  Applications in photodetection, photochemistry, and active metamaterials and
metasurfaces require fundamental understanding of ultrafast nonthermal and
thermal electron processes in metallic nanosystems. Significant progress has
been recently achieved in synthesis and investigation of low-loss
monocrystalline gold, opening up opportunities for its use in ultrathin
nanophotonic architectures. Here, we reveal fundamental differences in
hot-electron thermalisation dynamics between monocrystalline and
polycrystalline ultrathin (down to 10 nm thickness) gold films. Comparison of
weak and strong excitation regimes showcases a counterintuitive unique
interplay between thermalised and non-thermalised electron dynamics in
mesoscopic gold with the important influence of the X-point interband
transitions on the intraband electron relaxation. We also experimentally
demonstrate the effect of hot-electron transfer into a substrate and the
substrate thermal properties on electron-electron and electron-phonon
scattering in ultrathin films. The hot-electron injection efficiency from
monocrystalline gold into TiO2, approaching 9% is measured, close to the
theoretical limit. These experimental and modelling results reveal the
important role of crystallinity and interfaces on the microscopic electronic
processes important in numerous applications.
