A new paper by a research team at the Harvard University School of Engineering and Applied Sciences reports on a new device that is an exceptionally efficient, perfect absorber of infrared light. They expect it to be useful for a range of applications such as temperature measurement, spectroscopy, tunable filters, thermal emitters, radiation detectors, energy harvesting and high-sensitivity thermal cameras.
Optical absorbers trap and absorb light. A common example Fabry-Pérot optical cavities, which are used in laser optics. They are comprised of an absorbing material sandwiched between two reflective surfaces. The light simply bounces between the reflective surfaces until it is all consumed by the absorbing material.
The new device is composed of a nanoscale thin film of vanadium dioxide on sapphire, and its light absorbing functionality come from two sources.
First, VO2 undergoes a phase transformation at about 68˚C that affects its optical properties. Below 68˚C, the material is an electrical insulator. At the transition, VO2 undergoes an extremely rapid, polymorphic insulator-to-metal phase transition (the fastest known phase transition, as we reported earlier). According to the press release, the transition involves the formation of “metallic islands,” and eventually, the material becomes “uniformly metallic.” The metal-like electronic properties arise from the collapse of the material’s band gap during the monoclinic-to-tetragonal transition, which creates free electrons.
In the press release, associate professor and coauthor, Shriram Ramanathan says that near the transition the film has a “very complex and rich microstructure in terms of its electronic properties, and it has very unusual optical properties.” With the phase transition, the optical properties of the film change from transparent to reflective. The substrate, sapphire, it turns out, is opaque to certain infrared wavelengths and reflects it back. Thus, the VO2-sapphire interface is, itself, an optical trap.
The film, which is about 180 nanometers thick, is about 100 times smaller than the wavelength of the infrared radiation it is absorbing. So how, exactly, is the light absorbed? The answer is rooted in the nature of optically lossy materials. Graduate student and lead author Mikhail Kats explains in the press release, “… when light reflects between lossy materials, instead of transparent or highly reflective ones, you get strange interface reflections. When you combine all of those resulting waves, you can coax them to destructively interfere and completely cancel out. The net effect is that a film one hundred times thinner than the wavelength of the incident light can create perfect absorption.”
In a phone interview, Ramanathan explained that the optical effect in VO2 films is very sensitive to the quality or purity of the film. The films are synthesized by physical vapor deposition and, he says, that the film composition is very sensitive to the processing parameters. “As with all transition metal oxides, there are many compounds that are possible, and we want to make phase-pure samples with as pure a composition as possible,” he says. “It is only recently that we have been able to synthesize exceptionally high-quality films. Stabilizing the phase is a very hard problem.” The films can be epitaxial, depending on the substrate. The group has grown films on several substrates, including sapphire and titania.
According to the abstract, the tuning range for a device such as this is huge: from ∼80% to 0.25% in reflectivity. Ramanathan explained that the transition threshold of the film can be controlled several ways including electric fields, doping with charge carriers or adjusting the lattice constant with dopants such as W, Cr and Nb or lattice strain during film deposition.
In addition to a wide tuning range, the phase transition is “very reproducible through millions of cycles, with the right composition,” according to Ramanathan. The dielectric constant of VO2 also undergoes rapid change with the VO2 phase transition, so his group has been studying the material for high-speed switches and other ultrahigh-speed electronic applications. This new ability to control the optical characteristics opens up new possibilities. “We are starting to think about opto-electronic platforms. What kind of oxide electronic devices can we make?”
The paper is “Ultra-thin perfect absorber employing a tunable phase change material,” M.A. Kats, et al., Appl. Phys. Lett., doi:10.1063/1.4767646.