Discussion of the mechanism of extraordinary optical transmission in metallic gratings

The excitation of a Fabry-Perot resonant cavity mode (FP resonance) and the resonant coupling between surface plasmons wave on both sides of the grating (CSP) are two main theories to explain the extraordinary optical transmission effect in transmission metallic gratings with very narrow rectangle slits. In this letter, specific modals are designed to weaken the CSP and the FP resonance. Concretely speaking, we have changed the interface situation of gratings and vacuum to weaken the CSP and changed the sharp of gratings to avoid the FP resonance. The transmittance spectrums of these modals are computed by finite difference time domain (FDTD) method. By analyzing these transmittance spectrums, we found that the extraordinary optical transmission effect is still obvious. SO it is concluded that the appearance of transmission resonance does not depend on whether or not the existence of the FP resonance or the CSP. But the physical origin of this phenomenon needs to be studied thorough yet.