoantennas with varying protein film thicknesses Finally, an advanced model based on nonequilibrium Green's function formalism is introduced, which explains the observed Fano-type absorption line-shapes and tuning of the absorption strengths with the antenna resonance

https://www.pnas.org/content/pnas/106/46/19227.full.pdf

Ultra-sensitive vibrational spectroscopy of protein monolayers with plasmonic nanoantenna arrays.

We report the effect of metal-island size variation in nanoparticle-enhanced photodetectors. Nanoparticle size was controlled by varying the deposition and annealing conditions used to produce the metal-island films. Increasing the size of silver-island particles fabricated onto 165 nm thick silicon-on-insulator (SOI) photodetectors resulted in a dramatic increase in the observed photocurrent. A nearly factor-of-20 photocurrent enhancement was observed for light of wavelength 800 nm, a significant improvement over previously reported results. The improvement is linked to two physical effects: the increased scattering efficiency of the larger nanoparticles and a qualitative change in the resonance characteristics of the metal-island film due to radiative coupling to the SOI waveguide modes.

Island size effects in nanoparticle-enhanced photodetectors

We report on the optical properties of single isolated silver nanodisks and pairs of disks fabricated by electron beam lithography. By systematically varying the disk size and surface separation and recording elastic scattering spectra in different polarization configurations, we found evidence for extremely strong interparticle interactions. The dipolar surface plasmon resonance for polarization parallel to the dimer axis exhibited a red shift as the interdimer separation was decreased; as expected from previous work, an extremely strong shift was observed. The scattering spectra of single particles and pairs separated by more than one particle radius can be well described by the coupled dipole approximation (CDA), where the particles are approximated as point dipoles using a modified dipole polarizability for oblate spheroids. For smaller particle separations (d < 20 nm), the simple dipole model severely underestimates the particle interaction, indicating the importance of multipolar fields and finite-s...

Confined Plasmons in Nanofabricated Single Silver Particle Pairs: Experimental Observations of Strong Interparticle Interactions

We demonstrate that coupling between grating diffraction and localized surface plasmons in two-dimensional gold nanoparticle arrays in water leads to narrow near-infrared resonance peaks in measured far field extinction spectra. Good agreement is obtained between finite difference time domain (FDTD) calculations and experimental extinction spectra. The FDTD calculations predict that the gold nanoparticle arrays exhibit near-field electric field intensity (E2) enhancements approximately one order of magnitude greater than those of single isolated gold nanoparticles.

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Experimental observation of narrow surface plasmon resonances in gold nanoparticle arrays

Amplified electromagnetic fields generated by a surface of finitely sized metal or dielectric particles are calculated. Regular arrays of particles produced by lithographic techniques and stochastic particle distributions that occur, e.g., in island films, are discussed. Retarded dipolar interactions between the particles are explicitly taken into account. Particles of finite size are considered for which dynamic depolarization and radiation damping effects are important. Limits of validity of the present approach are indicated. The total electromagnetic field from the surface is calculated by superposition: Scalar potentials characterizing a single particle are convoluted with a distribution function describing the particle positions. The surface Hertz vector is obtained from the single-particle Hertz vector by convolution with a two-dimensional Shah function representing the array or with the autocorrelation function of the stochastic surface. A plane-wave description of the dipolar fields is used, whereby the convolution is transformed into a simple multiplication in Fourier space. Cylindrical, general spheroidal, and spherical shapes are considered for the individual particle. Particle dipole moments are obtained by a self-consistent procedure. Dipolar interactions result in shifts and broadening of the particle plasmon resonances, which are responsible for the local intensity enhancement. A set of universal curves is given from which shift and broadening can be calculated for particles of all sizes and shapes. Extrema in the dipolar interactions arise when grating orders change from radiative to evanescent character. The strong variation of the Raman enhancement with angle and wavelength in the vicinity of these extrema is clearly predicted from the Hertz-vector calculation. The formalism described permits one to calculate electromagnetic properties of the surface and enhancement factors for any electromagnetic process occurring at or near the surface. As examples, the calculation of reflectivities for s-and p-polarized excitation and surface-enhanced Raman-scattering cross sections are discussed.

A. Wokaun

Papers

Enhanced fields on rough surfaces: dipolar interactions among particles of sizes exceeding the Rayleigh limit