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The optical conductivity ${\sigma }_1\left(\omega \right)$ and the spectral weight $SW$ of four topological insulators with increasing chemical compensation (${\mathrm{Bi}}_2{\mathrm{Se}}_3,{\mathrm{Bi}}_2{\mathrm{Se}}_2\mathrm{Te},{\mathrm{Bi}}_{2-x}{\mathrm{Ca}}_x{\mathrm{Se}}_3$, and ${\mathrm{Bi}}_2{\mathrm{Te}}_2\mathrm{Se}$) have been measured from 5 to 300 K and from subterahertz to visible frequencies. The effect of compensation is clearly observed in the infrared spectra through the suppression of an extrinsic Drude term and the appearance of strong absorption peaks that we assign to electronic transitions among localized states. From the far-infrared spectral weight $SW$ of the most compensated sample (${\mathrm{Bi}}_2{\mathrm{Te}}_2\mathrm{Se}$), one can estimate a density of charge carriers on the order of ${10}^{17}/{\mathrm{cm}}^3$ in good agreement with transport data. Those results demonstrate that the low-energy electrodynamics in single crystals of topological insulators, even at the highest degree of compensation presently achieved, is still influenced by three-dimensional charge excitations.

Transmission measurements have been performed in reduced ${\mathrm{SrTiO}}_3$ between 300 and 17 K for different concentrations of oxygen vacancies. A midinfrared absorption band has been observed, whose behavior with doping and temperature is accounted for by an intraband scattering process, phonon or disorder assisted. Connections with the family of high-${\mathit{T}}_{\mathit{c}}$ superconducting oxides are demonstrated.

Low-resolution transmittance and reflectance spectra of high-quality chemical-vapor deposition (CVD) diamond windows were measured in the infrared in the 2.5–500-μm wavelength range (20–4000 cm^-1). High-resolution measurements on a window with nearly parallel surfaces show well defined interference fringes at low frequencies. By standard procedures the optical constants n and k of CVD diamond were determined, for the first time to the author’s knowledge, in the far-infrared region. It is shown that a window with a large wedge angle, close to 1°, does not produce appreciable interference fringes. Modeling of these results confirms that interference fringes can be avoided by use of properly wedged CVD diamond windows. This result is of considerable relevance to the use of CVD diamond windows in spectroscopic applications for which fringe suppression is a major requirement.

Polaronic features similar to those previously observed in the photoinduced spectra of cuprates have been detected in the reflectivity spectra of chemically doped parent compounds of high-critical-temperature superconductors, both n type and p type. In ${\mathrm{Nd}}_2$ ${\mathrm{CuO}}_{4\mathrm{-}\mathit{y}}$ these features, whose intensities depend both on doping and temperature, include local vibrational modes in the far infrared and a broad band centered at ∼ 1000 ${\mathrm{cm}}^{\mathrm{-}1}$. The latter band is produced by the overtones of two (or three) local modes and is well described in terms of a small-polaron model, with a binding energy of about 500 ${\mathrm{cm}}^{\mathrm{-}1}$. Most of the above infrared features are shown to survive in the metallic phase of ${\mathrm{Nd}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Ce}}_{\mathit{x}}$ ${\mathrm{CuO}}_{4\mathrm{-}\mathit{y}}$, ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CuO}}_6$, and ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathit{y}}$, where they appear as extra-Drude peaks. The occurrence of polarons is attributed to local modes strongly coupled to carriers, as shown by a comparison with tunneling results. © 1996 The American Physical Society.