Imaging Strongly Coupled Plasmon–Phonon Modes in Mid-Infrared Double Antennas

Strong-coupling effects alter the polaritonic states of metal–dielectric nanostructures and, therefore, can be used to tailor the response of infrared (IR) photonic elements. We report on the development of a simple IR double-nanoantenna platform that achieves strong coupling between plasmon and phonon polaritons using extreme aspect ratio Al and amorphous SiO2 antennas of modest quality-factor, equaling coupling performance of platforms using novel materials. Our spatially resolved spectroscopy studies reveal Rabi splitting (26 meV) and offer a detailed study of the strongly coupled mode structure within the nanoantenna, with unprecedented spatial sensitivity (<10 nm). We imaged the spatial distribution of strongly coupled modes, accessing the local electromagnetic density of states of the system, obtaining key information on photonic population. Our study obtains strong-coupling behavior in hybrid nanosystems using geometry, rather than materials properties, promising high performance using cheaper and more abundant materials.