Giant vacuum-field Rabi splitting of intersubband transitions in multiple quantum wells

A large, 42 meV, vacuum-field Rabi splitting was observed between the upper and lower intersubband polariton branches of a moderately n-doped GaAs-AlGaAs multiple quantum well sample. For this structure, the Rabi energy represents 17% of the bare intersubband resonance, a situation corresponding to the ultrastrong coupling regime between the photon and intersubband excitations. A different phenomenon is also demonstrated: the occurrence of a higher mode polariton. The large tunability of the intersubband absorption with carrier concentration may lead to new optoelectronic devices.