Vanadium dioxide (VO2) displays a well-known metal-insulator (MI) transition at a temperature of 68°C. In this study we use terahertz time-domain spectroscopy (THz-TDS) to investigate the optical properties of VO2 thin films in the vicinity of the MI transition temperature in the frequency range 0.1 – 1.5 THz. We observe the interesting effect that the phase of the transmitted THz field through the conducting VO2 film is delayed in comparison to the phase of the same THz signal transmitted through the insulating VO2 film. This is in contrast to the expected behavior of a homogeneous, conducting film. This observation shows that even at temperatures significantly above the transition temperature, the formation of a homogeneous, conducting film is incomplete. We demonstrate that effective-medium theory (EMT) in combination with a Drude model accounting for the conductivity of metallic domains formed in the VO2 film accounts for all our observations. We show that the Maxwell-Garnett EMT is consistent with our observations, whereas the Bruggeman EMT fails to account for our observations.
