  The insulator-to-metal transition (IMT) in vanadium dioxide (VO2) can enable
a variety of optics applications, including switching and modulation, optical
limiting, and tuning of optical resonators. Despite the widespread interest in
optics, the optical properties of VO2 across its IMT are scattered throughout
the literature, and are not available in some wavelength regions. We
characterized the complex refractive index of VO2 thin films across the IMT for
free-space wavelengths from 300 nm to 30 {\mu}m, using broadband spectroscopic
ellipsometry, reflection spectroscopy, and the application of effective-medium
theory.
  We studied VO2 thin films of different thickness, on two different substrates
(silicon and sapphire), and grown using different synthesis methods (sputtering
and sol gel). While there are differences in the optical properties of VO2
synthesized under different conditions, they are relatively minor compared to
the change resulting from the IMT, most notably in the ~2 - 11 {\mu}m range
where the insulating phase of VO2 has relatively low optical loss. We found
that the macroscopic optical properties of VO2 are much more robust to
sample-to-sample variation compared to the electrical properties, making the
refractive-index datasets from this article broadly useful for modeling and
design of VO2-based optical and optoelectronic components.
