Hyperbolic metamaterials (HMMs) have attracted much attention because they allow for broadband enhancement of spontaneous emission and imaging below the diffraction limit. However, HMMs with traditional metals as metallic component are not suitable for applications in the infrared spectral range. Using Ga-doped ZnO, we demonstrate monolithic HMMs operating at infrared wavelengths. We identify the material's hyperbolic character by various optical measurements in combination with theoretical calculations. In particular, negative refraction of the extraordinary wave and propagation of light with wave vector values exceeding that of free-space are demonstrated in the entire telecommunication window. These findings reveal a considerable potential for creating novel functional elements at telecommunication wavelengths.

We report on controllable tuning of intersubband transitions in ZnO/Zn0.60Mg0.40O multiple quantum well structures grown by molecular beam epitaxy on sapphire. The transitions from the first to the second electronic energy state within the conduction band are directly observed by infrared spectroscopy. By variation of the quantum well width, the intersubband transition energies are tuned from 290 to 370 meV. The experimental results are in good agreement with theoretical calculations assuming the presence of internal electric fields of 2 MV·cm−1.

Sn‐doped In2 O3 films with free‐electron concentration varied up to 1.7×1021cm−3 are prepared by molecular beam epitaxy. In this way, a metallic Drude‐type dielectric function with a negative real part extending beyond λc= 1050 nm is created. Despite essentially polycrystalline structure of the layers, the plasmonic damping Γ is found not to exceed 70 meV in the entire doping range making excitation of low‐loss surface plasmon polaritons at frequencies fully covering the telecommunication band feasible. A monotonically increasing discrepancy between the carrier concentration obtained from Hall‐effect and the concentration extracted from fitting the optical spectra hints at a change of the band‐structure related parameters of In2 O3 with increasing Sn‐doping.

(In1−xErx)2O3 ternary alloys are grown on A‐plane sapphire wafers by plasma‐assisted molecular beam epitaxy. The layers crystallize in the cubic bixbyite structure with true〈111true〉 axis aligned parallel to the substrate normal. The d‐spacing of the {111} lattice planes linearly increases with increasing the Er molar fraction in accordance with Vegard's law. Incorporation of Er in the In2O3 matrix is accompanied by the widening of the optical gap and linear increase of the peak absorption coefficient reaching 380 cm−1 at the telecommunication wavelength of 1.54 µm. The samples exhibit Er3+‐related emission at indirect excitation via the crystalline host at room temperature.