We consider nonlinear selective reflection on the interface between di
electric media and dilute vapors of three-level atoms. Assuming a pump
-probe scheme with cascade-type excitation, we study the modification
Delta R of the reflection coefficient of the probe induced by the pres
ence of the pump. We extend previous calculations of the frequency spe
ctrum of Delta R made for low probe power to the case of arbitrary pro
be power. One then obtains a strong resonance which has already been o
bserved experimentally. Moreover, for the two-photon resonance present
in the low-intensity spectrum, a power splitting can be predicted wit
h increasing intensity