Direct Observation of Plasmon Band Formation and Delocalization in Quasi-Infinite Nanoparticle Chains

Abstract

Chains of metallic nanoparticles sustain strongly confined surface plasmons with relatively low dielectric losses. To exploit these properties in applications, such as waveguides, the fabrication of long chains of low disorder and a thorough understanding of the plasmon-mode properties, such as dispersion relations, are indispensable. Here, we use a wrinkled template for directed self-assembly to assemble chains of gold nanoparticles. With this up-scalable method, chain lengths from two particles (140 nm) to 20 particles (1500 nm) and beyond can be fabricated. Electron energy-loss spectroscopy supported by boundary element simulations, finite-difference time-domain, and a simplified dipole coupling model reveal the evolution of a band of plasmonic waveguide modes from degenerated single-particle modes in detail. In striking difference from plasmonic rod-like structures, the plasmon band is confined in excitation energy, which allows light manipulations below the diffraction limit. The non-degenerated surface plasmon modes show suppressed radiative losses for efficient energy propagation over a distance of 1500 nm.

Plasmonic Oligomers With Tunable Conductive Nanojunctions

“…By increasing further the number of particles in the chain, the L1 mode energy converges asymptotically to a finite value due to the so-called infinite chain limit effect 22,[33][34] and the addition of new modes within a finite energy range results in a broad plasmon band. 20 Those results are consistent with previous STEM-EELS studies describing the collective modes of spherical NPs chains, which observe as many L modes as nanoparticles in the chain. [19][20]23 Bridging the oligomers with silver has a dramatic influence on the position of the energy loss peaks (Figure 4).…”

Complex-k modes of plasmonic chain waveguides

“…Plasmonic chain waveguide in its simple setting has been studied mostly through theoretical modeling [1,[6][7][8][9][10][11][12][13] and, in fewer occasions, through experiments [14][15][16][17][18][19][20][21]. There are several ways to prepare chain waveguides.…”

“…Alternatively, chemists can routinely synthesize noble metal nanoparticles of preferred geometry and then let them self-organize on a pre-patterned (chemically or topographically) substrate [17,19,21,22]. Especially, self-assembly of colloidal particles on wrinkled elastomeric template [21,22] has produced chains with 20 and even more gold nanoparticles with fairly homogeneous particle spacings (<2 nm). Very recently, based on the DNA origami method [23], there have been several work demonstrating well-structured plasmonic chains with gap size precisely controlled at sub-10 nm level [20,[24][25][26].…”

“…It shall be pointed out that most experimental and some theoretical work focused on chains made of a few particles (e.g. [16,20,21,29]). Such short chains, owing to reflections at two ends, support Fabry-Pérot resonances; consequently, a single propagating mode in an originally infinitely periodic waveguide becomes several modes appearing at distinct resonant frequencies.…”

“…In other words, a single Bloch mode becomes quantized into several modes. This phenomenon happens particularly for the L mode in short gold chains [16,20,21,29], where quantized L modes are referred to as L 1 , L 2 , L 3 , etc, depending on how many nodes in their field profiles. Furthermore, the quantized modes are labeled as super-radiant, sub-radiant or dark modes, depending on their overall dipole moments.…”