Harnessing The Infrared: Materials and Structures for Breaking Reciprocity and Control of Thermal Radiation

In this talk I will discuss materials and photonic design concepts that allow us to experimentally observe breaking of optical reciprocity for thermal radiation, as well as metastructures that allow for considerable control of the thermal emission angular distribution. Thermal emission—the process through which all objects with a finite temperature radiate electromagnetic energy—has generally been thought to obey reciprocity, where the absorbed and emitted radiation from a body are equal for a given wavelength and angular channel. This equality, formalized by Gustav Kirchhoff in 1860, is known as Kirchhoff’s law of thermal radiation and has long guided designs to control the emitted radiation. There is considerable interest and numerous theoretical proposals for design of nonreciprocal absorbers that violate the Kirchhoff thermal radiation law. Until recently however, there were no experiments demonstrating this concept. I will discuss direct observation of the inequality between the spectral directional emissivity and absorptivity for an InAs photonic metastructure arising from the non-diagonal permittivity tensor of InAs at the epsilon-near-zero condition under an externally applied magnetic field. The magneto-optic response of magnetic Weyl semimetals is characterized by non-diagonal permittivity governed by the nontrivial Berry curvature that exists between recombinant Weyl nodes. I will also discuss reciprocity breaking in the magnetic Weyl semimetal Co₃Sn₂S₂, confirmed via observation of a net reflectance modulation that is nearly an order of magnitude higher than that of the typical transverse magneto-optical Kerr effect in ferromagnets, without the concurrent application of any external magnetic field, and discuss implications of these findings.