Hybrid longitudinal-transverse phonon polaritons

Abstract

Phonon polaritons, hybrid light-matter quasiparticles resulting from strong coupling of the electromagnetic field with the lattice vibrations of polar crystals are a promising platform for mid-infrared photonics but for the moment there has been no proposal allowing for their electrical pumping. Electrical currents in fact mainly generate longitudinal optical phonons, while only transverse ones participate in the creation of phonon polaritons. We demonstrate how to exploit long-cell polytypes of silicon carbide to achieve strong coupling between transverse phonon polaritons and zone-folded longitudinal optical phonons. We develop a microscopic theory predicting the existence of the resulting hybrid longitudinal-transverse excitations. We then provide an experimental observation by tuning the resonance of a nanopillar array through the folded longitudinal optical mode, obtaining a clear spectral anti-crossing. The hybridisation of phonon polaritons with longitudinal phonons could represent an important step toward the development of phonon polariton-based electrically pumped mid-infrared emitters.

Optical Nonlocality in Polar Dielectrics

“…Here the Fröhlich resonance, radiative in the far field, plays the part of the bright state while longitudinal modes are dark. This behavior is analogous to the weak phonon coupling previously observed in larger 4H-SiC particles, where the weak phonon frequency was determined by the lattice periodicity rather than particle morphology [9,27]. Fano interference acts to reduce the bright state's far-field scattering, while increasing absorption as energy is funneled into nonradiative dark states [43,44].…”

“…We demonstrated that the negative dispersion of optical phonons, and the existence of a finite transverse frequency bounding the reststrahlen band from below, leads to a rich and unique nonlocal phenomenology. In particular, opening the possibility to resonantly couple photonic excitations to propagative longitudinal modes makes it possible to engineer hybrid longitudinal-transverse SPhPs, recently highlighted as an important stepping stone toward the realization of electrically pumped SPhP devices [27], in generic nanoscale objects.…”

“…As SPhP concentrators grow smaller the dielectric local theory used to model them is also expected to break down, as LO phonons hybridize with the photon field and perturb the system response. Far from being a nuisance, hybridization of LO phonons and SPhPs has been suggested as a possible path toward electrical injection of SPhPs, and was recently demonstrated by using elongated silicon carbide (SiC) polytypes [27], where a weak phonon mode [28] naturally crosses the SPhP dispersion.…”

Image polaritons in boron nitride for extreme polariton confinement with low losses

“…This band is characterized by a negative real part of the permittivity. A characteristic feature of SPhPs 1 , 8 , 9 is their ability to confine radiation to subdiffractional length scales 10 , 11 , benefitting various applications such as surface-enhanced infrared spectroscopy 4 , 12 , 13 , thermal energy harvesting 14 , and nanoscale heat transfer 3 , among others. In contrast to diffraction-limited electromagnetic modes in dielectric waveguides, the polariton wavelength can be continuously reduced by shrinking the dimensions of polariton-supporting materials.…”

Polariton panorama

“…Hybrid modes produced by plasmons in graphene and molecular vibrations of absorbates on the graphene surface may enable high-selectivity sensing mechanisms [216,217]. A special case of hybrid modes is hybrid longitudinal-transverse phonon polaritons [218]. Polaritonic heterostructures with phase change materials enable persistent switching of polaritonic response under thermal and optical stimuli [219].…”