Optical Picosecond Sum-Frequency Generation Diagnostics of Plasma Discharges

Recent progress in diagnostics of low-temperature plasmas highlights capabilities of measuring gas temperatures and electric fields on picosecond temporal scales. Such advances were made possible by novel approaches leveraging hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (CARS) and electric field-induced second harmonic generation (E-FISH), respectively. However, a recent study hypothesized kinetic mechanisms supporting the existence of regimes of operation of pulsed discharges within which ultrafast changes of gas pressure and temperature (hence changes of the gas density) could occur simultaneously. The developed theory predicted ultrafast changes of the density of a few key species. More specifically, up to 3 orders of magnitude variation within less than 500 ps was predicted. The investigation of such a phenomenon calls for diagnostics capable of selectively measuring species number densities with high temporal resolution. In the present study, we report on the development of picosecond sum-frequency generation diagnostics (four-wave mixing) in the gas phase, for simultaneous measurements of the total gas number density, species densities and electric fields. Our results show that this novel approach can cover a wide range of density measurements relevant to low and high pressure plasmas. Moreover, the effectiveness of the diagnostic is compared with theoretical predictions.