Coherent phonon spectroscopy of the phase transition in VO2 single crystals and thin films

With performing time-resolved pump-probe experiments using femtosecond optical pulses, we investigate temperature dependent transition of the structural phase in single crystalline (SC) VO2 plates and polycrystalline films. Analysis of coherent optical phonons with the temperature change has revealed the M1-M2 transition within the insulating phase of the SC VO2. In stark contrast, no evidence on the M2 phase was found in a VO2 film composed of multi-grains. The results demonstrate that the strain is an important parameter in the structural transitions of VO2, which determines not only the appearance of the intermediate M2 phase but also the transition temperatures.