Gallium oxide is being widely studied, mainly for high-power electronics applications. It is a very promising material for photonic/optoelectronic applications, such as solar-blind UV detectors and light emitters. In this work, we study the temperature-dependent behavior of the optical properties of microcavities based on luminescent β-Ga2O3:Cr nanowires that emit an intense red-infrared band. Two distributed Bragg reflectors (DBR), when milled with a focused ion beam (FIB) and separated some microns, result in an optical microcavity that confines the light longitudinally. Both chromium R lines and Fabry-Perot spectral resonances (FPSR) are observed to shift as temperature varies, making these DBRs a valuable thermometer in a wide temperature range, due to both luminescent and interferometric transducing mechanisms. The underlying origin of this shift, in the case of the FPSR, is mainly the variation of the refractive index with temperature and the thermal expansion of the cavity. Ellipsometry studies carried out at different temperatures in bulk β-Ga2O3 yielded the dispersion relations for the three main crystal axes, i.e. its temperature-dependent anisotropic refractive index. These results were implemented in finite-difference time-domain (FDTD) simulations to compare the expected spectral shift of the FPSR in the modelled system with the experimental shift in the DBR cavities, as obtained experimentally by micro-photoluminescence. The results from these two approximations, and a third one based on solving the relevant analytical equations, are compared.
In this work, we present our recent results on the applicability of optical microcavities based on Cr doped Ga2O3 wires to operate as a nanothermometer in a wide temperature range (at least from 150 up to 550 K) and achieving a temperature precision of around 1 K. To this purpose, DBR (distributed Bragg reflectors) have been used to enhance the reflectivity at the lateral ends of the wires. The transduction mechanism encompasses both the luminescence features of the characteristic R-lines of Cr3+ ions in this host as well as the interferometric effects of the Fabry-Perot resonances within the cavity.
Ga2O3 micro- and nanowires-based optical microcavities have been obtained by patterning pairs of distributed Bragg reflectors (DBRs) with a focused ion beam (FIB) microscope. DBRs result in widely tunable high reflectivity bands. The microcavities have been designed and optimized with the aid of simulations and optically characterized by micro-photoluminescence. Tunable strong modulations are confirmed in the NUV-blue as well as in the red-NIR ranges for unintentionally doped and chromium doped wires, respectively. Experimental, analytical and simulations results will be compared and some possible applications of these cavities will be assessed.
On one hand, interest on the tunability of the optical microcavities has increased in the last few years due to the need for selective nano- and microscale light sources to be used as photonic building blocks in several applications. On the other, transparent conductive oxide (TCO) β-Ga2O3 is attracting attention in the optoelectronics area due to its ultra wide band gap and high breakdown field. However, at the micro- and nanoscale there are still some challenges to face up, namely the control and tuning of the optical and electrical properties of this oxide. In this work, Cr doped Ga2O3 elongated microwires are grown using the vapor-solid (VS) mechanism. Focused Ion Beam (FIB) etching forms Distributed Bragg Reflector (DBR)-based resonant microcavities. Room temperature microphotoluminescence (μ-PL) spectra show strong modulations in the red-NIR range on five cavities with different lengths. Selectivity of the peak wavelengths is obtained, proving the tunability of this kind of optical systems. The confined modes are analyzed experimentally, analytically and via finite difference time domain (FDTD) simulations. Experimental reflectivities up to 78% are observed.
The synthesis of complex nanostructures that combine materials and dimensionality, promises the ability to identify novel designs and architectures with enhanced properties that could be used in new devices. One of the building blocks in nanomaterials are nanowires, which offer several possibilities to get complex nanostructures. We present two kinds of morphologies based on oxide nanowires obtained by a thermal evaporation method. The common feature of both morphologies is a central oxide nanowire and, depending on the growth parameters, nanowires with either nanocrystallites or nano/microrods attached to the central wire are obtained. We have previously reported the fabrication of several single oxide nanowires and in particular, gallium oxide (β-Ga2O3) and zinc germanate oxide (Zn2GeO4) nanowires. Here we report the shape evolution of these nanowires by the suitable modification of the growth parameters. The addition of tin oxide (SnO2) to the precursors and variation of the thermal treatments duration result in the formation of the above-mentioned complex nanostructures. Structural and chemical characterizations were performed by electron microscopy techniques and Raman spectroscopy. The results shed light on the understanding of the driving mechanisms that lead to the formation of complex oxide nanostructures.
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