Coated Sapphire Is Invisible to Infrared Cameras
Infrared detectors are used to see objects otherwise hidden under cover of darkness. The dectors pick up the heat given off by living bodies, warm buildings and vehicles and reveal them as glowing objects when viewed through infrared goggles or cameras. If a building, body or vehicle is cold, the detector typically can't visualize it.
Now new technology from researchers at Harvard School of Engineering and
Applied Sciences could turn infrared detection on its head. The technique not only makes hot objects appear cold to infrared detectors — which could help hide soliders from their enemies at night — but it can be also used to make an infrared camera so sensitive that even cold objects would look relatively bright.
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The researchers coated a millimeter-thick sheet of
sapphire with a 180-nanometer layer of
vanadium dioxide, which is used as an insulator. Next, they heated the layered material to 154 degrees Fahrenheit (68
degrees Celsius). At that temperature, the crystal structure of the vanadium dioxide became altered, changing it from an insulator to a metallic conductor.
When the scientists shone infrared light onto the altered material, they found it was a near-perfect absorber, soaking up 99 percent of the infrared light that hit it. It worked because the infrared light waves bounced off the sapphire get absorbed by the vanadium dioxide, and any light waves that escape destructively interfere with each other so that they cancel out.
When the scientists cooled the layers down, the materials returned to their former states.
Mikhail Kats, the lead author of the research, told Discovery News that
vanadium dioxide, unlike other materials, absorbs infrared radiation
differently depending on its temperature — it can be tuned.
If one could coat a vehicle or building with this material, it would make the objects invisible — or at least black — to an infrared camera, since any infrared radiation emitted by the objects would be absorbed by the material before it could escape.
And because the material is so sensitive to infrared light, it could also work as a detector. Any imaging device has to absorb light in order to translate
it into electrical signals and make a picture. The more sensitive the sensor, the better the detector.
One big challenge was making the vanadium dioxide crystals
pure enough. Any flaws would mean it lost its unique reaction to temperature.
The research appears in the latest issue of Applied Physics Letters.
Credit: Harvard University / Kirill Nadtochiy