A double-layer subwavelength dielectric grating has been designed, fabricated, and characterized for terahertz (THz) wave asymmetric transmission. The subwavelength grating resonance effect and the first-order diffraction effect of the two different grating structures lead to the different light paths of the forward and backward waves, and thus the one-way transmission can be realized. The transmission and resonance properties of this grating with different polarization and propagation directions have been demonstrated experimentally and theoretically, which show that this is a nonmagnetic one-way transmission device with an extinction ratio of over 15 dB and insertion loss of lower than 5 dB at 1.45 THz. We provide an effective way to nonmagnetic THz asymmetric transmission devices operating at room temperature without rotating or converting the polarization state of incident wave.
Our recent research work on artificial birefringence and broadband polarization converter in terahertz (THz) functional devices was reviewed in this paper, we proposed the subwavelength dielectric gradient grating structure with artificial high birefringence, broadband and low dispersion, and the dielectric metasurface with line-square compound lattice which can realize polarization dependent EIT effect with a large artificial birefringence effect. On the basis, we presented a compound metasurface and a coupled dielectric-metal grating for broadband THz wave polarization conversion and asymmetric transmission. Moreover, we introduced two-dimensional materials into THz polarization devices, and proposed a switchable quarter-wave plate based on graphene grating and a carbon nanotube attached subwavelength grating for broadband THz polarization conversion and dispersion control. This work has greatly promoted the development and practical application of THz polarization devices.
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