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The concept of strong light–matter coupling has been demonstrated in semiconductor structures, and it is poised to revolutionize the design and implementation of components, including solid state lasers and detectors. We demonstrate an original nanospectroscopy technique that permits the study of the light–matter interaction in single subwavelength-sized nanocavities where far-field spectroscopy is not possible using conventional techniques. We inserted a thin (∼150 nm) polymer layer with negligible absorption in the mid-infrared range (5 μm < λ < 12 μm) inside a metal–insulator–metal resonant cavity, where a photonic mode and the intersubband transition of a semiconductor quantum well are strongly coupled. The intersubband transition peaks at λ = 8.3 μm, and the nanocavity is overall 270 nm thick. Acting as a nonperturbative transducer, the polymer layer introduces only a limited alteration of the optical response while allowing to reveal the optical power absorbed inside the concealed cavity. Spectroscopy of the cavity losses is enabled by the polymer thermal expansion due to heat dissipation in the active part of the cavity, and performed using atomic force microscopy (AFM). This innovative approach allows the typical anticrossing characteristic of the polaritonic dispersion to be identified in the cavity loss spectra at the single nanoresonator level. Results also suggest that near-field coupling of the external drive field to the top metal patch mediated by a metal-coated AFM probe tip is possible, and it enables the near-field mapping of the cavity mode symmetry including in the presence of a strong light–matter interaction.

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