Abstract

It is well known that an intense pump laser at frequency ω_Linteracting with a two-level system of resonance frequency ω₀strongly modifies the frequency ω₀dependence of the linear refractive index of the weak probe.¹The refractive index spectrum shows characteristic features situated at ω_Land at the Rabi sideband frequencies ω_L±Ω′ where Ω′ is the generalized Rabi frequency $\Omega \text{'}={\left[{\left({\omega }_L-{\omega }_0\right)}^2+{\Omega }_L^2\right]}^{1/2}$and Ω_L, is the Rabi frequency associated with the pump laser. This triplet structure, which is consistent with the atomic energy structure modified by the ac Stark effect, shows up in the resonance flourescence, spectrum,² in the weak probe absorption profile,³and in the weak probe-induced four-wave mixing excitation spectrum¹of a strongly driven two-level system. As long as the probe intensity is weak, a probe field at ω_Lor at ω_L± Ω′ will not modify the refractive index of the pump laser. However, as we shall show, this behaviour changes if the probe radiation is allowed to become strong. Then, a strong probe near a Rabi sideband frequency appreciably modifies the nonlinear refractive index of the pump laser. The effect is stronger for the Rabi sideband close to the resonance frequency than for the sideband near the three-photon scattering frequency. An even smaller effect is obtained when the probe is at the pump frequency.

© 1994 IEEE