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...

This study examines the influence of lithium intercalation on the optical properties of vanadium pentoxide films. The films with a thickness between 400 and 1000 nm were prepared by DC magnetron sputter deposition. Cyclic voltammetry and chronopotentiometry were used to set a well defined lithiation state of the LixV2O5 films between x = 0 and x = 1. The optical properties of these films were measured by optical reflectometry in the wavelength range between 500 and 1700 nm. From the reflectance data, the refractive index and the extinction coefficient of the films were finally calculated as a function of the wavelength using Cauchy's dispersion model. The results confirm that the optical behavior of LixV2O5 films varies significantly upon lithium insertion. It is demonstrated that the changes produced in the optical properties are completely reversible within the limits of permanent structure changes.

A new approach for an optical actuator system based on mixed ionic–electronic conductor materials is proposed. The system actuates on light propagating in a waveguide implemented on a photonic integrated circuit by electrochemically changing the composition of the MIEC material, using the characteristic dependence of the optical properties upon stoichiometry. To realize this actuator, a multilayer stack was sputtered and characterized forming a battery-like system where ions reversibly travel from a Li-ion source to a Li x V2O5 layer, producing the desired change of the optical properties. Modal field FEM simulations were carried out to estimate the influence of the formed actuator on a waveguide fundamental mode implemented on the silicon on insulator platform. The time resolution of the actuator is estimated solving the diffusion profile of Li inside the Li x V2O5 coating and its evolution with time. Through simulations and measurements, promising results for a potential actuator system are shown, like small device length (<20 μm), low power consumption (∼10 pW per switch), reversibility, and long time stability.

No abstract

This work examines the proton intercalation in vanadium pentoxide (V 2 O 5) thin films and its optical properties in the near-infrared (near-IR) region. Samples were prepared via direct current magnetron sputter deposition and cyclic voltammetry was used to characterize the insertion and extraction behavior of protons in V 2 O 5 in a trifluoroacetic acid containing electrolyte. With the same setup chronopotentiometry was done to intercalate a well-defined number of protons in the H x V 2 O 5 system in the range of x = 0 and x = 1. These films were characterized with optical reflectometry in the near-IR region (between 700 and 1700 nm wavelength) and the refractive index n and extinction coefficient k were determined using Cauchy's dispersion model. The results show a clear correlation between proton concentration and n and k.