Cascaded coherent anti-Stokes Raman scattering for high-sensitivity number density determination in the gas phase

Cascaded coherent anti-Stokes Raman scattering (CARS) signals can be efficiently generated from CARS signals when propagating collinearly with the pump/Stokes and probe beams. This effect can be seen as the CARS beam acting as the probe beam and being inelastically scattered a ‘second time’ from the Raman coherence induced along the focus of the pump/Stokes beam axis. Although much weaker, this additionally scattered signal co-propagates with the CARS signal and may complicate the analysis of the CARS spectrum used for diagnostics in the gas phase. In particular, the occurrence of the cascaded CARS process needs to be taken into account analysing minor spectral signatures at relatively high number density of scattering molecules. Here we show how polarization control can be employed to generate CARS and cascaded CARS signals with orthogonal linear polarization and how, in this way, the cascaded CARS signals can be efficiently suppressed. However, instead of rejecting this signal, we collect both the generated CARS and cascaded CARS signals on the same detector frame, and we explore the use of these counterparts for absolute concentration measurements of the Raman-active species. The cascaded CARS signal has exponential-order higher sensitivity to the number density of the scattering molecules in the mixture. We demonstrate that the ratio of the CARS and the cascaded CARS signals is independent of the probe pulse energy in use, which can be a promising approach for wide-range absolute concentration measurements in gas-phase media.