Fig. 4: Ultrafast pump–probe response of Au on silicon and hBN substrates using a 520-nm pump and 2.1-μm probe.
At this probe wavelength, the Drude response in the thermoreflectivity of the probe results in sensitivity to the lattice temperature changes in the Au53,75. a, The solid squares and open circles show the measured transient lattice temperature changes in Au when on a Si substrate and hBN substrate, respectively. The Au lattice temperature changes on hBN during and after the 520 nm pump heating is suppressed at shorter pump–probe delay times and takes longer to rise and equilibrate when coupling to the excited electrons in the Au as compared to the case when Au is on a Si substrate. This is owing to an additional energy loss pathway of the hot electrons in the Au to the hBN substrate, the trends of which we capture with a 3TM that accounts for a gold electron to hBN HPhP–optical phonon energy transport pathway (hHPhP), shown as a solid blue line assuming this pathway’s TBC is 500 MW m−2 K−1. Note, these ultrafast Au thermoreflectance trends are not captured with a TTM, which shows a much faster lattice temperature rise when this Au electron to substrate TBC is not considered and only electron-phonon coupling in the Au (Gep) is considered, indicated by the red solid line. b, Un-normalized thermoreflectance signal from the data presented in a, where the reduced magnitude in the signal indicates the overall enhanced cooling of the Au surface. c, The relative sensitivity of the presented measurements to the HPhP interface coupling.
