Now you see it, now you don’t.
A new device invented at the Harvard School of Engineering and Applied Sciences (SEAS) can absorb 99.75 percent of infrared light that shines on it. When activated, it appears black to infrared cameras.
Composed of just a 180-nanometer-thick layer of vanadium dioxide (VO2) on top of a sheet of sapphire, the device reacts to temperature changes by reflecting dramatically more or less infrared light. This perfect absorber is ultrathin, tunable, and exceptionally well suited for use in a range of infrared optical devices.
Perfect absorbers have been created many times before, but not with such versatile properties. In a Fabry-Pérot cavity, for instance, two mirrors sandwich an absorbing material, and light simply reflects light back and forth until it’s mostly all gone. Other devices incorporate surfaces with nanoscale metallic patterns that trap and eventually absorb the light.
“Our structure uses a highly unusual approach, with better results,” says 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.”
With collaborators at Harvard and at the University of California, San Diego, Capasso’s research group took advantage of surprising properties in both of the materials they used.
Vanadium dioxide is normally an insulating material, meaning that it does not conduct electricity well. Take it from room temperature up to about 68 degrees Celsius, however, and it undergoes a dramatic transition. The crystal quickly rearranges itself as the temperature approaches a critical value. Metallic islands appear as specks, scattered throughout the material, with more and more appearing until it has become uniformly metallic.
Via Dr. Stefan Gruenwald