On Observability of the Ac Stark Effect Line Splitting in Case of Beams of Finite Size

The small signal gain distortions in the field of a Gaussian pump beam were investigated theoretically and experimentally in a Λ-type quantum system depending on the size and power of the pump beam. The strong pump beam induces a change of refraction index of a methanol model material, depending on the frequency of the signal. It leads to a nonlinear active waveguiding effect causing distortion (narrowing and shift) of the signal line shape. In case of Stark splitting, the induced nonlinear active waveguiding effect manifests itself in the disappearance of the line split, for narrow pump beams. Collision induced frequency shift of the signal line depending on the pressure of the methanol gas was observed.