Implications of thermal lensing and four-wave mixing on stimulated Raman scattering in an aqueous solution of sodium nitrate

Stimulated Raman scattering (SRS) is an effective technique for imaging, spectroscopy and for frequency conversion of lasers in all states of matter. Yet, controlling SRS frequency conversion efficacy is a great challenge, due to ensuing competing scattering processes and laser-induced thermal lensing (TL). Here, we focus on both SRS and the associated processes occurring during frequency conversion in an aqueous solution of sodium nitrate. It is demonstrated that SRS efficiently converts 532 nm nanosecond laser pulses, via a single pass through the solution. In this light-matter interaction, SRS along with four-wave mixing (FWM) converted the pump beam to collinear beams of the 1^st [yellow (564 nm)] and 2^nd [orange (599 nm)] Stokes of the NO₃ ⁻ ion symmetric stretch. The experimental results were well reproduced by a newly developed numerical model. Based on the experimental and numerical results, it was found that the 1^st Stokes is generated through SRS of the pump laser, while the onset and growth of the 2^nd Stokes were obtained through FWM. The results also reveal the significant role played by SRS induced TL, growing over many laser pulses, suggesting its substantial contribution and importance.