  We have systematically studied a variety of vanadium dioxide (VO2)
crystalline forms, including bulk single crystals and oriented thin films,
using infrared (IR) near-field spectroscopic imaging techniques. By measuring
the IR spectroscopic responses of electrons and phonons in VO2 with
sub-grain-size spatial resolution (~20 nm), we show that epitaxial strain in
VO2 thin films not only triggers spontaneous local phase separations but also
leads to intermediate electronic and lattice states that are intrinsically
different from those found in bulk. Generalized rules of strain and symmetry
dependent mesoscopic phase inhomogeneity are also discussed. These results set
the stage for a comprehensive understanding of complex energy landscapes that
may not be readily determined by macroscopic approaches.
