Spin-Momentum Locked Thermal Metasurfaces

Incandescence, i.e., thermal emission, is the most ubiquituous form of light source, characterized by incoherent, broadband and unpolarized emission. Despite this, it remains widely used, especially in the mid-infrared spectrum, for its low cost and easy access. However, for many applications it is vital to control and select polarization, wavelength, temporal and spatial coherence and/or wavefront of emission. A stack of optical components, e.g., band pass filters, pinholes, polarizers, waveplates and refractive lenses may be used to tailor and control the features of thermal light, but at the cost of efficiency, footprint and setup complexity. Recently, we have been demonstrating that nonlocality engineering may offer a powerful approach to engineer spatially and temporally the coherence of thermal emission, offering powerful opportunities for thermal emission manipulation. In particular, quasi-bound states in the continuum (q-BIC) can be employed in thermal emission engineering, enabling high Q-factors and narrow emission angles. Moreover, the polarization states of these q-BIC thermal states can be arbitrarily defined by manipulating the scattering matrix. However, controlling the wavefront of thermal emission remains challenging, particularly when a specific polarization state of interest is required. One of the underlying reasons is that the wavefront control requires engineering local geometrical phases, and the spatial and temporal coherence must be maintained across these different phases. It is challenging to realize such a task in a single-layer metasurface, as these tuning knobs are frequently correlated.<br/>Here, we introduce and demonstrate spin-momentum locked wavefront engineering of thermal emission in an aperiodic metasurface platform. We leverage deliberate local perturbations of BIC thermal states, in a way that the non-local properties (spatial and temporal coherence) are preserved even when the local features (geometrical phase) are manipulated. The high Q-factor of the q-BIC enables temporal coherence, while spatial coherence is upheld through the modal dispersion of the unperturbed periodic structure. The resulting thermal emission is spin-momentum locked, i.e., opposite handednesses have reversed dispersion because of the single-layer geometry of choice. We use this platform to demonstrate beam steering of thermal emission by introducing linear phase gradients of geometric phase, and confirm that the non-local properties are preserved. Then, we explore advanced local phase control to demonstrate a cylindrical lens design, where the unit cells along one of the axes are completely aperiodic. Because of spin-momentum locking, the left-handed polarization is focused as in a convex lens geometry, while the opposite handedness diverges as in a concave lens. Additionally, different wavelengths of emission are all focused on the same focal plane, yet at different positions, mimicking a rainbow pattern with highly coherent focused light. In summary, our platform and the experimental demonstrations showcase a locally controlled non-local metasurface for spin-momentum locked thermal emission, which exemplifies exciting opportunities for thermal emission engineering and low-cost distributed thermal sources.