Non-Resonant Picosecond Three-Wave Mixing in the Gas Phase

We report on the experimental observation of non-resonant, second-order optical Sum-Frequency Generation (SFG) in five different atomic and molecular gases. The measured signal is attributed to a SFG process by characterizing its intensity scaling and its polarization behavior. We show that the electric quadrupole mechanism cannot explain the observed trends. Our results demonstrate that SFG in the gas phase is about four orders of magnitude stronger than Third Harmonic Generation (THG) and independent from any externally-applied electric fields. These features make this method suitable for gas number density measurements at the picosecond timescale in reactive flows and plasmas.