Coherent four-photon spectroscopy of excited atoms in a laser-produced plasma: From point-by-point to multipoint two-dimensional mapping

An experimental technique for the two-dimensional mapping of the relative populations of excited states of atoms and ions in a low-temperature plasma of optical breakdown is developed on the basis of coherent four-wave mixing (FWM) with hyper-Raman resonances. Conditions when FWM in a plasma occurs in the phase-matched regime and is not subject to significant influence of one-photon absorption are experimentally determined. A folded FWM scheme for single-pulse two-dimensional imaging of spatial distributions of atoms and ions in a laser-produced plasma is discussed.