Propagation speed calculation of a plasmonic THz wave trapping system

A graded metallic grating structure acts as a wave trapping system. Different frequencies of THz waves are trapped at different positions along this structured metal surface grating. The real wave propagation speed of such a system is reduced gradually from the light speed in vacuum to zero, which is demonstrated by calculation and simulation. Different frequencies of THz waves are propagated at a designed propagation speed by a partial graded grating according to the practical demand.