Chiral Metalens And Circular Dichroism Device Based On Sub-Wavelength Structure

In the trend of miniaturization and system integration,optical systems require ultra-thin and multi-functional compatible optical components.As an artificial structure,metamaterials can flexibly manipulate phase,amplitude and polarization state at sub-wavelength scale,which is an ideal structure for capturing multiple information and implementing multifunctional optical devices.In this thesis,theoretical design and experimental demonstration of circular polarized dichroism and focusing devices with sub-wavelength chiral structures are studied.The detailed work is as follows:(1)A chirality metalens of circular polarization dichroism was proposed and demonstrated both theoretically and experimentally.The metalens consists of azimuthal angle varied sub-wavelength helical surface arrays covered by gold thin film on a silicon dioxide substrate.Theoretical studies show that the phase of the transmission of the helical surface array is sensitive to the azimuthal angle of each helical surface unit,and a full phase shift(-π～π)can be obtained by manipulating the azimuthal angle.With this finding,arbitrary phase distribution either in the transmission and reflection can be obtained by controlling the azimuthal angle of the helical surface.Based on this characteristic of the structure,a chiral metalens of circular dichroism(CMCD)is designed in the mid-infrared region of 3-5μm,in which one of the circularly polarized light(either left-handed or right-handed)is transmissively focused at a designed focal length while the other is reflectively focused.The circular dichroism(CD=TRCP-TLCP)of the metalens can reach 70%at a broadband of 3-5μm theoretically.The designed focal length is in good agreement with the simulated one,and the size of the focal spot is close to the theoretical value of diffraction limit.(2)Experimental fabrication and demonstration of the proposed 3D CMCD device are conducted.A CMCD based on gradient sub-wavelength helical surface array fully covered with metal is fabricated by three-dimensional laser writing technique and electron beam evaporation technique.Experimental measurement results of the fabricated structures are consistent with the designed parameters.The experimentally obtained circular dichroism is 40%,and trend is consistent with the theoretical simulation results in the wavelength of 3-5μm.The focusing function of the device is verified by the lock-in thermography technique.The experimental results are in good agreement with the theoretical simulation results.(3)A high-efficiency circular dichroism metasurface device formed by an array of multi-sized chiral slits etched in a silicon layer on a silicon dioxide substrate is proposed.Theoretical results show that high circular dichroism can be obtained at a specific wavelength by adjusting the scale ratio of the multi-sized slits in a unit.The number of formants can be controlled by changing the relative positions of the slits with different sizes in the unit structure to meet the needs of multi-band operation.Based on the unique optical characteristics,a high-efficiency circular dichroism device with multi-sized chiral structure is designed.Numerical simulation results show that the device has high circular dichroism in the range of 1500-1550 nm,and at the resonance wavelength of 1540 nm,the highest circular dichroism(CD=TRCP-TLCP)can reach 97%,and the extinction ratio(ER=TRCP/TLCP)is as high as 1999:1.The proposed and demonstrated chiral metalens of circular dichroism based on the sub-wavelength three-dimensional gradient helical surface can flexibly manipulate the phase of electromagnetic waves either in transmission or reflection,and focusing and circular dichroism can be simultaneously realized with a single device,which provides new ideas for the study of integrated multi-functional devices.The proposed circular dichroism device based on two-dimensional multi-sized chiral metasurface can flexibly adjust the wavebands,and has the advantages of high circular dichroism,easy-fabrication and high compatibility with linear polarized components,which provides a possible solution for pixelated full-Stokes polarization imaging.