  Dielectric metalens has emerged as an attractive device for advanced imaging
system because of its powerful manipulation ability of light beam, small
volume, and light weight. However, the applications of silicon nitride (Si3N4)
metalens are limited by the low refraction index of Si3N4, and multi-foci
metalens has not been realized based on a Si3N4 metalens. Here, we deeply
explore the working mechanism of a truncated waveguide meta-atom and obtain a
Si3N4 metalens with longitudinal three diffraction-limited focal points. By
utilizing the metalens sample as a condenser lens, a commercial microscope can
obtain three magnifications based on a single objective lens. Finally, an
infinity-corrected microscope with three high magnifications (9.5X, 10X, and
29X) and diffraction-limited resolution is integrated into centimetre-dimension
for the first time by using the tri-foci metalens sample as an objective lens.
This research would boost the scaling up of metalens microscope as well as the
multifunctional application of Si3N4 metalens.
