Parallel-chain optical transmission line for a low-loss ultraconfined light beam

Linear arrays of plasmonic nanoparticles have been suggested by various groups as optical waveguides. Here, inspired by the concept of optical nanocircuits, we show that it is possible to improve the performance of such waveguide chains by pairing two of them, forming a two-wire optical transmission line. We show that choosing the operation regime near the light line may greatly reduce the effect of material absorption and disorder, still allowing field confinement in between the paired chains. Application for low-loss optical interconnects and subwavelength imaging devices are envisioned.

Figure 1

(Color online) Propagation properties of linear plasmonic waveguides. (a) Frequency dispersion of propagation length and (b) guided wave number for a cylindrical nanorod made of silver and for a linear chain of spherical silver nanoparticles, as in the inset of panel (a). Both geometries have radius $a=10\text{nm}$, the chain period is $d=2.1a$. In the inset of panel (b): nanocircuit model of a chain of plasmonic nanoparticles. At the crossing of the wave number with the light line, the nanoinductors and nanocapacitors support a series resonance, with guidance properties analogous to those of an ideally conducting wire at low frequencies.

Figure 2

(Color online) Optical two-wire nanotransmission lines. (a) Dispersion of the guided wave number and (b) propagation length for two parallel chains as in Fig. 1, with separation $l=3a$, comparing the antisymmetric (red lighter line) and symmetric (black solid) modes, together with the isolated chain (black dotted). The thin lines on the top and bottom of panel (a) indicate the first Bragg resonance and the light line, respectively. Panel (c) shows the propagation length for $l=6a$ (due to much reduced coupling, the wave number in this case is very similar in coupled and uncoupled geometries and it has not been reported).

Figure 3

(Color online) (a) Amplitude of the magnetic field and (b) power flux density distribution on a transverse cut across the line connecting the two chains for the geometry of Figs. 2a, 2b, comparing the three different modes under analysis. For simplicity of model, the chains have been represented in these field plots with their averaged current density along their axis, which model well the field distribution in the background region. The strong field and power flow confinement provided by the antisymmetric (nanotransmission-line) operation is evident here.

Figure 4

(Color online) Full-wave simulations of the magnitude of the transverse electric field along a finite twin chains, two wavelengths long, as evaluated with commercial software, Ref. 22, for the geometry of Figs. 2a, 2b, considering antisymmetric (nanotransmission-line) excitation.