Manipulating light at subwavelength scale by exploiting defect-guided spoof plasmon modes

We study the defect-guided modes supported by a set of metallic rods structured at the subwavelength scale. Following the idea of photonic crystal waveguide, we show that spoof plasmon surface waves can be manipulated at subwavelength scale. We demonstrate that these waves can propagate without leakage along a row of rods having a different length than the surrounding medium and we provide the corresponding dispersion relation. The principle of this subwavelength colored guide is validated experimentally. This allows us to propose the design of a wavelength demultiplexer whose efficiency is illustrated in the microwave regime.

Figure 1

2D geometry of metallic layers between a ground plane and the air, spoof plasmons can propagate according to (1) [Left side]. 3D geometry of a structured waveguide of width $w$, guided modes can propagate according to Eq. (6) [Right side].

Figure 2

Theoretical dispersion relations of the guided modes, Eq. (6), in plain blue lines for the W1 and W3 guides (see text), respectively, in (a) and (b), defining the frequency range FR (blue colored areas) where guided modes can propagate without leakage. The gray and the dashed black curves show, respectively, the dispersion relation of the surrounding medium and the light line. For comparison, dotted blue lines report the behavior of the classical spoof plasmons, Eq. (1).

Figure 3

(a) Photography of the experimental setup designed for the microwave range. A zoom on a W1 waveguide is presented on the right side. Simulated (b) and measured (c) transmission spectra of the W1 and W3 channels as a function of the frequency.

Figure 4

Typical fields measured just above the rods in the colored guide W1.

Figure 5

(a) Schematic view of the multichannel demultiplexing device operating at subwavelength scale. (b) Dispersion relations of the four colored waveguides. The dispersion relation of the surrounded medium and the light line are also plotted for comparison in gray and dashed black, respectively. (c) Transmission spectra calculated at the exits of the three colored W1 waveguides.

Figure 6

Electric fields scanned above the structure at three frequencies chosen, respectively, in the red ($f=3:1$ GHz), green ($f=3:4$ GHz), and blue ($f=3:8$ GHz) frequency range $[fc-;fc+]$.