Study On Terahertz Slow Light Characteristics Of Dielectric Grating Coupled Graphene Plasmonic Structure

Terahertz wave communication has advantages of larger transmission capacity,faster speed and higher confidentiality,and has become a research hotspot in the industry.Slow light technology,which can perform controllable optical delay and all-optical caching,is an important basis for realizing terahertz communication.At present,terahertz slow light can be realized by constructing waveguide and photonic crystal structure,but the active regulation of slow light characteristics is not enough.As a new two-dimensional material with adjustable conductivity and excellent electrical tuning characteristics,graphene is an ideal choice for the design of dynamic control devices.Especially in the terahertz band,the hybrid coupling of different configurations of graphene plasmon structure to generate slow light effect is an effective way to solve the problem of adjustable slow light devices for terahertz waves.In this thesis,the continuous graphene sheet,the graphene ribbon arrays,the graphene ribbon arrays coupling graphene sheet structures are simulated to study the plasmon of them.There is no resonance peak in the terahertz transmission spectrum of the continuous graphene sheet.It is difficult to be excited directly by incident wave.The graphene ribbon arrays exhibit a resonance transmission effect,and its resonance frequency can be regulated by changing the structural period and Fermi level of the graphene.When graphene ribbon arrays coupling continuous graphene sheet,the transmission spectrum can produce an electromagnetic induced transparent-like window.The surface plasmon of the continuous graphene sheet are excited by the localized plasmon of the graphene ribbon arrays in near field,and plasmon-induced transparency effect is generated by the near field anti-phase coherent coupling.By properly introducing the dielectric grating,the surface plasmon polarization of the continuous graphene sheet can be excited efficiently,and it can resonance propagation.It can be regulated by changing the geometric structure parameters of grating and the Fermi level of graphene.An interval layer is introduced to adjust the coupling distance between the grating graphene layer and the graphene ribbon arrays.The coupling strength is inversely proportional to the thickness of the spacer layer.Changing the Fermi energy level of graphene can alter the plasmon-induced transparency spectrum of the hybrid system,generate different degrees of detuning,and then control the phase delay and dispersion relationship.The terahertz slow light characteristics of dielectric grating-coupled graphene plasmonic hybrid systems are numerically studied.The effective method to reduce the propagation speed of terahertz wave pulse is to enhance the anomalous dispersion effect of the refractive index of the medium on the base of keeping the signal low loss transmission.The plasmon-induced transparency effect,which can make the refractive index dispersion curve change violently under the condition of keeping the medium transparent,is an effective method to obtain the terahertz slow light transmission.In this thesis,the mechanism of plasmon-induced transparency by dielectric grating coupled graphene hybrid system is discussed.The surface plasmon resonance of continuous graphene sheet and the localized plasmon resonance mechanism of graphene ribbon arrays are analyzed.The dielectric grating compensates the wave vector mismatch of the graphene plasmon.So that the graphene surface plasmon and localized plasmon resonance modes can be directly and efficiently excited by the incident wave.Through the near field coherent coupling of the double bright mode,the plasmon-induced transparency effect can be excited at the resonance frequency.Then we discuss the relationship between the phase shift,the light delay and the group index of the dielectric grating coupled graphene hybrid system with different geometric structure.The maximum of light delay of the proposed hybrid system is 0.22 ps and the maximum of group index is 177.This design not only has the characteristics of high efficiency excitation,but also can independently adjust two plasmon resonance modes of coherent coupling by virtue of the applied gate voltage,and then obtain the adjustable terahertz slow light transmission,which has the potential application prospect in terahertz slow light technology and light delay devices.