1 of 6) 1600732 technology of mechanically reconfigurable metasurfaces is generating significant outcomes, [6] there are many applications which would benefit from a tunable device without any moving part. Being permeability in ordinary materials essentially equal to that in vacuum, researchers have intensively looked for processes enabling to tune the permittivity of some of the components which constitute the metasurface. A number of mechanisms and materials have been explored, spanning from free charge in ordinary semiconductors [7,8] and in graphene [9] up to phase-change materials, [10] materials based on insulatorto-metal or phase transitions, [11,12] gas adsorption into metals, [13] and liquid crystals. [14] An intrinsically different mechanism for tuning the permittivity of a material is electrochromism. In essence, electrochromism consists of a modification of the optical properties of the material occurring when it undergoes an electrochemical reaction. While being well known since many decades, its application to photonic devices is not yet fully explored. Most of the research developed to date is indeed based on conducting polymers; [15][16][17][18][19][20] however, electrochromism occurs also in transition metal oxides, which, compared to polymers, are in general more stable with respect to optical excitation.Among transition metal oxides, vanadium pentoxide (V 2 O 5 ) has the remarkable property to withstand the intercalation of huge amounts of lithium ions in its lattice [21][22][23] undergoing an intense investigation from the rechargeable battery community. Concurrently with lithium intercalation, the nearinfrared optical properties of V 2 O 5 are strongly modified, thus enabling the development of compact reconfigurable photo nic devices operating at telecommunication wavelengths. As a prototypical device we developed a metasurface comprising an array of aluminum nanoantennas directly placed on top of a V 2 O 5 layer (Figure 1a,b). The full layer stack (see the Methods section within the Supporting Information for details about the fabrication) also includes a platinum back plane, which has the main function of collecting/ injecting the balancing charge carried by electrons during the electrochemical intercalation process. This process took place in an electrochemical cell, following the procedure detailed in the Supporting Information. As an effect of intercalating a molar concentration x of Li ions, the V 2 O 5 material (which hence becomes Li x V 2 O 5 ) changes its permittivity according to Figure 1c: the major effect is observed on the imaginary part ε″, with a broad peak extending across the whole near-infrared spectral region, and values exceeding unity. This change of the complex permittivity is related to the mechanism of small polaron hopping, that is to the transfer of conduction electrons bound to the lattice ions to the neighboring orbitals (see the Supporting Information). It should be noticed that In the latest years the optical engineer's toolbox has welcomed a new concept, the m...