Polarization-modulated dual-wavelength metalens to overcome the diffraction limit

Two-photon stimulated emission depletion (STED) fluorescence microscopy has demonstrated advantages in ultrahigh resolution biological imaging. However, the bulky setup for generating excitation and depletion beams hinders the system development. Here, a miniaturized single-layer polarization-modulated dual-wavelength metalens is proposed for applications in STED. The metalens can achieve achromatic focusing at 532 nm and 632 nm, and can generate both a Gaussian-shaped spot and a donut-shaped depletion spot across different polarization states. Based on the theoretical analysis, we obtained an effective fluorescence spot that overcomes the diffraction limit. The multifunctional imaging capability of the metalens is also demonstrated, including bright- field and edge-detection imaging. The proposed metalens offers several advantages such as flexibility, single- layer, light-weight and multifunctionality. These features make it an ideal candidate for enabling super- resolution in minimally invasive imaging.