Engineering the Spectral and Spatial Dispersion of Thermal Emission via Strong Coupling

Phonon polaritons are quasiparticles comprising a photon and a coherently oscillating charge on a polar lattice, which are supported in the form of propagating (SPhP) and localized surface phonon polaritons (LSPhP). The promising properties of LSPhP modes are exceptionally high predicted Purcell enhancements and narrow resonance linewidths, with the potential for near-unity absorption (emissivity). However, one drawback is that as a highly localized mode, they offer no significant degree of spatial coherence (directionality) for thermal emission applications. Alternatively, high spatial coherence can be achieved using propagating SPhPs launched by grating elements. However, the non-localized nature of such propagating modes yields thermal emission into frequency-specific angles across the entire Reststrahlen band where such modes can be supported. The introduction of strong coupling between different polaritonic modes, therefore, provides us an opportunity to combine the virtues of the narrowband LSPhP resonances with the high spatial coherence associated with propagating SPhPs into a novel, mixed character polariton. Further, it has been proposed that strong coupling between LSPhPs with zone-folded longitudinal optic phonons (ZFLO) could provide a mechanism to use the longitudinal fields of an electrical bias to stimulate the transverse fields of SPhPs through Ohmic loss. Thus, we propose that through inducing strong coupling between LSPhPs, propagating SPhPs, and ZFLO phonons, that realization of a narrow-band, spatially coherent emitter amenable to electrically driven emission could be possible. Additionally, through coupling to such a ZFLO mode, the extremely narrow linewidths could be employed via strong coupling to further reduce the linewidths of the SPhP modes. In this work, we report on three-oscillator strong coupling within a SPhP platform using nanopillar arrays fabricated into a 4H-SiC substrate. Here, we experimentally manipulate the dispersion relation of coupled SPhP modes by strongly coupling LSPhPs, propagating SPhPs, with the ZFLO. In the strong coupling regime, the formation of such hybrid modes with mixed character is expected. Furthermore, the strength of the interactions between such optical modes can be precisely controlled through the hybridization of three oscillators. We further report on the influence of such strong coupling upon thermal emission within the long-wave-IR (LWIR), demonstrating significant narrowing of the spectral and spatial dispersion of the individual modes within this strongly coupled regime. In our three-oscillator strong coupling platform, we simultaneously demonstrate a five-fold reduction in the angular spread of the thermally emitted light and a three-fold enhancement of the quality factor over that of the uncoupled LSPhP mode at the anti-crossing point where the splitting occurs. Furthermore, the high Q-factors (over 200) achieved are realized using traditional photolithography, enabling such devices to be produced at large-scale and reasonable costs. Our results demonstrate that by leveraging three-oscillator strong coupling that the spectral and spatial dispersion of thermal emission can be engineered for a variety of LWIR applications extending from spectroscopy, sensing, to free-space communications