Accurate and Robust Wide-Range Luminescent Microthermometer Based on ALD-Encapsulated Ga2O3:Cr DBR Microcavities

The high spatial resolution and contactless optical readout capabilities of luminescence thermometry offer significant advantages in numerous fields, including biomedicine, space exploration and optoelectronics. In addition, robust, reproducible, and accurate temperature measurements are essential in these areas. The ultra-wide band gap semiconductor material Ga₂O₃ is a suitable host for optical sensing in harsh environments due to its high stability. In this work, the thermometric operation of Ga₂O₃:Cr-based microcavities are evaluated. They are designed as follows: Ga₂O₃:Cr microwires are encapsulated in multilayers fabricated by atomic layer deposition (ALD), which act as both Bragg reflectors and protective layers for the thermometric sensor. Prior to the ALD encapsulation step, focused ion beam carved trenches at the microwire ends are necessary to accommodate the multilayer coating. The structural and optical properties of the devices are assessed experimentally, analytically and by simulations. The developed microthermometers can be easily calibrated using a cubic polynomial for the temperature-dependent resonant peak position shift. A better than 0.5 °C temperature resolution and accuracy for temperatures above −80 °C is demonstrated. Additionally, the devices show robustness against excitation laser densities of at least 34 W mm⁻², can operate at temperatures up to 600 °C and remain functional in liquids.