Dual‐Wavelength Switchable Focusing via a Fabry‐Pérot Resonance‐Enabled Hydrogel Metalens

ABSTRACT Active metalenses overcome the static properties of traditional devices, offering a promising platform for realizing tunable focusing with versatile functionalities. Despite various dynamic strategies for focal length tuning having been studied, achieving switchable wavelength‐selective focusing remains a challenge. Here, a hydrogel metalens is demonstrated for dual‐wavelength switchable focusing based on Fabry‐Pérot (FP) resonance. By exploiting the morphological changes of FP‐type hydrogel resonators, the metalens enables humidity‐tunable wavelength‐dependent phase modulation due to the cavity resonance alternation. Under the low relative humidity, the hydrogel metalens exclusively focuses light with a resonance wavelength of 470 nm and directly reflects light of other wavelengths. As the relative humidity increases, the focusing wavelength of the hydrogel metalens is actively tuned from 470 to 640 nm with a distinct intensity contrast. Moreover, the operating wavelength channel of hydrogel metalens can be arbitrarily tailored in the entire visible range by simply adjusting the initial cavity length of hydrogel resonators. The proposed hydrogel metalens provides a universal route for wavelength‐switchable focusing and suggests practical applications in ultra‐compact optical imaging and augmented reality display.