Laser repetition rate in time-domain thermoreflectance techniques

Abstract

Pump-probe thermoreflectance techniques have been utilized extensively to measure the thermal properties; such as, thermal conductivity and interfacial conductances of thin film systems. While there have been many implementations of transient measurements, the most commonly operated systems utilize a near-IR femtosecond pulsed system as the fundamental laser source. The repetition rate of these femtosecond pulsed systems can vary from kHz to MHz. Thus, we perform an in-depth comparison of the implementation and analysis of low-repetition rate and high-repetition rate time-domain thermoreflectance (TDTR), demonstrating that despite varying the repetitionrate, the measured thermal properties are consistent. To illustrate the universality of these systems, we selected a range of materials to study on both TDTR setups , including the thermal conductivity of a SiC film, a SiO 2 film, and the interfacial conductances of Al/SiO 2 and Al/SiC systems.

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“…As shown in experimental schematic of TDTR ( Fig. 2a), a laser beam started from laser source is split into two laser beams [46]. One is a high energy pump pulsed laser used to heat the sample and the other is lower energy probe beam for probing the change in reflectance of laser beam induced by pump to measure surface temperature of the sample.…”

“…One is a high energy pump pulsed laser used to heat the sample and the other is lower energy probe beam for probing the change in reflectance of laser beam induced by pump to measure surface temperature of the sample. The pump laser is modulated on the high order of frequencies and focused on the sample [46]. The probing laser directly enters a mechanical delay stage that adjusts the relative optical path between the pump laser and the probe laser and occurs time delay.…”

“…The probing laser directly enters a mechanical delay stage that adjusts the relative optical path between the pump laser and the probe laser and occurs time delay. After the probing laser is focused on same spot with pump laser, then the reflected probing laser enters the detector [46]. The voltage output from the lock-in amplifier is proportional to the change in sample reflectivity, which is dependent on temperature of sample.…”

“…The voltage output from the lock-in amplifier is proportional to the change in sample reflectivity, which is dependent on temperature of sample. If the voltage output is recorded when the time delay is changed, the recording gives the sample reflectivity depending on pulse-probe time delay [46]. Thus, the surface temperature of the sample is monitored by analyzing the reflected laser beam.…”

Tutorial: Time-domain thermoreflectance (TDTR) for thermal property characterization of bulk and thin film materials

“…A schematic diagram of a typical TDTR setup is shown in Figure 1(b), while similar systems can also be found in some other References. 23,25,39,95,96 In a typical TDTR setup, a mode-locked Ti:sapphire laser oscillator is used as the light source, which generates a train of 150 fs laser pulses at an 80-MHz repetition rate, with wavelengths centered around 800 nm. Other types of pulsed lasers may also be used as the light source, such as a dye laser with wavelengths centered at 632 nm 97 and a Yb:doped fiber laser with wavelengths centered at 1030 nm 98 .…”