纳秒激光脉冲在空气中聚焦的临界自由电子密度问题

实验研究了高功率纳秒量级激光脉冲在空气中聚焦时的能量透过率随输入激光脉冲能量变化的规律，发现在纳秒激光脉冲聚焦半径相同的情况下，激光脉冲的能量透过率随入射激光脉冲能量的变化可分为三种情况：当入射激光脉冲能量较低时，激光脉冲能量全部通过；当入射激光脉冲能量增大后，激光脉冲的能量透过率由近100%迅速减小；当入射激光脉冲的能量进一步增加时，激光脉冲的能量透过率继续缓慢变小.用临界自由电子密度以及所对应的临界时间点对上述实验现象进行了理论分析得到了如下结论：当自由电子密度未达到临界自由电子密度时，多光子电离过程起主要作用，而当自由电子密度超过临界自由电子密度后，逆韧致吸收过程起主要作用，临界时间点是入射激光脉冲与空气作用过程中自由电子密度达到临界自由电子密度的时刻.入射激光脉冲能量决定了临界时间点在脉冲作用时间上的位置，临界时间点的位置决定了激光脉冲的能量透过率.可以通过测量激光脉冲的能量透过率来计算出临界自由电子密度，从而确定出激光脉冲在空气中聚焦时的能量透过特性. 关键词： 临界自由电子密度 临界时间点 多光子电离 逆韧致吸收

Study on the critical free electron density for nanosecond laser pulse focusing in air

Experimental study on the energy transmissivity of high -power nanosecond laser pulse focusing in air versus input laser energy has been carried out. There are three stages: when the input laser energy is relatively low，all of the laser energy can pass through the focusing point; when the input laser energy increases gradually，energy transmissivity decreases steeply from nearly 100% to lower values; when the input laser energy keeps on increasing，the transmissivity decreases slowly further. By defining the critical electron density and the corresponding critical time point，successful theoretical analysis has been carried out for the above experimental phenomena. When the free-electron density is less than the critical value，multiphoton ionization plays the dominating role; when the free-electron density is higher than the critical value, the inverse bremsstrahlung process dominates. The critical time point is the time when laser pulse reaches critical electron density in the process of laser pulse passing through the air; its position on the laser pulse is determined by the input laser pulse energy，meanwhile this position determines the energy transmissivity of the laser pulse. So we can calculate the critical electron density by measuring the transmissivity versus input energy and comprehend the characteristics of transmissivity for laser pulse focusing in the air.