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Enhanced Quality Factors of Surface Lattice Resonances in Plasmonic Arrays of Nanoparticles

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The experimental demonstration of narrow resonances in arrays of metallic nanoparticles was more elusive due to limitations in the quality of samples and the use of focused beams. Kravets et al. reported ultranarrow plasmonic resonances in asymmetric (different refractive indexes in the upper and lower media) arrays of Au nanoparticles. Shortly after, Auguié and Barnes and Chu et al. reported narrow resonances in symmetric arrays. The potential of narrow plasmonic resonances in arrays of nanoparticles for modifying the emission of fluorophores was also demonstrated. The origin of the narrow resonances, which are known as surface lattice resonances (SLRs), is the diffractive coupling of LSPRs through in-plane diffraction orders in symmetric media or evanescent
diffraction orders—the so-called Rayleigh anomalies (RAs)—in asymmetric media. SLRs can be described as a driven damped coupled oscillator system in which one oscillator has the natural frequency of the LSPR while the other has the frequency of the diffraction order. Nanoparticle arrays are open cavities that are easy to fabricate and offer the possibility of integration with thin films or planar structures. The remarkable properties of SLRs have led to improved surface-enhanced Raman scattering, sensitive bio/chemical sensing, plasmonic band-edge lasing, strong light–matter coupling, Bose–Einstein condensation, and optoelectronic devices. The multidisciplinary impact of SLRs has stimulated the quest toward modes with the highest possible quality (Q) factor. One strategy to obtain narrow linewidths with SLRs is by coupling multipolar resonances with different diffraction orders. However, Q-factors by these approaches vary significantly over momentum space.

a) Figure of merit of silver films, defined as the ratio of the real and imaginary components of the permittivity. b) AFM map of the surface of silver films deposited at 10 Å s−1; the surface roughness, defined in terms of the root mean square of the surface profile, is 0.567 nm. c) AFM map of a silver film deposited at 30 Å s−1 with RMS = 3.817 nm.

We have demonstrated high quality factor plasmonic resonances in arrays of Ag nanoparticles (Q > 300). These resonances, known as surface lattice resonances, emerge from the coupling of localized surface plasmon polaritons to diffraction orders in the plane of the array. The quadratic dispersion of SLRs leads to a nearly constant Q-factor over a wide range
of wave vectors or angles of incidence. We have investigated the role of the intrinsic quality of the metal in the Q-factor of SLRs. We have also iscussed the effect of the adhesion layer used between the substrate and the metal on the SLRs. The suppression of this layer can lead to SLRs with Q-factors larger than 1500. These extremely high Q-factors render arrays of metallic nanoparticles very interesting systems for plasmonic applications such in sensors, for enhanced light–matter interaction and nonlinear phenomena.

a) Extinction spectra as a function of the wave vector parallel to the surface, showing the dispersion of the degenerate SLRs resulting from the coupling of LSPRs in Ag nanoparticles to (0, ±1) RAs. The LSPR at λ = 475 nm is independent of the wave vector, whereas the white curve corresponds to the (0, ±1) RAs. b) Extinction spectra showing the dispersion of the diagonal SLRs arising from the coupling of the LSPRs to the (±1, 0) RAs (indicated
by the white lines). c) Extinction spectra from part (a), measured at different angles. d) Extinction spectra from part (b), measured at different angles. The insets in parts (a) and (b) show a schematic representation of the angle-dependent extinction measurements, where the rectangles represent the nanorods, the orange double arrow indicates the polarization of the incident light, and the white curve illustrates the rotation direction of the sample.

For more information: DOI: 10.1002/adom.201801451

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