Terahertz Plasma-induced Transparency Achieved By Metamaterials

Terahertz technology is a cross-cutting frontier science and technology that has developed rapidly in recent years.As a part of the electromagnetic spectrum,the special frequency range of the terahertz wave makes it have superior characteristics,which provides an important means to promote the science and technology innovation,national economic development,and national security.However,the materials that can respond in the terahertz band are very limited in nature.As a new type of composite material,metamaterials,can exhibit special electromagnetic properties that many materials in nature cannot achieve.Their appearance solves the problem of lack of materials in the terahertz band.Among them,plasma-induced transparency（PIT）using metamaterials in the terahertz band is one of the representative work.This thesis mainly conducts research on the PIT effect realized by metamaterials in the terahertz band,and analyzes the proposed structural performance parameters by combining research methods of simulations,experiments and theoretical fittings.The main research contents are:Firstly,a kind of PIT metamaterial based on superradiant mode and subradiant mode is studied.Through numerical simulation of the metamaterial structure composed of a double ring dimer connected by a conductive junction（DSRJ）and a pair of split ring resonators（SRRs）,the destructive interference phenomenon between the two resonators is analyzed.When the Fermi level and the carrier relaxation time of the graphene integrated in the gap of the connecting split ring are changed,the conductivity of the graphene changes accordingly.It is found that the transparent window gradually weakens to disappear with the decrease of graphene conductivity,thus realizing the active modulation of the transmission amplitude.Secondly,a PIT metamaterial based on cut wire（CW）and double U-shaped resonator（DUR）was designed and fabricated.The significant terahertz modulation behavior was observed by transferring an unpatterned monolayer of graphene onto the surface of a metamaterial structure.At the same time,the transmission curves before transfer of graphene were successfully fitted using a Lorentz oscillation model.And the coupling effect between the two resonators is analyzed by numerical simulation,revealing the potential mechanism of PIT formation.Finally,a PIT metamaterial based on a meanderline resonator（MLR）and a double U-shaped resonator（DUR）is proposed.The transmission characteristics of the three metamaterials were investigated by experiments and simulations.At the same time,the group delay of the terahertz wave transmitted through the sample was extracted to illustrate the slow light capability of the structure,and the experimental results were theoretically fitted.In addition,the resonance characteristics of the metamaterials under different structural parameters were studied by numerical simulation,and it was observed that the PIT response hardly change with the change of parameters,thereby indicating the stability of the structure.The essence of this stability was analyzed by using the equivalent circuit model.Due to the characteristics of the continuity of the MLR structure,the phase change materials vanadium dioxide（VO₂）was directly integrated into the bright mold MLR.The different VO₂conductivity was set to adjust the resonance intensity of the bright mode MLR,and then the coupling effect between the two modes was controlled,so as to realize the active modulation of PIT effect.Moreover,the group delay at the PIT window was also observed to change accordingly,which illustrates the ability and advantage of the structure in realizing the electronic control,and shows the unique PIT modulation behavior.