Objective  Femtosecond laser micromachining application scenarios commonly occur in atmospheric environments. When the femtosecond laser is focused and interacts with air, it ionizes to produce air plasma, which has a direct impact on the whole machining process. Among other things, the interaction of Kerr self-focusing with plasma scattering leads to filamentation, which changes the light field distribution, and air ionization can significantly affect the laser energy acting on the material. Studying the interaction between femtosecond laser and air, especially the process of ionization of air by laser pulses, is the key to leapfrogging to enhanced applications. To deeply understand the laser micromachining process in atmospheric environment, the transient evolution characteristics of air plasma generated by focused femtosecond laser pulses are studied by building a femtosecond time-resolved pump-probe shadow imaging system, and the temporal characteristics of air plasma under different focusing conditions are numerically simulated.

  Methods  A high time-resolved pump-probe shadow imaging system was built. The laser beam is focused in the air by a microscopic objective and imaged by another laser beam for detection(Fig.1). A 20× microscope objective was used for high-resolution imaging of the plasma to record the time-space evolution of the air plasma. The optical range difference between the pump light and the detection light is adjusted to determine the detection time interval, and the spatial morphology of the air plasma is characterized by the shadow image on the CCD. In the numerical simulation, the ionization model in atmospheric environment is established to obtain the complete process of plasma generation and dissipation in light calling ionization.

  Results and Discussions   Under the microscope focusing conditions at 20×, the plasma is growing and moving rapidly from 0 fs to 59 fs; After 64 fs, it enters a slow evolutionary stage, with the shadow moving speed and propagation distance slowly decreasing; And at 135 fs, the plasma enters a saturation stage and the shadow signal intensity stops growing (Fig.3). The wave front velocity of its ionization keeps decreasing with time and is overall less than the speed of light (Fig.4). From the computational model, it is obtained that the transition times between multiphoton ionization and tunneling ionization are −1.158τ and −1.26τ for 20-fold and 40-fold focusing conditions, and the times of complete dissipation of free electrons are 23.2 ps and 13.1 ps, respectively.

  Conclusions  A laser pulse with a pulse width of 290 fs, a single pulse energy of 160 μJ, and a central wavelength of 1 026 nm was used as the pump light pulse, and the time-space process of the focused femtosecond pulse transport ionization in air was investigated by building a femtosecond time-resolved pump-probe shadow imaging experiment. The time course of air plasma generation and disappearance was obtained by solving the free electron rate equation for different injection energies and focusing conditions numerically. The experimental results show that the ionized air plasma is shuttle-shaped, and the transient electron number density increases and then decreases as the delay time increases, while the extension velocity of the plasma gradually decreases from 2.9×10⁸ m/s to 0. Solving the free electron rate equation shows that the transient electron number density of the air plasma ionized by the femtosecond laser is higher under the focusing conditions of the high-NA objective, and the tunneling ionization contributes to a higher electron number density in the whole ionization process. The number of electrons contributing to the whole ionization process is higher; And considering the diffusion and compounding of ions, the decay of plasma density is faster under high NA focusing conditions, and the ionization and diffusion processes present a high degree of temporal asymmetry. The process optimization parameters in femtosecond laser micromachining are numerous and extensive, and often require a lot of labor and resources for experimental exploration. The existing micromachining computational models are unable to reproduce the actual processing results due to the lack of key physical parameters such as transient electron eigenvalues in the simulation model. Femtosecond time-resolved pumping probe technology can provide an important tool for transient measurement of physical feature parameters and improve the simulation model, thus increasing the accuracy of simulation results. Overall, femtosecond time-resolved pump-probe shadow imaging can provide a means to observe transient physical processes in different processing environments and is expected to become a powerful tool for online monitoring of high-end femtosecond processing equipment.