Direct Sensing of Interfacial Heat Transfer Between Identical Atomic Layers Using Femtosecond Diffraction

Understanding and controlling interfacial thermal transport is crucial for applications in microelectronics and photonics. Over the past few decades, much effort has been devoted to measuring and modeling the physics of heat transport at interfaces between vibrationally dissimilar materials, i.e. at hetero junctions. These studies have shown that vibrational mismatch plays a key role in determining the rate of heat exchange. However, little is understood about how heat flows across homo junctions, i.e. interfaces formed between identical materials. What is the effect of lattice registry on the phononic Kapitza resistance of a homo junction? What is the resistance in the limit where the contacting materials are atomically thin? These questions have thus far remained unanswered because of the lack of experimental techniques that can directly measure and distinguish the temperatures of identical atomic layers that are in contact (with an interlayer distance of only a few Å). Existing thermal metrology techniques are either indirect (e.g. thermoreflectance) in that they only measure the temperature of a remote transducer but not the temperatures of the buried atomic layers adjoining the interface, or even when they are direct, cannot distinguish the temperatures of atomic layers made of the same material (e.g. Raman spectroscopy).

To address this challenge, here we develop a quantitative ultrafast thermometry technique using femtosecond time-resolved electron diffraction – a pump-probe technique that uses 100-fs optical pump and time-delayed 4 MeV 100-fs electron probe pulses. By exploiting the relationship between Bragg peak intensity and atomic mean-square-displacement, we perform the first direct measurements, to our knowledge, of the temperatures of identical atomic lattices when heat flows between them on picosecond timescales. Using this technique, we directly measure the Kapitza resistance of a series of van der Waals 2D homo-bilayers as a function of their interlayer twist angle, revealing non-intuitive trends in the resistance. This illustrates the power of atomic-layer-resolved thermometry, as well as demonstrates twist angle as a new degree of freedom for tuning heat transport. Our work marks a key step forward in nanoscale thermal metrology, enabling direct measurements of temperature with picosecond resolution.