Thermally tunable nonreciprocal radiation in lithography-free vanadium dioxide-dielectric-Weyl semimetal stack

Abstract

Nonreciprocal thermal radiation (NTR) holds great promise in revolutionizing energy harvesting and conversion process, has become a hot topic of various research fields ranging from renewable energy, molecular sensing, to thermal circuitry and camouflage. Despite intensive effort into enhance the nonreciprocal response, static manipulation of NTR has limited the potential application, dynamic control of thermal radiation is highly demanded. Here, by using a multilayered stack comprised of vanadium dioxide (VO2) thin film, germanium (Ge) spacer layer, Weyl semimetal (WSM) layer terminated by molybdenum (Mo) substrate, we propose a thermally tunable nonreciprocal emitter. The effects of the incident angle (θi), azimuthal angle (ϕi), and the thickness of each layer for the VO2/Ge/WSM stack on the characteristics of thermal radiation have been discussed thoroughly. Dominant peaks and dips of nonreciprocity have been obtained with an incident angle of θi=60° and azimuthal angle of ϕi=0°. A switching effect between the near-unity nonreciprocity (”ON”) and zero nonreciprocity (”OFF”) states are induced when the temperature increases from 20 °C to 68 °C. It is shown that the magnitude and sign of the nonreciprocity can be managed simultaneously for transverse electric (s-) and transverse magnetic (p-) polarizations. The proposed structure shows good near-unity nonreciprocity stability in a wide range of θi and ϕi. This structurally lithography-free nonreciprocal thermal emitter not only provides the ability to tune nonreciprocal radiative properties, but also leads to the possible development of power scavenging and conversion devices.