Research On Terahertz Active And Passive Modulators Based On Graphene And Metamaterial

Recently,with many research groups joining in the terahertz community,terahertz has become a new hot spot of research frontiers,and a plethora of terahertz sources and detectors have been designed and proposed.However,the techniques to modulate terahertz waves are lagging behind.The main reason is that many materials in nature do not respond to terahertz radiation.In this thesis,we studied several kind of terahertz active and passive modulators based on the metamaterial and graphene by combining the theories,simulations and experiments.The main content is listed as follows:(1)The first work is the passive terahertz bandstop filters.High performance of tunability of ~0.3 THz was achieved by manipulating the parameters of relative distance between two U-shaped structures,etc.A LMC effective circuit model was proposed and successfully predicted the resonance frequencies and the corresponding capacitance,self-inductance and mutual inductance.(2)The second work is the passive terahertz modulator based on metamaterial and graphene.Combining the asymmetric SRR structure and monolayer graphene,we observed that the terahertz transmission variation of Fano resonance was higher than that of the dipolar resonance.The reason is that the electric fields could be tightly confined in the capacitive gaps at the Fano resonance according to the distributions of surface current and electric field in the simulation.This phenomenon also reveals the extreme sensitivity of Fano resonance to monolayer graphene.(3)The third work is the active terahertz modulators based on silicon and graphene.We transferred two layers of CVD graphene on an N-type silicon substrate in sequence.With simultaneous continuous wave optical pump and low gate voltages,we experimentally obtained a diode-like terahertz modulation effect: the modulator transmitted terahertz waves when biased with a positive voltage,whereas it attenuated the waves under a negative voltage.The highest modulation depth of 83% was achieved with the optical pump power at 420 m W and the gate voltage at-4 V.By applying the CST software,thin film theory and Drude model,the measured results were well reproduced,and the distribution of Fermi levels and permittivity were also obtained.(4)The fourth work is the active terahertz modulators based on metamaterial and graphene.A monolayer graphene was transferred on the U-shaped subwavelength arrays.With the excitations of continuous wave optical pump and gate voltage,the conductivity of graphene was changed,and thus the LC resonance and terahertz transmission were modulated accordingly.