Spectral modulation of stimulated Raman scattering signal: Beyond weak Raman pump limit

Abstract

Stimulated Raman scattering has recently been used in label‐free vibrational imaging studies of live cells. Important attempts to develop super‐resolution Raman imaging with intense Raman pump or decoherence beam have been made. However, the intense beams beyond weak stimulated Raman scattering limit could complicate spectral characteristics of Raman gain or loss signals. Here, the dependences of stimulated Raman loss signal on Raman pump intensity and pump‐probe delay time (Δt) are specifically investigated through both experimental and simulation studies. In the strong pump regime, a pronounced spectral modulation in stimulated Raman loss is observed at the Raman peaks of aromatic compounds with relatively large optical Kerr non‐linearity. We perform a numerical simulation for the coupled wave propagation to investigate the effect of cross‐phase modulation (XPM) on the stimulated Raman scattering (SRS). Our simulation involving both vibrationally resonant SRS and non‐resonant XPM effects in part accounts for the Δt‐dependent peak shift and spectral broadening as well as the appearance of a gain signal at negative time delays (Δt < 0). At the high pump intensity, it is found that the anti‐symmetric XPM signal with respect to the time zero (Δt = 0) cannot be selectively eliminated to produce pure SRS signal by time‐integration over Δt because of the increased interference term of the SRS and XPM at the Raman‐active frequency region. We believe that the experimental and simulation results discussed here would be of use in developing a super‐resolution coherent Raman imaging microscope in the near future.

Mitigating cross‐phase modulation artifacts in femtosecond stimulated Raman scattering

“…However, if the pump pulses arrived at the sample slightly before the Stokes pulses (Δ t < 0 in Figure 3), a delayed XPM effect around the Raman resonances could be measured, which can be explained by nuclear motion attributed to molecular vibrations. [ 14,15,27 ] In order to investigate the time delay dependence, SRL spectra were acquired for a constant peak intensity of 16 GW/cm 2 and different time delays between the pump and the Stokes pulses for ACN (Figure 3). Please note, that different colorbars were chosen in Figure 3a–e to clearly visualize the spectral evolution.…”

“…XPM will become of relevance in FSRS, if ultrashort (<2 ps pulse duration) and/or high intensity light pulses (GW/cm 2 ‐regime) are used. [ 27 ]…”

“…In the recent years, experimental as well as numerical studies have been published, showing the impact of XPM in femtosecond stimulated Raman scattering (FSRS). [ 27,28 ] Lim et al [ 27 ] presented a combined numerical as well as experimental study about the influence of XPM, dependent on the intensity and on the molecule under investigation for a potential super‐resolution microscope. They found that the SRS lineshape became more distorted for increasing pulse energies and materials with a higher nonlinear refractive index.…”

Recent advances in linear and nonlinear Raman spectroscopy. Part XIII

“…Lim and co‐workers described the spectral modulation of SRS intensity beyond the weak Raman pump limit. They believe that their experimental and simulation results discussed would be of use in developing a super‐resolution coherent Raman imaging microscope in the near future . Martinez et al tested ab initio quantum scattering calculations for the D 2 –He benchmark system with SRS.…”

“…They believe that their experimental and simulation results discussed would be of use in developing a superresolution coherent Raman imaging microscope in the near future. [120] Martinez et al tested ab initio quantum scattering calculations for the D 2 -He benchmark system with SRS. Close coupling dynamical calculations were performed on the most recent ab initio D 2 -He potential energy surface.…”