Modeling of THz concentric circular metal grating with TM polarization

A rigorous coupled-wave model describing the diffraction characteristics of transverse-magnetic (TM) waves by a concentric-circular metal grating (CCMG) operating within terahertz regime is presented. The high-order diffracted cylindrical waves are taken into account by expanding the TM field as infinite spatially harmonic Floquet waves obey Hankel distribution. The proposed model is inherently convergent and the diffraction characteristics of the system can be stabilized under a small amount of diffraction waves. Results show that only a few of them can keep propagating after being reflected by the grating and the rest waves are evanescent. The incident frequency effect on the diffraction efficiency is investigated and the detailed field distribution in the injecting region is presented.