Fig. 1. Schematic of the meta-microscope based on metalens doublet and annular illumination. The optimized doublet enlarges the FOV and the annular illumination improves the resolution. Inset is the implemented meta-microscope prototype in a very compact form.

Fig. 2. Optical design of the metalens doublet. (a) Ray tracing simulation schematic diagram of the metalens doublet of $150\mu \mathrm{m}$ FOV at the working wavelength of 470 nm. The diameters of the metalens-I and metalens-II are 450 and $240\mu \mathrm{m}$, located on both sides of a $500\text{-}\mu \mathrm{m}$-thick fused silica substrate. The blue, green, and red optical paths and spot diagrams correspond to object heights of 0, 40, and $75\mu \mathrm{m}$, respectively. (b) The optimized phase profiles of the metalens-I (red) and metalens-II (blue). (c) Calculated PSFs of the metalens doublet at different object heights, with the PSFs of the single-layer metalens at varied object heights included at the bottom for comparison. Scale bar is $10\mu \mathrm{m}$. Calculated MTFs of different object heights of (d) the metalens doublet and (e) single metalens, respectively.

Fig. 3. Characterization of the metalens doublet. (a) Phase and transmittance of meta-atoms with 12 different structural parameters, simulated by FDTD solutions. The sizes (in nanometers) of the nano-fins are marked along the phase distribution line. (b) Schematic of a single unit cell, the width and length of the nano fins are 90 nm and 230 nm, respectively. (c) Optical and top-view scanning electron microscope (SEM) images of the fabricated ${\mathrm{SiN}}_x$ metalenses. Scale bar is $75\mu \mathrm{m}$ and 300 nm, respectively. (d) Annular oblique lighting can expand the spectrum pass-band so that high-frequency information can be received, thus improving the resolution. (e) Schematic drawing of the measurement setup.

Fig. 4. Microscopic imaging performance. (a) Image of the whole FOV captured by the CMOS. Scale bar is $15\mu \mathrm{m}$. (b) Image of resolution detail captured by the CMOS in the case of annular illumination. Element 5, group 10 can be recognized, indicating a resolution up to 310 nm. (c) Resolution characterization at object heights (i.e., distance from the optical axis) of 20, 40, 55, and $70\mu \mathrm{m}$. (d) Normalized intensity profiles of the vertical line pairs corresponding to the achievable resolution, where the highest-resolution features are highlighted with solid curves.

Fig. 5. Meta-microscope prototype and its application in bio-diagnostics. (a) Photographic image of the meta-microscope prototype, along with the optical and top-view SEM images of the fabricated illumination metasurface. Scale bar is $400\mu \mathrm{m}$ and 500 nm for the optical and SEM images, respectively. (b) Images of the USAF resolution test chart taken with the metalens microscope. Scale bar is $100\mu \mathrm{m}$. (c) Images of the cervical cancer cells. Scale bar is $80\mu \mathrm{m}$. Area 1: normal cell with nuclei approximately several micrometers in size. Area 2: pathological cell with significantly increased nuclear-cytoplasmic ratio. Area 3: pathological cell whose nuclei begin to divide. Scale bar is $10\mu \mathrm{m}$ for enlarged areas 1–3.

Fig. 6. Parameters and performance of some meta-based microscopes reported.