由于飞秒激光脉冲宽度小于靶材电子—晶格热弛豫时间, 飞秒激光烧蚀靶材过程以及诱导击穿产生的等离子体膨胀动力学过程与纳秒激光作用过程不同, 因此研究飞秒激光诱导等离子体发射光谱特性对于研究飞秒激光烧蚀机制以及飞秒激光诱导等离子体的膨胀动力学过程非常重要。 Ge材料是一种常用的中远红外探测器以及光学元器件材料, 对中心波长为800 nm, 脉宽为50 fs的激光脉冲烧蚀空气中Ge靶材产生的等离子体发射光谱强度的时间和空间演化规律研究, 并探讨了飞秒激光脉冲能量对等离子体发射光谱强度的影响规律。 实验结果表明在等离子体羽膨胀初期, 飞秒激光诱导Ge等离子体发射光谱主要由线状光谱和连续光谱构成, 在200 ns时间内连续光谱强度逐渐减弱, 线状光谱开始占主导地位。 通过探测Ge等离子体的时间分辨发射光谱, 随着等离子体的快速膨胀, 等离子体发射光谱强度随着时间的增加呈现先增加后下降变化, 在335 ns达到最大。 通过探测Ge等离子体的空间分辨发射光谱, 随着距离Ge靶材表面的位置增加, 等离子体发射光谱强度随远离Ge靶材表面距离增加呈现先增加后下降变化, 在0.8 mm位置达到最大。 由于存在等离子体自吸收机制, 等离子体发射光谱强度随着脉冲能量的增加而增加, 在脉冲能量为0.627 mJ时, 飞秒激光诱导Ge等离子体存在自吸收现象, 从而使等离子体发射光谱强度出现下降变化。

For the pulse width of a femtosecond laser is shorter than the electron-lattice thermal relaxation time of the medium, and the processes of femtosecond laser ablation and the dynamic process of plasma expansion are different from that of a nanosecond pulse laser, it is very important to study the emission spectrum characteristics of femtosecond laser-induced plasma for studying the femtosecond laser ablation mechanism and the femtosecond laser-induced plasma expansion dynamics. Ge material is commonly used for mid-far infrared detectors and optical components. In this paper, the processes of the temporal and spatial evolution of plasma emission spectroscopy intensity produced by femtosecond pulsed laser ablated Ge material with center wavelength of 800 nm and pulse width of 50 fs are studied in air, and the effect of laser pulse energy on plasma emission spectral intensity is discussed. The experimental results show thatthe Ge plasma emission spectrum induced by the femtosecond pulsed laser is mainly composed of line spectrum and continuous spectrum at the early stage of plasma plume expansion, and the continuous spectrum weakens gradually. In contrast,the line spectrum becomes dominating within the delay time of 200ns. With the rapid expansion of the plasma plume, the plasma emission spectral intensity increases first and then decreases with the delay time increasing, and plasma emission spectral intensity reaches the maximum at a delay time of 335 ns. With the distance increasingto the Ge target surface at the delay time of 335 ns, the plasma emission spectral intensity increases first and then decreases and reaches the maximum at a distance of 0.8 mm to the Ge target surface. In the process of plume expansion, the existence time of the ion spectrum line is shorter than that of the atom spectrum line. Due to the existence of the self-absorption mechanism of the plasma plume, the plasma emission spectral intensity increases with the increasing of femtosecond pulse energy. When the pulse energy is 0.627 mJ, femtosecond laser-induced Ge plasma has a self-absorption phenomenon, which decreasesthe plasma emission spectral intensity.