  Wavelength-selective thermal emitters (WS-EMs) hold considerable appeal due
to the scarcity of cost-effective, narrow-band sources in the mid-to-long-wave
infrared spectrum. WS-EMs achieved via dielectric materials typically exhibit
thermal emission peaks with high quality factors (Q factors), but their optical
responses are prone to temperature fluctuations. Metallic EMs, on the other
hand, show negligible drifts with temperature changes, but their Q factors
usually hover around 10. In this study, we introduce and experimentally verify
a novel EM grounded in plasmonic quasi-bound states in the continuum (BICs)
within a mirror-coupled system. Our design numerically delivers an
ultra-narrowband single peak with a Q factor of approximately 64, and
near-unity absorptance that can be freely tuned within an expansive band of
more than 10 {\mu}m. By introducing air slots symmetrically, the Q factor can
be further augmented to around 100. Multipolar analysis and phase diagrams are
presented to elucidate the operational principle. Importantly, our infrared
spectral measurements affirm the remarkable resilience of our designs'
resonance frequency in the face of temperature fluctuations over 300 degrees
Celsius. Additionally, we develop an effective impedance model based on the
optical nanoantenna theory to understand how further tuning of the emission
properties is achieved through precise engineering of the slot. This research
thus heralds the potential of applying plasmonic quasi-BICs in designing
ultra-narrowband, temperature-stable thermal emitters in mid-infrared.
Moreover, such a concept may be adaptable to other frequency ranges, such as
near-infrared, Terahertz, and Gigahertz.
