Scientists have developed a new device capable of absorbing 99.75 percent of infrared light that shines on it, appearing black to infrared cameras when activated.
Invented at the Harvard School of Engineering and Applied Sciences (SEAS), the device is composed of a 180-nanometer-thick layer of vanadium dioxide (VO2) on top of a sheet of sapphire. The gadget reacts to temperature changes by reflecting considerably more or less infrared light.
This perfect absorber, announced Monday, Nov. 26 in the journal Applied Physics Letters, is ultra-thin, tunable, and very well-suited for use in various infrared optical devices. While scientists have created many perfect absorbers before, none of those had such versatile properties.
When two mirrors sandwich an absorbing material in a Fabry-Perot cavity, for instance, light simply reflects light back and forth until it's mostly gone. Other devices, meanwhile, employ surfaces with nanoscale metallic patterns that trap and eventually absorb the light.
"Our structure uses a highly unusual approach, with better results," said principal investigator Federico Capasso, Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at SEAS.
"We exploit a kind of naturally disordered metamaterial, along with thin-film interference effects, to achieve one of the highest absorption rates we've ever seen. Yet our perfect absorber is structurally simpler than anything tried before, which is important for many device applications."
Capasso's research team worked with researchers at Harvard and the University of California, San Diego, taking advantage of the surprising properties of the two materials they used.
Vanadium dioxide is typically an insulating material, i.e. it does not conduct electricity well, but it undergoes a dramatic transition when taken from room temperature to about 68 degrees Celsius (154.4 degrees Fahrenheit). As the temperature approaches a critical value, the crystal rearranges itself and metallic islands appear as specks, scattered within the material. Increasingly more metallic islands appear until the material becomes uniformly metallic.
According to co-author Shriram Ramanathan, SEAS Associate Professor of Materials Science, an intriguing medium made of both insulating and metallic phases forms right near this transition from insulator to metal. Ramanathan synthesized the thin film. In terms of electronic properties, he explained, it is a very complex and rich microstructure. The material also delivers unusual optical properties. When manipulated properly, those properties are reportedly ideal for infrared absorption.
The underlying sapphire substrate, meanwhile, has a major role as well. While usually transparent, its crystal structure actually makes it reflective and opaque like a metal to a narrow subset of infrared wavelengths. As a result, the combination of materials internally reflects and "devours" incident infrared light.
"Both of these materials have lots of optical losses, and we've demonstrated that when light reflects between glossy materials, instead of transparent or highly reflective ones, you get strange interface reflections," added lead author Mikhail Kats, a SEAS graduate student. "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."
The great challenge was not only to understand this behavior, but also to discover how to fabricate pure enough samples of the vanadium dioxide.
As Ramanathan explains, vanadium oxide can exist in many oxidation states, but it only goes through a metal-insulator transition with VO2 close to room temperature. To grow such complex films, the scientists developed a number of techniques in the lab to allow "exquisite compositional and structural control, almost at the atomic scale." The resulting phase purity allowed the team to observe the remarkable properties, which otherwise would have been very difficult to notice.
Moreover, the device can be easily switched between its absorbent and non-absorbent states, which allows for a wide range of possible applications, including thermal imaging devices (bolometers) with tunable absorption, spectroscopy devices, tunable filters, thermal emitters, radiation detectors, and energy harvesting equipment.
According to Kats, an ideal bolometer design must absorb all of the infrared light that falls on it, turning it to heat, while its resistance should correspondingly change per degree change in temperature. The new absorber could theoretically be used to make incredibly sensitive thermal cameras, added Kats. The Office of Technology Development at Harvard has filed patent applications on the invention and is actively seeking licensing and commercialization opportunities for the novel device.
