Femtosecond lasers, characterized by ultrashort pulse durations and extremely high peak power, facilitate strong nonlinear optical interactions with matter on femtosecond timescales. These features not only significantly enhance temporal resolution and dynamic control precision in the manipulation of micro and nanoparticles, but also provide new pathways for ultrafast non-contact precision manipulation.
Within the framework of the optical Bloch equation, the optomechanical behavior of a cascade three-level atom driven by a femtosecond laser field is investigated under multiphoton interaction processes, with particular attention paid to the evolution characteristics and control mechanism of the optical force. The results show that both the transverse and longitudinal optical forces exhibit rapid oscillations with approximately twice the carrier frequency, but their impulse accumulation mechanisms are different. In the lower-intensity region, the transverse impulse accumulates continuously with increasing pulse interaction time or electric-field amplitude, corresponding to either a focusing effect toward the optical axis or a defocusing effect away from the optical axis. In contrast, the longitudinal impulse shows periodic oscillations, and its evolution is consistent with the population dynamics of the atomic levels. For atoms initially prepared in the ground state, the longitudinal impulse remains positive. Different multiphoton resonance conditions correspond to different dominant transition pathways, leading to different directions and magnitudes of the transverse impulse. Under strong-field conditions, the transverse impulse no longer varies monotonically, but exhibits complex fluctuations and oscillations associated with population redistribution.
Carrier-frequency detuning can further regulate the magnitude and direction of the transverse impulse, causing the transverse optical action to switch between focusing and defocusing. Near the single-photon resonance region, the transverse impulse can change from negative to positive with increasing electric field amplitude, indicating a transition from focusing to defocusing. Near the two-photon resonance region, the transverse impulse is mainly accumulated in the negative direction, while blue detuning weakens the focusing effect and may even lead to a reversal of the impulse direction. Near the three-photon resonance region, the transverse impulse mainly remains negative and shows a relatively stable focusing feature.
The results indicate that effective control of both the magnitude and direction of the optical force can be achieved by tuning the carrier frequency and electric-field amplitude. This work provides theoretical guidance and parameter-design references for applications of femtosecond lasers in quantum control, cold-atom physics, and ultrafast precision manipulation.