Recent advancements in metamaterials and plasmonics have promised a number of exciting applications, in particular at terahertz and optical frequencies. Unfortunately, the noble metals used in these photonic structures are not particularly good conductors at high frequencies, resulting in significant dissipative loss. Here, we address the question of what is a good conductor for metamaterials and plasmonics. For resonant metamaterials, we develop a figure-of-merit for conductors that allows for a straightforward classification of conducting materials according to the resulting dissipative loss in the metamaterial. Application of our method predicts that graphene and high-Tc superconductors are not viable alternatives for metals in metamaterials. We also provide an overview of a number of transition metals, alkali metals and transparent conducting oxides. For plasmonic systems, we predict that graphene and high-Tc superconductors cannot outperform gold as a platform for surface plasmon polaritons, because graphene has a smaller propagation length-towavelength ratio.

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A comparison of graphene, superconductors and metals as conductors for metamaterials and plasmonics

Optical Properties of HfO2 Thin Films Deposited by Magnetron Sputtering: From the Visible to the Far-Infrared

Spatial and temporal coherence of thermal radiation in asymmetric Fabry–Perot resonance cavities

Optical and radiative properties of semiconductor materials and structures are often critical to the functionality and performance of many devices, such as semiconductor lasers, radiation detectors, tunable optical filters, waveguides, solar cells, selective emitters and absorbers, etc. This chapter reviews the optical and thermal radiative properties of semiconductor materials related to the recent technological advancements that are playing a vital role in the integrated-circuit manufacturing, optoelectronics, and radiative energy conversion devices. Some fundamental aspects related to important micro/nanoscale processes in semiconductor optoelectronics are presented. These include the electronic band structure, energy gap, interband and intraband transitions, free-carrier absorption, optical and acoustic phonons, and the effects of impurities and temperature. Theoretical and experimental studies on the radiative properties of thin films and multilayer systems, rough surfaces, and nanostructured surfaces are summarized. Quantum confinement in nanomaterials is described, followed by a review of the radiative properties of photonic crystals and porous silicon. Potential applications and future developments are outlined.

Optical and thermal radiative properties of semiconductors related to micro/nanotechnology

Spectral and directional control of thermal radiation is a challenging yet important task for a number of applications, including thermophotovoltaic energy conversion, solar energy utilization, space thermal management, and high-efficiency incandescent lamps. A number of structures have been proposed as coherent emission sources, with spectral emission peaks in well-defined directions. The grating structure has been commonly used in which coherent emission is achieved by the excitation of surface polaritons or surface waves. Along with the development of metamaterials, magnetic polaritons have also received much attention lately and have been demonstrated to be useful for controlling the radiative properties of periodic structures. Furthermore, planar structures can also support surface waves or optical resonance for thermal emission control, such as the truncated one-dimensional photonic crystals and asymmetric Fabry–Perot structures. This article provides an overview of theoretical and experimental research activities, performed in the Nanoscale Thermal Radiation Lab at the Georgia Institute of Technology, on tailoring the spectral and directional thermal radiative properties.

Measurements and Modeling of the Spectral and Directional Radiative Properties of Micro/Nanostructured Materials

The transmittance and reflectance of superconductive YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} (YBCO) thin films deposited on Si substrates have been measured in the far-infrared frequency region from 10 to 100 cm{sup {minus}1} (wavelength from 1,000 to 100 {micro}m) at temperatures between 10 and 300 K. The effects of interference, optical resonance, and antireflection on the radiative properties of high-temperature superconducting (HTSC) films are observed and quantitatively analyzed. Furthermore, the authors have measured the reflectance of the HTSC film-substrate composites for radiation incident on the substrate side (backside reflectance) for the first time. The backside reflectance increases significantly from the normal state to the superconducting state at certain frequencies; this experimentally demonstrates that HTSC films can be used to build far-infrared intensity modulators. The complex refractive index of the YBCO films is determined from the measured transmittance using the Drude model in the normal state and a two-fluid model in the superconducting state. The complex refractive index obtained from this study is useful for various applications of YBCO films, including radiation modulators, detectors, and Fabry-Perot resonators.

