Fig. 1. Symmetry-based optical spin control of thermal radiation.
(A) Thermal radiation originates from fluctuating dipoles and is thus considered an incoherent signal. It is naturally broadband and omnidirectional and carries no SAM. (B and C) Recent research efforts aim to impart temporal (B) and spatial (C) coherence in thermal radiation, where narrow-band and directional thermal radiation are demonstrated. (D) In this work, by imparting spin coherence, we achieve effective tailoring of thermal emission in its spectral and spin properties. (E) Schematic demonstrates that the photon spin characteristics are governed by the symmetries in the 2D system. When both inversion (î) and mirror () symmetries are preserved, the photon spin of thermally radiated photons is degenerate in energy-momentum space. Thus, the spin and helicity both vanish. (F) When inversion symmetry is broken, spinning thermal radiation arises at oblique angles. However, the antisymmetric spin pattern guarantees the spin degeneracy at surface normal and a total-zero optical helicity. (G) In this work, we show that photon spin arises in an asymmetric pattern when both inversion and mirror symmetries are broken, and the nonvanishing optical helicity is observed.