Non-destructive and high-sensitivity plasma diagnostics based on third-harmonic generation of ultrashort-pulsed laser

We report a principle-of-proof approach for non-destructive and high-sensitivity plasma diagnosis based on third-harmonic generation (THG) of an ultrashort-pulsed laser. We show that the on- and off-axial spectral intensities of the generated TH depend strongly on the electron density of a discharged plasma, even when the laser is non-contact to the plasma, and that the two TH components exhibit distinctly different detection sensitivities, in which the sensitivity measured by the on-axial TH component is about one order of magnitude higher than that by the off-axial one. This difference is ascribed to their different generation mechanisms, where the on- and off-axial components require quasi-phase- and phase-matching conditions, respectively. The quasi-phase-matching condition for on-axial THG is improved in the plasma, benefiting from the change in nonlinear properties related to the electron density. Our results open up a viable route for high spatiotemporal resolution diagnosis of various types of plasmas.