Temperature-Dependent Anisotropic Refractive Index in β-Ga2O3: Application in Interferometric Thermometers

An accurate knowledge of the optical properties of β-Ga2O3 is key to developing the full potential of this oxide for photonics applications. In particular, the dependence of these properties on temperature is still being studied. Optical micro- and nanocavities are promising for a wide range of applications. They can be created within microwires and nanowires via distributed Bragg reflectors (DBR), i.e., periodic patterns of the refractive index in dielectric materials, acting as tunable mirrors. In this work, the effect of temperature on the anisotropic refractive index of β-Ga2O3 n(λ,T) was analyzed with ellipsometry in a bulk crystal, and temperature-dependent dispersion relations were obtained, with them being fitted to Sellmeier formalism in the visible range. Micro-photoluminescence (μ-PL) spectroscopy of microcavities that developed within Cr-doped β-Ga2O3 nanowires shows the characteristic thermal shift of red–infrared Fabry–Perot optical resonances when excited with different laser powers. The origin of this shift is mainly related to the variation in the temperature of the refractive index. A comparison of these two experimental results was performed by finite-difference time-domain (FDTD) simulations, considering the exact morphology of the wires and the temperature-dependent, anisotropic refractive index. The shifts caused by temperature variations observed by μ-PL are similar, though slightly larger than those obtained with FDTD when implementing the n(λ,T) obtained with ellipsometry. The thermo-optic coefficient was calculated.This work was supported by MICINN projects (RTI2018-097195-B-I00, RTI2018-096918-B-C41, PID2021-122562NB-I00 and PID2021-123190OB-I00/AEI/10.13039/501100011033/FEDER, UE). The authors acknowledge the financial support of the excellence research network RED2018-102609-T by MINECO. The authors acknowledge the support from the Air Force Office of Scientific Research under Award No. FA8655-20-1-7013 (Program Manager: Ali Sayir). M.A.-O. acknowledges financial support from MICINN (FPU contract No. FPU15/01982) and thanks the Central Research Development Fund (CRDF) of the University of Bremen for funding (ZF04/2021). J.S.M. and J.J. were supported by the Air Force Office of Scientific Research under award number FA9550–21–1–0507, monitored by Dr. Ali Sayir. Any opinions, finding, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the United States Air Force

Characterization of 2D α-MoO3 microcrystals deposited by pulsed laser deposition based process

2D-Transition Metal oxides (TMOs), although less studied than the well-known 2D transition metal dichalcogenides(2D-TMCs) are appealing due to some specific advantages. Like the TMCs they are semiconductors, however they are less contaminant and offer a large potential for tuning their electro-optical properties by varying their stoichiometry. In particular, MoO3 is transparent and shows a wide bandgap (>3 eV) and a high dielectric constant k ~ 500. Additionally, orthorhombic α-MoO3 possesses the well-known layered crystal structure of MoO3 which offers the possibility to create two dimensional (2D) morphologies (de Castro et al., 2020). In this context, 2D MoO3 has shown extraordinary properties such as anisotropic polariton propagation (Ma et al, 2019) and is an ideal material for electronic applications for high power electronics and short wavelength optoelectronics.Depto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEpu

Building nanoplatelet α-MoO3 films: A high quality crystal anisotropic 2D material for integration

The successful development of nanoplatelet α-MoO3 −films with wavelength-dependent in-plane and out-of-plane birefringence both in the visible and in the medium infrared is demonstrated. The films are prepared by a two-step process starting from structurally amorphous and continuous substoichiometric MoO3-X followed by isothermal annealing at low temperature (250 °C) to yield the formation of a dense network of large α-MoO3 2D nanoplatelets lying in-plane with no overlapping. These α-MoO3 crystals form a film with excellent thickness uniformity (20 nm) and are oriented with the [0 1 0] crystallographic direction parallel to the substrate normal. Finally, we report the film anisotropic optical complex refractive index, both parallel and perpendicular to the plane the full spectral range from 0.7 to 5 eV as determined by spectroscopic ellipsometry, and their characteristic in-plane phonon mode in the IR from FTIR measurements. These results show a promising pathway to the creation of highly functional anisotropic α-MoO3 2D films suitable for the development of integrated nanostructured photonic components.Agencia Estatal de InvestigaciónEuropean Regional Development FundCSICMCIN/AEI/10.13039/501100011033Universidad Nacional Autónoma de MéxicoEuropean UnionDepto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEpu