Hertzian plasmonic nanodimer as an efficient optical nanoantenna

Inspired by the geometry and shape of the classical radio-frequency radiator, the Hertzian dipole, here we analyze the design of a plasmonic optical dimer nanoantenna. We show how it may be possible to operate a pair of closely spaced spherical nanoparticles as an efficient optical nanoradiator, and how its tuning and matching properties may be tailored with great degree of freedom by designing suitable nanoloads placed at the dimer gap. In this sense, we successfully apply nanocircuit concepts to model the loading nanoparticles. High levels of optical radiation efficiency are achieved, even considering the realistic absorption of optical metals, thanks to this specific geometry and design.

Figure 1

(Color online) Analogy between two dimer antennas: (a) an early Hertzian antenna to operate at microwave frequencies (sketch from Ref. 3, reprinted with permission); (b) the nanodimer antenna under analysis here.

Figure 2

(Color online) Peak in the radiated electric field versus frequency for the geometry of Fig. 1b with $L=100\text{nm}$, $g=3\text{nm}$, and $r=3\text{nm}$, fed by a $10\text{k}\Omega$ port.

Figure 3

(Color online) Input impedance $Z_{\text{in}}$ at the port gap for the geometries of Fig. 2 varying the load permittivity (thick lines) and corresponding extraction of $Z_{\text{dimer}}$ (thin).

Figure 4

(Color online) (a) Electric and (b) magnetic field distributions (snapshots in time) on the $E$ plane of the nanodimer antenna fed at its gap at the resonance frequency $f=665\text{THz}$.

Figure 5

(Color online) (a) Gain and (b) radiation efficiency as a function of frequency for the antennas of Figs. 2, 3. (Here, gain and radiation efficiency are evaluated without including the effect of input impedance mismatch, i.e., considering the effective power accepted at the antenna terminal.)

Figure 6

(Color online) Consistent with Fig. 5, radiation efficiency as a function of frequency varying the size of the load from $r=0$ (black line), $r=3\text{nm}$ (red), to $r=6\text{nm}$ (blue).

Figure 7

(Color online) Scattering cross section for the nanodimers of Figs. 2, 3 varying the permittivity of the nanoloads.