Being able to dynamically shape light at the nanoscale is one of the ultimate goals in nanooptics 1 . Resonant light-matter interaction can be achieved using conventional plasmonics based on metal nanostructures, but their tunability is highly limited due to fixed permittivity 2 . Materials with switchable states and methods for dynamic control of lightmatter interaction at the nanoscale are therefore desired. Here we show that nanodisks of a conductive polymer can support localised surface plasmon resonances in the near-infrared and function as dynamic nanooptical antennas, with their resonance behaviour tuneable by chemical redox reactions. These plasmons originate from the mobile polaronic charge carriers of a poly[3,4-ethylenedioxythiophene:sulfate (PEDOT:Sulf) polymer network. We demonstrate complete and reversible switching of the optical response of the nanoantennas by chemical tuning of their redox state, which modulates the material permittivity between plasmonic and dielectric regimes via non-volatile changes in the mobile charge carrier density. Further research may study different conductive polymers and nanostructures and explore their use in various applications, such as dynamic metaoptics and reflective displays.We prepared thin conductive polymer films of poly [3,4-ethylenedioxythiophene:sulfate] (PEDOT:Sulf, see Fig. 1a), which can provide high electrical conductivity and metallic character 3,4 . Using vapour phase polymerization and sulfuric acid treatment (see Methods), we obtained films with electrical conductivity exceeding 5000 S/cm (see Supplementary Table.1). Their complex and anisotropic permittivity was determined by ultrawide spectral range ellipsometry, employing an anisotropic Drude-Lorentz model as described previously (see Supplementary Table . 2) 5 . Fig. 1b shows the resulting in-plane permittivity of a thin PEDOT:Sulf film with thickness of 32 nm (Supplementary Fig. 1 presents the raw data). The shaded area highlights a spectral region (0.8 to 3.6 μm) in which the film has negative real permittivity and lower magnitude imaginary permittivity, which we define as plasmonic regime. This optically metallic and plasmonic character is related to the high conductivity within the thin film due to high concentration (2.6 ×10 21 cm -3 , determined by ellipsometry, see Supplementary Table.1and Supplementary Information for details) of mobile positive polaronic charge carriers. We also note that the mobility is highly anisotropic 5,6 and the out-of-plane real permittivity (Supplementary Fig. 2a) is primarily positive throughout the measured range, making the conductive polymer thin film a natural hyperbolic material 7 (Supplementary Fig. 3).

Precise manipulation of light–matter interactions has enabled a wide variety of approaches to create bright and vivid structural colors. Techniques utilizing photonic crystals, Fabry–Pérot cavities, plasmonics, or high‐refractive‐index dielectric metasurfaces have been studied for applications ranging from optical coatings to reflective displays. However, complicated fabrication procedures for sub‐wavelength nanostructures, limited active areas, and inherent absence of tunability of these approaches impede their further development toward flexible, large‐scale, and switchable devices compatible with facile and cost‐effective production. Here, a novel method is presented to generate structural color images based on monochromic conducting polymer films prepared on metallic surfaces via vapor phase polymerization and ultraviolet (UV) light patterning. Varying the UV dose enables synergistic control of both nanoscale film thickness and polymer permittivity, which generates controllable structural colors from violet to red. Together with grayscale photomasks this enables facile fabrication of high‐resolution structural color images. Dynamic tuning of colored surfaces and images via electrochemical modulation of the polymer redox state is further demonstrated. The simple structure, facile fabrication, wide color gamut, and dynamic color tuning make this concept competitive for applications like multifunctional displays.

Nanostructures of conventional metals offer manipulation of light at the nanoscale but are largely limited to static behavior due to fixed material properties. To develop the next frontier of dynamic nano‐optics and metasurfaces, this study utilizes the redox‐tunable optical properties of conducting polymers, as recently shown to be capable of sustaining plasmons in their most conducting oxidized state. Electrically tunable conducting polymer nano‐optical antennas are presented, using nanodisks of poly(3,4‐ethylenedioxythiophene:sulfate) (PEDOT:Sulf) as a model system. In addition to repeated on/off switching of the polymeric nanoantennas, the concept enables gradual electrical tuning of the nano‐optical response, which was found to be related to the modulation of both density and mobility of the mobile polaronic charge carriers in the polymer. The resonance position of the PEDOT:Sulf nanoantennas can be conveniently controlled by disk size, here reported down to a wavelength of around 1270 nm. The presented concept may be used for electrically tunable metasurfaces, with tunable farfield as well as nearfield. The work thereby opens for applications ranging from tunable flat meta‐optics to adaptable smart windows.

