Raman parametric four-wave mixing mechanism of the first-order Stokes generated in CO₂ and H₂

Without the axial component, an annular spatial profile of the first-order Stokes (S1) was observed during the SRS process in low-energy pumped CO 2 gas, which is supposed to be generated by a parametric four-wave mixing process (PFWM), i.e., 2 ω P = ω AS 1 + ω S 1 . In order to verify such a mechanism, similar experiments were conducted in H 2 , and the annular S1 intensity distribution was also noticed. Furthermore, simulations of S1 radial intensity distributions were carried out based on the proposed PFWM phase matching geometry. The PFWM has been verified to be a process that directly annihilates two pump photons and simultaneously produces one AS1 photon and one S1 photon.