What is Thermoreflectance?

by Dr. Patrick Hopkins

About Thermoreflectance

Over the past few decades, modulated thermoreflectance techniques have been used as a tool to back out the thermal properties of materials based on measurements of the thermoreflectance change on the surface of a sample. In a typical modulated pump-probe thermoreflectance experiment, a pump beam is used to thermally excite the surface of the sample at some frequency, f, while the change in reflectivity of a separate probe beam is related to the temperature change on the surface as function of either f, or in the case of short pulsed-based experiments, the time delay between the pump and probe pulses. In a typical modulated thermoreflectance experiment, the surface of a sample is coated with a metal, which acts as a reference thermal mass in which the change in reflectivity is directly related to the change in temperature (i.e., the detected change in reflectivity at frequency f is the thermoreflectivity).

In commonly employed pump-probe thermoreflectance experiments, a pump beam is used to thermally excite the surface of the sample at some frequency, f. This leads to a change in temperature of the material, and since the optical properties of materials are related to temperature, the change in the optical properties of the material due to this modulated temperature change from the pump can be used as a probe of thermal properties. In practice, the change in the intensity of a reflected probe beam is related to the temperature change on the surface.

In a typical modulated pump-probe thermoreflectance experiment such time- or frequency-domain thermoreflectance (TDTR or FDTR, respectively), the change in the intensity of the reflected probe beam is related to the temperature change on the surface as a function of either frequency of the pump (in FDTR) or the delay time between the pump and probe pulses in the case of short pulsed-based experiments such as TDTR.

Where TDTR utilizes short, typically subpicosecond, pulses to monitor the thermoreflectance decay as a function of delay time after pump pulse heating as well as the phase shift induced from the modulated temperature change at f, FDTR can utilize a variety of pulsed or continuous wave (cw) lasers to monitor the phase shift in thermoreflectance signals solely as a function of f . When f becomes low enough, the material of interest will reach steady-state conditions during periods of the modulation event. In this regime, a third technique has recently emerged. “Steady-State Thermoreflectance” (SSTR) operates like FDTR only in the low frequency limit, monitoring the thermoreflectance of the surface at increasing pump powers and inducing a Fourier-like response in the material. Ultimately, SSTR offers a direct measure the thermal conductivity of materials via optical pump-probe metrologies, where as the transient nature of the temperature in FDTR and TDTR usually result in these techniques measuring a combination of both thermal conductivity and heat capacity of the material. The characteristic pump excitations and responses for each of these techniques are presented in the figure below.

In addition to their noncontact nature, these optical metrologies are advantageous relative to many other thermometry platforms since TDTR, FDTR and SSTR can probe relatively small volumes and near-surface regions. The thermal penetration depth (i.e., the depth beneath the surface in which these techniques measure the thermal properties), δthermal, can be limited to the focused spot size, or much less, depending on the modulation frequency. Furthermore, given that the pump and probe spot sizes can be readily focused to length scales on the order of micrometers, thermoreflectance techniques allow for spatially resolved surface measurements of thermal properties with micrometer-resolution and the ability to create thermal property areal “maps” or “images.”