Transient Symmetry Breaking in a Pumped Crystal via the Electric-Field-Dependent Stark Effect

We have investigated the transient properties of a ZnO crystal under a pump pulse (pumped crystal) using a pump-probe methodology. By monitoring the second harmonic of the weak probe pulse generated from the pumped crystal, we observe that the angular distribution of the second-harmonic yield becomes asymmetric compared to that without the pump. Importantly, the asymmetry of the second-harmonic generation (SHG) appears to be retarded by nearly 25 fs relative to the pump pulse, and its scaling shows a highly nonlinear dependence on the pump intensity. In addition, by using a two-color pump pulse, we further demonstrate that the asymmetry of SHG is controllable by adjusting the electric field of the two-color pulse. These phenomena can be attributed to the strong-field-induced electric-field-dependent Stark effect. Unlike the conventional intensity-dependent Stark effect, where only energy levels of the pumped target move, electric-field-dependent Stark effect indicates that the excitation induced by the pump pulse can distort the orbital and plays a vital role on the transient property of the pumped crystal. Our work suggests a new freedom for rationally designing the transient property of crystals by controlling the electric field of the pump pulse, which paves the way for realizing petahertz high-speed signal processing.