Effect of Two-Photon Stark Shift on the Multi-Frequency Raman Spectra

Abstract

High order Raman generation has received considerable attention as a possible method for generating ultrashort pulses. A large number of Raman orders can be generated when the Raman-active medium is pumped by two laser pulses that have a frequency separation equal to the Raman transition frequency. High order Raman generation has been studied in the different temporal regimes, namely: adiabatic, where the pump pulses are much longer than the coherence time of the transition; transient, where the pulse duration is comparable to the coherence time; and impulsive, where the bandwidth of the ultrashort pulse is wider than the transition frequency. To date, almost all of the work has been concerned with generating as broad a spectrum as possible, but we are interested in studying the spectra of the individual orders when pumped in the transient regime.We concentrate on looking at extra peaks that are generated when the Raman medium is pumped with linearly chirped pulses. The extra peaks are generated on the low frequency side of the Raman orders. We discuss how linear Raman scattering from two-photon dressed states can lead to the generation of these extra peaks.

Time-Dependent, Two-Photon Rabi Frequency Shift Observed in Multi-Frequency Raman Spectra

“…The shifted shoulder is consistently longer in duration than the Raman pulse and, except for the timed case, is higher in amplitude than the Raman pulse. The higher amplitude of the red-shifted pulse with a time delay of 333 fs is consistent with the observations in our earlier work [10], which showed that the amplitude of the red-shifted peak grew with increasing anti-Stokes order until there was only the shoulder at the highest orders for this time delay between pump pulses. We expect that this is due to the pump pulse instantaneous frequency separation being on resonance with the dressed states for this delay time.…”

“…We have investigated these red-shifted spectra under different instantaneous frequency separations and input energies. The results showed that the red-shifted spectrum is not a result of four-wave mixing but could be the result of a two-photon dressed-state Rabi frequency shift [10].…”

“…Our group has been investigating MRG in the transient regime, where the pump pulse duration is similar to the dephasing time of the Raman material. In transient MRG experiments, the pumps are linearly chirped to either reduce nonlinear effects such as self-phase modulation [7,8] or to have the ability to easily change the instantaneous frequency separation of the pumps by varying the time delay [9,10].…”

Generation of three-octave-spanning transient Raman comb in hydrogen-filled hollow-core PCF

“…The anti-Stokes lines, which lie in the normal dispersion region over most of the fiber length, appear to be slightly red-shifted relative to their expected wavelengths, especially for the higher order lines (a similar effect was observed in [17], where it was ascribed to a Stark shift of the Raman levels).…”

Multicolored Femtosecond Pulse Synthesis Using Coherent Raman Sidebands in a Reflection Scheme

“…Katsuragawa's group reported the carrier envelope offset control of octave-spanning Raman comb by using dual-frequency laser radiation locked on a single laser cavity and, simultaneously, its second harmonic [11]. Broadband spectra based on multifrequency Raman generation were also studied in photonic crystal fiber [12] and hollow fibers filled with SF6 [13]. Meanwhile, the molecular modulation method has been extended to the continuous-wave (CW) domain.…”