Limitations and Advances in Optical Thermal Transport Measurements: Extremes in Properties, Length Scales, and Temperature

Conductive and radiative thermal transport play a critical role in the design, development, and performance of a wide array of technologies and applications. In this review, we focus on the challenges associated with nano- and microscale thermal measurements and the strategies developed thus far to overcome them. For measurements below ∼1,000°C, numerous thermoreflectance techniques are already in wide use; however, uncertainty and measurement error may limit the measurement of samples in certain regimes. These regimes include materials of high thermal conductivity (≳2,000 W/m·K), thin films (≲100 nm), or interfaces located well below the sample surface. A rigorous treatment of uncertainty and error is thus required for measuring these samples and for the development of future metrology tools. At higher temperatures, pyrometry techniques are being developed; however, several physical and experimental limitations exist. Some methods rely on a known emissivity for the measurement of temperature, and significant radiative transport can introduce error in modeling. Both of these mean that knowledge of spectrally dependent and temperature-dependent emissivity properties may be required.