摘要： The optical Stark effect (OSE) stems from a coherent, nonlinear interaction between a transition and a non-resonant light field, which usually causes a blue-shift of the transition energy. This shift can be understood using the picture of so-called photon-dressed states or Floquet states. The perturbation induced by a light filed which is periodical in time results in a series of quasi-static Floquet eigenstates periodically spaced in units of the photon energy. The repulsion (hybridization) between the Floquet and equilibrium states causes a blueshift of the transition between the equilibrium states, as schematically shown in Fig. 1a. The OSE effect is intrinsically helicity-selective, that is, a left circularly polarized light couples only to the |0> to |+1> but not the |0> to |-1> transition and vice versa (the number in the ket is the azimuthal quantum number of the total angular momentum). Experimentally, the OSE can be conveniently observed using circularly polarized transient absorption (TA) spectroscopy. With a co-circularly polarized pump-probe configuration, the probe pulse measures a blue-shifted transition in the duration of the pump pulse and a derivative-like difference spectrum (with respect to the unpumped spectrum) can be detected (Fig. 1b), whereas with a counter-circularly polarized configuration nothing can be detected if multi-photon absorption is negligible.