Optical Measurement of Thermal Conductivity Using Fiber Aligned Frequency Domain Thermoreflectance

Fiber aligned frequency domain thermoreflectance (FAFDTR) is a simple noncontact optical technique for accurately measuring the thermal conductivity of thin films and bulk samples for a wide range of materials, including electrically conducting samples. FAFDTR is a single-sided measurement that requires minimal sample preparation and no microfabrication. Like existing thermoreflectance techniques, a modulated pump laser heats the sample surface, and a probe laser monitors the resultant thermal wave via the temperature dependent reflectance of the surface. Via the use of inexpensive fiber coupled diode lasers and common mode rejection, FAFDTR addresses three challenges of existing optical methods: complexity in setup, uncertainty in pump-probe alignment, and noise in the probe laser. FAFDTR was validated for thermal conductivities spanning three orders of magnitude (0.1–100 W/m K), and thin film thermal conductances greater than 10 W/m2 K. Uncertainties of 10–15% were typical, and were dominated by uncertainties in the laser spot size. A parametric study of sensitivity for thin film samples shows that high thermal conductivity contrast between film and substrate is essential for making accurate measurements.