Dielectric Metasurfaces Magnetic Mirriors

Metasurfaces are special two-dimensional planar thin metamaterial.By introducing the discontinuous varying of electromagnetic wave responses to both sides of the meta-atom,the phase,amplitude and polarization of reflected and transmitted waves can be efficiently manipulated as expected.Compared with three-dimensional metamaterials,the subwavelength metasurfaces have shorter propagation distance leading to less absorption loss,and easier to integrate and fabricate since the light weight and small volume.Metasurfaces have shown great application potential in antennas,sensors,active components,and integration technologies.However,the metallic metasurfaces exist high intrinsic ohmic losses in the high frequency band,especially in the optical band,while dielectric materials exhibit smaller losses resulting in the high-performance dielectric metasurfaces have developed rapidly.By adjusting the geometrical parameters of subwavelength high refractive index dielectric meta-atoms,it is possible to provide rich electric and magnetic Mie resonances with similar intensity.The different electric and magnetic resonant scattering modes excited by the dielectric metasurfaces can be constructive or destructive interference in the space to manipulate the phase,amplitude,and polarization of electromagnetic waves.Therefore,the dielectric metasurfaces have been extensively researched and applied in topological photonics,optical antennas,and many novel functional photonic components and devices.Recently,optical induced magnetic mirrors have made significant breakthroughs in different wavebands.The excitation of electric and magnetic Mie resonances is one of the main ways to fabricate the dielectric metasurface magnetic mirrors.The zero phase change of the reflected electric field makes the perfect magnetic mirrors eliminate the half-wave loss commonly existing in the metal electric mirror,so as to enhance the electric field near the interface between the magnetic mirror and the environment.It,thus,can significantly enhance the interaction between light and matter near the interface and promote the development of biosensor,antenna and optical components.Although the dielectric metasurface magnetic mirrors have been abundantly researched,it also needs to explore that the physical mechanism of the magnetic mirrors,tunable and retroreflective perfect magnetic mirrors,and periodic disordered metasurface magnetic mirrors.This thesis is mainly to solve the above-mentioned shortcomings of dielectric metasurface magnetic mirrors.The main research contents of this thesis include:1.Magnetic mirror by exciting magnetic quadrupole in two-dimensional LiTaO₃dielectric spherical array metasurface at 15.5 THz.Based on the Mie scattering theory,the scattering characteristics of the dielectric spheres of the metasurface are analyzed.It is concluded that the physical origination of the magnetic mirror is the magnetic quadrupole resonance of the metasurface at 15.5 THz.The zero reflected electric field phase change near the interface of the magnetic mirror is obtained,and the magnetic mirror shows an obvious near-field electric field enhancement phenomenon.By adjusting the period of the metasurface,the zero reflected electric field phase change frequency of the perfect magnetic mirror remains unchanged,and we optimize to achieve a higher reflectivity.2.Tunable perfect magnetic mirrors and retroreflectors with multiple frequencies and multiple incidence angles in the terahertz band.A magnetic mirror operating in the terahertz band is designed based on the functional reflective metasurface,which is simply constructed by a one-dimensional periodic Li TaO₃ micro cylindrical rod array upon a Teflon layer coated with a metal substrate at the bottom.Magnetic dipole resonance of the micro cylindrical rod excited in the metasurface is the reason for the retroreflective perfect magnetic mirror at the frequency of 0.286 THz with the reflectivity of R=0.97 for a normal incidence electromagnetic wave.By real-time varying the direction from normal（0）to 28.45 degrees and the frequency from 0.286 to 0.382 THz of the incident wave,the metasurface can still behave as a perfect magnetic mirror with the reflectivity as high as 0.99.Most interestingly,in this case,the metasurface possesses the property of a retroreflector that the reflected wave returns along the direction of the incident wave,which is consistent with the grating equation.The tunable perfect magnetic mirror effect and the retroreflector property may provide ways in novel photonic devices and sensing applications.3.Terahertz periodic disordered metasurface magnetic mirror modulated by anapole mode.One dimension Cu-LiTaO3 core-shell composite cylinders are arranged upon a metal plate to form a reflective metasurface.According to Mie theory analysis,the metasurface is modulated by anapole mode combining with multiple resonance modes including magnetic dipole,electric dipole,and electric quadrupole.The multiple modes result in the reflective full2πphase at 0.38 THz by simply continuously vary the radius of the inner cylinder.Due to a fixed and non-radiating anapole mode,the reflected electric field phase change and reflectivity curves of the metasurfaces are not almost affected by the change of period and exhibit good periodic disorder immune.By comparing the metasurfaces without anapole mode,the periodic disordered metasurfaces modulated by anapole mode can obtain stable perfect magnetic mirror response.This work breaks through the limitation of the fixed period magnetic mirror and facilitates the design and manufacture and practical application of the magnetic mirrors.