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L. D. Negro et al.
9.2.6 Hot Spot Engineering with Aperiodic Plasmon Arrays
In this section, we will discuss the distinctive features of local electromagnetic field
engineering using aperiodic arrays of metal nanoparticles. Let us first recall that
arbitrary arrays of resonant metallic nanostructures can resonantly couple at multiple length scales due to near-field (i.e., quasi-static) interactions and radiative electromagnetic interactions (i.e., multiple scattering). Nanoplasmonics is mostly concerned with the engineering of longitudinal near-fields around metallic nanostructures and near-field coupling between nanostructures in order to boost the intensity of
incident electromagnetic fields over nanoscale regions, known as “electromagnetic
hot spots” [87]. However, when the electromagnetic radiation is incident on multiparticle arrays with separations comparable or larger than the wavelength of light,
nanoparticles additionally couple by radiative interactions and give rise to collective scattering resonances that are largely tunable by the array geometry. Therefore,
when engineering multi-particle arrays, long-range photonic–plasmonic coupling
effects must always be carefully accounted for. A simple example of “array driven”
photonic–plasmonic coupling is provided by the Fano-type resonances observed in
metal nanoparticle gratings [88–92]. Periodic gratings support a discrete spectrum of
narrow photonic resonances, known as diffractive or grating modes that when spectrally/spatially overlapped with the broader LSP modes of individual nanoparticles,
under appropriate excitation wavelengths and incidence angles, lead to more intense
hot spots and larger optical cross sections.
On the other hand, in aperiodic multi-particle arrays a large number of spatial frequencies (wavenumbers) are available to match in-plane photonic scattering
processes, resulting in photonic–plasmonic coupling effects distributed over a larger
wavelength range [93]. Therefore, we can generally expect that aperiodic arrays featuring a large density of spatial frequencies will result in a higher density of enhanced
hot spots over a broader frequency range compared to structures with more regular
geometry.
Consistently, roughened metal surfaces and random media, characterized by deltacorrelated white spectra, have demonstrated dramatic enhancement of hot spot intensities for single molecule detection [94]. We have recently confirmed the qualitative
picture discussed above by coupled dipole and semi-analytical multiple scattering
calculations of near-fields and scattering spectra in deterministic aperiodic arrays
of varying structural complexity (i.e., Fourier spectral properties) [80, 93, 95]. It is
important to notice that the ability to engineer aperiodic structures with structural
complexity in between random and periodic media enables the optimization of hot
spots in complex photonic–plasmonic media, and can unveil distinctive engineering
design rules. This point is illustrated by Forestiere et al. [80], who systematically
investigated the near-field plasmonic behavior and far-field scattering response of Ag
nanoparticle arrays generated according to prime numbers distributions in two spatial dimensions. Using rigorous coupled dipole analysis for dipolar nanoparticles,
this study demonstrates how the local intensity of plasmon fields can be strongly
enhanced over broad frequency spectra by engineering closely packed (i.e., large
L. D. Negro et al.
9.2.6 Hot Spot Engineering with Aperiodic Plasmon Arrays
In this section, we will discuss the distinctive features of local electromagnetic field
engineering using aperiodic arrays of metal nanoparticles. Let us first recall that
arbitrary arrays of resonant metallic nanostructures can resonantly couple at multiple length scales due to near-field (i.e., quasi-static) interactions and radiative electromagnetic interactions (i.e., multiple scattering). Nanoplasmonics is mostly concerned with the engineering of longitudinal near-fields around metallic nanostructures and near-field coupling between nanostructures in order to boost the intensity of
incident electromagnetic fields over nanoscale regions, known as “electromagnetic
hot spots” [87]. However, when the electromagnetic radiation is incident on multiparticle arrays with separations comparable or larger than the wavelength of light,
nanoparticles additionally couple by radiative interactions and give rise to collective scattering resonances that are largely tunable by the array geometry. Therefore,
when engineering multi-particle arrays, long-range photonic–plasmonic coupling
effects must always be carefully accounted for. A simple example of “array driven”
photonic–plasmonic coupling is provided by the Fano-type resonances observed in
metal nanoparticle gratings [88–92]. Periodic gratings support a discrete spectrum of
narrow photonic resonances, known as diffractive or grating modes that when spectrally/spatially overlapped with the broader LSP modes of individual nanoparticles,
under appropriate excitation wavelengths and incidence angles, lead to more intense
hot spots and larger optical cross sections.
On the other hand, in aperiodic multi-particle arrays a large number of spatial frequencies (wavenumbers) are available to match in-plane photonic scattering
processes, resulting in photonic–plasmonic coupling effects distributed over a larger
wavelength range [93]. Therefore, we can generally expect that aperiodic arrays featuring a large density of spatial frequencies will result in a higher density of enhanced
hot spots over a broader frequency range compared to structures with more regular
geometry.
Consistently, roughened metal surfaces and random media, characterized by deltacorrelated white spectra, have demonstrated dramatic enhancement of hot spot intensities for single molecule detection [94]. We have recently confirmed the qualitative
picture discussed above by coupled dipole and semi-analytical multiple scattering
calculations of near-fields and scattering spectra in deterministic aperiodic arrays
of varying structural complexity (i.e., Fourier spectral properties) [80, 93, 95]. It is
important to notice that the ability to engineer aperiodic structures with structural
complexity in between random and periodic media enables the optimization of hot
spots in complex photonic–plasmonic media, and can unveil distinctive engineering
design rules. This point is illustrated by Forestiere et al. [80], who systematically
investigated the near-field plasmonic behavior and far-field scattering response of Ag
nanoparticle arrays generated according to prime numbers distributions in two spatial dimensions. Using rigorous coupled dipole analysis for dipolar nanoparticles,
this study demonstrates how the local intensity of plasmon fields can be strongly
enhanced over broad frequency spectra by engineering closely packed (i.e., large