Kübler, Carl
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Ultrafast insulator-metal phase transition in VO2 studied by multiterahertz spectroscopy
The ultrafast photoinduced insulator-metal transition in VO2 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.
THz Slow Motion of an Ultrafast Insulator-Metal Transition in VO2 : Coherent Structural Dynamics and Electronic Correlations
The multi-THz conductivity of VO2 recorded during a photoinduced insulator-metal transition directly reveals the femtosecond dynamics of V-V stretching modes and electronic correlations. We suggest a novel qualitative model for the nonthermal phase transition.
Coherent structural dynamics and electronic correlations during an ultrafast insulator-to-metal phase transition in VO2
We directly trace the multi-THz conductivity of VO2 during an insulator-metal transition triggered by a 12-fs light pulse. The femtosecond dynamics of lattice and electronic degrees of freedom are spectrally discriminated. A coherent wave packet motion of V-V dimers at 6 THz modulates the lattice polarizability for approximately 1 ps. In contrast, the electronic conductivity settles to a constant value already after one V-V oscillation cycle. Based on our findings, we propose a qualitative model for the nonthermal phase transition.