Figure 4
(a) Schematic illustrations of the psWGP(top) and the fsWGP (bottom) before (left) and after (right) applying EMA method. (b) Complex refractive index (Nx) of the Al WGP for TM wave at 4000-nm wavelength as a function of the air gap between nanowires. (c) Admittance diagram showing the loci for the psWGP of [air|EMA Al nanowires|EMA Si nanopatterns|Si substrate] and the fsWGP of [air|EMA Al nanowires|Si substrate]. At the normal TM wave incidence, the admittance of the [air|EMA Al nanowires|EMA Si nanopatterns|Si substrate] starts from a, arrives at b, and ends at c, while the modified admittance at the 45° incidence angle follows a′, b′, and c′. The admittance of the fsWGP of [air|EMA Al nanowires|Si substrate] at the normal TM wave incidence starts from a and ends at d. Both the Al nanowires with the 6-nm air gap and the nanopatterned Si substrate with the 50-nm air gap are modeled as anisotropic uniaxial thin films by the EMA at the 4000-nm wavelength. The EMA Al-nanowire layer thickness is 44 nm, and the EMA Si-nanopattern layer thickness is 140 nm. (d) The dispersion characteristics of the Al WGP is presented as a function of the incident wave vector in the x direction. In the case of the Al WGP with the air gap of 6 nm explored in this paper, the dispersion curve is very close to the light line, suggesting that the TM wave can even pass through a narrow air gap with high transmittance.
