Optimizing Interparticle Gaps in Large‐Scale Gold Nanoparticle Supercrystals for Flexible Light‐Matter Coupling

Abstract

order on scales larger than optical wavelengths, new optical excitations can emerge. The plasmons of different NPs interact, creating coherent collective excitations that propagate through the lattice. [4][5][6] The collective plasmonic modes couple to photons forming hybrid quasiparticles, plasmon polaritons, with properties different from the excitations of the individual NPs. Depending on the supercrystal geometry, polaritons in different lightmatter coupling regimes can be realized.The reduced coupling strength g η ω =with the coupling strength g normalized to the frequency of the confined mode ω is an established quantity to classify lightmatter interaction. [7] The ultrastrong coupling regime, where 0.1 < η < 1, and the deep strong coupling regime, where η > 1 have only emerged in the past decade. The lower limit of η = 0.1 has been by now well established as the regime where observable effects of lightmatter interaction beyond the Purcell effect emerge. These strong light-matter coupling regimes promote a wide range of interesting optical effects, nonlinear and quantum. [8][9][10] Plasmonic supercrystals present the first realization of a deepstrong coupling material at optical frequencies. [4] In plasmonic supercrystals, the effective coupling strength and polariton properties depend on the fill factor, that is, the particle sizes and gap sizes between the particles; cf. Figure 1. [11] Effects such as enhanced Raman scattering scale with the resulting local fields. [13] In principle, the geometry-dependence of the polaritonic properties allows accessing different regimes of light-matter coupling by material design. However, to be able to benefit from the tunability granted by the supercrystal geometry, supercrystals composed of uniform constituents with long-range order and controlled gap sizes are required on large (>1 µm 2 ) dimensions. Fundamental studies as well as applications require robust, reproducible materials with small geometry variations.Herein, we present guidelines for the preparation of high quality plasmonic supercrystals with controlled gap sizes for commonly available gold nanoparticle (AuNP) sizes. The correct choice of the stabilizing organic ligand molecule is very important for obtaining defined supercrystals. It facilitates self-organization into large supercrystals and governs the gap sizes.Periodic arrangements of plasmonic nanoparticles, supercrystals, feature strong light-matter interaction. The light-matter coupling strength depends on the particle diameter and on the interparticle gaps, which provides a lever for controlling it. To facilitate material design, experimental data is analyzed with focus on how the reproducibility and tunability of the gap sizes change with the employed nanoparticles (particle size and molecular weight of the stabilizing organic ligands). A different behavior of the polystyrene-based ligands was found depending on their molecular weight with important consequences for the correlation of nanoparticle diameter and resulting gaps in the supercrystals...

Far-Field Petahertz Sampling of Plasmonic Fields

“…The detailed protocol for the synthesis of AuNS is stated in ref. [28,29]. The protocol for AuNRs is stated in ref.…”