  Miniature lenses with tunable focus are essential components for many modern
applications involving compact optical systems. While several tunable lenses
have been reported with various tuning mechanisms, they often still face
challenges in power consumption, tuning speed, fabrication cost, or production
scalability. In this work, we have adapted the mechanism of an Alvarez lens - a
varifocal composite lens in which lateral shifts of two optical elements with
cubic phase surfaces give rise to a change in optical power - to construct a
miniature, MEMS-actuated metasurface Alvarez lens. The implementation based on
electrostatic microelectromechanical systems (MEMS) generates fast and
controllable actuation with low power consumption. The utilization of
metasurfaces, ultrathin and subwavelength-patterned diffractive optics, as the
optical elements greatly reduces the device volume compared to systems using
conventional freeform lenses. The entire MEMS Alvarez metalens is fully
compatible with modern semiconductor fabrication technologies, granting it the
potential to be mass-produced at a low unit cost. In the reported prototype to
operate at 1550 nm wavelength, a total uniaxial displacement of 6.3 um is
achieved in the Alvarez metalens with direct-current (DC) voltage application
up to 20 V, modulating the focal position within a total tuning range of 68 um,
producing more than an order of magnitude change in focal length and 1460
diopters change in optical power. The MEMS Alvarez metalens has a robust design
that can potentially generate a much larger tuning range without substantially
increasing device volume or energy consumption, making it desirable for a wide
range of imaging and display applications.
