Metasurface lens that is converging or diverging depending on transmission direction enables ultra-compact MEMS tunable reflective lens

A conventional refractive lens surface can act as a positive (converging) or negative (diverging) lens, but the same surface cannot act as both. We show that a geometric phase metasurface lens can have the unique property of acting both as a positive or negative lens upon transmission through its front or rear side, respectively. This offers certain freedom in compound lens design, where one combines focusing and defocusing operations. We utilize this property to make an ultra-compact, varifocal reflective lens, where a metasurface is placed in front of a novel long-stroke piezoelectric MEMS-micromirror. A large theoretical diopter tunability of 6330 m$^{-1}$ is enabled due to innovative thin-film piezoelectric MEMS design, offering 62 $\mu$m displacement at 40V and low power, along with rapid actuation in the kHz region. The achieved MEMS-displacement is an order of magnitude larger than previously reported out-of-plane mechanical metasurface actuation. Since both metasurface and micromirror are flat, the presented reflective lens can be assembled without need for a spacer. It is therefore well suited for wafer-level silicon fabrication at high volumes and low cost. A proof-of-concept implementation using a 1550nm NIR metalens is demonstrated, attaining on the order of 1121 m$^{-1}$ diopter change for a focal length shift of 270 $\mu$m caused by a 53 $\mu$m micromirror displacement.