Understanding plasmon resonances of metal-coated colloidal crystal monolayers

Abstract

Metal films deposited over two-dimensional colloidal crystals (MFoCC) constitute a low-cost periodic structure with interesting photonic and plasmonic properties. It has previously been shown that this structure exhibits a behaviour similar to the well-known Extraordinary Optical Transmission (EOT) of metallic hole arrays in planar films. Here, we explore the transmission characteristics of AgFoCC by systematic comparison with that of the bare CC. Furthermore with additional reflectivity measurements we evaluate the AgFoCC overall plasmonic response, which, notably, exhibits a strong plasmon absorption band at wavelengths larger than those of the transmitted maximum. By corroborating these results with finite-difference time-domain electromagnetic simulations, we identify a hybrid metal-dielectric propagative mode in the transmission mechanism. On the contrary a strongly localized mode is responsible for the maximum light absorption by this structure. These results shed new light on the current understanding of this highly promising plasmonic structure, being useful for the design of surface-enhanced Raman scattering and enhanced fluorescence substrates.

Far-field disentanglement of modes in hybrid plasmonic-photonic crystals by fluorescence nano-reporters

“…Guided (Bloch) modes and cavity resonances of all dielectric 2D CC become more pronounced due to stronger light localization in the metal terminated diffraction gratings. This relevance was corroborated by the computed electromagnetic field distributions, which present strong field confinement at the location of the spheres, both within the sphere and near the metal external boundary, evidencing the occurrence of a Bloch-SPP mode at the transmission maximum [11,19]. Recently, an emission enhancement of 28 times was demonstrated for fluorophores placed at about 1 nm above the hybrid structure [20].…”

“…A strong plasmon excitation manifests itself through a strong absorption of incident light. The inflection point around 705 nm, exhibited in the transmission spectrum of Figure 2 A and considerably red-shifted with respect to the transmission maximum, might be attributed to localized surface plasmon polariton (LSPP) resonances in the silver half-shells, which are coupled and strongly interacting at the HS rim junctions of the hybrid structure [11,32].…”

“…The large electromagnetic field intensity achieved due to the field confinement in a Bloch mode promotes the excitation of SPPs, which are coupled out. The transmission minimum found on the short-wavelength side of the main transmission peak is associated to light diffracted parallel to the lattice surface, and is known as Wood anomaly or Rayleigh-Wood (RW) anomaly [11,31]. Another resonance in the metal-on-monolayer hybrid structure refers to localized plasmons excited in individual halfshells (HSs) [12].…”

“…Right: SEM top views of the hybrid structure at two different magnifications. According to the recently published litterature [11,12,18], the transmission peak maximum, here located at 640 nm, is attributed to a Bloch-SPP mode. The Bloch mode wavelength is given by λ max = 2 π n eff / G kl , where n eff is the effective refractive index of the PS array and…”

“…A periodically nanostructured noble metal obtained by combining a continuous metal film to a dielectric photonic crystal both supports SPPs and LSPPs and allows to control the interplay between them [11]. Furthermore, the coupling strength between the photonic and plasmonic components of such a hybrid structure is strongly increased if the plasmonic component possesses its own resonances, one/some of them overlapping the resonances of the photonic crystal [12].…”