采用具有旋转对称性的三维空间电离模型, 对紧聚焦条件下中心波长为800 nm、脉冲宽度为50 fs的激光脉冲产生的空气等离子体的时间和空间特性进行了数值模拟, 揭示了整个等离子体区折射率的动态演化过程。计算结果表明：当激光脉冲开始聚焦时, 靠近焦点前方的空气首先被电离, 随后产生一个中心区域达到饱和、边缘具有很高折射率梯度的水滴状等离子区域; 随着激光脉冲继续向焦点传播并越过焦点, 等离子体区进一步向前延伸, 直至在焦点前后形成一个近乎对称的双卵形结构; 脉冲通过焦点区域后, 空气等离子体强度沿传播方向逐渐减弱, 直到最终完全消失; 在10倍物镜聚焦下, 脉冲能量为500 μJ、脉冲宽度为50 fs的激光脉冲产生的空气等离子体由产生到消失的整个过程约持续3 ps。理论计算结果与30 kPa低气压环境下的实验观测结果相吻合。

Both temporal and spatial characteristics of air plasma induced by tightly-focused laser pulse with central wavelength of 800 nm and pulse width of 50 fs are numerically simulated on the basis of a three-dimensional spatial ionization model with rotational symmetry, and the dynamic evolution of refractive index in whole plasma area is revealed. The simulation results clearly show that, when the laser pulse begins to focus, the ionization of air firstly appears in the region near the focal point. The initially ionized air gradually grows to form a water droplet-like plasma region with saturated electron density at the center and a high refractive index gradient in the peripheral. As the laser pulse propagates through the focal plane, the plasma zone expands forwards and forms an ionization region with almost symmetrical double-oval-shape. After the laser pulse propagates out of the focal region, the intensity of air plasma decreases along the direction of transmission and varnishes finally. Focused by a 10-fold microscope objective, the whole evolution process of the air plasma induced by laser pulse with pulse energy of 500 μJ and pulse width of 50 fs lasts about 3 ps. The simulation results can fit the experimental results obtained under low air pressure of 30 kPa.