Ultrafast dynamics of phase transition in Vanadium Dioxide thin films
There is a class of materials that undergo an insulator metal transition when heated past a critical temperature. We are working with VO2, which has a phase transition from insulator to metal when heated up to 68oC, making it easy to transition from room temperature. Previous work has shown that the insulator to metal transition (IMT) can also be induced with a pulsed laser beam, on time scales faster than what can be explained by a thermal transition. We are working with the femtosecond laser system, and have it currently set up a pump probe experiment to look at the IMT of a VO2 thin film sample over time.
For this experiments we use a powerful ultrafast laser system (Mantis + Legend Elite amplifier, Coherent, Inc.) that outputs 100 fs pulses at 785 nm central wavelength. The output of the laser is sent through a beam splitter and sent down two paths. The weaker beam (probe) bounces off a retroreflector that is mounted on a motorized adjustable track, so as to control the time delay between the two beams. Both beams are focused to hit the sample, where we measure the reflectivity of the probe beam. By changing the delay of the probe beam we can see how the reflectivity of VO2 changes over time during the transition. We know from previous study that when VO2 makes the transition from the insulator phase to the metal phase it reflects less of an 800nm beam. We have been able to see the reflectivity drop sharply after the pump beam hits the sample. In the future we hope to look at the Raman spectroscopy of the sample during the transition, in order to get a better understanding of how the crystal structure changes over time during the transition, and compare the rate of change in the crystalline structure compares with the rate of change of the electronic properties.
Future work will likely include studying surface polaritons on thin films of VO2, and looking at how surface polaritons propagate through layers of thin films of crystalline VO2 and RuO2 , and layers of polycrystalline VO2 and gold.