Heat accumulation and phase transition induction in a VO2 thin film by a femtosecond pulse-periodic radiation.

ABSTRACT

The kinetics of optical switching due to the insulator-metal phase transition in a VO2 thin film is studied experimentally at different laser pulse repetition frequencies (PRFs) in the NIR range and compared with temperature kinetics obtained through the thermal conductance calculations. Two switching processes have been found with characteristic times <2 ms and <15 ms depending on the PRF; the former is explained by the accumulation of metallic domains remaining after a single-pulse phase transition, and the latter is referred to the heat accumulation in the film. Consequently, the dynamics of the microscopic domains is leading in the initiation of phase transition under pulse-periodic conditions compared to the macroscopic heat transfer. The reverse transition at the radiation turn-off depends on the PRF with a time coefficient of 17.5 µs/kHz and is determined by the metallic domains' decay in the film. The results are important for understanding the nature of the insulator-metal transition in thin films of VO2 as well as using them in all-optical switches of pulse-periodic laser radiation.