Temporal analysis and control of Raman scattering dynamics

Abstract: Raman scattering underlies a broad range of spectroscopic and light-generation techniques, yet its conventional description, based on the Raman gain spectrum, accurately describes only long-pulse, steady-state dynamics. We present and develop a time-domain theoretical approach that provides a unified and physically-transparent description of Raman interactions across all temporal regimes. It enables direct visualization of Raman temporal dynamics and accounts for spectrotemporal aspects of Raman phenomena. We apply this theory specifically to Raman-shifting with ultrashort light pulses in gases, where the excitation is in the impulsive Raman regime and dephasing of Raman transitions is weak. The analysis, for the first time, exposes temporal and spectral distortions that arise from Raman scattering and which impact frequency-shifting performance detrimentally. Crucially, it also identifies how these distortions can be suppressed through temporal control of the nonlinear response. Numerical simulations of the soliton self-frequency shift (SSFS) in gas-filled hollow-core fibers show that molecules with strong Raman responses do not yield efficient frequency conversion, and predict that reducing the relative Raman contribution (compared to the electronic response) enhances the process. Experiments using gas mixtures with tunable Raman fraction of the nonlinear response confirm these predictions. An analytic expression for impulsive SSFS in gases, which departs significantly from the well-known formula for glasses, predicts the observed behavior when Raman-induced temporal distortion is suppressed. The new time-domain framework uncovers phenomena and provides physical insight that are inaccessible through the decades-old frequency-domain treatment of Raman scattering.