In this work, we investigate dielectric materials for thermoreflectance-based thermal sensing by extracting key optical parameters using temperature-dependent spectroscopic ellipsometry in the midinfrared regime. Leveraging optical phonon resonances, we demonstrate that the thermoreflectance coefficients in infrared-active polar dielectrics rival, and in some cases exceed by an order of magnitude, those observed in commonly used metals that are typically used as temperature transducers in thermoreflectance measurements. We introduce a transducer figure of merit (FOM) that combines pump absorption and probe reflectance modulation at different wavelengths, serving as a design-oriented screening metric for comparing thermoreflectance transducer performance across materials and spectral regions. Our results show that polar materials can exhibit material-level FOM values up to eight times those of metal transducers. To demonstrate practical capability, we perform transient thermoreflectance measurements on a 100-nm thermally grown ${\mathrm{SiO}}_2$ film on silicon. These results position dielectric materials as compelling candidates for next-generation thermal metrology, broadening the design space for optical thermometry, with strong implications for high-resolution thermal mapping and characterization of layered device structures based on phonon probing.

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• Dielectric properties
• Optical & microwave phenomena
• Phonons
• Thermal properties
• Dielectrics
• Insulators
• Ellipsometry
• Infrared techniques
• Optical techniques
• Thermometry
• Thermoreflectance
• Time-resolved infrared spectroscopy