Modifying Plasmonic-Field Enhancement and Resonance Characteristics of Spherical Nanoparticles on Metallic Film: Effects of Faceting Spherical Nanoparticle Morphology

Abstract

A three-dimensional finite-difference time-domain study of the plasmonic structure of nanoparticles on metallic film (NPOM) is presented in this work. An introduction to nanoparticle (NP) faceting in the NPOM structure produced a variety of complex transverse cavity modes, which were labeled S11 to S13. We observed that the dominant S11 mode resonance could be tuned to the desired wavelength within a broadband range of ~800 nm, with a maximum resonance up to ~1.42 µm, as a function of NP facet width. Despite being tuned at the broad spectral range, the S11 mode demonstrated minimal decrease in its near field enhancement characteristics, which can be advantageous for surface-enhanced spectroscopy applications and device fabrication perspectives. The identification of mode order was interpreted using cross-sectional electric field profiles and three-dimensional surface charge mapping. We realized larger local field enhancement in the order of ~109, even for smaller NP diameters of 50 nm, as function of the NP faceting effect. The number of radial modes were dependent upon the combination of NP diameter and faceting length. We hope that, by exploring the sub-wavelength complex optical properties of the plasmonic structures of NPOM, a variety of exciting applications will be revealed in the fields of sensors, non-linear optics, device engineering/processing, broadband tunable plasmonic devices, near-infrared plasmonics, and surface-enhanced spectroscopy.

Plasmonic Nanocavity Modes: From Near-Field to Far-Field Radiation

“…To visualize how the QNMs evolve with varying facet width w, Fig. 3(d To further explore the radiative nature of each QNM, we estimate the radiation efficiency of the ( m) mode as Figure 4 shows the radiation efficiencies of (10), (20), (30), (40), (11), (21), (31), (41), (22) and (33) modes. Overall, the ( 0) modes are the dominant radiative channels of NPoMs, as one would expect.…”

“…1(d, e) and 2(f-j) for the w = 20 nm NPoM. Nonetheless, the symmetries of these QNMs are preserved for all facet widths, validating the nomenclature used in this article.Previous studies where the far-field spectra of NPoMs were analysed[17][18][19]21 have identified only the ( 0) and ( 1) modes. The ( 0) modes are bright and can be efficiently excited by the vertical (z-axis) component of an exciting plane wave.…”

“…Radiation efficiencies η m of (10),(20),(30),(40),(11),(21),(31),(41),(22) and (33) modes of NPoMs with facet width from 0 to 40 nm. The ( 0), ( 1),(22) and(33)modes are shown as blue, red, orange and green lines, respectively.…”

Defining the plasmonic cavity performance based on mode transitions to realize highly efficient device design

“…The refractive index of gold was taken from the Johnson and Christy database and was modeled using Lorentz-Drude dispersion fitting. [44][45][46][47][48] e w ð Þ ¼ 1…”

“…These systems display rich plasmonic modes and have a high potential technological utility. 11,16,19,27,35,41,44,45 These NPOM designs mimic theoretical free-space dimer (FSD) designs, in which two metallic nanoparticles (NPs) are separated by a nanogap distance to create a hot spot. 36,44,45 The dielectric layer inserted between the NP and mirror acts as a virtual hot-spot region, in which attractive plasmonic properties can be realized.…”