Tracking the evolution of electronic and structural properties of VO${}_2$ during the ultrafast photoinduced insulator-metal transition

We present a detailed study of the photoinduced insulator-metal transition in VO${}_2$ with broadband time-resolved reflection spectroscopy. This allows us to separate the response of the lattice vibrations from the electronic dynamics and observe their individual evolution. When we excite VO${}_2$ above the photoinduced phase transition threshold, we find that the restoring forces that describe the ground-state monoclinic structure are lost during the excitation process, suggesting that an ultrafast change in the lattice potential drives the structural transition. However, by performing a series of pump-probe measurements during the nonequilibrium transition, we observe that the electronic properties of the material evolve on a different, slower time scale. This separation of time scales suggests that the early state of VO${}_2$, immediately after photoexcitation, is a nonequilibrium state that is not well defined by either the insulating or the metallic phase.

Figure 1

(a) Reflectivity hysteresis measured at 800 nm and corresponding crystal structures. (b) Wavelength-dependent change in reflectivity on heating, relative to the reflectivity at 300 K. (c) Cuts in the differential reflectivity spectra, corresponding to the dashed lines in (b), above and below $T_c$ (solid lines) and expected reflectivity change based on the optical data of Ref. 21 (dashed line).

Figure 2

Broadband reflectivity change, measured at room temperature (a) below the photoinduced IM transition threshold pumping and (b) above.[19] Change in reflectivity at (c) 800 and (d) 525  nm measured for a range of fluences above and below threshold. Blue lines correspond to pump powers below threshold, orange above, and red in saturation. (e) Power dependence of the reflectivity change at 1 ps measured at 525 and 800 nm plotted against a normalized fluence scale. Vertical dashed lines indicate the transition threshold and saturation regimes. (f) Comparison of the reflectivity change at 3.5 ps, when the coherent phonon amplitude has reduced, to the “amplitude” of the phonon signal, as measured by the difference in reflectivity between the first trough and the following peak.

Figure 3

(a) Examples of fits in the below threshold regime for 800-nm (upper) and 525-nm (lower) transient reflectivities (circles: data, red lines: fits, purple lines: residuals). The insert shows the Fourier transform of the residual signal exhibiting modes at 4.3 and 10 THz. (b)–(d) The fluence dependence of the phonon mode parameters: $A$ is a phonon amplitude, $\zeta$ is a damping ratio, and $f$ is a phonon frequency in the low-fluence regime. Filled (unfilled) markers correspond to parameters from 525(800)-nm data. Circles correspond to the parameters associated to the $Q_1$ mode, squares correspond to $Q_2$. The errors in the fit parameters are discussed in the text.

Figure 4

(a) Example fit in the above threshold regime for a 525-nm probe wavelength (pump fluence $4.7F_{\mathrm{TH}}$), (circles data, red line fits, purple line residual). (b) The fluence dependence of the heating term, $H$ (filled squares), $C_1$ (open circles), $C_2$ (open triangles) from fitting Eq. (5) on a linear scale. (c) Half-rise time ${\tau }_{1/2}$ (solid squares), time constants ${\tau }_{C1}$ (open circles), and ${\tau }_{C2}$ (open triangles) are displayed on a log-log scale. Dashed red lines correspond to the time scales observed in the 800-nm data below threshold. The solid lines correspond to the time for a half period, T${}_p$/2 $=$ $1/2f$ of the 5.7- and 6.7-THz phonon mode and the pulse duration. Dashed black line, ${\tau }_p=40$ fs, corresponds to the pulse duration.

Figure 5

Broadband pump-probe signal of metallic VO${}_2$ at 400 K. The response is characterized by a positive spikelike change in reflectivity at all wavelengths and a slower scale dynamics on longer time scales which is larger at longer wavelengths. Note that the wavelength axis has been swapped compared to Fig. 2.

Figure 6

(a) Pulse sequence used to probe the formation of the metallic state. P${}_1$ is above the threshold fluence and triggers the transition, P${}_2$ excites the transient state after a time $t_{12}$, and the response is probed by P${}_3$. (b) Pump-probe measurements on the induced reflectivity change in the transient state compared to the metallic state response at 525 nm. The numbers correspond to the delay between the P${}_1$, which creates the transient state and P${}_2$, which excites it. Dotted line corresponds to pump-probe signal in the metallic state. (c) Formation of a metallic peak response, data points are obtained by integrating the area between the dashed lines in (b).