Investigation Of Achromatic Sub-diffraction Lens Based On Metasurface

The focusing lens is the basic element of the optical imaging system,it is widely used in research and industrial manufacturing of many fields such as biology,medicine,materials science,and microelectronics.However,the imaging resolution of conventional focusing lens is limited due to the phenomenon of light diffraction.Therefore,breaking the diffraction limit to obtain a higher imaging solution is great significance for the research in micro-nano field.In recent years,optical superoscillation phenomena have been demonstrated to achieve non-invasive far-field superresolution imaging.This novel phenomenon is essentially the result of delicate interference of light field.It can effectively avoid complex near-field manipulation and specific dye labeling.As an ultra-thin,ultra-light,and easy-to-integrate artificial two-dimensional metamaterial,metasurface can realize the arbitrary control of the phase,amplitude and polarization of electromagnetic waves.Based on this flexible wavefront modulation,metasurfaces have been widely employed to construct super-oscillation fields for subdiffraction focusing.In this thesis,the investigation method of achromatic subdiffraction focusing based on metasurface is carried out.The main contents of the investigation include:1.A multi-wavelength achromatic super-oscillatory lens based on geometric phase modulation is designed,and the design method does not involve spatial multiplexing and stacking technology,which can effectively avoid complicated manufacturing processes.According to the performance parameters of the super-oscillatory light field,a multi-objective and multi-constrained optimization model is established to reversely optimize the required phase profile.With the aid of the modulation mechanism of geometric phase under circularly polarized light,a super-oscillatory lens is constructed to focus visible light wave with the wavelengths of 473 nm,532 nm and 632.8 nm on the same axial plane.Through the simulation calculations,the focal spots formed at three wavelengths are 0.706,0.722 and 0.750 times of the diffraction limit,respectively.2.Focusing on the problem of expanding the working bandwidth of the superoscillation lens,a design method is proposed to realize the broadband achromatic subdiffraction focusing lens.Based on the required parameters of the sub-diffraction focal spot,the corresponding inverse optimization is established.And then the required phase profile for sub-diffraction focusing can be obtained by linear programming method.The transmission phases of different wavelengths are folded into 0-2π and independently controlled by changing the geometric parameters of the unit structure.The particle swarm optimization algorithm can be used to optimize the full-mode structure that can match well the desired phase distribution.Then,the constructed metasurface is verified by electromagnetic simulation,and focal spots with the size of0.8 times the diffraction limit are precisely formed at the same axial plane in the visible range from 492 nm to 592 nm.