Research On Multi-functional Polarization Converter And Absorber Based On Surface Plasmons

Surface plasmons(SPs)are collective oscillations of free electrons in metal nanostructures.With the development of nanomaterials and the novel optical properties of SPs,researchers have applied the theory of SPs to the structure of metamaterials,and obtained excellent applications in perfect absorbers,polarization converters,highly sensitive sensors,solar absorbers and other aspects.In this thesis,we completed theoretical design and simulation research based on the SPs mechanism.Two metamaterial structures are designed and simulated by using finite-difference time-domain method.The specific research contents are as follows:Firstly,we designed and simulated a three-layer composite structure of graphene metasurface/SiO₂dielectric layer/Cu substrate.The structure realized the effects of multi-function polarization conversion and polarization conversion functions switching in mid-infrared region.Under a wide incidence angle range(0⁰?45⁰)of x linear polarized light,the structure in the mid-infrared band(30?50THz)to achieve a variety of functions,including broadband x-to-y cross polarization conversion(x-to-y CPC)function of x linear polarized light convert to y linear polarized light,dual-frequency(35.81 THz and 39.89 THz)linear-to-circular polarization conversion(LTC-PC)function of x linear polarized light convert to circularly polarized light,and switching between different polarization conversion functions(x-to-y CPC and LTC-PC).In addition,we also realize the circular-to-linear polarization conversion(CTL-PC)function that the circularly polarized light can be converted to x linear polarized light.This structure has potential applications in optical polarization control components such as integrated compact polarization sensors and polarization switches.Secondly,we designed and simulated a monolayer refractory titanium oxide and nitride(Ti N/TiO₂)core-shell nanowires grating array on the opaque Ti N substrate,and obtained a novel all-dielectric optical absorber.The absorber observed multiple resonant absorption bands in the adjacent wavelength range,thus forming an ultra-broadband absorption window from visible to infrared(500-1800 nm).By tuning the oblique incident angle or the polarization state,the bandwidth of wideband absorption can be affected.In addition,the absorption window can shift in the wavelength range by changing the refractive index of the TiO₂ shell.By introducing a feasible method to manipulate the absorption characteristics,it has a promising application in metal-free photoelectric equipment,such as solar energy absorption.Thirdly,combining colloidal crystals and refractory materials,a Ti cavity/SiO₂spherical array/Ti substrate three-layer structure is proposed,which realizes narrow-band absorption and wide-band absorption simultaneously.The maximum absorption efficiency is 99.8%(at 0.336?m)for narrowband absorption and 99.1%(at 1.382?m)for wideband absorption.The absorption bandwidths of the two bands are 81 nm and 1237 nm,respectively.By artificial adjusting the structural parameters,the absorber's absorption performance,such as absorption bandwidth,absorption efficiency and working wavelengths,can be adjusted.The absorber is also insensitive to incident Angle and independent of polarization,which are important for applications in complex practical environments.