Stimulated Raman spectroscopy using chirped pulses

We present a detailed theoretical and experimental characterization of a new methodology for stimulated Raman spectroscopy using two duplicates of a chirped, broadband laser pulse. Because of the linear variation of laser frequency with time (chirp'), when the pulses are delayed relative to one another, there exists a narrow bandwidth, instantaneous frequency difference between them, which, when resonant with a Raman-active vibration in the sample, generates stimulated Raman gain in one pulse and inverse Raman loss in the other. This method has previously been used for coherent Raman imaging and termed spectral focusing'. Here, gain and loss signals are spectrally resolved, and the spectrally integrated signals are used to determine the spectral resolution of the measured Raman spectrum. Material dispersion is used to generate a range of pulse durations, and it is shown that there is only a small change in the magnitude of the signal and the spectral resolution as the pulse is stretched from 800 to 1800fs in duration. A quantitative theory of the technique is developed, which reproduces both the magnitude and linewidth of the experimental signals when third-order dispersion and phase-matching efficiency are included. The theoretical calculations show that both spectral resolution and signal magnitude are severely hampered by the third-order dispersion in the laser pulse, and hence, a minimal amount of chirp produces the most signal with only a slight loss of spectral resolution. Copyright (c) 2014 John Wiley & Sons, Ltd.