Plasmonic Band Gaps and Trapped Plasmons on Nanostructured Metal Surfaces

Nanostructured metal surfaces comprised of periodically arranged spherical voids are grown by electrochemical deposition through a self-assembled template. Detailed measurements of the angle- and orientation-dependent reflectivity reveal the spectral dispersion, from which we identify the presence of both delocalized Bragg and localized Mie plasmons. These couple strongly producing bonding and antibonding mixed plasmons with anomalous dispersion properties. Appropriate plasmon engineering of the void morphology selects the plasmon spatial and spectral positions, allowing these plasmonic crystal films to be optimized for a wide range of sensing applications.

Figure 1

(a) Dispersion of zone-folded surface plasmon polaritons, for voids in Au spaced by 600 nm. Images show surface electric field distribution for the corresponding standing wave modes at $k=1/\mu \mathrm{m}$ close to $\Gamma M$, outline circles (red online) mark dishes. (b) Localized plasmon energies vs normalized sample thickness $\overline{t}$ for a random array of separated voids. Calculated $xy$ field distributions for encapsulated spheres, $\overline{t}=1$. Inset: scanning electron micrograph of sample at $\overline{t}=0.9$.

Figure 2

Measured energy dispersion of reflectivity for TM polarized light as a function of in-plane wave vector for increasing void thickness, $\overline{t}$ (a)–(c). Log color scale, white dotted lines show zone-folded plasmon dispersion, sample orientations of $\varphi =30°$ in all cases. (i–iv) $k$ space cuts through dispersion relation at (i) $(\overline{t},E)=(0.25,2.2\mathrm{eV})$; (ii) $(\overline{t},E)=(0.4,2.2\mathrm{eV})$; (iii) $(\overline{t},E)=(0.4,1.7\mathrm{eV})$; (iv) $(\overline{t},E)=(0.6,2.2\mathrm{eV})$, symmetery shown above (i). Light shade corresponds to absorption features.

Figure 3

(a) Extracted energies of Bragg and Mie plasmons as a function of normalized sample thickness for $\theta =0°,30°$, showing anticrossing (dashed and solid lines show experimentally determined pure Bragg (for $\overline{t}=0$) and Mie mode behavior, respectively). (b)–(d) Reflection image plots of energy vs normalized thickness for (b) $\theta =0°$, $\varphi =0°$; (c) $\theta =30°$, $\varphi =0°$; (d) $\theta =30°$, $\varphi =30°$.

Figure 4

(a) Energy dispersion vs thickness of Au nanovoid samples of different diameter $d$, mapped by reflectivity of TM polarized light for sample orientations $\theta =30°$, $\varphi =0°$. Light shade correspond to absorption features. Vertical white line shows position of Bragg mode collapse; dark (blue online) line indicates separation between Bragg modes (low energy) and Mie modes (high energy). (b) Extracted sample thickness (normalized) of Bragg mode collapse with increasing void diameter.