Nanospectroscopy of a single patch antenna strongly coupled to a mid-infrared intersubband transition in a quantum well

Scanning-probe-assisted mid-infrared nano-spectroscopy is employed to reveal the polaritonic dispersion of individual MIM (metal-insulator-metal) square patch antennas whose modes can be strongly coupled to a mid-infrared intersubband transition. The patch antenna side length L sets the resonances between lambda =5.5 mu m and 12.5 mu m. The active region consists of a highly doped AlInAs/InGaAs/AlInAs single quantum well that presents an intersubband transition at 1190cm(-1) (lambda =8.4 mu m). When the patch antenna optical resonance approaches and matches the intersubband transition frequency (L similar to 1.8 mu m), a clear anticrossing behavior-evidence of strong coupling-is observed in the near-field scattering phase spectra of individual antennas. The measured Rabi splitting is 4.5THz. The near-field scattering spectra agree with the far-field extinction spectra acquired on arrays of identical antennas.