AC Stark shift ($\Delta E_g\simeq \hslash |\Omega {|}^2/(4\Delta )$)

The energy shift produced by an oscillating electric field, often called a light shift. For a far-detuned two-level drive with coupling $\hslash \Omega /2$ and $\Delta ={\omega }_L-{\omega }_{eg}$, the ground-state shift is approximately $\hslash |\Omega {|}^2/(4\Delta )$. Multilevel atoms can have polarization-dependent differential shifts.