Broadband and giant nonreciprocity at the subwavelength scale in magnetoplasmonic materials

We unveil a previously overlooked wave propagation regime in magnetized plasmonic (gyrotropic) materials with comparable plasma and cyclotron frequencies, which enables a giant and broadband (nondispersive) nonreciprocal response. We show that this effect is due to a natural form of dispersion compensation that ultimately originates from the subtle implications of the principle of causality for gyrotropic plasmonic media, which allows the existence of a low-loss frequency window with anomalous nonmonotonic dispersion for the extraordinary mode. This is in stark contrast with conventional nongyrotropic passive materials, for which the frequency derivative of the permittivity dispersion function is always positive in low-loss regions. These findings pave the way for superior nonreciprocal components in terms of bandwidth of operation and compactness, with orders-of-magnitude reductions in size. As a relevant example, we consider indium antimonide (InSb) to theoretically demonstrate a deeply subwavelength, broadband, THz isolator operating under moderate magnetic bias and at room temperature.