| Polarization is one of the fundamental properties of light.The generation and manipulation of polarization have important applications in imaging,display,quantum computing,biomedical sensing and many other fields.As an ultra-thin artificial material,metasurfaces can fully control parameters such as amplitude,phase,and polarization of light with subwavelength structural thicknesses,overcoming the shortcomings of traditional optical devices that are bulky and single-function,and meeting the demands of modern optical systems for ultra-thin,ultra-compact and multifunctional optical components.In this thesis,two types of metasurface-based polarization devices in the optical communication band are investigated,in which tunable waveplates based on graphene metal metasurfaces and broadband circular dichroism devices based on all-dielectric two-dimensional chiral metasurfaces are proposed,respectively.The main content and results of the thesis are as follows:(1)A tunable reflective waveplate device based on graphene and ultrathin silver nanoslit arrays at a working wavelength of 1.55 μm is proposed.The device is composed of SiO2 substrate,silver reflecting film,SiO2 layer,single-layer grapheme and ultrathin silver nanoslits.By adjusting the Fermi level of graphene to change the surface plasmon resonance frequency,the properties of the waveplate can be dynamically tuned.Numerical simulation results show that:For a linearly polarized normal incident light,when the Fermi level of graphene is adjusted to 0.53 eV,0.71 eV,and 1.00 eV,respectively,by applying a gate voltage,the functions of 1/4 waveplate(which makes they-axis polarized component a 90°-phase retardation with respect to the x-axis polarized component),half waveplate(which makes they-axis polarized component a 180°-phase retardation with respect to the x-axis polarized component)and-1/4 waveplate(which makes the y-axis polarized component a-90°-phase retardation with respect to the x-axis polarized component)can be realized.Namely,the polarization state of the reflected light can be adjusted between right-handed circular polarization(RCP),linear polarization orthogonal to the incident light,and left-handed circular polarization(LCP).Meanwhile,by continuously changing the Fermi level of graphene between 0.53-1.00 eV,arbitrary left and right elliptically polarized reflected light with arbitrary ellipticity can be obtained.(2)A broadband circular dichroism device based on all-dielectric two-dimensional chiral metasurfaces is proposed and experimentally demonstrated.The device consists of a silicon dioxide substrate and an array of silicon two-dimensional chiral structures on the substrate.Theoretical studies show that the structure can generate a circular dichroism of more than 0.9(up to 0.993)and a circular polarization extinction ratio of more than 20 dB(up to 35 dB)in the transmission within the entire wavelength band of 1.40-1.60 μm.The working bandwidth and the circularly polarized extinction ratio are the highest reported so far.The properties of the reflected and transmitted light field corresponding to incident light with different circular polarization states are analyzed in detail.For a normally incident right-circularly polarized light,a right-handed metasurface acts as a magnetic mirror,reflecting almost all the incident energy,and the polarization state of the reflected light is still close to right-circularly polarized light.For a normally incident left-circularly polarized light on the right-handed metasurface,the transmittance is close to 1,and most of the LCP incident light is converted to right-handed circular polarization.Finally,the proposed devices were fabricated by chemical vapor deposition,electron beam lithography,inductively coupled plasma etching and other processes and the performances of the fabricated device were evaluated.The measured results are in excellent agreement with the theoretical design,which validates the design of the device.This work provides a new idea for designing broadband circularly polarized dichroic devices of two-dimensional chiral metasurface,which has broad application prospects in the fields of polarization imaging,life science,and drug chirality detection. |
