Switchable Visible-Frequency Monochromatic Focusing via a Liquid-Tunable Metalens.

Active metalenses offer a promising route for dynamic focusing and versatile functionalities, overcoming the static imaging limitations. However, despite various tunable metalenses having been studied, achieving continuous wavelength-switchable focusing with convenient tuning strategies remains a challenge. Here, we demonstrate a microfluidic-integrable metalens for liquid-tunable wavelength-selective focusing in the entire visible range. By injecting common liquids onto the resonant metalens, the operating wavelength can be continuously tailored for switchable monochromatic focusing due to ambient refractive-index-induced resonance variation. As the refractive index of the immersion liquid increases from 1 (air) to 1.58 (methylaniline), the focusing wavelength of the metalens shifts from blue to red, with a tuning range exceeding 180 nm. In addition, we demonstrated that the initial operating wavelength of the metalens can be flexibly selected by engineering the geometry of the resonator, providing wavelength design freedom for tunable monochromatic focusing. Since liquid immersion is a commercial microscopy-compatible strategy, the proposed metalens with monochromatic tunability can promote practical technologies of dynamic focusing and multispectral imaging.