ultra-thin equivalents, but may also introduce new functionalities (phase discontinuities, anomalous reflection, and refraction, etc.). [2,3] However, the shape of the wavefront is defined by the metasurface design, including material selection and geometry, and is fixed after fabrication. Active postfabrication control requires the tunability of the optical response of each metasurface element. [4,5] One common approach for active metasurfaces is to capitalize on the change of the material polarizabilityeither of the scatterer or its surroundings. This can be achieved, for example, by modulation of the charge density in doped semiconductors (e.g., GaAs) or graphene, [6] by changing the state of liquid crystals adjacent to metallic antennas, [7] or by including phase-transition (e.g., vanadium dioxide VO 2) [8-10] or phase-change [e.g., germanium antimony telluride (GeTe) x (Sb 2 Te 3) 1−x ] [11] materials in the metasurface. Mechanical tuning with stretchable substrates or actuators, [12] chemical reactions at the scatterers, [13] or enhanced optical nonlinearities [14] are other possible approaches for post-fabrication tunability. Phase-change materials (PCMs) are among the best-suited materials to provide tunability of metasurfaces due to the property contrast between their amorphous (A) and crystalline (C) phase. [15] In contrast to phase-transition materials (e.g., VO 2), PCMs feature non-volatile states and thus, no energy is needed to maintain the material properties. The structural change is accompanied with a refractive index change Δn = |n C − n A | Metasurfaces allow for the spatiotemporal variation of amplitude, phase, and polarization of optical wavefronts. Implementation of active tunability of metasurfaces promises compact flat optics capable of reconfigurable wavefront shaping. Phase-change materials (PCMs) are a prominent material class enabling reconfigurable metasurfaces due to their large refractive index change upon structural transition. However, commonly employed laser-induced switching of PCMs limits the achievable feature sizes and restricts device miniaturization. Thermal scanning-probe-induced local switching of the PCM germanium telluride is proposed to realize near-infrared metasurfaces with feature sizes far below what is achievable with diffraction-limited optical switching. The design is based on a planar multilayer and does not require fabrication of protruding resonators as commonly applied in the literature. Instead, it is numerically demonstrated that a broad-band tuning of perfect absorption can be realized by the localized tip-induced crystallization of the PCM. The spectral response of the metasurface is explained using resonance mode analysis and numerical simulations. To facilitate experimental realization, a theoretical description of the tip-induced crystallization employing multiphysics simulations is provided to demonstrate the great potential for fabricating compact reconfigurable metasurfaces. The concept can be applied not only for plasmonic sensing and spatial freq...
