Time-domain thermoreflectance (TDTR) is a technique used to characterize the thermal properties of thin films or bulky materials. The measured data can help improve the understanding of transport processes driven by phonons and electrons. In typical TDTR experiments using ultrafast laser sources, a modulated pump pulse and a subsequent time-delayed probe pulse are shone onto the sample surface. The induced thermoreflectance response is then measured as a function of the time delay between the pump and probe pulses. Taking advantage of ultrashort laser pulses and pump-probe detections allows quantities like thermal conductivity or volumetric heat capacity to be measured with ps or even sub-ps time resolution.
Frequency-domain thermoreflectance (FDTR) is a variation of TDTR in which the thermoreflectance signal is measured as a function of the modulation frequency of the pump beam, rather than the delay time between the pump beam and the probe beam. FDTR can measure the same thermal properties as TDTR using either pulsed lasers or CW lasers. The main advantages of CW-FDTR measurements are avoiding expensive and delicate pulsed laser systems and eliminating error-prone mechanically moving parts in the setup. Additionally, proper selection of the modulation frequency is essential for accurate TDTR measurements but less crucial for FDTR.
For both FDTR and TDTR, different heat transfer processes can be investigated by changing the pump beam modulation frequency.
The measured signals in TDTR and FDTR are periodic and remarkably small; therefore a fast and high-quality lock-in amplifier is a key requirement for achieving a large signal-to-noise ratio (SNR). Another attractive measurement approach for the pulsed laser schemes consists in employing fast photodetectors in combination with boxcar averaging. This allows the signal to be recorded only during the short duty cycle of the experiment and therefore excludes the vast amounts of data recording time during which only noise is present.
As a result of the periodic heat flux induced by the pump beam, the probe beam detects the corresponding temperature change through the change in sample reflectance in a time-resolved manner. The pump beam is usually modulated at a frequency in the range 0.2 - 20 MHz using an electro-optic modulator (EOM). It is then directed onto the sample through an objective lens, together with the delayed, collinear probe pulses. A photodetector and a lock-in amplifier then measure the reflected signal. The typical experimental configuration is depicted in Figure 1.
Due to the small signals, TDTR turns out to be technically challenging and requires sensitive electronics and heavy averaging to achieve good SNR.