Research On Terahertz Wave Modulation Based On Metasurfaces

Terahertz(THz)wave is an electromagnetic wave with a frequency ranging from 0.1 to 10 THz,which is located between the microwave and the infrared radiation.It has attracted extensive interests because of its significant scientific and technological potential in imaging,wireless communication and so on.However,miniaturized modulating devices in THz frequency ranging are still lacking,which greatly affected the further exploration of terahertz waves.More recently,metasurfaces composed of an array of subwavelength artificial structure,have attracted great study in the optical and infrared regimes,owing to their high ability in locally manipulating the wavefront of electromagnetic(EM)waves.They can also be introduced into the THz range to solve the problem thatthe spatial modulation devices are still insufficient.In addition,it is a novel modality that has drawn board attention from physics and engineering communities,which is also suitable for thin optical elements that could directly replace traditional bulk optical components or provide novel functionalities.Firstly,there is a strong requirement for modulating the performance of existing bulk devices without changing their physical geometries in their practical applications.Here,a flexible metasurface is bonded to a non-planar device,the curved surface of the cylindrical lens,to form a conformal metasurface for polarization division multiplexing(PDM).The transferable conformal metasurface combines with the lens to enable the cross-polarized waves to achieve the control functions of focusing,generating nondiffracting Bessel beam and phase hologram,while the co-polarized waves are focused into a line by the cylindrical lens.The transferable conformal metasurface can be easily fabricated into two layers or even multi-layers via a cost-effective standard flexible printed circuit(FCP)technique,which is very challenging for the conventional conformal metasurface.Furthermore,it is very easy for the device to switch between two operating modes,i.e.,with and without the metasurface,and different metasurfaces can be integrated with the same conventional bulk device for attaining various modulations.These advantages make the flexible conformal metasurface very promising in wearable functional components and compact functional devices.In addition,we propose an all-dielectric metasurface based on fused silica resonators,consisting of a rectangular-shaped bar placed in the center of a cross netshaped structure for wavefront manipulating.As proof of concept,several transmissiontype devices for spatial modulation based on above basic resonator cells were designed for the target frequency of 0.14 THz.The on-axis and off-axis focusing with 60% and 62% efficiencies of simulation can be realized.Simulated efficiency of non-diffracting Bessel beam is 45%.Furthermore,the multi-focus lens(TMFL)is designed on the basis of the Gerchberg-Saxton(G-S)retrieval algorithm based on Fresnel diffraction.The THz waves are focused into two and three spots in one dimension with focal length of 5 cm.The total modulation efficiency is 49% and 55% respectively.This novel approach for manipulating arbitrary wavefronts of a THz beam also can be used for information storage and other phase-related techniques in the rapid development of THz applications.Finally,we propose an electromagnetic antireflection method to increase the transmittance of terahertz wave.Compared with other antireflection methods,the method is simple to process and can be realized by choosing the grating parameters flexibly.