Near-perfect optical absorber developed

A tunable device coated with a film 100 times thinner than the wavelength of incident light absorbs 99.75 percent of a specific mid-infrared band on demand, potentially expanding thermal detection and energy harvesting applications.

Designed by scientists at the School of Engineering and Applied Sciences (SEAS) and the University of California, San Diego, the near-perfect absorber is composed of a 180-nm-thick layer of vanadium dioxide (VO₂) on a sheet of sapphire, a standard substrate for growing vanadium dioxide.

Scientists previously have created perfect absorbers, but none have had such versatile properties, the team said. When two mirrors sandwich an absorbing material in a Fabry-Perot cavity, for instance, light simply reflects back and forth until it is mostly gone. Other devices employ surfaces with nanoscale metallic patterns that trap and eventually absorb the light.

Left: The experimental setup used for measuring the reflectivity of the vanadium-sapphire device. The vanadium oxide layer is only 180 nm thick, much thinner than the wavelength of the incident infrared light. Right: At just the right temperature (light blue line), the reflectivity of the device drops almost to zero (99.75 percent absorbance) for infrared light at a wavelength of 11.6 µm.

The SEAS device – structurally simpler than anything tried before – uses a highly unusual approach to achieve better results. The team exploited a “kind of naturally disordered metamaterial, along with thin-film interference effects, to achieve one of the highest absorption rates we’ve ever seen,” said Federico Capasso, Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in electrical engineering at SEAS.

“By changing temperature, one reaches a ‘perfect’ absorbing state with 99.75 percent absorption at a specific mid-infrared wavelength,” Capasso, principal investigator, told Photonics Spectra. “We were surprised because we did not expect that this could happen for a film so much thinner (180 nm) than the wavelength (λ/65). Then my student Mikhail Kats realized that an absorption resonance can exist for a film that is much thinner than the wavelength of light, when the imaginary part of its refractive index is comparable to its real part, and that resonance yields a perfectly absorbing state due to destructive interference effects.”

Capasso’s team took advantage of some surprising properties of VO₂ and sapphire.

Although VO₂ is typically an insulating material, it undergoes a dramatic transition when taken from room temperature to about 68 °C. As the temperature increases, the crystal rearranges itself, and metallic islands appear as specks, scattered within the material. More appear until the material becomes uniformly metallic. When manipulated correctly, those properties are ideal for infrared absorption.

“We wanted to investigate the optical properties across the insulator-to-metal transition in vanadium oxide, suspecting that some interesting properties might arise since the transition is not abrupt as the temperature is raised,” Capasso said. “In fact, we found out that in the transition region, the vanadium oxide behaves as a meta-material with tunable refractive index.”

The underlying sapphire substrate also plays a significant role. 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.

An artist’s rendition of the experimental setup used to measure the reflectivity of the Harvard SEAS and University of California, San Diego, perfect absorber researchers created using unusual materials and interference effects.

Both materials have significant optical losses, and the researchers demonstrated that when light reflects between lossy materials, instead of transparent or highly reflective ones, strange interface reflections occur, Kats said. “When you combine all of those resulting waves, you can coax them to destructively interfere and completely cancel out.”

Because the device can be switched easily between its absorbent and nonabsorbent states, it is suitable for a variety of applications, including thermal imaging devices (bolometers) with tunable absorption, spectroscopy devices, tunable filters, thermal emitters, radiation detectors and energy-harvesting equipment.

The team would next like to achieve the same effect by electrically tuning the insulator-to-metal transition, rather than using thermal tuning, Capasso said. “By periodically turning on and off the bias, one should then be able to make a fast infrared modulator.”

Details of the device were published in Applied Physics Letters (doi: 10.1063/1.4767646).