Temperature-dependent infrared ellipsometry of Mo-doped VO2 thin films across the insulator to metal transition.

We present a spectroscopic ellipsometry study of Mo-doped VO2 thin films deposited on silicon substrates for the mid-infrared range. The dielectric functions and conductivity were extracted from analytical fittings of Ψ and Δ ellipsometric angles showing a strong dependence on the dopant concentration and the temperature. Insulator-to-metal transition (IMT) temperature is found to decrease linearly with increasing doping level. A correction to the classical Drude model (termed Drude-Smith) has been shown to provide excellent fits to the experimental measurements of dielectric constants of doped/undoped films and the extracted parameters offer an adequate explanation for the IMT based on the carriers backscattering across the percolation transition. The smoother IMT observed in the hysteresis loops as the doping concentration is increased, is explained by charge density accumulation, which we quantify through the integral of optical conductivity. In addition, we describe the physics behind a localized Fano resonance that has not yet been demonstrated and explained in the literature for doped/undoped VO2 films.

Digital holographic interferometry applied to the investigation of ignition process.

We use the digital holographic interferometry (DHI) technique to display the early ignition process for a butane-air mixture flame. Because such an event occurs in a short time (few milliseconds), a fast CCD camera is used to study the event. As more detail is required for monitoring the temporal evolution of the process, less light coming from the combustion is captured by the CCD camera, resulting in a deficient and underexposed image. Therefore, the CCD's direct observation of the combustion process is limited (down to 1000 frames per second). To overcome this drawback, we propose the use of DHI along with a high power laser in order to supply enough light to increase the speed capture, thus improving the visualization of the phenomenon in the initial moments. An experimental optical setup based on DHI is used to obtain a large sequence of phase maps that allows us to observe two transitory stages in the ignition process: a first explosion which slightly emits visible light, and a second stage induced by variations in temperature when the flame is emerging. While the last stage can be directly monitored by the CCD camera, the first stage is hardly detected by direct observation, and DHI clearly evidences this process. Furthermore, our method can be easily adapted for visualizing other types of fast processes.

A new blue, green and red upconversion emission nanophosphor: BaZrO3:Er,Yb.

Strong Blue, green, and red upconversion emission of Er3+ in nanocrystalline BaZrO3:(Yb3+Er3+) is observed. Powder samples were obtained by a facile hydrothermal process at 100 degrees C. The as synthesized nanocrystallites preserve a stable cubic perovskite phase under subsequent annealing treatment up to 1000 degrees C. No other phase or segregation of other compounds was detected. Crystallites sizes were around 115 nm and well faceted. Under IR excitation in the range between 900 and 1050 nm the Er3+ blue emission was almost not present in single Er3+ doped BaZrO3, whereas it became easily observable when Yb3+ was added as codopant. Besides, both green and red upconversion emission or upconverted signal of Er3+ are enhanced by around three orders of magnitude in comparison with the single Er3+ doped BaZrO3. The strong blue emission presents dependence on both excitation power and excitation wavelength. This is the first time that upconversion emission is observed in BaZrO3. A possible mechanism for the upconversion process that leads to the observed blue, green and red emissions under NIR excitation is suggested based on the experimental results.