Trapping and releasing bidirectional rainbow at terahertz frequencies

Rainbow trapping arisen from slow light effect has been attracted lots of attention for the past two decades. However, until now only unidirectional rainbow trapping has been proposed, which relies on a complicated configuration or technique. Here we propose and investigate a metal–dielectric-semiconductor-dielectric-metal structure, where the semiconductor layer is applied with an external DC magnetic field. We show that in such a waveguide configuration there exist branches of dispersion curves showing slow-light bands at finite propagation constants. The positions of the slow-light bands can be tuned by varying the thickness of the semiconductor or the dielectric layers in the waveguide. By tapering the waveguide, we first demonstrate bidirectional rainbow trapping in such a simple waveguide configuration, and more importantly the trapped rainbow can be easily released by changing the external magnetic field.