Waveguide-Plasmon Polaritons: Strong Coupling of Photonic and Electronic Resonances in a Metallic Photonic Crystal Slab

Strong coupling between localized particle plasmons and optical waveguide modes leads to drastic modifications of the transmission of metallic nanowire arrays on dielectric waveguide substrates. The coupling results in the formation of a new quasiparticle, a waveguide-plasmon polariton, with a surprisingly large Rabi splitting of 250 meV. Our experimental results agree well with scattering-matrix calculations and a polariton-type model. The effect provides an efficient tool for photonic band gap engineering in metallodielectric photonic crystal slabs. We show evidence of a full one-dimensional photonic band gap in resonant plasmon-waveguide structures.

Figure 1

Schematic view (a), electron micrograph (b), and measured extinction [$-\mathrm{l}\mathrm{n}(T)$, $T$: transmission] spectra (c) of the gold wire array on top of a 140-nm-thick ITO waveguide; (d) same as (c) with a 15-nm-thick ITO layer. The dashed arrows in (a) show the electric (magnetic) field direction for TE (TM) polarization. ${\mathbf{k}}_0$ is the incident light wave vector. The extinction spectra are represented by solid lines and dashed lines for TM and TE polarization, respectively.

Figure 2

Measured (top panels) and calculated (bottom panels) extinction spectra: (a) normal light incidence and nanowire arrays with periods from 375 to 575 nm; (b),(c) light incidence with angle $\vartheta$ from $0°$ to $20°$ in nanowire arrays with periods of 450 and 375 nm, respectively.

Figure 3

Measured and calculated positions of the extinction spectra maxima (solid symbols and open symbols in top panels) in TE (stars) and TM (circles) polarization. The dependences vs $d_x$ and $\vartheta$ are given in (a) and (b),(c), respectively. Thick lines are the dispersions of the three TM waveguide-plasmon polariton modes near the center of the first BZ: real and imaginary parts of the eigenenergies of Eq. (2) are shown in the top and bottom panels, respectively. The thin lines are the corresponding two TE quasiguided modes (shown only in the top panels). The arrows show the polariton Rabi splitting and the full 1D PBG.