Dual-polarization narrowband thermal vertical emitter with ultrahigh directionality

Thermal radiation is inherently temporally and spatial incoherent. Precise control over thermal emission, especially achieving narrowband and highly directional radiation, is crucial for enhancing the energy conversion efficiency of photovoltaic systems, gas sensing, etc. However, the realization of a perfect narrowband thermal emitter capable of vertical emission with dual-polarization support remains a fundamental challenge. In this work, we demonstrate a dual-polarization narrowband thermal emitter operating in the vertical direction, leveraging the mechanism of lattice resonance. The proposed structure comprises an array of germanium (Ge) nanocylinders disposed on a silicon dioxide (SiO2) spacer layer and a gold substrate. It supports efficient narrowband thermal emission at 3222 nm for both transverse-electric (TE) and transverse-magnetic (TM) polarizations at normal incidence, with peak emissivities exceeding 97% in both cases. The emissivity exhibits a sharp decline when the angular deviation exceeds merely 0.2° (for TE) and 1° (for TM) from the surface normal, underscoring its exceptional emission directionality. Furthermore, we demonstrate the potential of this emitter for highly sensitive detection of methane and ammonia gases, as well as for refractive index sensing, offering new strategies for the design of high-efficiency thermal photonic devices.