The development of artificially structured electromagnetic materials, termed metamaterials, has led to the realization of phenomena that cannot be obtained with natural materials¹. This is especially important for the technologically relevant terahertz (1 THz = 10¹² Hz) frequency regime; many materials inherently do not respond to THz radiation, and the tools that are necessary to construct devices operating within this range—sources, lenses, switches, modulators and detectors—largely do not exist. Considerable efforts are underway to fill this ‘THz gap’ in view of the useful potential applications of THz radiation^2,3,4,5,6,7. Moderate progress has been made in THz generation and detection⁸; THz quantum cascade lasers are a recent example⁹. However, techniques to control and manipulate THz waves are lagging behind. Here we demonstrate an active metamaterial device capable of efficient real-time control and manipulation of THz radiation. The device consists of an array of gold electric resonator elements (the metamaterial) fabricated on a semiconductor substrate. The metamaterial array and substrate together effectively form a Schottky diode, which enables modulation of THz transmission by 50 per cent, an order of magnitude improvement over existing devices¹⁰.

Atomic substitution in alloys can efficiently scatter phonons, thereby reducing the thermal conductivity in crystalline solids to the “alloy limit.” Using ${\mathrm{In}}_{0.53}{\mathrm{Ga}}_{0.47}\mathrm{As}$ containing ErAs nanoparticles, we demonstrate thermal conductivity reduction by almost a factor of 2 below the alloy limit and a corresponding increase in the thermoelectric figure of merit by a factor of 2. A theoretical model suggests that while point defects in alloys efficiently scatter short-wavelength phonons, the ErAs nanoparticles provide an additional scattering mechanism for the mid-to-long-wavelength phonons.

In this paper, we explore electron filtering as a technique to increase the Seebeck coefficient and the thermoelectric power factor of heterostructured materials over that of the bulk. We present a theoretical model in which the Seebeck coefficient and the power factor can be increased in an ${\mathrm{In}}_{0.53}{\mathrm{Ga}}_{0.47}\mathrm{As}$-based composite material. Experimental measurements of the cross-plane Seebeck coefficient are presented and confirm the importance of the electron filtering technique to decouple the electrical conductivity and Seebeck coefficient to increase the thermoelectric power factor.

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The $3\omega$ technique and time-domain thermoreflectance (TDTR) are two experimental methods capable of measuring the cross-plane thermal conductivity of thin films. We compare the cross-plane thermal conductivity measured by the $3\omega$ method and TDTR on epitaxial ${\left({\text{In}}_{0.52}{\text{Al}}_{0.48}\right)}_x{\left({\text{In}}_{0.53}{\text{Ga}}_{0.47}\right)}_{1-x}\text{As}$ alloy layers with embedded ErAs nanoparticles. Thermal conductivities measured by TDTR at low modulation frequencies $\left(\sim 1\text{MHz}\right)$ are typically in good agreement with thermal conductivities measured by the $3\omega$ method. We discuss the accuracy and limitations of both methods and provide guidelines for estimating uncertainties for each approach.

The effect of nanoparticles on the thermoelectric power factor is investigated using the relaxation time approximation. The partial-wave technique is used for calculating the nanoparticle scattering cross section exactly. We validate our model by comparing its results to the experimental data obtained for ErAs:InGaAlAs samples. We use the theory to maximize the power factor with respect to nanoparticle and electron concentrations as well as the barrier height. We found that at the optimum of the power factor, the electron concentration is usually higher in the sample with nanoparticles, implying that Seebeck is usually unchanged and conductivity is increased.