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Electrically conducting polymers (ECPs) are becoming increasingly important in areas such as optoelectronics, biomedical devices, and energy systems. Still, their detailed charge transport properties produce an anomalous optical conductivity dispersion that is not yet fully understood in terms of physical model equations for the broad range optical response. Several modifications to the classical Drude model have been proposed to account for a strong non-Drude behavior from terahertz (THz) to infrared (IR) ranges, typically by implementing negative amplitude oscillator functions to the model dielectric function that effectively reduce the conductivity in those ranges. Here we present an alternative description that modifies the Drude model via addition of positive-amplitude Lorentz oscillator functions. We evaluate this so-called Drude-Lorentz (DL) model based on the first ultra-wide spectral range ellipsometry study of ECPs, spanning over four orders of magnitude: from 0.41 meV in the THz range to 5.90 eV in the ultraviolet range, using thin films of poly(3,4-ethylenedioxythiophene):tosylate (PEDOT:Tos) as a model system. The model could accurately fit the experimental data in the whole ultrawide spectral range and provide the complex anisotropic optical conductivity of the material. Examining the resonance frequencies and widths of the Lorentz oscillators reveals that both spectrally narrow vibrational resonances and broader resonances due to localization processes contribute significantly to the deviation from the Drude optical conductivity dispersion. As verified by independent electrical measurements, the DL model accurately determines the electrical properties of the thin film, including DC conductivity, charge density, and (anisotropic) mobility. The ellipsometric method combined with the DL model may thereby become an effective and reliable tool in determining both optical and electrical properties of ECPs, indicating its future potential as a contact-free alternative to traditional electrical characterization.Fig. 4 (a) In-plane real optical conductivity dispersion of PEDOT:Tos represented by multiple Lorentz oscillators (the Drude contributions are removed) derived from the DL model. (b) The contributions of the real optical conductivity from THz (blue curve), broad oscillators (brown curve), interband transitions (green curve), and molecular vibrations (grey curves) are indicated. (c and d) Display the amplitude and broadening parameters for each Lorentz oscillator as well as their resonance energy (frequency).Their out-of-plane counterparts are shown in Fig. S10 (ESI †). The parameters for these Lorentz oscillators are listed in Table S2 (ESI †).

Dynamic control of structural colors across the visible spectrum with high brightness has proven to be a difficult challenge. Here, this is addressed with a tuneable reflective nano‐optical cavity that uses an electroactive conducting polymer (poly(thieno[3,4‐b]thiophene)) as spacer layer. Electrochemical doping and dedoping of the polymer spacer layer provides reversible tuning of the cavity's structural color throughout the entire visible range and beyond. Furthermore, the cavity provides high peak reflectance that varies only slightly between the reduced and oxidized states of the polymer. The results indicate that the polymer undergoes large reversible thickness changes upon redox tuning, aided by changes in optical properties and low visible absorption. The electroactive cavity concept may find particular use in reflective displays, by opening for tuneable monopixels that eliminate limitations in brightness of traditional subpixel‐based systems.

Plasmons and excitons can interact to form new hybridized light–matter states, with a multitude of potential applications including optical logic circuits and single-photon switches. Here, we report the first observation of strong coupling based on optically thin plasmonic nanohole films. The absorptive plasmon resonances of these nanohole films lead to suppressed transmission and Fano-shaped extinction peaks. We prepared silver nanohole films by colloidal lithography, which enables large-scale fabrication of nanoholes distributed in a short-range order. When coated with J-aggregate molecules, both extinction and absorption spectra show clear formation of two separated polariton resonances, with vacuum Rabi splitting on the order of 300 meV determined from anticrossing experiments. In accordance with strong coupling theory, the splitting magnitude increases linearly with the square root of molecular concentration. The extinction peak positions are blue-shifted from the absorption polariton positions, as explained by additional Fano interference between the hybridized states and the metal film. This highlights that absorption measurements are important not only to prove strong coupling but also to correctly determine hybridized polariton positions and splitting magnitudes in hybrid plasmonic nanohole systems. The polariton absorption peaks also show strong dependence on illumination direction, as found related to inherent directionality of the plasmonic nanohole metasurface and differences in light interaction with nonhybridized molecules. Importantly, optical simulations could successfully reproduce the experimental results and all coupling features. Furthermore, simulated spatial distribution of the absorption provides additional evidence of strong coupling in the hybrid nanohole system. The work paves the way toward strong coupling applications based on optically thin nanohole systems, as further promoted by the scalable fabrication.

Certain bird species have evolved spectacular colors that arise from organized nanostructures of melanin. Its high refractive index (∼1.8) and broadband absorptive properties enable vivid structural colors that are nonsusceptible to photobleaching. Mimicking natural melanin structural coloration could enable several important applications, in particular, for noniridescent systems with colors that are independent of incidence angle. Here, we address this by forming melanin photonic crystal microdomes by inkjet printing. Owing to their curved nature, the microdomes exhibit noniridescent vivid structural coloration, tunable throughout the visible range via the size of the nanoparticles. Large-area arrays (>1 cm 2 ) of high-quality photonic microdomes could be printed on both rigid and flexible substrates. Combined with scalable fabrication and the nontoxicity of melanin, the presented photonic microdomes with noniridescent structural coloration may find use in a variety of applications, including sensing, displays, and anticounterfeit holograms.