Groove-regulated light wave resonance inside a metallic subwavelength slit

Jian-Shiung Hong^1*, Kuan-Ren Chen¹

¹Department of Physics, National Cheng Kung University, Tainan, Taiwan

* Presenter:Jian-Shiung Hong, email:hongjs@phys.ncku.edu.tw

The forward coupling during an incident light wave through a subwavelength slit in a metal film can cause the transmission to be enhanced; as coupling grooves are patterned at the exit side of the film, the surface wave radiated from the slit is scattered to form a light beam in free space. We study the groove effect with an analytical method based on the conventional Green’s theorem and find that there is an underneath backward coupling mechanism: as the surface wave is scattered, it can flow back into the slit. Since the slit width is much smaller than the wavelength, the scattered wave mode re-entering the slit is again the fundamental mode and alters the resonance inside the slit. Our analysis shows an effective phase delay of the reflection coefficient at the slit exit in our case studied; the phase delay will result in a reduction in the film thickness at resonance. For a film thickness slightly smaller, the more in-phase roundtrips of the traveling wave in the slit causes more incident energy transmitted.

Keywords: subwavelength structures, forward scattering, backscattering, phase shift