Theoretical study of nonreciprocity in gyromagnetic material based metasurface

Abstract

Reciprocity is a common phenomenon that the measurement remains invariant after the positions of source and receiver are exchanged. Breaking electromagnetic reciprocity has been a huge demand in the application of compact electromagnetic devices. This thesis numerically and analytically studies a class of nonreciprocal non-Hermitian metasurfaces composed of gyromagnetic cylinders and dielectric cylinders. First, a dimer cylinder metasurface exhibits a pair of anti-symmetric transmittance and absorption spectra, arising from unequal excitation of lossy dielectric cylinders. The unequal excitation can be elucidated by the roles of Lorentz nonreciprocal medium, material loss and symmetry breaking in the dual-dipole model from truncated multiple scattering theory (MST), or unequal excitation of loss-dominant eigenmode based on biorthogonal analysis. Second, an eccentrically coated cylinder metasurface consisting of inner gyromagnetic cylinders embedded in outer lossy dielectric cylinders can demonstrate a strong nonreciprocal transmittance with low reflectance. Based on biorthogonal analysis, unequal excitation of incidence-reactive eigenmode is associated with the center-to-center distance between outer and inner cylinders. The relationship between the unequal excitation and nonreciprocal mechanism can be illustrated in the optimized metasurface. Apart from single nonreciprocal metasurface, a homogeneous nonreciprocal photonic crystal composed of trimer cylinder metasurfaces can exhibit spectral nonreciprocity of complex band structure and nonreciprocal isolation in a frequency range of dual-resonance, arising from unequal excitation of lossy dielectric cylinders along the monolayers. The decrease in interlayer distance of the crystal can achieve the strong reflectance with no transmittance by changing Bragg's condition for interaction of zero-th order diffraction. The further reduction of interlayer distance can obtain different nonreciprocal characteristics, arising from the cooperative effect of interaction of zero-th and first order diffractions. The emergence of nonreciprocal isolation with the cooperative effect can be related to the transition of dissipative and non-dissipative bulk modes between real bands of complex band structure. This research may develop the understanding of light-matter interaction in nonreciprocal photonics, as well as inspire the design of simple and thin nonreciprocal units and compacted integrated nonreciprocal systems.