The Malen Laboratory

Thermoreflectance Metrology

Frequency Domain Thermoreflectance (FDTR):  FDTR is used in our lab to measure thermal conductivity/conductance in a range of material systems.  We are constantly improving the technique to achieve higher accuracy and deeper understanding of phonon physics.  FDTR is a non-contact optical method that uses lasers to locally heat (spot diameter 10-20 µm) a surface and sense thermal response based on its temperature-dependent reflectance.

An intensity modulated pump laser (488nm) periodically heats the sample surface and a probe laser (532nm) monitors the resultant thermal wave via the thermoreflectance of a thin Au coating (50-100 nm thick). The measured phase lag, relative to the pumped heat flux, of the surface temperature is a function the sample’s thermal transport properties. Phase lag data for the frequency range 0.1 < f < 200 MHz are fit by a multi-layer heat conduction model to determine the thermal properties of the thin film or interface (thickness and thermal properties of the Au film are measured separately).  It is also possible to induce non-diffusive (i.e., quasi-ballistic) behavior with FDTR to discern phonon mean free path dependent contributions to thermal conductivity (i.e., the thermal conductivity accumulation function).  Schematics of FDTR (a) and Broadband FDTR (b), which has an expanded frequency range, are shown below.  For a description of these techniques we recommend Regner, et al.  Review of Scientific Instruments, 2013.  84, 064901.