Theoretical Study On Optical Absorption Properties Of Twisted Graphene Systems

Graphene,as a two-dimensional honeycomb material composed of carbon atoms,can form a twisted graphene material by introducing a relative twist angle between its adjacent layers.At present,most of studies focuses on twisted bilayer graphene（TBG）,have found that the rotation angle changes the periodicity and symmetry of the original graphene lattice,so that the interlayer periodic potential field is modulated,and thus changes the carrier energy state,resulting in a series of new physical properties.In order to enrich the existing theories,this paper theoretically studies the optical absorption characteristics of three twisted graphene systems（twisted bilayer graphene TBG,twisted trilayer graphene TTG,twisted double-bilayer graphene TDBG）with rotation angle and field dependence.In this thesis,the band distribution of twisted graphene systems was firstly studied under the efficient continuous approximation model:The effective Hamiltonian near K point and near van Hove singularities point（VHS）with or without an external electric field are set up respectively,and use theoretical derivation or numerical calculation method to obtain the eigenvalue and eigenfunction function of the corresponding energy of each system.And then,according to the obtained results,the influence of rotation angle and applied field strength on the band structure of the system is analyzed.Combined with the electron-photon interaction Hamiltonian,the analytical results for the light absorption coefficient of the system near the K point and the numerical results for the light absorption coefficient near the VHS point are obtained,respectively.The obtained light absorption coefficient is used to plot the absorption spectrum of different twisted systems,so as to analyze the influence of rotation angle,applied electric field and incident light wavelength range on the light absorption characteristics of twisted graphene systems.The results show that:（1）In the absence of external electric field,the rotation angle will not change the symmetry of the energy band,but it will cause the band gap near the VHS singularities to increase.When the applied vertical electric field,the symmetry of the original band structure will be destroyed.（2）Considering the optical transition near point K:the one-photon absorption coefficientαare independent of the rotation angle,and the coefficient of TBG is constant,about 2.15×10~8m^-1,while the coefficients of TTG and TDBG vary in segments with the wavelength of incident light,showing decreasing trend as a whole.However,the two-photon absorption coefficientβis affected by both the incident wavelength and the rotation angle,where absorption coefficient value of each system increases slightly with the increase of the angle.The coefficientβof TBG increases monotonically slowly with the incident light wavelength.The two-photon absorption spectra of TTG and TDBG show peaks at light wavelengths.（3）Considering the optical transitions near VHS singularities with and without applied electric fields:in the absence of external field,there is only one resonant peak in the intraband and interband transition of the three systems,and the position and value of the peak of different systems are affected by the rotation angle differently.After the applied electric field,the interband transition absorption peaks of all systems split into two sub-peaks,and the relative positions of the two peaks move in reverse direction with the increase of electric field intensity,but the intraband transition spectra of the three systems are regulated differently by electric field intensity.The above research results are of great significance both for broadening the basic theory of the optical absorption properties of graphene materials and for improving the performance of graphene optoelectronic devices.