Frequency Domain Thermoreflectance (FDTR) utilises a continuous pump laser that is modulated, using a lock-in amplifier, to transiently heat a sample of interest.
The oscillations in sample temperature are not precisely aligned with the pump laser modulation frequency (fmod), as there is an intrinsic phase-delay in the temperature response of the sample.
This shift in phase between the pump laser modulation and sample temperature is dependant on the thermal properties of the sample.
An unmodulated continuous wave probe laser observes the variations in phase shift between pump laser and sample temperature, as the reflected probe laser flux oscillations replicate the sample temperature oscillations.
The thermal penetration depth of the pump laser is dependant on its modulation frequency, a phase-shift measurement acquired using a low pump laser modulation frequency will heat the sample at greater depths than a measurement acquired using a higher pump laser modulation frequency.
Acquiring phase shifts through a range of modulation frequencies allows depth sensitive measurement of multi-layered samples to be performed, making Frequency Domain Thermoreflectance an ideal technique for thermal characterization for bulk materials and multi-layered structures alike.