Vanadium dioxide is new tunable-index material, study says

Vanadium dioxide (VO₂) reversibly switches back and forth between transparent and reflective states near 68°C because of a metal-insulator phase change—this characteristic makes it attractive as an ultrafast optical switch, for example. In the region around its transition temperature, the refractive index of VO₂ also changes, meaning that it could also be used as a tunable-index material. Scientists at the Naval Research Laboratory (Washington, DC) have characterized the tunable refractive index of VO₂ by measuring its optical transmittance and reflectance as a function of temperature, and from this data creating wavelength- and temperature-dependent models of the material's complex optical refractive index. The information could be used in the design of future optical devices requiring index tunability.

The researchers grew a high-quality, 35-nm-thick film of VO₂ on sapphire via atomic-layer deposition, producing a layer in a monoclinic (020) orientation aligned with the sapphire substrate. A tungsten lamp was then shone on the film at an angle of 6° from normal and temperature incrementally varied between 24° and 90°C, with two minutes of stabilization time between measurements. Changes in transmittance and reflectance were noted in three spectral regions: visible (400–800 nm), near-infrared (800–1100 nm), and infrared (1100–2200 nm). There was negligible scattering loss in the VO₂ film. The film's optical permittivity was then modeled throughout the transition region from the metallic to the insulating state. The researchers note that the possibility of conformal and/or flexible VO₂ coating with tunable refractive index could lead to novel optical devices. Reference: M. Currie et al., Opt. Mater. Express (2017); https://doi.org/10.1364/ome.7.001697.