Single-wavelength Focusing Metalens for Multichannel Two-photon Microscopy

The miniatured two-photon microscope is promising for monitoring brain activities of free-behaving animals or in-situ endoscope diagnosis. Ultrathin and lightweight metalens presents a potential alternative to bulky refractive lenses for a miniaturized optical system. However, designing and fabricating achromatic metalens covering the near-infrared and visible light spectrum range is challenging. This paper proposes a metalens focusing at one wavelength while capable of multichannel microscopy due to the nonlinear properties of two-photon excitation. The metalens was designed solely to focus near-infrared excitation laser (1040 nm) using geometric phase regardless of the chromatic correction for visible emission light. Our method simplifies the design process, reduces manufacturing costs, and increases the focusing efficiency for the excitation light while maintaining a high transmittance for visible light. Using the metalens, two-photon biological imaging with three different labels was experimentally demonstrated with subcellular resolution.