Fig. 1

Thermal photonics with broken symmetries. Without nanophotonic engineering (central panel), the general thermal emission exhibits omnidirectional, incoherent, unpolarized, and reciprocal features. The spectral density of an ideal thermal emitter with unity emissivity can be described by Planck’s law of blackbody emission. With broken geometrical symmetries (top panel), using anisotropic metastructures can tailor emission angle and polarization; Aperiodic or randomized structures may expand the emission band; Chiral structures show differential responses for circularly polarized thermal emission; Twist-optical metastructures are responsible for twist-angle sensitive narrow band thermal emission. With engineered mode symmetries (left panel), Fano resonance, bound states in the continuum, and non-Hermitian at the exceptional point can be formed as peculiar optical states. The mode symmetries play an important role in the formation of each optical state, enabling unconventional thermal emission control such as asymmetric spectral line-shape of emissivity, ultrahigh quality factor, and the transition of emissivity near the exceptional point, respectively. To break reciprocity (lower right panel), magneto-optical materials and spatiotemporal modulation can be exploited for nonreciprocal thermal emission control, indicating the ability to violate Kirchhoff’s law of thermal radiation