The interaction between cavity modes and optical transitions leads to new coupled light-matter states in which the energy is periodically exchanged between the matter states and the optical mode. Here we present experimental evidence of optical strong coupling between modes of individual sub-wavelength metamaterial nanocavities and engineered optical transitions in semiconductor heterostructures. We show that this behaviour is generic by extending the results from the mid-infrared to the near-infrared. Using mid-infrared structures, we demonstrate that the light-matter coupling occurs at the (...) single resonator level and with extremely small interaction volumes. We calculate a mode volume of 4.9 × 10 from which we infer that only ~2,400 electrons per resonator participate in this energy exchange process. (shrink)