Study On Controllable Terahertz Metamaterials Based On Graphene

Metamaterials are artificial materials which can be designed to effectively manipulate the amplitude,phase and polarization of electromagnetic waves.Active and tunable terahertz metamaterials provide great support for the practical applications of terahertz waves.The two-dimensional material graphene possesses excellent photoelectric properties,whose conductivity can be adjusted through optical pumping,electrical doping and chemical doping.In this thesis,we studied active THz metamaterials integrated with monolayer graphene and investigated the modulation mechanism behind them.The main research contents are as follows.（1）A time-resolved optical-pump（800 nm or 400 nm）terahertz-probe system based on LiNbO₃ crystal which also could deliver terahertz pulses with a peak electric field>100 k V/cm was constructed.The system was utilized to modulate graphene with strong terahertz electric field and optical pumping.（2）Mode strength and mode coupling modulated by strong terahertz electric fields:The decrease of the conductivity of graphene under strong terahertz fields was experimentally realized.Based on this feature,a metamaterial composed of strongly coupled LC resonators and dipole resonators with a monolayer graphene on it was designed,fabricated,and experimentally demonstrated to show a modulation depth of 21.7%in the transmission under the application of a strong terahertz electricfield.Electromagnetic simulations and coupled mode theory calculations revealed the mechanism of the terahertz field induced modulation of the resonance strength and mode coupling.（3）Optical pumping controlled PT symmetry phase transition in metamaterials:Based on the modulation of graphene conductivity through optical pumping,a metal-graphene hybrid metamaterials with PT-symmetry was designed.The condition of the existence of PT-symmetry,the PT phase transition and the exceptional point in the polarization space of the transmission were investigated through electromagnetic simulations and theoretical calculations.The feature that a 180° sharp change in the transmission phase from right-handed circularly polarized light to left-handed circularly polarized light near the exceptional point could make the metamaterial have important application prospects in gas sensing.（4）Gate-controlled metal-graphene hybrid terahertz absorbers:A broadband terahertz absorber with the absorption band 0.7-1.6 THz and a dual band terahertz absorber at 0.85 THz and 1.52 THz were experimentally demonstrated in the metamaterials composed of split“I”shaped metal structures covered with a monolayer graphene.The modulation behavior of the absorption induced by the change of graphene conductivity and the effect on the absorption performance caused by the distribution of graphene surround the metal structure were systematically studied.