  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.
