Enhancement of the extraordinary optical transmission in a subwavelength metal slit dressed by a metal grating

We propose a dressed subwavelength metal slit structure, which is formed by adding a thin metallic grating near the slit entrance with a distance. The simulation computation by means of the finite-difference time-domain method shows that the transmissivity can be substantially enhanced by 30 times compared with the conventional metal slit structure. The transmission spectrum exhibits two characters of the Fabry-Perot resonance and a left-shift. The introduced grating has two effects: (1) more energy is collected from the incident plane, and (2) the effective dielectric region in the slit is extended to the region out of the entrance.