  Active metasurfaces, whose optical properties can be modulated
post-fabrication, have emerged as an intensively explored field in recent
years. The efforts to date, however, still face major performance limitations
in tuning range, optical quality, and efficiency especially for non mechanical
actuation mechanisms. In this paper, we introduce an active metasurface
platform combining phase tuning covering the full 2$\pi$ range and
diffraction-limited performance using an all-dielectric, low-loss architecture
based on optical phase change materials (O-PCMs). We present a generic design
principle enabling switching of metasurfaces between two arbitrary phase
profiles and propose a new figure-of-merit (FOM) tailored for active
meta-optics. We implement the approach to realize a high-performance varifocal
metalens operating at 5.2 $\mu$m wavelength. The metalens is constructed using
Ge2Sb2Se4Te1 (GSST), an O-PCM with a large refractive index contrast ($\Delta$
n > 1) and unique broadband low-loss characteristics in both amorphous and
crystalline states. The reconfigurable metalens features focusing efficiencies
above 20% at both states for linearly polarized light and a record large
switching contrast ratio of 29.5 dB. We further validated aberration-free
imaging using the metalens at both optical states, which represents the first
experimental demonstration of a non-mechanical active metalens with
diffraction-limited performance.
