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

Abstract

In the past decade, advances in nanotechnology have led to the development of plasmonic nanocavities which facilitate light-matter strong coupling in ambient conditions. The most robust example is the nanoparticle-on-mirror (NPoM) structure whose geometry is controlled with subnanometer precision. The excited plasmons in such nanocavities are extremely sensitive to the exact morphology of the nanocavity, giving rise to unexpected optical behaviors. So far, most theoretical and experimental studies on such nanocavities have been based solely on their scattering and absorption properties. However, these methods do not provide a complete optical description of a NPoM. Here, the NPoM is treated as an open non-conservative system supporting a set of photonic quasinormal modes (QNMs). By investigating the morphology-dependent 1 arXiv:1910.02273v1 [physics.optics] 5 Oct 2019 optical properties of nanocavities, we propose a simple yet comprehensive nomenclature based on spherical harmonics and report spectrally overlapping bright and dark nanogap eigenmodes. The near-field and far-field optical properties of NPoMs are explored and reveal intricate multi-modal interactions. Introduction Metallic nanostructures have the ability to confine light below the diffraction limit via the collective excitation of conduction electrons, called localized surface plasmons. Through recent advances in nanofabrication techniques, gaps of just 1-2 nm between nanostructures have been achieved. 1,2 At such extreme nanogaps, the plasmonic modes of two nanostructures hybridize to allow an unprecedented light confinement, 3,4 making coupled nanostructures an ideal platform for field-enhanced spectroscopy, 5,6 photocatalysis 7 and nano-optoelectronics. 8One such nanostructure is the nanoparticle-on-mirror (NPoM) geometry where a nanoparticle is separated from an underlying metal film by a molecular mono-layer. 9,10 This geometry (which resembles the prototypical dimer but is more reliable and robust to fabricate) has attracted considerable interest since it enables light-matter strong-coupling of a single molecule at room temperature, 11 and it has many potential applications, including biosensing 12 and quantum information. 13,14 A wide range of theoretical and experimental studies have been conducted to investigate the optical properties of NPoM nanocavities. 12,15,16 Several studies examine resonances of NPoMs 17-21 and their influence on optical emission of single molecules in the nanogaps. [22][23][24] However, most studies on the nanocavities have so far described their optical response via a scattering method and infer their resonances from resulting far-field spectral peaks. Although

Controlling Optically Driven Atomic Migration Using Crystal-Facet Control in Plasmonic Nanocavities

“…The optics of such cubic nanoconstructs have been thoroughly characterized previously. 25 − 37 Based on a spherical harmonics description, the two dominating radiative modes for such small gaps are labeled (10) and (20) 22 , 38 and are accessed from large-angle excitation using high NA optics. 39 We emphasize that these modes are different from the (11) x , y modes typically accessed at normal incidence in NCoMs with larger gaps (>5 nm), 34 which for the small gaps here red-tune beyond 1 μm wavelength and possess negligible optical coupling efficiency.…”

“…Detailed full-wave simulations, analytic models, and experiments show that 1% redshift in the (10) mode (Δλ = 8 nm) corresponds to the arrival of only ∼500 atoms onto the facet, or a change in facet radius of less than 1 atom. 38 , 40 , 42 This allows real-time dark-field measurements to detect nanoscale reconstruction by comparing between NPoMs and NCoMs before (orange line) and after 30 s (blue line) of continuous laser illumination ( Figure 2 a,b). These show similar scattering strength at the 633 nm pump wavelength (red arrows).…”

“…This is expected for the larger cube facets where more of the mode tuning is transferred onto the longer wavelength (10) mode. 22 , 38 , 46 For different NCoMs, the power thresholds differ. This is quantified by fitting the power threshold ( I th ) using a sigmoidal dependence λ = λ o + K [1 + exp{ – ( I – I th )/ R }] −1 ( Figure 4 b,c).…”

Nanoscopy through a plasmonic nanolens

“…We first explored this complexity by solving Maxwell's equations using finite element methods (FEMs; Methods and ref. 35). We obtained the far-field emission image of a dipole emitting at λ = 660 nm inside an 80-nm NPoM with gap d = 1 nm and facet diameter w = 20 nm, as it was shifted along the x direction by up to 15 nm ( Fig.…”

Optical Scattering of Liquid Gallium Nanoparticles Coupled to Thin Metal Films

“…Recently, the plasmonic properties of a metallic NP placed on a thin metal film [ 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 ], has attracted great interest. The plasmonic properties of a gold (Au) NP placed on a thin Au film was numerically studied [ 36 ].…”