Enhanced asymmetric transmission in hyperbolic epsilon-near-zero slabs

We investigate the asymmetric transmission for forward and backward propagation of tilted circular polarized optical waves in subwavelength epsilon-near-zero (ENZ) hyperbolic slabs. This chiral-optical effect is solely triggered by anisotropy without resorting to any breaking of reciprocity and chiral symmetries or spatial nonlocal effects. Remarkably, we show that the asymmetric transmission undergoes a dramatic enhancement near the ENZ condition. This happens since, close to the zero-crossing point, the extraordinary waves can accumulate the desired propagation phase even though the slab is ultrathin and, by varying excitation angles and slab thickness, we engineer this phase thus achieving a huge asymmetric transmission. The proposed strategy holds promise for realizing ultra-compact and efficient polarization devices in different frequency range even at very high frequencies (ultraviolet) since the effect is merely due to anisotropy and it is available without resorting to nanofabrication processes.