  Metasurface optics provide an ultra-thin 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 TiO2 nanostructures to
induce a polarization-dependent optical response. By rotating the polarization
of linearly-polarized input light, the focal length of a 40 micrometer-diameter
metalens is tuned from 220-550 micrometers with nearly diffraction-limited
performance. 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. Tunable achromatic
metalenses may be useful for applications in imaging and display.
