Mode splitting transmission effect of surface wave excitation through a metal hole array

The resonant frequencies of the excited surface waves on a metal hole array with respect to the incident angle were studied in the terahertz region. The experimental and theoretical results demonstrate that the resonant peak of surface wave excitation splits into two when transmitted through a metal hole array off-normally. The high-order mode with resonant frequency above the cutoff frequency f(c) (plasma frequency effect) has a shorter attenuation length than that of the low-order mode whose resonant frequency is below f(c). The reason is that the high-order mode is a coupled mode consisting of surface wave and hole modes, while the low-order mode is just an excited surface wave (which can be considered as the spoof surface plasmons). Our investigation may open a door to distinguish the spoof surface plasmons and the coupled modes of surface waves and hole modes.