Christopher R. Gubbin

Low-voltage polariton electroluminescence from an ultrastrongly coupled organic light-emitting diode

We demonstrate electroluminescence from Frenkel molecular excitons ultrastrongly coupled to photons of a metal-clad microcavity containing a 2,7-bis[9,9-di(4-methylphenyl)-fluoren-2-yl]-9,9-di(4-methylphenyl)fluorene emissive layer. Thin layers of molybdenum oxide and 4,7-diphenyl-1,10-phenanthroline are used as hole and electron injection layers, respectively. The fabricated devices exhibit an electroluminescence threshold of 3.1 V, a value that is below the bare exciton energy. This result is found to be independent of detuning and consistent with a two-step process for polariton formation. Moreover, we investigate the quantum efficiency of carrier to polariton to photon conversion and obtain an external quantum efficiency of 0.1% for the fabricated structures, an improvement of 5 orders of magnitude over previous reports.

Strong and Coherent Coupling between Localized and Propagating Phonon Polaritons.

Following the recent observation of localized phonon polaritons in user-defined silicon carbide nanoresonators, here we demonstrate strong and coherent coupling between those localized modes and propagating phonon polaritons bound to the surface of the nanoresonators substrate. In order to obtain phase matching, the nanoresonators have been fabricated to serve the double function of hosting the localized modes, while also acting as a grating for the propagating ones. The coherent coupling between long lived, optically accessible localized modes, and low-loss propagative ones, opens the way to the design and realization of phonon-polariton based coherent circuits.

Theoretical investigation of phonon polaritons in SiC micropillar resonators

Of late there has been a surge of interest in localized phonon polariton resonators which allow for the subdiffraction confinement of light in the midinfrared spectral region by coupling to optical phonons at the surface of polar dielectrics. Resonators are generally etched on deep substrates which support propagative surface phonon polariton resonances. Recent experimental work has shown that understanding the coupling between localized and propagative surface phonon polaritons in these systems is vital to correctly describing the system resonances. In this paper we comprehensively investigate resonators composed of arrays of cylindrical SiC resonators on SiC substrates. Our bottom-up approach, starting from the resonances of single, free-standing cylinders and isolated substrates, and exploiting both numerical and analytical techniques, allows us to develop a consistent understanding of the parameter space of those resonators, putting on firmer ground this blossoming technology.

Real-space Hopfield diagonalization of inhomogeneous dispersive media

We introduce a real-space technique able to extend the standard Hopfield approach commonly used in quantum polaritonics to the case of inhomogeneous lossless materials interacting with the electromagnetic field. We derive the creation and annihilation polaritonic operators for the system normal modes as linear, space-dependent superpositions of the microscopic light and matter fields. We prove their completeness and invert the Hopfield transformation expressing the microscopic fields as functions of the polaritonic operators. As an example, we apply our approach to the case of a planar interface between vacuum and a polar dielectric, showing how we can consistently treat both propagative and surface modes, and express their nonlinear interactions, arising from phonon anharmonicity, as polaritonic scattering terms. We also show that our theory, including the proof of completeness, can be naturally extended to the case of dissipative materials.

Strong Coupling of Epsilon-Near-Zero Phonon Polaritons in Polar Dielectric Heterostructures.

We report the first observation of epsilon-near-zero (ENZ) phonon polaritons in an ultrathin AlN film fully hybridized with surface phonon polaritons (SPhP) supported by the adjacent SiC substrate. Employing a strong coupling model for the analysis of the dispersion and electric field distribution in these hybridized modes, we show that they share the most prominent features of the two precursor modes. The novel ENZ-SPhP coupled polaritons with a highly propagative character and deeply subwavelength light confinement can be utilized as building blocks for future infrared and terahertz nanophotonic integration and communication devices.

Nonlinear Response and Strong Coupling of Surface Phonon Polaritons

Analogous to surface plasmon polaritons in metals, polar dielectrics support surface phonon polaritons (SPhP) in the mid infrared (MIR) spectral region. In contrast to their plasmonic counterpart, however, SPhPs exhibit much longer lifetimes, and hence constitute a promising alternative in the development of nanophotonics. Employing an MIR free-electron laser, we here experimentally reveal resonant second harmonic generation from SPhPs, excited in the Otto geometry for prism coupling. In a second system featuring an ultrathin layer on top of the SPhP active material, we observe strong coupling of the bulk SPhP to the epsilon near zero mode supported by the thin layer. Our experimental findings are corroborated using a specifically developed matrix formalism for anisotropic multilayer wave propagation.