Enhanced Photosusceptibility near $T_c$ for the Light-Induced Insulator-to-Metal Phase Transition in Vanadium Dioxide

We use optical-pump terahertz-probe spectroscopy to investigate the near-threshold behavior of the photoinduced insulator-to-metal (IM) transition in vanadium dioxide thin films. Upon approaching $T_c$ a reduction in the fluence required to drive the IM transition is observed, consistent with a softening of the insulating state due to an increasing metallic volume fraction (below the percolation limit). This phase coexistence facilitates the growth of a homogeneous metallic conducting phase following superheating via photoexcitation. A simple dynamic model using Bruggeman effective medium theory describes the observed initial condition sensitivity.

Figure 1

(a) THz waveform transmitted through the sample when heated to 340 from 300 K ($T_{\uparrow }$) and when cooled from 370 K ($T_{\downarrow }$), offset for clarity. (b) The optical conductivity vs frequency as determined from the transmitted waveforms in (a) and at 300 K. (c) The temperature-dependent hysteresis of the THz conductivity (left scale, ◆ for $T_{\uparrow }$, ● for $T_{\downarrow }$) and of the conductance measured using standard dc electrical techniques (right scale: dashed line for $T_{\uparrow }$; solid line for $T_{\downarrow }$).

Figure 2

(a) Photoinduced conductivity change at 300 K for various fluences where $\sigma (T_f)$ is the conductivity in the full metallic state. (b) Magnitude of the conductivity change as a function of fluence at 300 K.

Figure 3

(a) Induced conductivity change as a function of time at a fluence of $12.8\mathrm{mJ}{\mathrm{cm}}^{-2}$ for various initial temperatures. (b) Magnitude of the induced conductivity (■) and the maximum possible conductivity change (●). The black lines are a fit as described in the text. (c) Fluence threshold as a function of base temperature.

Figure 4

The volume fraction, $f_m$, responsible for the observed conductivity shown in Fig. 1 (◆ for $T_{\uparrow }$ and ● for $T_{\downarrow }$) and the extracted volume fraction in the insulating state (▾) as determined from the dynamic BEMT described in the text.