Temperature-dependent anisotropic refractive index in beta-Ga_(2)O_(3): application in interferometric thermometers

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An accurate knowledge of the optical properties of beta-Ga_(2)O_(3) 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 beta-Ga_(2)O_(3) n(lambda,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 (mu-PL) spectroscopy of microcavities that developed within Cr-doped beta-Ga_(2)O_(3) 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 mu-PL are similar, though slightly larger than those obtained with FDTD when implementing the n(lambda,T) obtained with ellipsometry. The thermo-optic coefficient was calculated.
© 2023 by the authors. Licensee MDPI 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.