Novel technique for measuring the thermal diffusivity of thin films

Published in Rev. Sci. Instrum. as Editor’s Pick.

A paper recently published in the journal ‘Review of Scientific Instruments’ introduces a new all-optical technique for measuring thermal diffusion in nanoscale systems. The study was conducted by a team of researchers led by Prof. Klaas-Jan Tielrooij, senior group leader at ICN2. This “pre-time-zero spatiotemporal pump-probe microscopy” has several important benefits over existing techniques and provides an excellent tool for studying heat transport at the nanoscale.

Thermal transport in materials plays a critical role in many technological applications in electronics, optoelectronics, energy conversion, and more. Overheating is actually a major concern for many devices, since it can hinder their proper operation. Therefore, understanding the heat flow at the nanoscale is crucial to optimizing device performance. There are several optical techniques available for measuring heat transport, but they often require knowledge of multiple parameters of the material under study, and strong heating of the sample. 

Researchers from the ICN2 Ultrafast Dynamics in Nanoscale Systems Group, led by Prof. Klaas-Jan Tielrooij, have developed a novel all-optical technique for measuring thermal transport properties of thin films with high sensitivity (down to one degree) and without the need for any material input parameters. This work has been described in a paper recently published in Review of Scientific Instruments and selected as an Editor’s Pick.As explained in the article, the authors demonstrate the advantages of this technique using various semiconducting 2D materials –MoSe2, WSe2, MoS2 and WS2– as test cases.

The technique is based on spatiotemporal pump-probe microscopy, in which a fixed laser beam (the pump pulse) is used to excite the material and a second one (the probe) is used to scan the sample spatially and in time –i.e., at variable time delays after the excitation. Phonon heat diffusion is a slow process (as compared to electronic diffusion), but the diffusing heat produced by the pump laser is long-lived, so its effects can be observed at larger delays, as long as the temporal spacing between subsequent pump pulses. Thus, the researchers used a small negative pump-probe time delay to observe heat flow in their samples just before the arrival of a new pulse.

The authors termed this new measurement technique “pre-time-zero spatiotemporal pump-probe microscopy“, because, as said, they obtain the thermal diffusivity by examining the spatial profile that results from scanning the probe beam over the pump beam at a negative pump-probe delay. This effectively corresponds to a large pump-probe delay – specifically, 13 ns –, matching the inverse of the repetition rate of the ultrafast laser they used. 

Sebin Varghese, PhD student at ICN2 and first author of the paper, explains: “Our novel technique  combines advantageous elements from several well-established methods: it exploits the long-lived heat signal in pump-probe measurements similar to time-domain thermoreflectance; it is sensitive to phonon heat through a temperature-sensitive mode similar to Raman thermometry; and it uses two laser beams that are spatially scanned with respect to each other, which has similarities with two-laser optothermal microscopy.”

By bringing together the advantages of each of these techniques into a single method, the researchers were able to directly measure the thermal diffusivity of a few samples of semiconducting materials, finding excellent agreement with earlier studies and with theoretical calculations performed in collaboration with the research groups led by Prof. Pablo Ordejón at ICN2 (Spain) –Theory and Simulation Group—, by Prof. Zeila Zanolli at Utrecht University (the Netherlands), and by Prof. Matthieu Verstraete at Liege University (Belgium), respectively.

Thanks to this spatiotemporal scanning, the newly developed method is in fact a super-resolution technique that can resolve spatial spreading with an accuracy around 10 nm, which is much smaller than the laser spot size (of around 1 micron). Being highly sensitive and non-invasive, and not requiring knowledge of material parameters, this method will enable the study of diffusion processes in a large variety of systems.  “This new technique has great potential for advancing our understanding of nanoscale thermal transport phenomena, including in cases where Fourier’s law might no longer hold,” concludes Prof. Klaas-Jan Tielrooij, corresponding author of the paper.