Fig. 1
From: Coherent anti-Stokes Raman spectroscopy of single and multi-layer graphene
Schematic of CARS and NRVB third-order nonlinear processes. Interaction with pulses ωP, ωS, results in blue-shifted (a) CARS and (b) NVRB contributions at ωas = 2ωP − ωS. Since in CARS a vibrational coherence is stimulated by two consecutive interactions with the pump and Stokes fields, their frequency difference must correspond to a Raman active mode, ωP − ωS = ωv. c, d Constraints for the temporal sequence of the field-matter interactions (represented by circles at the top of the pulse envelopes), for CARS and NVRB. In NVRB, the three interactions generating χ(3) happen within the few fs electronic dephasing time20. In CARS, the third interaction can occur over the much longer vibrational dephasing time (a few ps)20, within the pump pulse (PP) temporal envelope
