The Reasearch Of Terahertz Asymmetric Transmission Devices

The emergence of terahertz sources and detections,such as photoconductive antennas and quantum well photodetectors,has greatly promoted the development of terahertz technologies.In recent years,terahertz wave has been widely used in many fields,such as biomedicine,non-destructive test,environmental monitoring,security inspection and wireless communication.The phenomenon that electromagnetic waves show different transmission characteristics when incident into a device in two opposite directions is called asymmetric transmission(AT),and the device is called AT device.As an important terahertz function device,the diode-like AT device can be applied in circulators,polarization converters and other devices,and has important application value in terahertz systems.First,the Jones matrix and the conditions for achieving AT phenonenon are briefly introduced.And two methods are proposed to realize the terahertz asymmetric transmission,one of which is based on structural asymmetry and the other is based on direction-dependent polarization conversion.Then based on the above methods,five terahertz AT devices are proposed,which are double-layer dual L-shaped structure,dual L-strip structure,dual L-split rectangular ring structure,single layer grating type structure and double layer grating type structure,respectively.The unit cell of the first device consists of double-layer dual L-shaped structure,and its AT parameter(i.e.the difference between the forward and backward transmittance,and the symbol is ?)of x-polarized wave is beyond 0.60 with a bandwidth of 0.25 THz and beyond 0.80 with a bandwidth of 0.17 THz.Due to the lack of AT performance of circularly polarized wave,the first terahertz AT device is improved.After optimization,the AT parameter of x-polarized wave is greater than 0.50 with a bandwidth of 0.16 THz,and the AT parameter of lefthanded circularly polarized wave is greater than 0.30 with a bandwidth of 0.21 THz.The unit cell of the second AT device is a dual L-strip structure,and its AT parameter of xpolarized wave is over 0.60 with a bandwidth of 0.31 THz.Compared with the first device,the operating bandwidth of the second one is significantly improved.The unit cell of the third device consists of a dual-L pattern and a split rectangular ring(SRR),and its AT parameter of x-polarized wave is over 0.50 with a bandwidth of 0.56 THz.The fourth device combines an array of metallic pattern with single layer grating.Its AT parameter of y-polarized wave is beyond 0.60 with a bandwidth of 0.62 THz,and its polarization conversion rate(PCR)of y-polarized wave is beyond 0.98 with a bandwidth of 1.05 THz.The fifth device is an improved structure with the third one.It is a structure with two orthogonal gratings.Its AT paremeter of y-polarized wave is beyond 0.80 with a bandwidth of 1.36 THz,and PCR of y-polarized wave is beyond 0.99 with a bandwidth of 1.77 THz.Finally,the dual L-SRR terahertz AT device is fabricated and measured,and the measured results and simulation results are compared and analyzed.In summary,the great performance in wide frequency range indicates the proposed terahertz AT devices can play important role in direction-dependent polarization conversion,polarization-sensitive sensing and imaging.