  We present a detailed infrared study of the insulator-to-metal transition
(IMT) in vanadium dioxide (VO2) thin films. Conventional infrared spectroscopy
was employed to investigate the IMT in the far-field. Scanning near-field
infrared microscopy directly revealed the percolative IMT with increasing
temperature. We confirmed that the phase transition is also percolative with
cooling across the IMT. We present extensive near-field infrared images of
phase coexistence in the IMT regime in VO2. We find that the coexisting
insulating and metallic regions at a fixed temperature are static on the time
scale of our measurements. A novel approach for analyzing the far-field and
near-field infrared data within the Bruggeman effective medium theory was
employed to extract the optical constants of the incipient metallic puddles at
the onset of the IMT. We found divergent effective carrier mass in the metallic
puddles that demonstrates the importance of electronic correlations to the IMT
in VO2. We employ the extended dipole model for a quantitative analysis of the
observed near-field infrared amplitude contrast and compare the results with
those obtained with the basic dipole model.
