Intersubband polaritons are robust against inhomogeneous broadening induced by multiple quantum wells

We show that intersubband (ISB) polaritons are robust to inhomogeneous broadening. In a series of multiple quantum well samples with mid-infrared ISB absorption transitions with broadenings varying by a factor of 5 (from 4 meV to 20 meV), we observed polariton linewidths lying in the 4-7 meV range only. We have verified the dominantly inhomogeneous origin of the broadening of the ISB transition, and that the linewidth reduction of the modes persists up to room-temperature. The immunity to inhomogeneous broadening is a direct consequence of the coupling of the large number of ISB oscillators to a single photonic mode.