Multi-order stimulated Raman scattering in colliding droplets

We investigated the temporal dynamics of multi-order stimulated Raman scattering (SRS) generated in single and colliding liquid droplets of carbon tetrachloride. Liquid droplets function as high-quality factor (Q) optical cavities via whispering-gallery modes, facilitating intense nonlinear optical processes. Using a 532 nm pulsed laser, we conducted a comprehensive shot-to-shot analysis of over 103 SRS waveforms. We observed that the generation time of multi-order SRS in a single droplet was delayed by 10-20 ns relative to the incident laser pulse. Our results revealed a sequential buildup process in which the (i - 1)th order SRS pumped the ith order SRS, resulting in progressive time delays for successive order SRS. In contrast, colliding droplets exhibited a significantly shorter generation time and higher efficiency than single droplets. A numerical multi-order SRS generation model was employed to fit the experimental temporal profiles, yielding a Q of ∼3 × 107 and 5 × 106 for single and colliding droplets, respectively. The lower Q in colliding droplets, arising from the high surface curvature of the disk-like mixing region, enhances the coupling efficiency between the incident field and droplet cavity. These findings clarify the dynamic buildup of nonlinear processes in microspherical cavities and demonstrate that colliding droplets can function as highly efficient, tunable cavities for advanced molecular spectroscopy and interface analysis.