传统的等离子体闪光法, 是根据探测器是否接收到来自薄膜样片周围发射的闪光信号, 对薄膜是否发生损伤进行评判, 这样的评判方法极易把空气与薄膜的等离子体闪光混淆而发生误判。 为了消除这种误判, 提出通过比较空气和薄膜各自的等离子体闪光的点燃时间, 利用两者时间上的差异, 实现对传统等离子体闪光法误判现象的消除方法。 为了验证新方法的可靠性, 借助于多光子吸收和级联电离理论, 建立了空气等离体子体点燃时间的计算模型, 根据薄膜与激光的相互作用原理建立了薄膜被击穿时的等离子体点燃时间计算模型, 利用建立的模型仿真计算了空气和薄膜的等离子体闪光点燃时间分别为1.856和7.843 ns; 搭建实验装置以实现对传统等离子体闪光法的更新, 在装置中的不同位置设置三个光电探测器分别采集入射激光信号、 空气和薄膜等离子体闪光信号, 采集入射激光信号的光电探测器置于聚焦透镜的侧面, 另外两个探测器位于薄膜样片周围且左右对称放置, 分别用于采集薄膜的等离子体闪光信号和空气的等离子体闪光信号, 所有光电探测器采集的信号转换为电信号后同步传输至示波器, 以入射激光信号为基准信号, 其与空气和薄膜等离子体闪光信号的起始时刻之差, 分别为空气和薄膜等离子体闪光点燃时间。 脉宽为10 ns、 波长为1 064 nm的Nd∶YAG脉冲激光以0.015 cm的聚焦光斑半径、 82.4 mJ的入射能量作用于光学厚度为λ/4、 直径为20 mm的单层Al2O3薄膜样片上后, 采集上述激光作用条件下的各路信号, 经处理后得到的空气和薄膜的等离子体闪光点燃时间测试值分别为2.7和7.8 ns; 理论计算和实验测试结果表明, 空气的点燃时间总是小于薄膜的点燃时间, 二者有很好的一致性。 说明当强激光作用于单层Al2O3薄膜表面时, 空气等离子体闪光先于薄膜等离子体闪光发生。 基于空气和薄膜等离子体闪光点燃时间上的这种差异, 利用闪光信号时间上的差别就可准确分辨出薄膜是否发生损伤, 从而获得识别薄膜损伤与否的判据, 这种从时间差异上识别薄膜等离子体闪光损伤的新方法, 无论从理论上还是实验上均为传统等离子体闪光法误判现象的消除提供了技术基础。

Based on whether the detector receives a flash signal emitted from surrounding of the thin film sample, the traditional plasma flash method can evaluate whether the thin film is damaged, while it can be easy to confuse plasma flash of air and with that of thin film and make a misjudgment. The elimination of misjudgment on traditional plasma flash method can be realized through comparing the ignition time of the plasma flash of the air and thin film and using the difference of time between them to eliminate this misjudgment. In order to verify the reliability of the new method, a model for calculating the ignition time of air plasma was established by means of the theory of multiphoton absorption and cascade ionization. On the other hand, a calculation model of the same kind was established when the thin film was broken through, and the plasma flash ignition time of air and thin film was calculated to be 1.856 and 7.843 ns respectively by using these established model simulations. The experimental device was set up to update the traditional plasma flash method, and three photodetectors were set up at different positions in the device to collect incident laser signals, air and thin film plasma flash signals, respectively, and the photodetector collecting the incident laser signal was placed on the side of the focusing lens, and the other two detectors were placed around the thin film sample and stay bilateral symmetry, which were used to collect the plasma flash signal of the film and the air, respectively. The signals collected by all photoelectric detectors were converted into electrical signals and sent to oscilloscope synchronously, with the incident laser signal taken as the reference signal, the difference between the initial time of the incident laser signal and the flash signal of air and thin film plasma was the flash ignition time of air and thin film plasma, respectively. When the Nd∶YAG pulse laser, whose pulse width is 10 ns and the wavelength is 1 064 nm, with a radius of 0.015 cm and incident energy of 82.4 mJ laser acts on the sample of single layer Aluminum trioxide film with the optical thickness λ/4 and the diameter of 20 mm. Acquiring various signals under the above laser action, the measured values of plasma flash ignition time for air and thin film after treatment were 2.7 and 7.8 ns respectively. The theoretical calculations and experimental results showed that the ignition time of air is always smaller than that of the film, which is in good agreement with each other. The results showed that the flash of air plasma is earlier than that of thin film when strong laser acts on the surface of single layer Aluminum trioxide film. This difference between the plasma flashes ignition time of air and thin film can be used to accurately identify whether the film is damaged and obtain the criterion to identify if the film is damaged or not . This new method for identifying the flash damage of thin film plasma from time difference provides a technical basis for the elimination of misjudgment in conventional plasma flash method both theoretically and experimentally.