High-Efficiency Asymmetric Transmission of Circularly Polarized THz waves using a Dielectric Herringbone Metasurface

An interesting topic is that of metamaterials imparting chiral responses which invoke a disparity between opposite handednesses of circularly polarised (CP) light. Most chiral metamaterials are either 3D-helical structures [1] or stacked metallic structures with twisted orientations [2]. These structures allow selective transmission of one CP whilst prohibiting or reflecting the other, termed Circular Dichroism. However, for 2D chiral metamaterials, this is not so. Instead, the cross-polarisation conversion of one CP to another is different. The original work in [3] used an anisotropic lossy planar-chiral “fish-scale” structure to exhibit this effect, termed Asymmetric Transmission (AT). However, these responses are small with efficiencies less than 25%. Works to improve efficiency used 3D arrangements. Work in [4] achieved much higher efficiency than for the 2D planar-chiral structures, but due to the metallic construction absorption losses were unavoidable; such losses were given as 37%. Here, we propose a means of achieving AT using a loss-free mechanism at 1THz frequency by constructing Monolithic Herringbone metamaterials from a dielectric medium [5]. This device works by a spin-selective interference of CP light, due to Pancharatnam-Berry (PB) phases, in conjunction with a propagative dynamic phase (Fig. 2) causing constructive interference for TRL and destructive for TLR Jones matrix components. An analytical derivation (Fig. 1a) was found to agree well with numerical simulations (Fig. 1b) for the design. These results indicate a conversion efficiency of LCP to RCP (TRL) exceeding 80%. Fabrication of Intrinsic Silicon was used for the devices (Fig. 2) and THz Time Domain Spectroscopy (THz-TDS) was used to characterise the samples, showing a 60% spin-conversion efficiency (Fig. 3). Such a device is robust and is not easily degraded by errors in fabrication.