Thermal radiation is inherently broadband and incoherent, limiting its utility in applications requiring spectral selectivity such as molecular spectroscopy and free-space optical communications. Bound states in the continuum (BICs) provide a powerful route to confine light in open photonic systems and realize resonances with extremely high quality factors. Here we demonstrate high-$Q$ light emission from a photonic-phononic nonlocal metasurface composed of monocrystalline silicon on a 4H-SiC substrate. The platform supports BIC-enabled ultranarrow band emission for both TE and TM polarizations; specifically, we identify an off-$\mathrm{\Gamma }$ Friedrich-Wintgen BIC and a $\mathrm{\Gamma }$-point symmetry-protected BIC for TE polarization and a $\mathrm{\Gamma }$-point symmetry-protected BIC for TM polarization. Under TM polarization, coupling between the photonic mode of the metasurface and surface phonon polaritons in the SiC substrate produces strong field localization and a symmetry-enforced complementarity between the photonic and phononic resonances, in which the photonic $Q$ factor diverges while the phononic mode radiates maximally at the $\mathrm{\Gamma }$ point. Using a bonding-based fabrication approach that integrates monocrystalline silicon with SiC through an optically ultrathin interlayer, we experimentally realize a quasi-BIC thermal emitter with a measured $Q$ factor of 181. This represents the highest reported $Q$ factor for thermal emitters operating at wavelengths longer than $7\mathrm{\mu }\mathrm{m}$ and establishes photonic-phononic nonlocal metasurfaces as a platform for coherent and spectrally selective midinfrared thermal radiation.

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• Bound states in the continuum
• Dispersion engineering
• Light-matter interaction
• Metasurfaces
• Photonics
• Strong electromagnetic field effects
• Surface phonon polaritons
• Carbon-based materials
• Device fabrication
• Dielectrics