Metal nanoparticles with sharp corners: Universal properties of plasmon resonances

Abstract

We predict the simultaneous occurrence of two fundamental phenomena for metal nanoparticles possessing sharp corners: First, the main plasmonic dipolar mode experiences strong red shift with decreasing corner curvature radius; its resonant frequency is controlled by the apex angle of the corner and the normalized (to the particle size) corner curvature. Second, the split-off plasmonic mode experiences strong localization at the corners. Altogether, this paves the way for tailoring of metal nano-structures providing wavelength-selective excitation of localized plasmons and a strong near-field enhancement of linear and nonlinear optical phenomena.PACS numbers: 73.20. Mf, 42.25.Fx, 78.40.Kc Plasmon excitations of metal nanoparticles, including 2D particles (nanowires), is a vast and hot research area. Potential applications of nano-plasmons span from nanolasers (spasers) [1-3] to bio-sensors and sensors of single atoms and molecules [4,5]. It is highly important to apprehend the possibilities of tailoring of plasmonic resonances and the extends of concentration of the light energy deeply on the sub-wavelength scale.Analytical plasmonic solutions to Maxwell's equations are available only for metal nanoparticles of simplest 2D and 3D shapes -like circular cylinder, sphere, and ellipsoid [6,7]. They include the optical permittivity of the Using direct numerical methods, serious efforts have been undertaken to ascertain the phenomenology of the plasmonic response of 2D and 3D particles of more complicated shapes [8][9][10][11]. The general outcome is that the deviations from the most symmetric shapes result in enriching plasmon spectrum and in the appearance of new red-shifted resonances. In the 2D case, the strongest shape effects were found for triangular cross-sections.Regardless of the plasmonic effects, the presence of sharp tips and corners (metal or dielectric) is known to lead to the corner singularities, i.e., to a strong local enhancement of electromagnetic fields [12,13]. At the same time, the impact of tips and corners on the plasmonic properties of nanoparticles remains in essence unexplored. In numerical simulations, the sharp surface features are typically rounded to avoid numerical instabilities. Non-rounded π/2-corners lead to divergence of numerical methods for ε M → −3 [14]. Mathematical foundations of the plasmonic theory [15,16] refer to sufficiently smooth (Lyapunov) surfaces. A recent heuristic attempt to deal with plasmonic properties of nanoparticles possessing perfectly sharp features [17] is strongly unconvincing.Experimentally, the presence of sharp features of metal nanoparticles is far from exotic. Modern chemical methods for production such particles are able to provide almost atomically sharp edges in the crystallographic directions [18][19][20]. Synthesis of metal nanostars has allowed recently to strongly enhance the near fields at the plasmonic resonance [21].In this Letter, we predict and analyze a strong impact of corners of metal nanoparticles on fundamental plasmonic properties...

Selective excitation of plasmons superlocalized at sharp perturbations of metal nanoparticles

“…16 The simplest polyhedral 2D particles -wires of smoothed square, triangular and rhomboidal cross sections -exhibit highly specific plasmonic properties: The resonant values of the metal permittivity are determined by the geometric parameters of the corners -the apex angle and the curvature radius. 16,17 A similar behavior was predicted for 3D particles of smoothed cubic shapes. 18 Generally, as the sharpness increases, the plasmons become superlocalized, i.e.…”

“…1,2 With analytical solutions available only for the simplest (e.g., spherical and ellipsoidal) shapes, many efforts have been spent on numerical modeling of LPs of complex-shape particles. [9][10][11][12][13][14][15][16][17][18][19][20][21] It is recognized that adding new shape features enriches the LP spectrum considerably. In certain cases, it is possible to consider plasmons of complex particles as a result of hybridization of plasmons supported by constituents of simpler shape 14,15 similarly to hybridized diatomic electronic states.…”

Correlation of local strain and temperature measurements in confocal Raman microscopy

“…This increment can be attributed to an electric field enhancement normally observed in sharp edges of dielectric materials, where the sample geometry plays an important role in the magnitude of the enhancement. [57][58][59] In order to confirm the laser-induced thermal effects observed experimentally, numerical simulations of silicon cavities and copper-filled TSVs are conducted. As illustrated in Figure 8a, the increase of temperature observed in Figure 7a should also occur for measurements performed with a laser power as low as 1 mW.…”

Plasmonic Liquid Marbles: A Miniature Substrate‐less SERS Platform for Quantitative and Multiplex Ultratrace Molecular Detection

“…This high AEF is mainly attributed to the presence of high SERS hotspot densities in the Ag nanocube powder, in which Ag nanocubes are closely packed together. [19] Further to the quantitative ultratrace detection of an aqueous analyte encapsulated within the plasmonic liquid marble, the exterior of the plasmonic liquid marble can also be used for analyte detection in the organic phase (Figure 3 A). Figure 3 B shows a typical SERS spectrum of coumarin, an organic-soluble compound (see Table S2 for the assignment of the SERS bands).…”