  Varifocal lenses are essential components of dynamic optical systems with
applications in photography, mixed reality, and microscopy. Metasurface optics
has strong potential for creating tunable flat optics. Existing tunable
metalenses, however, typically require microelectromechanical actuators, which
cannot be scaled to large area devices, or rely on high voltages to stretch a
flexible substrate and achieve a sufficient tuning range. Here, we build a 1 cm
aperture varifocal metalens system at 1550 nm wavelength inspired by an Alvarez
lens, fabricated using high-throughput stepper photolithography. We demonstrate
a nonlinear change in focal length by minimally actuating two cubic phase
metasurfaces laterally, with focusing efficiency as high as 57% and a wide
focal length change of more than 6 cm (> 200%). We also test a lens design at
visible wavelength and conduct varifocal zoom imaging with a demonstrated 4x
zoom capability without any other optical elements in the imaging path.
