Enhanced Coherent Far-Field Thermal Emission in Bilayer Polar Dielectric Nanoribbons

Highly confined phonon polaritons have enabled a wide range of novel phenomena and applications such as enhanced thermal emission, nanophotonic devices and high-resolution imaging. Among these, the strong light-matter interactions of the phonon polariton allow the utilization of incandescent heat source as coherent thermal emission in both near-field and far-field regimes. Surface phonon polaritons (SPhPs) form evanescent waves confined along the surface of polar dielectrics, which can squeeze light beyond the diffraction limit, resulting in the coherent control of far-field thermal emission within the Reststrahlen band of the polar dielectrics. Earlier studies have utilized anisotropic SiO2 nanoribbons to demonstrate SPhPs-mediated, coherent far-field thermal emission, highlighting the significance of surface effects. To achieve further enhancement in emissivity, manipulations of energy dispersions of the emitters have been considered, including the addition of a low-loss dielectric layer to strengthen confinement and the sporadic coating of metal dots to broaden the emission spectrum through the hybridized polaritonic mode. Both modifications leave room for improvement in achieving high emissivity close to the blackbody limit, due to challenges in broadening the Reststrahlen bands while enhancing the resonance.

To address these challenges, we incorporate a monolayer of h-BN into polar dielectrics to investigate its potential for enhancing thermal emissions. Phonon polaritons in two-dimensional (2D) materials, such as hexagonal boron nitride (h-BN), exhibit strong confinement and ultralow loss, with a resonance peak in the mid-IR frequency range, near the thermal wavelength at room temperature (~10 μm). Specifically, h-BN exhibits two resonance frequencies at 1370 cm-1 and 780 cm-1, resulting in two Reststrahlen bands with negative permittivity: 780 to 830 cm-1 (Type I) and 1370 to 1610 cm-1 (Type II). Notably, these bands do not overlap with those of the SiO2. Recent developments also enable the production of high-quality, large-area 2D materials with variable atomic layer thickness. In addition to these unique properties in h-BN alone, studies also find that atomically thin dielectric layers on polar crystals construct ultra confined SPhPs far beyond free space wave vector. These characteristics present opportunities to enhance thermal emission by combining h-BN with polar dielectrics, leveraging the ultra-confined SPhPs modes and the broadened emission spectrum supported by h-BN.

We conducted an optical model to reveal the contribution with the presence of the h-BN layer which demonstrates over tenfold greater confinement compared to that of the bare SiO2 without the h-BN layer. In addition, hybridized modes are present within h-BN’s two Reststrahlen bands under the same configuration. Furthermore, we measured the emissivity of the individual h-BN/SiO2 nanoribbons incorporated into our sensitive thermometry platform. By introducing the h-BN layer, the emission spectrum is expanded across the mid-IR range, leading to an overall enhancement of emissivity up to 0.595 at 200 K.

This study provides direct experimental evidence of enhanced far-field thermal radiation originating from the combination of 2D materials and polar dielectrics. Such coherent thermal emitters, with specified spatial and spectral distribution, hold promise for applications in semiconductor thermal management and image sensing.