Path-Dependent Thermal Metadevice beyond Janus Functionalities

Janus metamaterials, metasurfaces, and monolayers have received intensive attention in nanophotonics and 2D materials. Their core concept is to introduce asymmetry along the wave propagation direction, by stacking different materials or layers of meta-atoms, or breaking out-of-plane mirror asymmetry with external biases. Nevertheless, it has been hitherto elusive to realize a diffusive Janus metadevice, since scalar diffusion systems such as heat conduction normally operate in the absence of polarization control, spin manipulation, or electric-field stimuli, which all are widely used in achieving optical Janus devices. It is even more challenging, if not impossible, for a single diffusive metadevice to exhibit more than two thermal functions. Here a path-dependent thermal metadevice beyond Janus characteristics is proposed, which can exhibit three distinct thermal behaviors (cloaking, concentrating, and transparency) under different directions of heat flow. The rotation transformation mechanism of thermal conductivity provides a robust platform to assign a specific thermal behavior in any direction. The proof-of-concept experiment of anisotropic in-plane conduction successfully validates such a path-dependent trifunction thermal metamaterial device. It is anticipated that this path-dependent strategy can provide a new dimension for multifunctional metamaterial devices in the thermal field, as well as for a more general diffusion process.