Third harmonic from air breakdown plasma induced by nanosecond laser pulses
Harmonic generation is a nonlinear optical effect consisting in frequency up-conversion of intense laser radiation when phase-matching conditions are fulfilled. Here, we study the mechanisms involved in the third harmonic (TH) generation process, the conversion efficiency, and the properties of TH radiation generated in air by focusing infrared linearly polarized nanosecond laser pulses at intensities of the order of TW/cm2. By analyzing the emission from the air breakdown plasma, we demonstrate that filamentary breakdown plasma containing molecular nitrogen ions acts as an optical nonlinear medium enabling generation of TH radiation in the axial direction. The data reveal important properties of the TH radiation: maximum conversion efficiency of 0.04%, sinc2 dependence of the TH intensity on the square root of the pump intensity, and three times smaller divergence and pulse duration of TH as compared to the pump radiation.
This research was supported by the Romanian Government through the CAPACITIES/RO-CERN project, Grant ELI-NP, E/04 HHGDE (contract number 04/27.06.2014), and through CNDI-UEFISCDI project, Grant PN-II-PT-PCCA-2011-3.1-0886 UFOUV (contract number 1/2012).
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