Résumé
We experimentally demonstrate an ultrastrong coupling regime between a gap plasmon and phonon in metallic-insulator-metallic antennas at far-infrared frequencies. These plasmonic antennas made of a silicondoped semiconductor (InAsSb) are versatile structures able to reveal the polaritonic modes due to the hybridization of a gap plasmon and phonon. The anticrossing behavior featuring the ultrastrong coupling is properly studied by varying the width of the metallic antenna array. The gap-plasmon resonance supported by the antennas is shown to be particularly sensitive to the presence of phonons in the GaSb insulator. The experimental data demonstrate a giant Rabi splitting of 30% of the GaSb transverse optical phonon energy and are in good agreement with both electromagnetic and semiclassical calculations.