Ultrafast electron-lattice coupling dynamics in ${\mathrm{VO}}_2$ and ${\mathrm{V}}_2{\mathrm{O}}_3$ 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.

Figure 1

Temperature dependence of the static far-infrared conductivity for the 75-nm-thick (a) ${\mathrm{VO}}_2$ and (b) ${\mathrm{V}}_2{\mathrm{O}}_3$ thin films. Open black (solid red) symbols indicate increasing (decreasing) temperature. Both samples display hysteresis, characteristic of first-order phase transitions happening at 360 K in ${\mathrm{VO}}_2$ and at 160 K in ${\mathrm{V}}_2{\mathrm{O}}_3$. The fully metallic region is shaded gray.

Figure 2

Transient $\mathrm{\Delta }\sigma$ of (a) 75-nm-thick ${\mathrm{VO}}_2$, (b) 75-nm-thick ${\mathrm{V}}_2{\mathrm{O}}_3$, and (c) 75-nm- and 95-nm-thick ${\mathrm{V}}_2{\mathrm{O}}_3$ for temperatures below $T_{\mathrm{IMT}}$ at a pump fluence of (a) 3.8 mJ/${\mathrm{cm}}^2$, (b) 1 mJ/${\mathrm{cm}}^2$, and (c) 3 mJ/${\mathrm{cm}}^2$. Starting in the insulating phase, the THz response consists of a transient conductivity increase which corresponds to the IMT. (c) Comparison of the normalized $\mathrm{\Delta }\sigma \left(t\right)$ for the 95 nm and 75 nm ${\mathrm{V}}_2{\mathrm{O}}_3$ films at 100 K, revealing a significant difference in IMT dynamics at short-time delays which arises from different defect densities in the films.

Figure 3

Transient (a),(c) $\mathrm{\Delta }\sigma$ and (b),(d) $\mathrm{\Delta }R/R$ of 75-nm-thick (a),(b) ${\mathrm{VO}}_2$ and (c),(d) ${\mathrm{V}}_2{\mathrm{O}}_3$ for temperatures above $T_{\mathrm{IMT}}$ at a pump fluence of (a),(b) 3.8 mJ/${\mathrm{cm}}^2$ and (c),(d) 1 mJ/${\mathrm{cm}}^2$. Shown are the metallic reflectivity responses, dominated by acoustic signatures, and the metallic conductivity responses, also with clear acoustic signatures.

Figure 4

Background-subtracted $\mathrm{\Delta }\sigma$ acoustic oscillations for (a) ${\mathrm{VO}}_2$ and (b) ${\mathrm{V}}_2{\mathrm{O}}_3$, corresponding to the data from Figs. 3 and 3. $\mathrm{\Delta }\sigma \left(t\right)$ data for the ${\mathrm{VO}}_2$ film were smoothed using a five-point (1 ps) moving average. Vertical gray bars indicate the time delays at which the values in (c) were calculated [34]. (c) Bottom: Temperature dependence of the maximum acoustic modulation of $\mathrm{\Delta }\sigma$, normalized by the static conductivity at the corresponding temperature (see Fig. 1 and Fig. S2 of the Supplemental Material [34]). Top: Temperature dependence of the maximum acoustic modulation of $\mathrm{\Delta }\sigma$. Data is shown for all four ${\mathrm{VO}}_2$ and ${\mathrm{V}}_2{\mathrm{O}}_3$ films. A fluence of 3.8 mJ/${\mathrm{cm}}^2$ was used for the 75-nm-thick ${\mathrm{VO}}_2$ film (blue), 4 mJ/${\mathrm{cm}}^2$ for the 50 nm ${\mathrm{VO}}_2$ (orange), 1 mJ/${\mathrm{cm}}^2$ for the 75 nm ${\mathrm{V}}_2{\mathrm{O}}_3$ (yellow), and 3 mJ/${\mathrm{cm}}^2$ for the 95 nm ${\mathrm{V}}_2{\mathrm{O}}_3$ (purple). The gray shaded region corresponds to temperatures $T<T_{\mathrm{IMT}}+20$ K, where the influence of the IMT hysteresis could affect the results.

Figure 5

Background-subtracted $\mathrm{\Delta }R/R$ acoustic oscillations for (a) insulating and (b) metallic ${\mathrm{VO}}_2$ and for (e) insulating and (f) metallic ${\mathrm{V}}_2{\mathrm{O}}_3$, corresponding to the data in Figs. 3, 3, and also Fig. S3 of the Supplemental Material [34]. Plots to the right show the temperature dependence of (c),(g) the acoustic oscillation period and the corresponding sound velocity, as well as of (d),(h) the damping time, for (c),(d) ${\mathrm{VO}}_2$ and for (g),(h) ${\mathrm{V}}_2{\mathrm{O}}_3$, extracted by fitting the shaded regions of (a), (b), (e), and (f) with a damped sinusoid. Red lines in (c) and (g) are linear fits [(c),(g): insulating phase], an exponential guide to the eye [(c): metallic phase], and an exponential fit [(g): metallic phase]. In (d) and (h), the fully metallic region is shaded gray, for comparison with Fig. 1.