Self-Similar Gold-Nanoparticle Antennas for a Cascaded Enhancement of the Optical Field

We experimentally demonstrate cascaded field enhancement by means of gold nanoparticle dimer and trimer antennas. The local field enhancement is probed by single-molecule fluorescence using fluorophores with high intrinsic quantum efficiency ($Q^0>80\%$). Using a self-similar trimer antenna consisting of 80, 40, and 20 nm gold nanoparticles, we demonstrate a fluorescence enhancement of 40 and a spatial confinement of 15 nm. Compared with a single gold nanoparticle, the self-similar trimer antenna improves the enhancement-confinement ratio by more than an order of magnitude. Self-similar antennas hold promise for high-resolution imaging and spectroscopy, ultrasensitive detection, and efficient single-photon sources.

Figure 1

(a) Illustration of the experimental configuration. A self-similar optical antenna made of discrete colloidal gold nanoparticles of decreasing size is placed next to a sample with randomly distributed Nile blue molecules. The antenna is irradiated by a tightly focused radially polarized laser beam with wavelength ${\lambda }_{\mathrm{exc}}=633\mathrm{nm}$. Fluorescence photons are recorded while the sample is raster scanned at a distance of $\sim 5\mathrm{nm}$ underneath the antenna. (b) Scanning electron micrograph of a colloidal trimer antenna.

Figure 2

Excitation of single-molecule fluorescence with a dimer antenna consisting of 80 and 40 nm gold nanoparticles. (a) Fluorescence image of the single-molecule sample. A line cut through the marked fluorescence spot indicates an optical confinement of 23 nm. The inset is a scanning electron micrograph of the dimer antenna. (b) Single-molecule emission spectrum with (upper curve) and without the dimer antenna (lower curve), revealing a fluorescence enhancement of $\sim 20$. Compared with a single 40 nm particle, the 80–40 nm dimer antenna provides a fourfold stronger fluorescence enhancement.

Figure 3

(a) Field distribution near a self-similar trimer antenna consisting of gold nanoparticles with sizes 80, 40, and 20 nm. Contours of constant $|\mathbf{E}{|}^2$. Logarithmic scaling with factor of two between contour lines. (b) Spectrum at the apex of the trimer antenna. The curves show the phase relative to the incident field and the intensity enhancement.

Figure 4

Excitation of single-molecule fluorescence with a trimer antenna consisting of 80, 40, and 20 nm gold nanoparticles. (a) Fluorescence image of the single-molecule sample. Inset: Line cut through the single fluorescence spot marked by the arrow. (b) Fluorescence from a single $z$-oriented molecule recorded as a function of distance from a trimer antenna. The steep rise of fluorescence counts for separations smaller than 15 nm is due to strong field localization along the $z$ axis at the apex of the trimer antenna.