  In nature adaptive coloration has been effectively utilized for concealment
and signaling. Various biological mechanisms have evolved that can tune the
reflectivity for visible and ultraviolet light. These examples inspire many
artificial systems for mimicking adaptive coloration to match the visual
appearance to their surroundings. Thermal camouflage, however, has been an
outstanding challenge which requires an ability to control of the emitted
thermal radiation from the surface. Here we report a new class of active
thermal surfaces capable of efficient real-time electrical-control of thermal
emission over the full infrared (IR) spectrum without changing the temperature
of the surface. Our approach relies on electro-modulation of IR absorptivity
and emissivity of multilayer graphene via reversible intercalation of
nonvolatile ionic liquids. The demonstrated devices are light (30 g/m2), thin
(<50 um) and ultra-flexible which can conformably coat their environment. In
addition, combining active thermal surfaces with a feedback mechanism, we
demonstrate realization of an adaptive thermal camouflage system which can
reconfigure its thermal appearance and blend itself with the thermal background
in a few seconds. Furthermore, we show that these devices can disguise hot
objects as cold and cold ones as hot in a thermal imaging system. We anticipate
that, the electrical control of thermal radiation will impact on a variety of
new technologies ranging from adaptive IR optics to heat management for outer
space applications.
