This method can be applied most notably to thin film materials (up to hundreds of nanometers thick), which have properties that vary greatly when compared to the same materials in bulk.
The idea behind this technique is that once a material is heated up, the change in the reflectance of the surface can be utilized to derive the thermal properties.
The reflectivity is measured with respect to time, and the data received can be matched to a model with coefficients that correspond to thermal properties.
The technique of this method is based on the monitoring of acoustic waves that are generated with a pulsed laser.
This stress build in a localized region causes an acoustic strain pulse.
[1] This temperature increase results in a strain that can be estimated by multiplying it with the linear coefficient of thermal expansion of the film.
Usually, a typical magnitude value of the acoustic pulse will be small, and for long propagation nonlinear effects could become important.
But propagation of such short duration pulses will suffer acoustic attenuation if the temperature is not very low.
To sense the piezo-optic effect of the reflected waves, fast monitoring is required due to the travel time of the acoustic wave and heat flow.
Second-harmonic generation may be utilized to achieve frequency of double or higher.
The output of the laser is split into pump and probe beams by a half-wave plate followed by a polarizing beam splitter leading to a cross-polarized pump and probe.
The probe beam is then focused with a lens onto the same spot on the sample as the pulse.
The reflected probe light is input to a high bandwidth photodetector.
The output is fed into a lock-in amplifier whose reference signal has the same frequency used to modulate the pump.
The voltage output from the lock-in will be proportional to the change in reflectivity (ΔR).
In a typical time-domain thermoreflectance experiment, the co-aligned laser beams have cylindrical symmetry, therefore the Hankel transform can be used to simplify the computation of the convolution of the equation with the distributions of the laser intensities.
is radially symmetric and by the definition of Hankel transform, Since the pump and probe beams used here have Gaussian distribution, the
The probe laser beam measures a weighted average of the temperature
Modeling of data acquired in time-domain thermoreflectance The acquired data from time-domain thermoreflectance experiments are required to be compared with the model.
Through this process of time-domain thermoreflectance, the thermal properties of many materials can be obtained.
Common test setups include having multiple metal blocks connected together in a diffusion multiple, where once subjected to high temperatures various compounds can be created as a result of the diffusion of two adjacent metal blocks.
[5] Lowest thermal conductivity for a thin film of solid, fully dense material (i.e. not porous) was also recently reported with measurements using this method.
The thermoreflected signal is then measured by a photodiode which is connected to a RF lock-in amplifier.