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Far-Infrared Transmittance and Reflectance of YBa2Cu3O7-δ Films on Si Substrates

The transmittance and reflectance of superconductive YBa 2 Cu 3 O 7-δ (YBCO) thin films deposited on Si substrates have been measured in the far-infrared frequency region from 10 to 100 cm -1 (wavelength from 1000 to 100 μm) at temperatures between 10 and 300 K. The effects of interference, optical resonance, and antireflection on the radiative properties of high-temperature superconducting (HTSC) films are observed and quantitatively analyzed. Furthermore, we have measured the reflectance of the HTSC film-substrate composites for radiation incident on the substrate side (backside reflectance) for the first time. The backside reflectance increases significantly from the normal state to the superconducting state at certain frequencies; this experimentally demonstrates that HTSC films can be used to build far-infrared intensity modulators. The complex refractive index of the YBCO films is determined from the measured transmittance using the Drude model in the normal state and a two-fluid model in the superconducting state. The complex refractive index obtained from this study is useful for various applications of YBCO films, including radiation modulators, detectors, and Fabry-Perot resonators.

Far-infrared transmittance and reflectance of YBa2Cu3O7-δ films on Si substrates

Fabry-Perot resonators were built from two superconductive YBa 2 Cu 3 O 7-δ (YBCO) films separated by a spacer Each film of 35-nm thickness was deposited on a Si substrate, about 204 μm thick A slow-scan Michelson interferometer was employed to measure the transmittance of the resonator in the far-infrared frequency region from 10 to 90 cm -1 at temperatures between 10 and 300 K Measurements showed that in the normal state the peak (or resonant) transmittance decreases as temperature is lowered, whereas in the superconducting state it can increase with decreasing temperature The transmittance of the resonator was calculated using properties of individual reflectors obtained previously When the effect of partial coherence is taken into consideration, the calculated transmittance is in good agreement with the experiments Furthermore, the maximum possible resonant transmittance was predicted based on an optimization analysis in which the cavity length is varied The effect of the YBCO film thickness on the transmittance peaks was also studied, showing that the resonant transmittance decreases but the finesse increases as the film thickness is increased This study should help improve the future design of Fabry-Perot resonators based on HTSC thin films

Fabry-Perot Resonators Built With YBa2Cu3O7−δ Films on Si Substrates

We have measured the far-infrared reflectance of YBa2Cu3O7−δ high Tc superconducting (HTS) films, for radiation incident on the substrate side (back-side illumination), in the spectral region from 15 to 650 cm−1 and at temperatures between 10 and 300 K. The HTS films were deposited on Si substrates (with YSZ/CeO2 buffer layers) by pulsed laser ablation. The extremely large temperature dependence of the reflectance experimentally demonstrates the feasibility of using HTS films to construct far-infrared intensity modulators. In this letter, we present the measured results, as well as an analysis based on thin-film optics and a simple two-fluid model.

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Back-side reflectance of high Tc superconducting thin films in the far infrared

This work presents the absorptance of high-temperature superconducting YBa2Cu3O7-delta (YBCO) films, deposited on Si substrates, in the far infrared from 15 to 95 cm - 1 (wavelength from 667 to 105 mu m) at temperatures of 100, 200, and 300 K (i.e., in the normal state). Our experiments show a significant difference in the absorptance for radiation incident on the film side as compared to radiation incident on the substrate side. Interference fringes associated with the Si substrate are observed from the measurement and used to analyze the interaction of radiation with the film - substrate composite at the interface. The film thickness is found to have a strong effect on the absorptance of the film - substrate composite, especially for radiation incident on the substrate side.

V. A. Boychev

Papers

Far-Infrared Transmittance and Reflectance of YBa2Cu3O7-δ Films on Si Substrates

Far-infrared transmittance and reflectance of YBa2Cu3O7-δ films on Si substrates

Fabry-Perot Resonators Built With YBa2Cu3O7−δ Films on Si Substrates

Back-side reflectance of high Tc superconducting thin films in the far infrared

TEMPERATURE-DEPENDENT FAR-INFRARED ABSORPTANCE OF THIN YBa2Cu3O7-delta FILMS IN THE NORMAL STATE