Design And Characterization Of Terahertz Metasurface Devices For Field Modulations

Terahertz(THz)radiation occupies the region between infrared and microwave radiation in the electromagnetic spectrum.With the developments of ultrafast laser techniques and detections of THz wave,THz technology has been widely paid attention.Owing to its appealing properties,such as low photon energy,high transparency,and high spectral resolution,THz technology has been widely applied in the fields of security inspection,biomedicine and communications.In recent years,the research of THz source and detector has been supported strongly by our counctry.Most of researchers are committed to the development of efficient sources and sensitive detectors,and have made some progress.In order to meet the needs of practical applications,researchers are working to develop miniaturized and integrated THz system.However,there are not enough suitable modulators in the THz waveband,and the existing devices,such as quartz waveplate,polyethylene lens,are bulky and low efficiency.This situation will hinder the development of THz technology.Therefore,it is a very essential work for developing a variety of THz functional devices with small size and lightweight.Metasurface,which is an artificial structure composed of subwavelength antennas,can greatly reduce the size of the optical devices and exhibit arbitrary values of effective permittivity and permeability that do not exist in the natural world.Based on the phenomenon of localized surface plasmon resonance,metasurface can be used to modulate both the phase and amplitude of the electromagnetic wave from the visible to microwave frequencies.This property provides an opportunity for the design of a variety of THz metasurface modulators.In this thesis,we present a general method for the design of THz metasurface field modulator,and summarize the modulation law of the amplitude and phase of the scattered wave by the C-shaped slot antenna.Based on the design priciple,three kinds of THz metasurfaces for field modulation and one tunable THz metasurface lens are designed and studied.The contents of the study are summarized as follows:Based on metasurface,ultrathin vortex phase plates are designed to achieve the THz vortices with different topological charges of 1,2,and 3.In the experiment,the amplitude and phase distributions of the generated THz vortex field on the initial plane are measured by utilizing a THz holographic imaging system.The vortex beam is focused by a high resistivity silicon lens,and its far field propagation properties are analyzed by a Z scanning in detail,including evolution of the vortex amplitude and phase.Ultrathin multi-focus metasurface lenses are designed on the basis of theYang-Gu phase retrieval algorithm for focusing the THz wave into four or nine spots with focal spacing of 2 or 3 mm at a frequency of 0.8 THz.The size of the lense is only 10×10 mm2,and does not increase as the number of the focus increases.The designed THz multi-focus lens demonstrates good focusing and imaging functions in the experiment.The designed four-focus lens exhibits a focusing bandwidth of 800 GHz,and its diffraction efficiency can be as high as 33.92 % at the designed frequency.Three Meta-holograms are designed for generating the abruptly autofocusing beam with different parameters in the THz waveband.The designed meta-hologram is composed of gold C-shaped slot antennas,which can record both phase and amplitude of the ring-Airy beam on the initial plane.In the experiment,the generated ring-Airy beam focus abruptly during the propagation,and the focal length agrees well with the theoretical value.The method of generating the abruptly autofocusing beam in the THz region also can be expanded to other wavebands,expecially the visible band.Replacing the spatial light modulator with meta-hologram will simplify the system and avoid the undesirable zero diffraction order effectively.Based on the insulator-to-metal phase transition of vanadium dioxide(VO2),a THz tunable metasurface lens(TML),which consists of a VO2 and a THz metasurface lens,is proposed and verified.The experiment results show that the focal intensity of the TML can be thermally controlled.By adjusting the TML temperature from 337 to 353 K,the focal intensity decreases or increases with the focal length and the focal spot size remains unchanged.When the temperature of the TML is lower than 337 K,the lens is turned on and the focal intensity is maximum.When its temperature is higher than 353 K,the lens is turned off totally.It means that there is no THz wave passes through the TML and the focus disappears.Our approach provides a simple design for realizing a tunable untralthin lens,which would be a promising candidate for the intelligent THz system or temperature sensor.