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Remote temperature sensing at the micro- and nanoscale is key in fields such as photonics, electronics, energy, or biomedicine, with optical properties being one of the most used transducing mechanisms for such sensors. Ga_2O_3presents very high chemical and thermal stability, as well as high radiation resistance, becoming of great interest to be used under extreme conditions, for example, electrical and/or optical high-power devices and harsh environments. In this work, a luminescent and interferometric thermometer is proposed based on Fabry-Perot (FP) optical microcavities built on Cr-doped Ga_2O_3 nanowires. It combines the optical features of the Cr^(3+) -related luminescence, greatly sensitive to temperature, and spatial confinement of light, which results in strong FP resonances within the Cr^(3+) broad band. While the chromium-related R lines energy shifts are adequate for low-temperature sensing, FP resonances extend the sensing range to high temperatures with excellent sensitivity. This thermometry system achieves micron-range spatial resolution, temperature precision of around 1 K, and a wide operational range, demonstrating to work at least in the 150-550 K temperature range. Besides, the temperature-dependent anisotropic refractive index and thermo-optic coefficient of this oxide have been further characterized by comparison to experimental, analytical, and finite-difference time-domain simulation results.

8 pags.; 8 figs., Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. The light emission properties of anatase TiO2/Eu3+ thin films prepared by electrophoretic deposition and of the corresponding precursor nanoparticulate sols and xerogels with different Eu3+ contents are reported. Transparent anatase TiO2 thin films were successfully obtained by electrophoretic deposition after dilution of the original sols with ethanol in order to inhibit the water hydrolysis and facilitate the drying process. The topography of the films shows a high homogeneous distribution in terms of grain size and roughness of the surface of the thin films. The film thickness varied from 50 nm to 800 nm and it was found to be strongly dependent on the Eu3+ content of the sols, probably because of uncompensated electrical charges provided by Eu3+ ions located at the surface of the nanoparticles. A spontaneous densification of the films occurs at a thickness above 400 nm and, concomitantly, a significant increase of the Eu3+ emission efficiency is detected. Transparent and nanoparticulate anatase TiO2/Eu3+ thin films were obtained by electrophoretic deposition. A densification of the films occurs at a thickness above 400 nm and, concomitantly, a significant increase of the Eu3+ emission efficiency is detected. Similar values of PL efficiency to those of the xerogels (bulk material) could be achieved., This study has been supported by the Spanish Ministry of Economy and Competitiveness (MINECO) through projects MAT2012- 31090, TEC2012-38901-C02-01 and MAT2012-37276-C03-01. M. B. thanks the Consejo Superior de Investigaciones Científicas (CSIC) for the concession of a grant (JAE-Pre 083).

The title compounds are prepared by slow evaporation of citric acid solutions of stoichiometric amounts of Ln2O3 (Ln: Nd, Sm, Eu, Gd, Y, Er, Lu), Cu(NO3)2, and MnCO3, followed by heating at 600 °C for 12 h and reaction with KClO4 (Au capsule, 2 GPa, 1000 °C, 1 h).