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 mid-infrared 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. 
A. Pashkin, C. K\"ubler, H. Ehrke, R. Lopez, A. Halabica, R. F.   Haglund, Jr., R. Huber, and A. Leitenstorfer
cond-mat.str-el