Narrowband mid-infrared thermal emitters based on the Fabry-Perot type of bound states in the continuum

The development of narrow-band thermal emitters operating at mid-infrared (MIR) wavelengths is vital in numerous research fields. However, the previously reported results obtained with metallic metamaterials were not successful in achieving narrow bandwidths in the MIR region, which suggests low temporal coherence of the obtained thermal emissions. In this work, we demonstrate a new design strategy to realize this target by employing the bound state in the continuum (BIC) modes of the Fabry-Perot (FP) type. When a disk array of high-index dielectric supporting Mie resonances is separated from a highly reflective substrate by a low refractive index spacer layer with appropriate thickness, the destructive interference between the disk array and its mirror with respect to the substrate leads to the formation of FP-type BIC. Quasi-BIC resonances with ultra-high Q-factor (>10³) are achievable by engineering the thickness of the buffer layer. This strategy is exemplified by an efficient thermal emitter operating at a wavelength of 4.587 µm with the on-resonance emissivity of near-unity and the full-width at half-maximum (FWHM) less than 5 nm even along with consideration of metal substrate dissipation. The new thermal radiation source proposed in this work offers ultra-narrow bandwidth and high temporal coherence along with the economic advantages required for practical applications, compared to those infrared sources made from III-V semiconductors.