Electron effective mass is a fundamental material parameter defining the free charge carrier transport properties, but it is very challenging to be experimentally determined at high temperatures relevant to device operation. In this work, we obtain the electron effective mass parameters in a Si-doped GaN bulk substrate and epitaxial layers from terahertz (THz) and mid-infrared (MIR) optical Hall effect (OHE) measurements in the temperature range of 38-340 K. The OHE data are analyzed using the well-accepted Drude model to account for the free charge carrier contributions. A strong temperature dependence of the electron effective mass parameter in both bulk and epitaxial GaN with values ranging from (0.18 +/- 0.02) m(0) to (0.34 +/- 0.01) m(0) at a low temperature (38 K) and room temperature, respectively, is obtained from the THz OHE analysis. The observed effective mass enhancement with temperature is evaluated and discussed in view of conduction band nonparabolicity, polaron effect, strain, and deviations from the classical Drude behavior. On the other hand, the electron effective mass parameter determined by MIR OHE is found to be temperature independent with a value of (0.200 +/- 0.002) m(0). A possible explanation for the different findings from THz OHE and MIR OHE is proposed. (c) 2024 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/)

Being able to dynamically shape light at the nanoscale is oneof the ultimate goals in nano-optics1. Resonant light–matterinteraction can be achieved using conventional plasmonicsbased on metal nanostructures, but their tunability is highlylimited due to a fixed permittivity2. Materials with switchablestates and methods for dynamic control of light–matterinteraction at the nanoscale are therefore desired. Here weshow that nanodisks of a conductive polymer can supportlocalized surface plasmon resonances in the near-infraredand function as dynamic nano-optical antennas, with their resonancebehaviour tunable by chemical redox reactions. Theseplasmons originate from the mobile polaronic charge carriersof a poly(3,4-ethylenedioxythiophene:sulfate) (PEDOT:Sulf)polymer network. We demonstrate complete and reversibleswitching of the optical response of the nanoantennasby chemical tuning of their redox state, which modulatesthe material permittivity between plasmonic and dielectricregimes via non-volatile changes in the mobile chargecarrier density. Further research may study different conductivepolymers and nanostructures and explore their usein various applications, such as dynamic meta-optics andreflective displays.

We present a terahertz (THz) frequency-domain spectroscopic ellipsometer design that suppresses formation of standing waves by use of stealth technology approaches. The strategy to suppress standing waves consists of three elements geometry, coating, and modulation. The instrument is based on the rotating analyzer ellipsometer principle and can incorporate various sample compartments, such as a superconducting magnet, in situ gas cells, or resonant sample cavities, for example. A backward wave oscillator and three detectors are employed, which permit operation in the spectral range of 0.1–1 THz (3.3–33 cm−1 or 0.4–4 meV). The THz frequency-domain ellipsometer allows for standard and generalized ellipsometry at variable angles of incidence in both reflection and transmission configurations. The methods used to suppress standing waves and strategies for an accurate frequency calibration are presented. Experimental results from dielectric constant determination in anisotropic materials, and free charge carrier determination in optical Hall effect (OHE), resonant-cavity enhanced OHE, and in situ OHE experiments are discussed. Examples include silicon and sapphire optical constants, free charge carrier properties of two-dimensional electron gas in a group III nitride high electron mobility transistor structure, and ambient effects on free electron mobility and density in epitaxial graphene.

The effect of a tunable, externally coupled Fabry-Perot cavity to resonantly enhance the optical Hall effect signatures at terahertz frequencies produced by a traditional Drude-like two-dimensional electron gas is shown and discussed in this Letter. As a result, the detection of optical Hall effect signatures at conveniently obtainable magnetic fields, for example, by neodymium permanent magnets, is demonstrated. An AlInN/GaN-based high-electron mobility transistor structure grown on a sapphire substrate is used for the experiment. The optical Hall effect signatures and their dispersions, which are governed by the frequency and the reflectance minima and maxima of the externally coupled Fabry-Perot cavity, are presented and discussed. Tuning the externally coupled Fabry-Perot cavity strongly modifies the optical Hall effect signatures, which provides a new degree of freedom for optical Hall effect experiments in addition to frequency, angle of incidence, and magnetic field direction and strength. (C) 2015 Optical Society of America

Precise manipulation of light-matter interaction has enabled a wide variety of approaches to create bright and vivid structural colours. 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 significantly impede their further development towards flexible, large-scale, and switchable devices compatible with facile and cost-effective production. Herein, we present a simple and efficient method to generate structural colours based on nanoscale conducting polymer films prepared on metallic surfaces via vapour 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 colours from violet to red. Together with greyscale photomasks this enables fabrication of high-resolution colour images using single exposure steps. We further demonstrate spatiotemporal tuning of the structurally coloured surfaces and images via electrochemical modulation of the polymer redox state. The simple structure, facile fabrication, wide colour gamut, and dynamic colour tuning make this concept competitive for future multi-functional and smart displays.