Hyperbolic phonon-polariton electroluminescence in 2D heterostructures

Phonon polaritons are quasiparticles resulting from the coherent coupling of photons with optical phonons in polar dielectrics1. Owing to their exceptional ability to confine electric fields to deep-subwavelength scales with low loss, they are uniquely poised to enable a suite of applications beyond the reach of conventional photonics, such as subdiffraction imaging2 and near-field energy transfer3, 4-5. The conventional approach to exciting phonon polaritons through optical methods, however, involves costly light sources along with near-field schemes6,7, and generally leads to low excitation efficiency owing to substantial momentum mismatch between phonon polaritons and free-space photons. Here we demonstrate that under proper conditions, phonon polaritons can be excited all-electrically by drifting charge carriers. Specifically, in hexagonal boron nitride (hBN)/graphene heterostructures, by electrically driving charge carriers in ultrahigh-mobility graphene out of equilibrium, we observe bright electroluminescence of hBN's hyperbolic phonon polaritons (HPhPs) at mid-infrared frequencies, which shows a temperature and carrier density dependence distinct from black-body thermal emission. Moreover, the carrier density dependence of the HPhP electroluminescence spectra reveals that HPhP electroluminescence can arise from both interband transition and intraband Cherenkov radiation8 of charge carriers in graphene. The HPhP electroluminescence offers avenues for realizing electrically pumped mid-infrared and terahertz phonon-polariton light sources.