Giant impact of self-photothermal on light-induced ultrafast insulator-to-metal transition in VO2 nanofilms at terahertz frequency.

Ultrafast detection and switching of light are key processes in high-speed optoelectronic devices. However, the performances of VO2-based optoelectronics are strongly degraded by photothermal. The mechanism of the latter is still unclear. Here, by using femtosecond-laser (fs-laser) driven kinetic terahertz wave absorption, we quantitatively separate slow photothermal response and ultrafast photodoping response (e.g. light-induced insulator-to-metal transition) from second- to picosecond-timescales, and discover the competing interplay between them. With self-photothermal (mainly determined by fs-laser pulse repetition rate and pump fluence), the ultrafast transition time was degraded by 190% from 50 ps to 95 ps, the ultrafast transition threshold was decreased to 82% from 11mJ/cm2 to 9mJ/cm2, while the amplitudes of the two photoresponse are competing. Percolation theory, along with the macroscopic conductivity response, is used to explain the competing interplay. Our findings are relevant for designing and optimizing VO2-based ultrafast optoelectronic devices.