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 (metalinsulator-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 k ¼ 5.5 lm and 12.5 lm. The active region consists of a highly doped AlInAs/InGaAs/ AlInAs single quantum well that presents an intersubband transition at 1190 cm1 (k ¼ 8.4 lm). When the patch antenna optical resonance approaches and matches the intersubband transition frequency (L 1.8 lm), 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 4.5 THz. The near-field scattering spectra agree with the far-field extinction spectra acquired on arrays of identical antennas.