Optical properties of thin-film vanadium dioxide from the visible to the far infrared

TL;DR

This study provides comprehensive measurements of the complex refractive index of VO2 thin films across the insulator-to-metal transition from 300 nm to 30 μm, aiding optical device design.

Contribution

It offers the first broad wavelength characterization of VO2's optical properties during the IMT, accounting for variations in synthesis and substrates.

Findings

Optical properties vary little with synthesis method or substrate.

Significant change in optical loss occurs in the 2-11 μm range during IMT.

Refractive index data are robust and useful for device modeling.

Abstract

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.