Chirp effects and retrieval of Raman spectra in femtosecond stimulated Raman scattering

Abstract

Chirp effects caused by the group delay dispersion (GDD) of optical pulses have a twofold meaning when applied to femtosecond stimulated Raman scattering (fs SRS). On the one hand, these effects are responsible for Raman mode modulation and are thus detrimental to the reconstruction of Raman spectra. On the other, they can be cleverly employed to turn GDD into an additional optical variable of great usefulness. Here, to master the whole subject, the classical approach to coherent Raman scattering is chosen for its simplicity and, with reference to a large class of measurements where electronically off-resonant Raman transitions are probed, fs SRS signals are readily found for linearly chirped Gaussian pulses that guarantee the solution to the nonlinear optical scattering problem without recourse to numerical methods. Thanks to this result, fundamental features of chirp-dependent fs SRS are explored by means of comparisons with experiments taken from the existing literature on the subject. The focus is on four fundamental manifestations of chirp dependence. They are (1) temporal resolution invariance in time modulation of Raman decay and its drift, (2) spectral focusing in Raman gain and loss, (3) line intensity modulation and Raman mode selection, and (4) single-shot time mapping of molecular dynamics. The findings of this work show that basic chirp-dependent Raman theory provides the necessary insights into the Raman phenomena appearing when chirp affects the laser pulses even in the extreme regime where the transform-limited fs pulse duration is just a few harmonic cycles of the laser fields.