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).

Two-dimensional (2D) transition metal carbides and/or nitrides (MXenes) are a new class of 2D materials, with extensive opportunities for property tailoring due to the numerous possibilities for varying chemistries and surface terminations. Here, Ti2AlC and Nb2AlC MAX phase epitaxial thin films were deposited on sapphire substrates by physical vapor deposition. The films were then etched in LiF/HCl solutions, yielding Li-intercalated, 2D Ti2CTz and Nb2CTz films, whose terminations, transport and optical properties were characterized. The former exhibits metallic conductivity, with weak localization below 50 K. In contrast, the Nb-based film exhibits an increase in resistivity with decreasing temperature from RT down to 40 K consistent with variable range hopping transport. The optical properties of both films were determined from spectroscopic ellipsometry in the 0.75 to 3.50 eV range. The results for Ti2CTz films confirm the metallic behavior. In contrast, no evidence of metallic behavior is observed for the Nb2CTz film. The present work therefore demonstrates that one fruitful approach to alter the electronic and optical properties of MXenes is to change the nature of the transition metal.

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.

We report on the development of the first integrated mid-infrared, far-infrared and terahertz optical Hall effect instrument, covering an ultra wide spectral range from 3 cm −1 to 7000 cm −1 (0.1-210 THz or 0.4-870 meV). The instrument comprises four sub-systems, where the magneto-cryostat-transfer sub-system enables the usage of the magneto-cryostat sub-system with the mid-infrared ellipsometer sub-system, and the far-infrared/terahertz ellipsometer sub-system. Both ellipsometer sub-systems can be used as variable angleof-incidence spectroscopic ellipsometers in reflection or transmission mode, and are equipped with multiple light sources and detectors. The ellipsometer sub-systems are operated in polarizer-sample-rotating-analyzer configuration granting access to the upper left 3 × 3 block of the normalized 4 × 4 Mueller matrix. The closed cycle magneto-cryostat sub-system provides sample temperatures between room temperature and 1.4 K and magnetic fields up to 8 T, enabling the detection of transverse and longitudinal magnetic field-induced birefringence. We discuss theoretical background and practical realization of the integrated mid-infrared, far-infrared and terahertz optical Hall effect instrument, as well as acquisition of optical Hall effect data and the corresponding model analysis procedures. Exemplarily, epitaxial graphene grown on 6H -SiC, a tellurium doped bulk GaAs sample and an AlGaN/GaN high electron mobility transistor structure are investigated. The selected experimental datasets display the full spectral, magnetic field and temperature range of the instrument and demonstrate data analysis strategies. Effects from free charge carriers in two dimensional confinement and in a volume material, as well as quantum mechanical effects (inter-Landau-level transitions) are observed and discussed exemplarily.

The optical Hall effect is a physical phenomenon that describes the occurrence of magnetic-field-induced dielectric displacement at optical wavelengths, transverse and longitudinal to the incident electric field, and analogous to the static electrical Hall effect. The electrical Hall effect and certain cases of the optical Hall effect observations can be explained by extensions of the classic Drude model for the transport of electrons in metals. The optical Hall effect is most useful for characterization of electrical properties in semiconductors. Among many advantages, while the optical Hall effect dispenses with the need of electrical contacts, electrical material properties such as effective mass and mobility parameters, including their anisotropy as well as carrier type and density, can be determined from the optical Hall effect. Measurement of the optical Hall effect can be performed within the concept of generalized ellipsometry at an oblique angle of incidence. In this paper, we review and discuss physical model equations, which can be used to calculate the optical Hall effect in single- and multiple-layered structures of semiconductor materials. We define the optical Hall effect dielectric function tensor, demonstrate diagonalization approaches, and show requirements for the optical Hall effect tensor from energy conservation. We discuss both continuum and quantum approaches, and we provide a brief description of the generalized ellipsometry concept, the Mueller matrix calculus, and a 4×4 matrix algebra to calculate data accessible by experiment. In a follow-up paper, we will discuss strategies and approaches for experimental data acquisition and analysis.

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 †).

The temperature-dependence of free-charge carrier mobility, sheet density, and effective mass of a two-dimensional electron gas in a AlGaN/GaN heterostructure deposited on SiC substrate is determined using the THz optical Hall effect in the spectral range from 0.22 to 0.32 THz for temperatures from 1.5 to 300 K. The THz optical Hall-effect measurements are combined with room temperature mid-infrared spectroscopic ellipsometry measurements to determine the layer thickness, phonon mode, and free-charge carrier parameters of the heterostructure constituents. An increase of the electron effective mass from (0.22±0.01)m0 at 1.5 K to (0.36±0.03)m0 at 300 K is observed, which is indicative for a reduction in spatial confinement of the two-dimensional electron gas at room temperature. The temperature-dependence of the mobility and the sheet density is in good agreement with electrical measurements reported in the literature.

The effective electron mass parameter in Si-doped Al0.72Ga0.28N is determined to be m* = (0.336 +/- 0.020) m(0) from mid-infrared optical Hall effect measurements. No significant anisotropy of the effective electron mass parameter is found supporting theoretical predictions. Assuming a linear change of the effective electron mass with the Al content in AlGaN alloys and m* = 0.232m(0) for GaN, an average effective electron mass of m* = 0.376m(0) can be extrapolated for AlN. The analysis of mid-infrared spectroscopic ellipsometry measurements further confirms the two phonon mode behavior of the E-1(TO) and one phonon mode behavior of the A(1)(LO) phonon mode in high-Al-content AlGaN alloys as seen in previous Raman scattering studies.

Funding Agencies|National Science Foundation|MRSEC DMR-0820521MRI DMR-0922937DMR-0907475EPS-1004094|Swedish Research Council (VR)|2010-3848|Swedish Governmental Agency for Innovation Systems (VINNOVA)|2011-03486|Linkoping Linnaeus Initiative for Novel Functionalized Materials (VR)||VINNOVA||