  Using light to control transient phases in quantum materials is an emerging
route to engineer new properties and functionality, with both thermal and
non-thermal phases observed out of equilibrium. Transient phases are expected
to be heterogeneous, either through photo-generated domain growth or by
generating topological defects, and this impacts the dynamics of the system.
However, this nanoscale heterogeneity has not been directly observed. Here we
use time- and spectrally resolved coherent X-ray imaging to track the
prototypical light induced insulator-to-metal phase transition in vanadium
dioxide on the nanoscale with femtosecond time resolution. We show that the
early-time dynamics are independent of the initial spatial heterogeneity and
observe a 200 fs switch to the metallic phase. A heterogeneous response emerges
only after hundreds of picoseconds. Through spectroscopic imaging, we reveal
that the transient metallic phase is a highly orthorhombically strained rutile
metallic phase, an interpretation that is in contrast to those based on
spatially averaged probes. Our results demonstrate the critical importance of
spatially and spectrally resolved measurements for understanding and
interpreting the transient phases of quantum materials.
