Multi-Bandpass Effect of Terahertz Filter Caused by Surface Plasmon Polariton Resonant Coupling

In the simulation of cross-type terahertz bandpass filter by using the finite-difference time-domain method, the main peaks narrow as the thickness of the Al foil increases, and several abnormal transmission peaks appear at high frequencies. A terahertz filter with Al foil thickness of 150 mu m is fabricated by femtosecond laser micro-machining; it is tested using a time-domain terahertz spectroscopy system. The results show that the experimental results are consistent with the finite-difference time-domain method simulation results. The anomalous transmission peaks result from the Fabry-Perot resonant coupling of the surface plasmon polariton waves on the sidewalls of the slits of the cross structure. This coupling effect may be useful in controlling surface plasmon polariton waves and fabricating new terahertz devices.