Study On Metalenses Based On Dispersion Engineering

Dispersion engineering has attracted great interest in numerous applications.Devices or systems with conventional dispersion-engineered strategies are bulky,heavy,and diffraction-limited.Metasurfaces are thin optical components that can flexibly control the amplitude,phase,and polarization of the light.However,the reported metasurfaces suffer from a small range of group delay,a limited bandwidth of sub-diffraction focusing,and limited dispersion.Therefore,it will be of great scientific significance for dispersion-engineered metalenses.Limited group delay range of meta-atoms greatly restricts the design of achromatic metalenses,and optimization approaches are usually employed to realize achromatic metalens with high-numerical-aperture.In this thesis,a group of high-index dielectric double-strip meta-atoms is proposed,which can achieve a group delay range as large as54.08 fs within the wavelength range of 1470 nm?1590 nm.An optimization-free method is proposed for the design of broadband achromatic metalenses.For the design wavelength?₀=1530 nm,an infrared broadband achromatic metalens is designed with a numerical aperture(NA)of 0.81,a radius of 20.9?₀,and a focal length of 15?₀.Simulation results show that the focal length variation is less than 0.27?₀,which is almost 17%of the theory depth of focus(?₀/??²);the focal spot size is less than 0.68?₀,which is close to the diffraction limit;and the actual depth of focus is smaller than2.85?₀.Compared with other reported achromatic metalenses,the designed metalens has a larger numerical aperture and smaller depth of focus.Various sub-diffraction metalenses only work at a single wavelength or multiple discrete wavelengths,while broadband achromatic metalenses are restricted by the diffraction limit.An approach of realizing broadband achromatic sub-diffraction focusing has been proposed by simultaneous dispersion engineering and amplitude manipulating.For the design wavelength?₀=118.8?m(2.52 THz),a broadband achromatic metalens is designed with a radius of 5.11 mm,a focal length of 40.39 mm,a NA of 0.125,and a bandwidth of 0.41 THz.In the frequency range of 2.29?2.70 THz,both simulation and experimental results reveal the broadband achromatic sub-diffraction focusing performance,and the focal length variation is less than 3.7?₀,which is almost 4%of the actual depth of focus.Traditional lenses are faced with limited dispersion.An approach of designing hyper-dispersive metalens is proposed.For the design wavelength?₀=118.8?m(2.52THz),a lens with larger-than-regular dispersion(n=2)is designed with a radius of 180?₀,a focal length of 330?₀,a numerical aperture of 0.48,a bandwidth of 0.21 THz,and a dispersion constant(C)of 155.73?m.rad.^-2.ps².Theoretical results show that the range of the normalized focal length shift is 16.4%with frequency increasing from 2.40 to2.61 THz,which is about 1.76 times the result(9.3%)for the traditional lens,and the constant C is 155.68?m.rad.^-2.ps².Experimental results demonstrated that the range of the normalized focal length shift is 6.3%(-0.22%?6.08%)with frequency increasing from 2.52 to 2.60 THz;the constant C is 151.65?m.rad.^-2.ps²,namely the dispersion being larger than the regular one(n=1).