  Long regarded as a model system for studying insulator-to-metal phase
transitions, the correlated electron material vanadium dioxide (VO$_2$) is now
finding novel uses in device applications. Two of its most appealing aspects
are its accessible transition temperature ($\sim$341 K) and its rich phase
diagram. Strain can be used to selectively stabilize different VO$_2$
insulating phases by tuning the competition between electron and lattice
degrees of freedom. It can even break the mesoscopic spatial symmetry of the
transition, leading to a quasi-periodic ordering of insulating and metallic
nanodomains. Nanostructuring of strained VO$_2$ could potentially yield unique
components for future devices. However, the most spectacular property of VO$_2$
- its ultrafast transition - has not yet been studied on the length scale of
its phase heterogeneity. Here, we use ultrafast near-field microscopy in the
mid-infrared to study individual, strained VO$_2$ nanobeams on the 10 nm scale.
We reveal a previously unseen correlation between the local steady-state
switching susceptibility and the local ultrafast response to below-threshold
photoexcitation. These results suggest that it may be possible to tailor the
local photo-response of VO$_2$ using strain and thereby realize new types of
ultrafast nano-optical devices.
