Simultaneous Switching of Spin Helicity and Band Dispersion in Reconfigurable Thermal Metasurfaces

ABSTRACT Integrating spin and momentum degrees of freedom in thermal photonics offers unprecedented control over light emission, yet achieving dynamic, simultaneous reconfiguration of both properties within a single planar platform remains elusive. Here, we present a reconfigurable spin‐thermophotonic platform based on bilaterally stub‐loaded germanium waveguide metasurfaces that break in‐plane mirror symmetry. Driven by the inherent thermo‐optic effect, a cooperative dual‐switching paradigm is demonstrated, simultaneously inverting the radiative spin helicity and transforming the underlying band profiles. Spectroscopic measurements firmly substantiate these coexisting states, experimentally revealing a 3.6‐fold enhancement in the optical density of states (DOS) within the flatband mode and an extended far‐field spatial coherence length of 0.09 mm sustained by the highly dispersive branch. Furthermore, while normal emission unlocks a broad raw spectral tuning capability of 88 nm across a 160 K temperature excursion, a momentum‐space compensation scheme at a 6° oblique angle suppresses loss‐induced cross‐talk, securing a high‐purity chiral switching window spanning 34 nm with high temporal coherence ( Q ∼ 90) and an emission circular dichroism exceeding 0.86. Our platform offers a compact, scalable pathway toward active, on‐chip coherent thermal light sources for advanced infrared technologies.