  We extend the recently developed technique of opto-optical modulation (OOM)
to probe state-resolved ac-Stark-induced phase variations of a coherently
excited ensemble of helium atoms. In a joint experimental and theoretical
study, we find that the spatial redirection of the resonant emission from the
OOM process is different for the low-lying 1s2p state as compared with the
higher-lying Rydberg states, and that this redirection can be controlled
through the spatial characteristics of the infrared (IR) probe beam. In
particular, we observe that the intensity dependence of the IR-induced Stark
phase on the 1s2p emission is nonlinear, and that the phase accumulation
changes sign for moderate intensities. Our results suggest that OOM, combined
with precise experimental shaping of the probe beam, could be used to measure
the Stark-induced phase shifts of excited states.
