A pre-time-zero spatiotemporal microscopy technique for the ultrasensitive determination of the thermal diffusivity of thin films

• Published in: Review of scientific instruments 2023-03, Vol.94 (3), p.034903-034903
• Main Authors: Varghese, Sebin, Mehew, Jake Dudley, Block, Alexander, Reig, David Saleta, Woźniak, Paweł, Farris, Roberta, Zanolli, Zeila, Ordejón, Pablo, Verstraete, Matthieu J., van Hulst, Niek F., Tielrooij, Klaas-Jan
• Format: Article
• Language: English
• Subjects: Diffusion; Diffusion pumps; Instrumentation; Layered materials; Microscopy; Microscopy technique; Physical, chemical, mathematical & earth Sciences; Physics; Physics - Mesoscopic Systems and Quantum Hall Effect; Physique; Physique, chimie, mathématiques & sciences de la terre; Probe pulse; Pump pulse; Pump-probe microscopies; Scientific apparatus & instruments; Space and time; Temperature profiles; Thermal diffusivity; Thickness; Thin films; Time lag; Transient reflectivity; Transport phenomena; Transport phenomenon; Ultrasensitive
• ISSN: 0034-6748; 1089-7623; 1089-7623
• DOI: 10.1063/5.0102855

Diffusion is one of the most ubiquitous transport phenomena in nature. Experimentally, it can be tracked by following point spreading in space and time. Here, we introduce a spatiotemporal pump–probe microscopy technique that exploits the residual spatial temperature profile obtained through the transient reflectivity when probe pulses arrive before pump pulses. This corresponds to an effective pump–probe time delay of 13 ns, determined by the repetition rate of our laser system (76 MHz). This pre-time-zero technique enables probing the diffusion of long-lived excitations created by previous pump pulses with nanometer accuracy and is particularly powerful for following in-plane heat diffusion in thin films. The particular advantage of this technique is that it enables quantifying thermal transport without requiring any material input parameters or strong heating. We demonstrate the direct determination of the thermal diffusivities of films with a thickness of around 15 nm, consisting of the layered materials MoSe2 (0.18 cm2/s), WSe2 (0.20 cm2/s), MoS2 (0.35 cm2/s), and WS2 (0.59 cm2/s). This technique paves the way for observing nanoscale thermal transport phenomena and tracking diffusion of a broad range of species.