Research On Optical Functional Elements Based On Metamaterials Supporting Propagation Modes

Metamaterials,artificial materials which utilize subwavelength structures to manipulate electromagnetic waves,can realize electromagnetic phenomena,such as negative refraction,super-resolution imaging,and cloaking.Metamaterials have attracted extensive attention,and made great progress in extensive fields.Previous works on metamaterials mostly utilize resonant structures to realize the manipulation of electromagnetic waves,and thus intrinsically suffer from the issue of narrow bandwidth.In this paper,hyperbolic metamaterials and all-dielectric metamaterials are mainly studied.Both of them do not rely on resonance mechanisms,but utilize propagation modes to realize the manipulation of electromagnetic waves,thus are promising to be used to construct broadband optical functional elements.Schemes based on hyperbolic metamaterials and alldielectric metamaterials supporting propagation modes,have been proposed to address the issues of narrow bandwidth in transmissive metamaterial wave plates and nonlinear metamaterial devices,and limited numerical apertures and uncorrected spherical aberration in achromatic metalenses.The main research contents and innovations of this paper are listed as follows:(1)Broadband wave plates based on hyperbolic metamaterials are proposed,to solve the issue of narrow bandwidth in metamaterial wave plates based on propagation modes due to the dispersive birefringence.The phase modulation mechanism of multiple-beam interference is theoretically derived,and a scheme to design broadband wave plates based on multiple-beam interference is summarized.Applying the the proposed scheme to hyperbolic metamaterials,broadband quarter-wave plates in the microwave,terahertz,and near-infrared regions are designed.The microwave sample is fabricated and tested,and the experimental results demonstrate a good performance of polarization conversion with a degree of linear polarization of over 0.95 within the broadband frequency range of 7.2⁸.6GHz.The simulated results demonstrate that the terahertz and near-infrared devices achieve a degree of linear polarization of over 0.98 within the band range of 1.51².19 THz and1503¹716 nm,respectively.(2)A full-color second harmonic generator based on hyperbolic metamaterials is proposed,to address the issue of narrow bandwidth in resonant nonlinear metamaterials due to the narrow-band resonant mechanisms.Tapered hyperbolic metamaterials with gradually varying widths,support broadband frequency-dependent spatially localized slow light(termed as “rainbow trapping”),and can thus realize broadband field enhancement to boost second harmonic generation in broadband.Samples of hyperbolic metamaterials composed of Au/Zn O multilayers are fabricated and tested experimentally.The enhancement factor of second harmonic generation is over 100 within a pump wavelength range of 800¹300 nm,as compared to the unpatterned Au/Zn O multilayers,and full-color second harmonic signals can be observed with a camera.A maximum second harmonic conversion efficiency of2.78×10^-6 is obtained with a 4.74 m W pump at a wavelength of 900 nm.(3)A tri-wavelength aplanatic and achromatic metalens based on silicon metasurfaces is proposed,to solve the issue of uncorrected spherical aberration in achromatic metalenses.Firstly,the principle of correcting spherical aberration is studied theoretically.By using monocrystalline silicon as the building material,phase retardances introduced by metasurface units at three wavelengths can be controlled independently,and a triwavelength aplanatic and achromatic metalens can thus be realized.Then,a sample of the tri-wavelength aplanatic and achromatic metalens with a focal length of 365 ?m and a numerical aperture of 0.635,is fabricated and tested.The experimental results show a good performance of achromatic focusing and color imaging at the wavelengths of 490,570,and700 nm.The imaging performance is superior to that of the non-aplanatic metalens with the same numerical aperture.The proposed metalens has the advantages of transmission type,polarization independence,a large numerical aperture,aplanatism,and achromatism.