Terahertz Properties Of Plasmonic Materials

Surface plasmon polaritons are group oscillations generated by carriers in metallic materials or some semiconductor-like materials under the excitation of electromagnetic waves.It can be divided in to two parts,optical SPPs and acoustic SPPs.Acoustic SPPs is more localized than optical SPPs.Polariton material,refers to materials that can excite SPPs,the real part of its permittivity must be negative,that means the imaginary part of the corresponding conductivity must be positive.Only when this condition is met can the fields be matched at the boundary.The theoretical development of SPPs is in-depth,but its physical observation is still an important topic,and most materials cannot support SPPs observations with more periods due to their short relaxation times.The emergence of single-layer graphene has slightly changed this situation.Since it was discovered by the physical exfoliation method,it has received extensive attention from scientists.In addition to its excellent physical properties and high concentration of carriers,it also has a long relaxation time that provides convenience for the observation of SPPs.Since then,polariton material such as Dirac semimetals and topological insulators,which have the same Dirac cone band structure as graphene,have been gradually predicted and verified.They play an important role in the field of polariton benefit from their ultra-high carrier mobility and long relaxation time.In order to extract the electrical properties of the material,we use a terahertz time-domain spectrometer(TDS)system for non-destructive testing of the material,which can achieve a high signal-to-noise ratio within its effective dynamic range,providing accurate data for the calculation of electrical conductivity.The traditional calculation of conductivity often ignores the errors caused by the inconsistency of the substrate,which directly leads to the inconsistency of the calculation results with the theory sometimes.In this paper,for the offset of the terahertz signal time-domain waveform caused by various errors,the Three solutions are proposed,namely the direct calculation method in the literature,the secondary transmission signal correction method and the reflection coefficient correction method derived by ourselves when the difference in substrate thickness cannot be accurately calculated,which further improves the accurate calculation method of conductivity.After calculated conductivity,the physical model can be used to fit it,and the material parameters,such as plasma frequency,relaxation time and other key information can be extracted,which lays the foundation for subsequent research work.Loading gratings on the Polariton materials film can excite the SPPs inside the dielectric layer,and at the same time,the metal thin layer is used for regulation,so that the SPPs dispersion curve of the structure is further depressed to show the characteristics of acoustic polaritons.The grating structure can be designed using the extracted model parameters.The simulation results show that the acoustic SPPs is excited inside the dielectric layer,the field is localized inside the dielectric layer,and its intensity enhancement can reach 10~5 magnitude.In this paper,several methods for calculating and correcting electrical conductivity using the signals measured by TDS are described in detail,and the experimental measurement,data processing and modeling simulation of Dirac semimetals and topological insulators are carried out certain reference value.