  Active metasurfaces promise reconfigurable optics with drastically improved
compactness, ruggedness, manufacturability, and functionality compared to their
traditional bulk counterparts. Optical phase change materials (O-PCMs) offer an
appealing material solution for active metasurface devices with their large
index contrast and nonvolatile switching characteristics. Here we report what
we believe to be the first electrically reconfigurable nonvolatile metasurfaces
based on O-PCMs. The O-PCM alloy used in the devices, Ge2Sb2Se4Te1 (GSST),
uniquely combines giant non-volatile index modulation capability, broadband low
optical loss, and a large reversible switching volume, enabling significantly
enhanced light-matter interactions within the active O-PCM medium. Capitalizing
on these favorable attributes, we demonstrated continuously tunable active
metasurfaces with record half-octave spectral tuning range and large optical
contrast of over 400%. We further prototyped a polarization-insensitive
phase-gradient metasurface to realize dynamic optical beam steering.
