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

TL;DR

This paper introduces a novel pre-time-zero spatiotemporal microscopy technique that enables ultrasensitive, non-invasive measurement of thermal diffusivity in thin films with nanometer precision, expanding capabilities for nanoscale thermal transport analysis.

Contribution

The paper presents a new pump-probe microscopy method that measures thermal diffusivity without requiring material parameters or strong heating, using residual temperature profiles before pump pulses.

Findings

Measured thermal diffusivities of MoSe₂, WSe₂, MoS₂, and WS₂.

Achieved nanometer spatial accuracy in heat diffusion tracking.

Demonstrated non-invasive, parameter-free thermal transport measurement.

Abstract

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. In contrast to existing techniques for quantifying thermal transport it does not require any material input parameters or strong heating. We demonstrate the direct determination of the thermal diffusivities of the layered materials MoSe$_2$ (0.18 cm$^2$/s), WSe$_2$ (0.20 cm$^2$/s), MoS$_2$ (0.35 cm$^2$/s), and WS$_2$ (0.59 cm$^2$/s). This technique paves the way for observing novel nanoscale thermal transport phenomena and tracking diffusion of a broad range of species.