Study On Imaging Quality Of Microlens With Sub-wavelength Structure

All-dielectric subwavelength structures can modulate light intensity,polarization and phase,can be fabricated as anti-reflection element and metasurface.In order to reduce the requirement on the etching precision to suitable for mass production,A two-dimensional cylindrical subwavelength structure with single step is proposed.Calculation the equivalent refractive index of cylindrical subwavelength structures with different radii by finite-difference time-domain method,design and simulated a diffraction Convergence Lens with Radial gradient of equivalent Refractive Index based on the principle of Equal Light path.The refractive index of the diffractive lens is 1.4,the aperture of the microlens is 4?m,in the 400nm-800 nm band,the beam convergence is realized.The maximum numerical aperture is 0.72.The diffraction lens array is integrated into a chip with a thickness of only 1?m.In order to study the effect of subwavelength structure on the imaging quality of microlens,The finite-difference time-domain(FDTD)method is used to simulate the uniformly distributed cylindrical subwavelength structure,regular gradual Convergence Cylindrical Sub-wavelength structure,regular gradient Convergence Circular Hole-shaped Sub-wavelength structure and the effect of regular gradual gradient on the microlens,and analyze the focal spot size and focal length of microlens.By analyzing the difficulty of the existing experimental conditions,the uniform distribution of cylindrical sub-wavelength structure and the regular gradual change of converged circular hole-shaped subwavelength structure are finally selected to be verified by experiments.The parameters of cylindrical subwavelength structure with uniform distribution are designed by using the thin film theory and the equivalent refractive index theory.The accuracy of the design results is verified based on the rigorous coupled wave theory(RCWA).The experimental results show that the structure has a good anti-transmission effect in the visible light region.The focal spot size of 100 microlenses with sub-wavelength structure and without sub-wavelength structure on the surface is measured.The results show that the uniform distribution of cylindrical sub-wavelength structure does not change the focal spot size.A regular converged circular aperture subwavelength structure is fabricated on the surface of microlens,and analyze the size of spot.The simulation results show that the structure has influence the energy distribution and focal length of microlens spot.A circular subwavelength structure is fabricated on the surface of a microlens with an aperture of 7?m and the focal length is 19?m.The measurement results show that the sub-wavelength structure reduces the focal length of the microlens from 19?m to 15?m in the visible light band,and the focal depth becomes longer,the focal spot size is reduced at the same time.The maximum energy at the center of the spot was increased 17.9%.