Asymmetric Multifunctional Photonic Devices Based On Metasurfaces

Metasurfaces can be regarded as two-dimensional metamaterials whose electromagnetic characteristics do not depend on material's own characteristics,but on the arrangement,shape and size of structural units,like gratings and photonic crystals,which are special types of metasurfaces.The unique properties of metasurfaces allow arbitrary control of the amplitude,phase and polarization of electromagnetic waves,so they can achieve wide functionalities like light absorption,anomalous reflection and refraction,meta-lens,excitation of surface plasmon polariton(SPP)and asymmetric transmission,etc.This thesis mainly focuses on asymmetric transmission device,and asymmetric multifunctional device.Asymmetric transmission devices,exhibiting different transmittances for lightwave with opposite propagation directions,play an important role in optical communication and information processing.Multifunctional device means integrating multiple independent functionalities into one single device,which can promote optoelectronic integration.Depending on different polarizations,propagation directions and wavelengths,multifunctional devices show different control on the wavefront of lightwave,thus performing asymmetric multifunctions.Asymmetric multifunctional devices meet the needs of miniaturizing and integrating optical systems.Focusing on the asymmetric transmission device and the asymmetric multifunctional device by using phase gradient metasurfaces,our works mainly include the following two respects:Firstly,complementary coupled gratings are utilized in asymmetric transmission.Results show that through unidirectional excitation of SPP by bilayer gratings and near-field coupling inherent in the complementary structures,the metasurface reaches large forward transmittance of 0.96 for X-polarized light at 1250 nm,while the backward transmittance is less than 0.16,and it has a large 3 d B operation bandwidth of 174 nm.In addition,changing the spacer thickness t_s or lateral displacement s between the upper and lower gratings can adjust the vertical and lateral coupling effects,thus modifying the asymmetric transmission performance.Secondly,we also propose a multifunctional device based on phase gradient metasurface.The asymmetric multifunctional device is composed of the upper and lower metasurfaces,which is also called meta-device.The device employs different light incident directions and light polarization states,making the gradient metasurface meet different phase responses,then achieving asymmetric multiple functions.Results show that a metadevice with several functions at the wavelength of 800 nm is obtained,which can be used as an asymmetric transmitter for X-polarized light,a flat reflective focusing lens for forwardly incident Y-polarized light,and a reflective beam splitter for arbitrary linearly polarized light under backward illumination.