Research On Terahertz Quasi Optical Modulators Based On Artificial Microstructure Array

After entering the 21st century,as the research on microwave and visible light bands has become more and more mature,researchers from all over the world are paying more and more attention to the technically immature terahertz?THz?frequency band.THz wireless communication and THz imaging technology have always been the focus of THz field.In recent years,the development of THz direct modulation technology has provided a new research direction for solving a series of problems in THz wireless communication and THz imaging technology,and provided a new technical way for realizing efficient and high-speed THz wireless communication,fast and high-resolution THz imaging.The THz quasi optical modulator is a core device in THz wireless communication system and THz imaging system based on external direct modulation technology.The improvement of device performance plays an important role in the development of related THz systems.Aiming at the difficult problems such as the low modulation rate and efficiency in the research of THz modulation technology at home and abroad,this paper combines the artificial microstructure with special resonance modes and semiconductor materials to study the optically controlled and electronically controlled THz quasi optical phase modulation technology,and realize large phase modulation of THz waves under different applied excitation conditions;High speed electronically controlled THz quasi optical amplitude modulators are developed to realize the real-time transmission of indoor THz wireless HD video;A subunit array THz multi-pixel wavefront modulator is developed and experimentally confirmed its multi-pixel modulation capability.The research results of this thesis lay a good foundation for THz high-speed wireless communication and THz high-resolution imaging technology.The main research contents and innovations of this thesis are as follows:1.Aiming at the problem of low phase modulation capability of THz quasi optical phase modulator in single-layer structure in transmission mode,a new method for increasing phase shift by LC-dipole coupling resonance mode is proposed.Based on the theory of Kramers-Kronig relation,this thesis analyzes the intrinsic relationship between phase dispersion and amplitude dispersion.It is found that enhancing the resonance strength of the resonance mode can significantly increase the phase jump.An artificial microstructure with coupling resonance mode is proposed.The results show that the coupling mode has significant resonance enhancement effect and can produce 140°phase jump.A scheme of large phase modulation via optical control or electronic control is proposed by combining VO₂ or GaN-HEMT with a coupling resonance artificial microstructure array.The simulation results show that both approaches can achieve phase modulation above 150°;The experimental results show that the maximum phase shift of the VO₂ optically controlled phase modulator?central working frequency 0.6THz?and the HEMT electronically controlled phase modulator?central working frequency0.352THz?is 138°and 137°,respectively,and the bandwidths with phase shifts above130°are 55GHz?0.575THz-0.63THz?and 30GHz?0.337THz-0.367THz?,respectively.Compared with the maximum phase shift of 90°for similar devices in the world,the phase shift is increased more than 40°.2.A staggered netlike composite metasurface nested HEMT array is proposed,which can effectively suppress the parasitic mode in artificial microstructure array,and realize the high efficiency and high speed of THz wave amplitude modulation.Simulation studies show that the new metasurface structure can generate a special resonance state under the induction of EM field.By changing the transport properties of carriers in 2DEG,the dynamic transition between different resonance states is controlled,and the parasitic mode can be effectively suppressed.The modulation depth obtained by static modulation experiment at the central working frequency 0.352THz is 93%,which is basically consistent with the 94%modulation depth obtained by simulation;In the dynamic modulation experiment,the switching rate of the modulator is 3GHz.Compared with the highest modulation speed 1GHz of similar devices in the world,the device performance is significantly improved,and the maximum-3dB bandwidth is increased from 200MHz to 1GHz.The modulator is used to construct the world's first THz wireless communication system based on external direct modulation technology.Real-time transmission of HD video with a communication rate of 100Mbps is realized at a distance of 9m indoors.The transmitted picture is clear,the voice is complete and there is no significant delay in the picture,which further verifies the performance and reliability of this quasi optical modulator.3.In order to solve the impedance mismatch problem of spatial quasi optical THz modulator under high speed conditions,an electronically controlled HEMT artificial microstructure modulator suitable for rectangular waveguides is proposed.This scheme reduces the number of HEMTs in the modulation chip from thousands to five,which greatly alleviates the impedance mismatch between the internal circuit of the chip and the external circuit.The simulation results show that the resonance mode conversion in the HEMT artificial microstructure array has a good control effect on the transmission characteristics of the THz wave in the waveguide,and the modulation depth can reach18dB;In the static experiment,the modulation depth of the device at the central working frequency 0.353THz reached 13.5dB,which was close to the simulation result;The modulation speed obtained in the dynamic experiment can reach more than 8GHz and the-3dB bandwidth can reach more than 5GHz,which shows better modulation performance than the spatial quasi optical THz modulator.This scheme can not only improve the modulation depth and speed,but also realize the integration and miniaturization of the system.4.Aiming at the hardware requirements of THz single-pixel imaging based on compression sensing algorithm,a THz multi-pixel wavefront modulator working at0.22THz is developed in this thesis.The device has a 4×4 subunit array structure,each of which constitutes a pixel of the modulator.The turn-on and turn-off of 16 pixels can be controlled separately by the external FPGA control module.The experimental results show that the modulator has a good switching effect on THz wave and a remarkable modulation ability of multipixel.In order to further improve the performance of the device,the cause of crosstalk between pixels and the improvement method are analyzed,which lays a good foundation for further optimization of the THz multipixel wavefront modulator and practical application in the imaging system.