Ultrafast insulator-metal phase transition in VO${}_2$ studied by multiterahertz spectroscopy

The ultrafast photoinduced insulator-metal transition in VO${}_2$ is studied at different temperatures and excitation fluences using multi-THz probe pulses. The spectrally resolved midinfrared response allows us to trace separately the dynamics of lattice and electronic degrees of freedom with a time resolution of 40 fs. The critical fluence of the optical pump pulse, which drives the system into a long-lived metallic state, is found to increase with decreasing temperature. Under all measurement conditions, we observe a modulation of the eigenfrequencies of the optical phonon modes induced by their anharmonic coupling to the coherent wave-packet motion of V-V dimers at 6.1 THz. Furthermore, we find a weak quadratic coupling of the electronic response to the coherent dimer oscillation resulting in a modulation of the electronic conductivity at twice the frequency of the wave-packet motion. The findings are discussed in the framework of a qualitative model based on an approximation of local photoexcitation of the vanadium dimers from the insulating state.

Figure 1

(a) Vanadium sublattice of the rutile (left) and monoclinic (right) crystal structure of VO${}_2$. (b) Schematic band structure according to Refs. 12 and 15. The red arrow denotes the photoexcitation by an optical pump pulse with an energy of 1.5 eV.

Figure 2

Schematic of the optical pump–multi-THz probe setup. The output of the amplified Ti:sapphire laser system is split into three branches: (1) The optical pump pulse with variable delay $\tau$; (2) the THz probe pulse with variable delay $T$; and (3) the gating pulse for field-resolved electro-optic detection. The sample is mounted in a liquid helium cryostat equipped with CVD diamond windows.

Figure 3

(a) Temperature dependence of the normalized transmittance through a VO${}_2$ thin film upon heating (red guide to the eye) and cooling (blue guide to the eye). (b) Real part of the optical conductivity of the VO${}_2$ sample below and above $T_c$.

Figure 4

(a) Spectrally integrated transient change of the THz conductivity after excitation by a 12-fs near-infrared laser pulse at $T_L$ $=$ 295 K for various pump fluences. (b) Fluence dependence of $\Delta {\sigma }_1(\tau )$ at $\tau$ $=$ 60 fs (blue crosses) and 1 ps (red triangles). (c) Extrapolation of $\Delta {\sigma }_1$ (1 ps) curves (red triangles: 295 K, magenta circles: 320 K) to a critical fluence of ${\Phi }_c$ (295 K) $=$ 4.6 mJ/cm${}^2$ and ${\Phi }_c$ (320 K) $=$ 3.5 mJ/cm${}^2$, respectively. (d) Dependence of threshold fluence ${\Phi }_c$ on lattice temperature $T_L$. The solid line is the thermodynamic energy difference between the metallic state and the insulating state at a given temperature calculated according to Ref. 3. The dashed line marks the phase-transition temperature $T_c$.

Figure 5

Decomposition of the decay dynamics. (a) Spectrally integrated dynamics of the THz conductivity after excitation at fluence $\Phi$ $=$ 14 mJ/cm${}^2$ and temperature $T_L$ $=$ 4 K (black circles). Subtracting the biexponential decay component (red line) reveals a coherent oscillation (blue circles) to which a cosine function with a frequency of $\omega /2\pi$ $=$ 6.1 THz (green line) was fitted. (b) Spectrum of the oscillatory component (blue circles) and fit by a Lorentzian function (blue line). (c) The spectrum of the oscillatory modulation observed in a degenerate pump-probe experiment[28] at 1.55 eV (red line) and the unpolarized spontaneous Raman spectrum[51] (gray line).

Figure 6

2D optical pump–multi-THz probe data: (a) Equilibrium conductivity of insulating VO${}_2$ at 295 K. Color plots of the pump-induced changes of the conductivity $\Delta {\sigma }_1(\omega ,\tau )$: (b) at $T_L$ $=$ 4 K and an incident fluence of $\Phi$ $=$ 7.5 mJ/cm${}^2$; at $T_L$ $=$ 250 K and pump fluence (c) $\Phi$ $=$ 3 mJ/cm${}^2$ and (d) $\Phi$ $=$ 7.5 mJ/cm${}^2$. The broken vertical lines indicate the frequency positions of cross sections reproduced in Fig. 7.

Figure 7

Cross sections of Fig. 6 along the time axis $\tau$ for a photon energy of (a) $\hslash \omega$ $=$ 60 meV and (b) $\hslash \omega$ $=$ 100 meV. The curves taken at $\Phi$ $=$ 3 mJ/cm${}^2$ are scaled up by a factor of 2.4. (c) and (d) The oscillating components of the cross sections through the 2D scan in Fig. 6d for a photon energy of (c) $\hslash \omega$ $=$ 60 meV and (d) $\hslash \omega$ $=$ 92 meV. Green solid lines: (c) fit of the oscillating component by a cosine function with frequency of 6.1 THz, (d) the fitting function shown in panel (c) squared with subtracted constant background. The oscillation occurs at a doubled frequency of 12.2 THz.

Figure 8

Schematic energy surfaces as a function of a dimer coordinate $Q$. The energy minima of the potential surfaces $Q_M$ and $Q_R$ correspond to spatial configurations of the V-V dimers in the monoclinic and rutile phases, respectively. Vertical dashed arrows denote Franck-Condon-type photoexcitation of spin singlets into a conductive state. (a) Structural relaxation and coherent vibrations about the new energy minimum below the pumping threshold $\Phi <{\Phi }_c$. (b) Intense excitation above the threshold $\Phi >{\Phi }_c$ leads to a structurally assisted collapse of the mobility edge.