Ultrafast electron-lattice coupling dynamics in VO 2 and V 2 O 3 thin films

Ultrafast optical pump--optical probe and optical pump--terahertz probe spectroscopy were performed on vanadium dioxide (${\mathrm{VO}}_{2}$) and vanadium sesquioxide (${\mathrm{V}}_{2}{\mathrm{O}}_{3}$) thin films over a wide temperature range. A comparison of the experimental data from these two different techniques and two different vanadium oxides, in particular a comparison of the spectral weight oscillations generated by the photoinduced longitudinal acoustic modulation, reveals the strong electron-phonon coupling that exists in both materials. The low-energy Drude response of ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ appears more amenable than ${\mathrm{VO}}_{2}$ to ultrafast strain control. Additionally, our results provide a measurement of the temperature dependence of the sound velocity in both systems, revealing a four- to fivefold increase in ${\mathrm{VO}}_{2}$ and a three- to fivefold increase in ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ across the insulator-to-metal phase transition. Our data also confirm observations of strong damping and phonon anharmonicity in the metallic phase of ${\mathrm{VO}}_{2}$, and suggest that a similar phenomenon might be at play in the metallic phase of ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$. More generally, our simple table-top approach provides relevant and detailed information about dynamical lattice properties of vanadium oxides, paving the way to similar studies in other complex materials.