We present a comprehensive study on the influence of the metamaterial geometry on ultra-strong coupling to intersubband transitions. The spatial overlap of a metamaterial cavity mode and quantum-well region shows the strongest effect. © 2013 OSA.

We present ultra-strong light-matter interaction of a metamaterial mode and an intersubband transition for normal incidence radiation in the mid-infrared spectral region. The anti-crossed lines show a splitting of 15% of the central frequency. © 2013 The Optical Society.

We present a comprehensive study on the influence of the metamaterial geometry on ultra-strong coupling to intersubband transitions. The spatial overlap of a metamaterial cavity mode and quantum-well region shows the strongest effect. © OSA 2013.

We present a comprehensive study on the influence of the metamaterial geometry on ultra-strong coupling to intersubband transitions. The spatial overlap of a metamaterial cavity mode and quantum-well region shows the strongest effect. © 2013 OSA.

We present a comprehensive study on the influence of the metamaterial geometry on ultra-strong coupling to intersubband transitions. The spatial overlap of a metamaterial cavity mode and quantum-well region shows the strongest effect. © 2013 The Optical Society.

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 (~10 μm) to the near-infrared (~1.5 μm). 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(-4) (λ/n)(3) from which we infer that only ~2,400 electrons per resonator participate in this energy exchange process.