Enhanced Far-Field Emission Via Dual Reststrahlen Bands in h-BN/SiO₂ Bilayer

Highly confined phonon polaritons enable strong light-matter interactions that tailor incandescent heat sources for enhanced thermal emission in both the near- and far-field regimes. However, single polar dielectric materials are limited in both the emission spectral range and achievable mode confinement. In this study, we employ a bilayer structure comprising monolayer hexagonal boron nitride (h-BN) integrated with silicon dioxide (SiO₂) to exploit confined phonon polariton modes across a broadened energy spectrum. The distinct, nonoverlapping Reststrahlen bands of h-BN and SiO₂ provide multiple spectral channels for polaritonic enhancement, improving far-field emission. We report a 3.4-fold enhancement in emissivity with the addition of h-BN to a SiO₂ nanoribbon. We identify the confined modes within the Reststrahlen bands with numerical modeling, revealing the enhancement mechanism. This effect is verified with direct thermal measurements by using a thermal bridge method, yielding a peak emissivity of 0.6. This work offers insights into engineering broad-band polaritonic thermal emitters.