Ultra-narrow bandwidth mid-infrared thermal emitters achieved with all-dielectric metasurfaces

Narrow bandwidth and highly efficient mid-infrared (MIR) thermal emitters with low manufacturing cost and simple geometries have been continuously pursued by scientists. Unfortunately, conventional MIR thermal emitters based on metallic metamaterials suffer from large bandwidths mainly due to dissipations losses. In this work, we present our experimental demonstration of MIR thermal emitters with ultra-narrow bandwidth by exploiting the quasi bound-state-in-the-continuum mode supported by an all-dielectric metasurface structure. The whole structure is composed of a zigzag array of germanium elliptical disks, separated from the gold conductive substrate by a dielectric spacer layer made from the refractory material of Al2O3. Our experimental results demonstrate that even with an amorphous form of evaporated materials for both dielectrics, an emission bandwidth around 80 nm in the MIR can be obtained, one order of magnitude narrower than those achieved with metallic metamaterials. The bandwidth can in principle be further narrowed by a large extent when dielectrics of better quality are used. We further demonstrate with numerical results that this thermal emitter features additional advantages including high directionality, linear polarization, and large spectral tunability. Our results represent a significant step towards the realization of high-performance and low-cost MIR thermal emitters.