We report on experimental evidence of unidirectional transmission of terahertz waves through a pair of metallic gratings with different periods. The gratings are optimized for a broadband transmission in one direction, accompanied with a high extinction rate in the opposite direction. In contrast to previous studies, we show that the zero-order nonreciprocity cannot be achieved. Nonetheless, we confirm that the structure can be used successfully as an asymmetric filter.
We report on unidirectional and asymmetric transmission of radially polarized THz radiation through a dual circular metallic grating with sub-wavelength slits. Unidirectional transmission is shown theoretically for a super-Gaussian incident beam, and an asymmetric transmission is demonstrated experimentally, when the radially polarized beam of 0.1 THz is obtained by converting a linearly polarized beam with a discontinuous phase retarder and a tapered waveguide. The dual grating does not include nonlinear materials, its operation is reciprocal, and analogous to that of some planar metallic gratings.
We derive periodic multilayer absorbers with effective uniaxial properties
similar to perfectly matched layers (PML). This approximate representation of
PML is based on the effective medium theory and we call it an effective medium
PML (EM-PML). We compare the spatial reflection spectrum of the layered
absorbers to that of a PML material and demonstrate that after neglecting gain
and magnetic properties, the absorber remains functional. This opens a route to
create electromagnetic absorbers for real and not only numerical applications
and as an example we introduce a layered absorber for the wavelength of
$8$~$\mu$m made of SiO$_2$ and NaCl. We also show that similar cylindrical
core-shell nanostructures derived from flat multilayers also exhibit very good
absorptive and reflective properties despite the different geometry
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