Enhanced Infrared Transmission through Subwavelength Coaxial Metallic Arrays

We report the observation of enhanced near-infrared transmission through arrays of subwavelength coaxial metallic structures compared with that through comparable diameter hole arrays as a result of localized electromagnetic modes supported by the complex coaxial unit cell. Polarization and angle-dependent transmission measurements clearly demonstrate the coupling between this localized mode and delocalized surface plasmon modes. A generalized, multiple discrete states Fano line shape provides a good fit to the experimental results.

Figure 1

(a) Measured transmission spectra for four samples. Inset: scanning electron microscopy of coaxial array with $1.03\mu \mathrm{m}$ pitch, 348 nm outer and 194 nm inner radii. (b) Angular dependence of TM transmission for $348/194\mathrm{n}\mathrm{m}$ coaxial sample.

Figure 2

Coaxial peak positions as a function of refractive index of fluids in the coaxial space (top curve for $348/194\mathrm{n}\mathrm{m}$ sample) and as a function of the outer radius for the three experimental coaxial samples (bottom curve).

Figure 3

(a) Experiment, RCWA, and Fano fit simulations for the $376/194\mathrm{n}\mathrm{m}$ coaxial sample at normal incidence, offset for clarity. (b) Experimental transmission and Fano fit for $348/194\mathrm{n}\mathrm{m}$ coaxial sample for TM polarization at an incident angle of 45°. The resonance positions for the Fano fit are indicated.

Figure 4

(a) Peak positions from experimental spectra (gray) and from Fano fits (black). At low angles (0° and 5°) the features corresponding to the metal-glass SPW are not simple peaks and the entire region is denoted by the shaded rectangular bars. (b) Spectral resonances extracted from Fano fits (black symbols), SPW energies (dashed lines), and the localized coaxial energy from perturbation calculations (hollow symbols).