Polaritonic hybrid-epsilon-near-zero modes: engineering strong optoelectronic coupling and dispersion in doped cadmium oxide bilayers

Polaritonic materials that support epsilon-near-zero (ENZ) modes offer the opportunity to design light-matter interactions at the nanoscale through phenomena like resonant perfect absorption and extreme sub-wavelength light concentration. To date, the utility of ENZ modes is limited in propagating polaritonic systems by a relatively flat spectral dispersion, which gives ENZ modes small group velocities and short propagation lengths. Here we overcome this constraint by coupling ENZ modes to surface plasmon polariton (SPP) modes in doped cadmium oxide ENZ-on-SPP bilayers. What results is a strongly coupled hybrid mode, characterized by strong anti-crossing and a large spectral splitting on the order of 1/3 of the mode frequency. The resonant frequencies, dispersion, and coupling of these polaritonic-hybrid-epsilon-near-zero (PH-ENZ) modes are controlled by tailoring the modal oscillator strength and the ENZ-SPP spectral overlap, which can potentially be utilized for actively tunable strong coupling at the nanoscale. PH-ENZ modes ultimately leverage the most desirable characteristics of both ENZ and SPP modes through simultaneous strong interior field confinement and mode propagation.