https://doi.org/10.1351/goldbook.08579
Measurement method of Raman spectroscopy that uses third-order susceptibility and is one of several four-wave mixing spectroscopies. The excitation is through lasers of wavenumber \(\tilde{\nu}_{1}\) and \(\tilde{\nu}_{2}\), which coincide spatially and temporally in the sample and produce an output laser of wavenumber \(\tilde{\nu}_{3} = 2\tilde{\nu}_{1} - \tilde{\nu}_{2}\).
Notes:
- Experimentally, \(\tilde{\nu}_{1}\) is kept constant and \(\tilde{\nu}_{2}\) is scanned. A laser beam output is observed at the wavenumber for anti-Stokes Raman scattering \(\tilde{\nu}_{1} + \tilde{\nu}_{\rm{M}}\) when \(\tilde{\nu}_{1} - \tilde{\nu}_{2} = \tilde{\nu}_{\rm{M}}\), where \(\tilde{\nu}_{\rm{M}}\) is the wavenumber of an active vibration in the sample.
- The Raman scattered radiation emerges as a laser beam instead of being scattered into three dimensions, thus greatly enhancing the sensitivity over that in normal Raman scattering.