Fig. 2: Characterizing the optical performance of our flat lens.

a Measurement of the focal spot. The experimental profiles of focal spots near the focal plane are measured by knife-edge scanning. Based on the profiles at the different z-cut planes, the lateral (x- and y-direction) intensity profiles of the real spot are retrieved by our homemade algorithm and then yield the spot size (FWHM) labeled by red (x-direction) and green (y-direction) circles. b, c Calculation of the spot sizes in the x-direction b and y-direction c based on the multi-Gaussian fitting method. d, e Depth of focus (DOF) in the x-direction d and y-direction e. Transversal (i.e., in the grating direction) scanning of a grating pattern is employed to measure the DOF along the depth, as sketched by Step 3 in a. The upper images display the evolution of the intensity distribution obtained in the laser transmission scan of the grating pattern. A small spot yields clear stripes along the x- or y-direction, while a large spot gives a uniform intensity. The lower images plot the effective optical resolution (EOR) index of the laser spot along the depth so that we can evaluate the DOF of the lens