Study on the duration of laser-induced air plasma flash near thin film surface

Abstract The precision of testing the damage threshold of thin film lasers has always been a limiting factor in the advancement of high-power laser systems. The conventional plasma flash method sometimes fails to differentiate between film and air plasma flashes, resulting in significant errors in determining the film damage threshold. By distinguishing between the different durations of thin film and air plasma flashes, misjudgment can be eliminated. The aim of this article is to determine both the calculated and experimental values of the duration of air plasma flash near the surface of thin films ( t c ) and to analyze the factors that influence it. First, a model is established to calculate the theoretical value of t c . For a sample consisting of a single-layer hafnium oxide film, we assume that the incident laser wavelength, focal spot diameter, energy, and pulse width are 1,064 nm, 0.08 cm, 57.21 mJ, and 10 ns, respectively. The focal length of the lens positioned in front of the film sample is set at 350 mm, with a distance of 5 mm between the film sample and this lens. Under these conditions, the theoretical value for t c is calculated to be <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"> <m:mn>4.24</m:mn> <m:mo>×</m:mo> <m:msup> <m:mrow> <m:mn>10</m:mn> </m:mrow> <m:mrow> <m:mo>−</m:mo> <m:mn>6</m:mn> </m:mrow> </m:msup> <m:mspace width=".25em"/> <m:mo>s</m:mo> </m:math> 4.24\times {10}^{-6}\hspace{.25em}s . Second, the device was used to conduct eight experiments to obtain experimental values of t c . The results are as follows: (1) An increase in incident laser energy leads to an increase in t c . (2) The length of t c is solely dependent on the incident laser energy, regardless of the sample material or placement (even when the sample is not placed and the laser directly acts on air). (3) When substituting experiment parameters into the model for calculating t c , there is good agreement between theoretical and experimental values. Third, it was observed that t c increases with increasing incident laser energy, pulse width, and focal length of the lens, while decreasing with an increase in distance between the film sample and the lens.