  We investigate the differences in the dynamics of the ultrafast photo-induced
metal-insulator transition (MIT) of two VO$_2$ thin films deposited on
different substrates, TiO$_2$ and Al$_2$O$_3$, and in particular the
temperature dependence of the threshold laser fluence values required to induce
various MIT stages in a wide range of sample temperatures (150 K - 320 K). We
identified that, although the general pattern of MIT evolution was similar for
the two samples, there were several differences. Most notably, the threshold
values of laser fluence required to reach the transition to a fully metallic
phase in the VO$_2$ film on the TiO$_2$ substrate were nearly constant in the
range of temperatures considered, whereas the VO$_2$/Al$_2$O$_3$ sample showed
clear temperature dependence. Our analysis qualitatively connects such behavior
to the structural differences in the two VO$_2$ films.
