Threshold characteristics of ultraviolet and near infrared nanosecond laser induced plasmas
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2016 Author(s). The present contribution compares the energy absorption, optical emission, temperature, and fluid dynamics of ultraviolet (UV) = 266 nm and near infrared (NIR) = 1064 nm nanosecond laser induced plasmas in ambient air. For UV pulses at the conditions studied, energy absorption by the plasmas increases relatively gradually with laser pulse energy starting at delivered energy of E 8 mJ. Corresponding measurements of plasma luminosity show that the absorption of UV radiation does not necessarily result in visible plasma emission. For the NIR induced plasmas, the energy absorption profile is far more abrupt and begins at 55 mJ. In contrast with UV, the absorption of NIR radiation is always accompanied by intense optical emission. The temperatures of both types of plasma have been measured with Rayleigh scattering thermometry (at times after the Thomson signal sufficiently diminishes). The UV plasmas can attain a wider range of temperatures, including lower temperatures, depending on the pulse energy (e.g., T 400-2000 K for E 7-35 mJ at t = 10 s after the pulse) while the NIR plasmas show only hotter temperatures (e.g., T 12 000 K for E = 75 mJ at t = 10 s after the pulse) as is consistent with the literature. Differences in the fluid dynamics for UV versus NIR pulses are shown with Schlieren imaging. The contrast in the UV and NIR plasma threshold behavior is attributed to differing roles of avalanche ionization and multiphoton ionization as is also illustrated by a simple numerical model.
