Design Of Broadband Achromatic Metalens In The Mid-infrared Region Based On The Principle Of Phase Manipulation With Nanostructures

As a new type of artificially designed optical element,the metasurface can realize the focusing function of the traditional lens.At the same time,it has the advantages of small size,easy integration,and high design freedom.The metalens is an important application of the metasurface.It is an efficient diffractive optical element.It also has the problem of dispersion.How to eliminate dispersion so that the metalens can be used in a broadband of incident light is an important direction to expand the application of metalens.At present,the research of broadband achromatic metalens is mainly focused on the visible light and the near-infrared light.This dissertation mainly based on the principle of phase manipulation with nanostructures.The design of broadband achromatic metalens in the mid-infrared region and plasmonic lenses detecting polarization state of arbitrary polarized light are studied as follows.1.There are two main methods applied in achromatic metalens,the first one using geometric phase to control the focusing phase of incident light,and then use the resonance phase of the meta-atoms to compensate for chromatic aberration.The advantage of this method is that the value of the geometric phase is determined by the rotation angle of the meta-atoms.The focusing phase of the lens can arbitrarily adjust and a larger numerical aperture can be achieved.The disadvantage is that circularly polarized light is required as the incident light.Another method is polarizationindependent achromatic metalens,which uses meta-atoms with symmetrical shape.This type of metalens has no requirement on the polarization state of incident light.Without using the geometric phase,the wavefront of the lens is limited by the refractive index of the material,and the numerical aperture is usually small.Based on the second method,we are looking for achromatic metalenses working in the mid-infrared region.Silicon is commonly used in infrared region with a high transmittance and a large refractive index.When designing the size and shape of the meta-atoms,by calculating its broadband phase response,the meta-atoms whose phase changes linearly with the incident light frequency is obtained.Based on these meta-atoms,we designed a polarization-independent achromatic metalens in the mid-infrared region,and calculated the focal field distribution of the lens using simulation software.The results verify its achromatic ability in the mid-infrared region.2.A metalens capable of detecting the polarization state of arbitrary polarized light is designed by using plasmon metasurfaces.The metalens is composed of a double-ring arrayed nanocrosses-based plasmonic lens and two Archimedes-spiral arrayed nanoslits-based plasmonic lenses with reverse rotating direction.The phase difference and amplitude ratio between two components of the incident polarized light can be detected by this metalens.The metalens is composed of a gold film and a silicon dioxide substrate.Nanoslits are etched on the gold film.The shape and geometric size of the nanoslits are designed by theoretical derivation.The numerical simulation software is used to simulate the condition of different polarized incident light.The polarization of the incident light can be obtained accurately and simply from the focusing field distribution.