Fig. 1. Illustration of the imaging system, birefringent metalens, and model of a meta-atom. (a) Simplified stereoscopic microscope imaging system with birefringent metalens. The system shows a secondary imaging process, where the two symmetrical images of the target are first generated by the metalens which is illuminated by green light in the transmission mode, and then imaged again in CCDs with a commercial stereoscopic microscope. In the inset, red and blue arrows are the borderlines of the corresponding left and right metalenses, which have tilted optical axes and operate for linearly x- and y-polarized light, respectively. The yellow area illustrates the region of overlap between the two-phase profiles. The lengths of the birefringent metalens along the x- and y-axes are w and l. Here, w is also the diameter of a single left or right metalens. (b) Schematic diagram of the metalens, and model of a meta-atom for the birefringent metalens. The cured hydrogen silsesquioxane (HSQ) and SiO2 films are used as the substrate and protective layers. The parameters are h1=375  nm, h2=300  nm, a=200  nm, Lx=50–180  nm, and Ly=50 –180  nm.

Fig. 2. Calculation of (a), (c) the phase and (b), (d) the transmission of the c-Si cuboid nanopillars with different sizes by the linearly x- and y-polarized plane wave incidence, respectively. The period and height of the meta-atom are set to be 200 nm and 300 nm. The colored symbols represent the chosen structures for achieving four phase levels spanning the full 2π phase coverage. Their correspondences are listed in Table 1.

Fig. 3. (a) Ideal phase profile (lines) of the proposed birefringent metalens and the actual phase responses (dots) of the chosen meta-atoms. Red and blue correspond to the phases for the x- and y-polarized light according to Eq. (1) and Eq. (2), respectively. The yellow region illustrates the overlapping area of the phase profiles of the birefringent metalenses. (b)–(e) FDTD simulation of the focusing ability of the metalens. (b) Electric field intensity distribution of the linearly x-polarized dipole source (the spot on the bottom) passing through the metalens and focusing in the target position (the spot on the top). (c) The normalized intensity distributions of (b) along the x-axis at the focal plane. (d), (e) The corresponding simulation results for a y-polarized dipole source.

Fig. 4. SEM images of the fabricated birefringent metalens. (a) Overall outlines of the metalenses. Red and blue dashed circles outline the effective regions of the left and right metalenses. Each of them has a diameter of 400 μm. (b) Top view and (c) 30° titled view of the metalens at a higher magnification. The HSQ resist is not removed.

Fig. 5. Resolution characterization using images of the 1951 USAF resolution chart. (a) Fringes imaged by stereoscopic microscope without the birefringent metalens, operating at 6.2× magnification. The fringes in Group 7 Element 6 enclosed by the white solid frame are just clear while the fringes in Group 9 enclosed by the yellow dotted frame are all blurry. The inset shows a zoom-in image of Group 9. (b) Fringes imaged by the stereoscopic microscope with the birefringent metalens and from the left light path of the system for the x-polarized light. In this situation, the fringes enclosed by the yellow dotted frame in Group 9 Element 2 can be distinguished. (c) The corresponding fringes image of (b) from the right light path of the system for the y-polarized light. The magnifications of (b) and (c) are both 13.6×, where the extra 2.2× magnification is enabled by the metalens.

Fig. 6. Imaging results of pollen grains and silk fibers. Images of pollen grains taken from (a) the right light path of stereoscopic microscope without metalens of NA of 0.15 and magnification of 6.2×, (b) left and (c) right light paths of stereoscopic microscope with metalens of NA of 0.4 and magnification of 13.6×, (d) conventional binocular microscope with NA of 0.45 and magnification of 20× in transmission mode. (e)–(h) Images of silk fibers taken by the same methods as in (a)–(d). Red–blue composite pictures of (i) pollen grains and (j) silk fibers from (b), (c) and (f), (g). A stereoscopic perception can be obtained by wearing red–blue glasses (see Visualization 1 and Visualization 2 for more results).

Fig. 7. Stereoscopic microscope (Yong Heng XTZ-05T) is of Greenough-type with NA of 0.15. It has two individual light paths and two objectives. The metalens is placed in front of the objective to enhance the NA and magnification of the stereoscopic microscope. The captured left and right parallax images from the metalens-based stereoscopic microscope are sent to the autostereoscopic display to perceive vivid and full high-definition stereoscopic microscopic images without wearing extra equipment.