Double-pulse Model for the Study of Red-shifted Spectrum in Multi-frequency Raman Generation

Abstract

Multi-frequency Raman Generation (MRG) is a promising technique for generating few-femtosecond to sub-femtosecond pulses with high energy conversion efficiency. During transient MRG experiments coupled with two chirped pulses, when the instantaneous frequency separation between the pump and Stokes pulses is red-detuned from resonance, the individual Raman orders become a double-peak structure. These secondary peaks were only observed on the red side of the main Raman peaks. A double pulse model (DPM) is used to describe the phenomenon of the Raman order: the Raman pulse and the red-shifted pulse. Comparison of FROG measured spectrograms with simulation using the DPM achieved lower errors compared to the Frequency Resolved Optical Gating (FROG) standard iterative program results. From the experimental results under various conditions of instantaneous frequency separations and input energies, the simulation suggested that the Raman pulse remains similar to the pump pulses, while the red-shifted pulse exhibits extra higher-order phase due to the intensity-dependent two-photon Stark shift during Raman scattering. The 4-wave mixing is more dominant in blue shifted case. DPM simulation results show that this theory of two-photon Rabi frequency shifts matches the experimental results.