Fig. 1

Schematic graph of different strategies of thermal camouflage. a An object (yellow) can be thermally detected against the background (green) through its thermal radiation with an IR camera. The inset shows a typical image obtained by the IR camera. b A thermal metamaterial camouflages the object inside a closed conductive system. The rainbow colour represents a varying temperature. However, for the IR camera outside the system, whether a device is present does not make much difference. What is imbedded inside the host (thermal conductivity κ₀) can be anything. c The performance of the device becomes observable to the IR camera when a cross-section of the system is made. For a thermal cloak, whose conductivity κ _ijk is directly determined by the host conductivity (κ₀) and the spatial coordinates (x, y, z), its exterior temperature profile is restored. However, the object is also detectable with the IR camera, as seen from the result of ref.¹⁴ in the inset, where the object is an aluminium cylinder. d For radiative heat, the radiation intensity of the object can be changed by a covering film with a tuned emissivity ε (see the inset cross-section). However, this will only render a uniform apparent surface temperature T of the object (green), which is still detectable against a background surface with a temperature gradient (rainbow). e A surface thermal metamaterial (whose thermal conductivity κ _ijk is anisotropic and inhomogeneous but does not depend on the host conductivity κ₀, see the inset cross-section) brings back the radiation signal from the surface of the background to the top of the object, thereby thermally concealing the object. The thermal radiation signal is represented with wave arrows whose lengths and colours vary along the surface