垂直电场下扭转双层石墨烯光学吸收性质的理论研究

层间扭转角度是对石墨烯物理性质宽波段可调谐的一个新参量.本文采用2°<θ<15°扭转角度下的连续近似模型,获得了不同扭转角度双层石墨烯分别在有、无电场下的能带结构,通过电子-光子相互作用跃迁速率,计算模拟了范霍夫奇点附近电子带内跃迁和带间跃迁所引起的光学吸收谱.结果表明,在无外加电场时,带间跃迁吸收峰的位置随着扭转角度的增大而发生从红外到可见光波段的蓝移,且吸收系数增大,带内跃迁的光学吸收系数相对于带间跃迁高出2个数量级;而存在外加电场时,两个范霍夫奇点在波矢空间的位置发生偏移,带间跃迁吸收峰发生分裂,且两个分裂的吸收峰位置随着电场强度的不断增大而反向行进.上述研究结果对石墨烯材料在光电器件方面的应用有一定指导作用.

Theoretical studies on optical absorption in twisted bilayer graphene under vertical electric field

The interlayer twist angle is an important parameter that can tune the physical properties of graphene in a wide wavelength range.In this paper,we employ an effective continuum model to calculate the band structure of twisted bilayer graphene with different twist angles in the presence and absence of vertical electric field.Based on the transition rate of the electron-photon interaction,we calculate and simulate the optical absorption spectra caused by the interband and intraband transitions around the van Hove singularities.The calculation results show that the optical absorption caused by the interband transitions occurs in the wavelength range from visible light to near-infrared while it appears in far-infrared for intraband transitions.The optical absorption coefficient of the intra-band transitions is almost two orders of magnitude larger than that of inter-band transitions.In the absence of an external electric field,as the twist angle increases,the absorption peak of the inter band transition moves from the infrared light band to the visible light band,but the resonant peak position of its intra-band transition does not change.At the same time,the absorption coefficient values corresponding to the above two transitions will increase.When an electric field is applied perpendicular to the twisted bilayer graphene,the symmetry of the initial band structure of bilayer graphene is destroyed,which results in the splitting of the absorption peaks associated the with interband transitions,and the distance between the two splitting peaks increases with the electric field intensity increasing;while the position and amplitude of the absorption peak associated with the intraband transition are completely unaffected by the applied electric field.The theoretical calculation results in this paper can provide the theoretical guidance for further applying twisted graphene to optoelectronic devices such as tunable dual-band filters.