Mid-infrared intersubband polaritons in dispersive metal-insulator-metal resonators

We demonstrate room-temperature strong coupling between a mid-infrared (λ = 9.9 μm) intersubband transition and the fundamental cavity mode of a metal-insulator-metal resonator. Patterning of the resonator surface enables surface-coupling of the radiation and introduces an energy dispersion which can be probed with angle-resolved reflectivity. In particular, the polaritonic dispersion presents an accessible energy minimum at k = 0 where—potentially—polaritons can accumulate. We also show that it is possible to maximize the coupling of photons into the polaritonic states and—simultaneously—to engineer the position of the minimum Rabi splitting at a desired value of the in-plane wavevector. This can be precisely accomplished via a simple post-processing technique. The results are confirmed using the temporal coupled mode theory formalism and their significance in the context of the strong critical coupling concept is highlighted.