Three-photon resonance and adiabatic passage in the large-detuning Rabi model

We investigate theoretically the quantum Rabi model in the large-detuning regime. When the frequency of the cavity field is near one-third of the atomic transition frequency, we show that there exists a resonant three-photon coupling via intermediate states connected by counter-rotating processes. We derive an effective Hamiltonian of the three-photon coupling, which determines the magnitude of the energy splitting and the position of the resonance analytically. In addition, we present a scheme of three-photon generation based on adiabatic passage across the resonance.