Ultra-Narrowband Mid-Infrared Thermal Emitters Based on Phononic Metasurfaces

In this work, we demonstrate a new class of ultra-narrowband coherent emitters in the mid-IR range using bound states in the continuum (BICs) [1]. Most of the existing sources in the mid-IR range are thermal emitters and consequently spontaneous, incoherent, and broadband. Manufacturing a narrowband coherent source is vital in many applications including sensing, lasing, and optical communications. In addition, the ultra-narrowband emitter investigated in this work can pave the way to design and fabricate Vertical Cavity Surface Emitting Lasers (VCSELs) in the 10-12 um wavelength range.<br/> <br/>Although several works have been done to design and fabricate narrowband emitters at visible and near-IR regimes, this goal is still challenging in mid-IR wavelengths. By utilizing polar materials like silicon carbide (SiC) supporting surface phonon polaritons (SPhPs) one can achieve partially coherent emitters with near-unity emissivity [2]. Indeed, the surface waves excited by SPhPs generate thermal radiation with enhanced intensity beyond blackbody radiation; however, the quality factor of the emission peak has thus far been limited due to the intrinsic losses of plasmonic and phononic materials. We have shown by utilizing Friedrich-Wintgen BICs [3], we can establish a high <i>Q-</i>factor coherent source with a near-unity emissivity For both TE and TM polarization. The emission peak of the phononic metasurface has a <i>Q-</i>factor as high as 3,400 which is two orders of magnitude higher than any plasmonic or photonic structure in this wavelength range. Moreover, our simulated results show that the proposed structure has a reasonable fabrication tolerance meaning that any small deviations from the desired construction parameters may not affect the amplitude and Q-factore of the emission peak drastically.<br/> <br/>References:<br/>1. Marinica, D., A. Borisov, and S. Shabanov, <i>Bound states in the continuum in photonics.</i> Physical review letters, 2008. <b>100</b>(18): p. 183902.<br/>2. Greffet, J.-J., et al., <i>Coherent emission of light by thermal sources.</i> Nature, 2002. <b>416</b>(6876): p. 61.<br/>3. Azzam, S.I., et al., <i>Formation of bound states in the continuum in hybrid plasmonic-photonic systems.</i> Physical review letters, 2018. <b>121</b>(25): p. 253901.