Plasmonic electron injection drives ultrafast phase transition by catastrophic phonon collapse I: experiment*

Phase transitions in quantum materials such as vanadium dioxide (VO$_{\mathrm{2}})$ can provide functionality in nanophotonic devices. Here we report on a novel all-optical mechanism to trigger phase transformation (PT) of VO$_{\mathrm{2}}$ faster than its intrinsic single phonon period. By optically exciting a spectrally resonant sparse mesh of plasmonic gold nanoparticles, hot electrons created are ballistically injected across the Au/VO$_{\mathrm{2}}$ interface to assist the sub-picosecond PT, lowering the switching threshold by a factor of five. As confirmed by density functional calculations, the injected electrons cause a catastrophic collapse of the 6 THz phonon mode in VO$_{\mathrm{2}}$, essential for triggering its PT (next abstract). This demonstration of plasmon-induced hot-electron-driven PT controlled by this ultrafast technique represents a critical step towards developing hybrid nanomaterials with optimal switching thresholds.