Achromatic Varifocal Metalens for the Visible Spectrum

Metasurface optics provide an ultrathin alternative to conventional refractive lenses. A present challenge is in realizing metasurfaces that exhibit tunable optical properties and achromatic behavior across the visible spectrum. Here, we report the design, fabrication, and characterization of metasurface lenses ("metalenses") that use asymmetric titanium dioxide (TiO₂) nanostructures to induce a polarization-dependent optical response. By rotating the polarization of linearly polarized input light, the focal length of a 40 μm diameter metalens is tuned from 220 to 550 μm. We show that imparting a wavelength-dependent polarization rotation on incident light enables achromatic focusing over a wide band of the visible spectrum, 483-620 nm. We use this property to demonstrate varifocal color imaging with white light from a halogen source. We also discuss how tunable achromatic metalenses may be useful for display applications.