Manipulating Of Polarization In Non-Hermitian Chiral Metasurface

Metasurface refers to an artificial layered structure with a thickness less than the incident wavelength,which can flexibly adjust the optical characteristics of electromagnetic wave such as polarization,amplitude,phase,and propagation mode.As an open system,the energy absorption and radiation leakage exist in the metasurface during the interaction with incident electromagnetic wave that is suitable for studying the physical properties of the exceptional point(EP).The metasurface possesses numerous novel optical properties at the exceptional point,such as sudden rotation of the main axis of the elliptical polarization state,unidirectional zero reflection of the incident light and the sensitivity of photodetector improved in terahertz band that promote the development of related fields.The article is mainly based on the modulation research of polarization state by non-Hermitian chiral metasurface which includes the following two aspects:(1)A graphene chiral metasurface based on the exceptional point to adjust the polarization state of incident light in the terahertz band is designed.Graphene with the atomic thickness is a new type of two-dimensional material discovered in recent years which is suitable for preparing the thinnest metasurface that can respond to incident light from infrared to terahertz band and reduce the energy loss on the premise of optical performance.The unit cell of the designed chiral metasurface contains two strips of graphene perpendicular to each other.Varying the parameters of the unit cell and the incident wavelength,exceptional points with chiral symmetry can be observed in the terahertz band,corresponding to the transmission of left-handed circularly polarized light and right-handed circularly polarized light.Impinging the incident light on the metasurface in opposite directions can invert the chirality of the exceptional point,and the asymmetric transmission effect of circularly polarized light can also be observed.(2)A bi-layered L-shaped metallic chiral metasurface based on the exceptional point to adjust the polarization state of incident light is designed.The unit cell of the metasurface with rotational symmetry about the coordinate axis of x can change the polarization state by adjusting the structural parameters and the incident wavelength.With the incident wavelength increasing,the outgoing light slowly approaches to the linearly polarized state from the elliptically polarized state.The transmission of linearly polarized light with a 45 degrees to the coordinate axes in the quadrants of second and fourth corresponds to the occurrence of exceptional point where the evolution rate from the linearly polarized state to the circularly polarized state begins to accelerate.After rotating the metasurface by 45 degrees around the incident light direction,the appearence of exceptional point is equivalent to the transmission of axial linearly polarized light and the asymmetric transmission effect of axial linearly polarized light can also be observed.