Direct observation of interlayer-coupling-induced anomalous energy-dependent ultrafast carrier relaxation in graphite.

Interlayer coupling governs the band structure and ultrafast carrier dynamics of van der Waals (vdW) materials, serving as a fundamental degree of freedom for manipulating optoelectronic performance. However, energy-resolved characterization of its impacts on photoexcited carriers remains scarce. In this work, we use femtosecond transient reflectance spectroscopy with near-infrared supercontinuum probing to explore energy-resolved carrier relaxation in a Bernal-stacked epitaxial graphite film on nickel. An abrupt variation of carrier cooling rate is observed at a probe energy of 1.21 eV, which directly manifests the π-band splitting induced by interlayer coupling in graphite. Such band splitting opens additional scattering channels, thus leading to a pronounced cooling rate enhancement once the carrier energy exceeds the π-band splitting threshold. In addition, the increase in relaxation time under high pump fluences can be attributed to the hot phonon effect. This study presents direct experimental observation of anomalous energy-resolved carrier relaxation stemming from interlayer-coupling-induced band reconstruction in graphite, demonstrating the capability of broadband ultrafast spectroscopy to resolve subtle band-structure features in diverse layered vdW materials.