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Active metasurfaces based on phase-change memory material digital metamolecules

Tunable metasurfaces are a promising candidate for the next generation of spatial light modulators which will require higher refresh rates, smaller pixel sizes, and compact form factors. Phase-change memory materials present a unique platform for nonvolatile reconfigurable metasurfaces which could undergo phase transitions at MHz frequencies if actuated electrically, more than three orders of magnitude higher than refresh rates of existing commercial SLMs. While stable intermediate phases of GeSbTe (GST) exist which can be used for imparting differential phase shifts, the stochasticity of the material properties would limit the robustness of such a phase shifter, whereas the fully crystalline and amorphous states exhibit more consistent behavior. To overcome this, we design GST digital metamolecules comprising constituent meta-atoms which individually are in either the SET or RESET state, but which together form a tunable metamolecule with a set of robust phase shifts. We simulate active metasurface lenses based on these metamolecules, showing successful focusing, and demonstrate nano-patterning of a GST film with isolated nanoposts of material which could be electrically actuated, unlike counterparts which must be optically reconfigured.