Coherent four-wave mixing in excited and ionized gas media: four-photon spectrochronography, ellipsometry, and nonlinear-optical imaging of atoms and ions

A review of the main directions of development, the most promising new techniques, and urgent applications of coherent four-photon methods of investigation of excited and ionized gas media is presented. It is demonstrated that the methods of coherent four-wave mixing (FWM) provide important spectrochronographic information on atoms and ions, allow selective studies of separate components of nonstationary gas and plasma systems with a complex composition using polarization-sensitive four-photon spectroscopy and coherent ellipsometry, and permit nonlinear-optical imaging of the spatial distribution of atoms and ions in excited gases and plasmas.