We report a comparative experimental investigation of femtosecond laser-induced third harmonic generation (THG) in an ethanol flame and in air. It was found that the third harmonic (TH) signal produced in the presence of a combustion field can be greatly enhanced, in comparison to that generated in air, and that the enhancement factor depends strongly on the experimental parameters, such as the focal length, relative position of the flame and filament, and laser repetition frequency. Moreover, by replacing the flame with a point temperature controller, a similar signal enhancement of THG was observed, and the TH signal exhibited a nonlinear growth with the heating temperature. Further analysis indicated that the observed enhancement of THG originates from the suppression of Gouy-phase-induced destructive interference due to the disturbed gas density under high-temperature heating. The high sensitivity of the TH signal to the combustion temperature helps formulate an effective and straightforward approach to achieve nonintrusive temperature measurements in the combustion field.

中文翻译：

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飞秒激光诱导三次谐波产生的燃烧场非侵入式温度测量

我们报告了乙醇火焰和空气中飞秒激光诱导三次谐波产生 (THG) 的比较实验研究。结果发现，与在空气中产生的信号相比，在存在燃烧场的情况下产生的三次谐波 (TH) 信号可以大大增强，并且增强因子在很大程度上取决于实验参数，例如焦距，火焰和灯丝的相对位置，以及激光重复频率。此外，通过用点温度控制器代替火焰，观察到类似的 THG 信号增强，并且 TH 信号表现出随加热温度的非线性增长。进一步分析表明，观察到的 THG 增强源于高温加热下扰动气体密度引起的 Gouy 相诱导相消干涉的抑制。TH 信号对燃烧温度的高灵敏度有助于制定一种有效且直接的方法来实现燃烧场中的非侵入式温度测量。