A chameleon in the physics lab

October 25, 2013
A chameleon in the physics lab

Shriram Ramanathan and Frederico Capasso make the news again!
See the news article: A chameleon in the physics lab -LOOKING COOLER WHEN HEATED, A THIN COATING TRICKS INFRARED CAMERAS
See the scientific article: Vanadium Dioxide as a Natural Disordered Metamaterial: Perfect Thermal Emission and Large Broadband Negative Differential Thermal Emittance
Mikhail A. Kats, Romain Blanchard, Shuyan Zhang, Patrice Genevet, Changhyun Ko, Shriram Ramanathan, and Federico Capasso* School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA
(Received 24 April 2013; published 21 October 2013)

We experimentally demonstrate that a thin (approximately 150-nm) film of vanadium dioxide (VO2) deposited on sapphire has an anomalous thermal emittance profile when heated, which arises because of the optical interaction between the film and the substrate when the VO2 is at an intermediate state of its insulator-metal transition (IMT). Within the IMT region, the VO2 film comprises nanoscale islands of the metal and dielectric phases and can thus be viewed as a natural, disordered metamaterial. This structure displays ‘‘perfect’’ blackbodylike thermal emissivity over a narrow wavelength range (approximately 40 cm1), surpassing the emissivity of our black-soot reference. We observe large broadband negative differential thermal emittance over a >10 C range: Upon heating, the VO2-sapphire structure emits less thermal radiation and appears colder on an infrared camera. Our experimental approach allows for a direct measurement and extraction of wavelength- and temperature-dependent thermal emittance. We anticipate that emissivity engineering with thin-film geometries comprising VO2 and other thermochromic materials will find applications in infrared camouflage, thermal regulation, and infrared tagging and labeling.