Quasi-ballistic thermal transport from a nanoscale hotspot observed using ultrafast coherent extreme ultraviolet beams

We study thermal transport from a nanoscale hotspot into a bulk material using ultrafast, coherent, extreme ultraviolet beams. When the size of the hotspot is smaller than the mean free path of the energy-carrying phonons in the substrate, we measure a decrease in energy transport compared with the diffusive Fourier law prediction. This is the first observation and quantitative measurement of quasi-ballistic thermal transport from a nanoscale heat source. Our results show that the Fourier law can be corrected to describe energy dissipation from nanostructures into the bulk by introducing a size-dependent ballistic thermal resistance. This finding could have significant impact on the thermal management and reliability of emerging nanoscale devices, and nano-enabled energy systems.