2.2 Susceptibility to Third-Order Cascades
Cascades of third-order signals have been recognized as a serious experimental
complication in off-resonant fifth-order Raman experiments conducted on pure
liquids [49–54]. As indicated in Fig. 6, this artifact represents a process in which the
four-wave mixing response on one molecule radiates a signal field that drives a fourwave mixing process on a second molecule. The second four-wave mixing signal
(i.e., the cascaded signal) is radiated in the same direction as the desired fifth-order
response and carries many of the same spectroscopic signatures. In contrast, all five
field-matter interactions take place with an individual molecule in a genuine sixwave mixing process, thereby yielding a direct fifth-order signal field. Under offresonant conditions, the cascaded response can be many orders of magnitude larger
than the fifth-order 2D Raman signal. Success in measuring the off-resonant 2D
Raman spectrum of CS 2 was finally achieved in 2002 when clever experimental
geometries and detection schemes were developed to suppress the cascaded signal
intensity [53, 55]. The possibility of conducting 2DRR spectroscopy without
contributions from cascaded artifacts was first studied by our group in 2014 [22].
Cascades were more recently shown to be negligible in a related fifth-order method
[33]. These investigations suggest potential for even higher-order nonlinear
spectroscopy techniques such as the Raman echo [19, 56].
Fig. 6 Cascades of four-wave mixing signals generally dominate in off-resonant 2D Raman experiments.
In the desired ‘‘direct’’ process, all 6 field-matter interactions take place with an individual molecule. The
cascaded response is generally negligible in 2DRR spectroscopy because it involves 8 field-matter
interactions, whereas the direct process only requires 6. Both types of nonlinearities are subject to the
same selection rules under electronically resonant conditions
Top Curr Chem (Z) (2017) 375:87
123
257
Reprinted from the journal
Cascades of third-order signals have been recognized as a serious experimental
complication in off-resonant fifth-order Raman experiments conducted on pure
liquids [49–54]. As indicated in Fig. 6, this artifact represents a process in which the
four-wave mixing response on one molecule radiates a signal field that drives a fourwave mixing process on a second molecule. The second four-wave mixing signal
(i.e., the cascaded signal) is radiated in the same direction as the desired fifth-order
response and carries many of the same spectroscopic signatures. In contrast, all five
field-matter interactions take place with an individual molecule in a genuine sixwave mixing process, thereby yielding a direct fifth-order signal field. Under offresonant conditions, the cascaded response can be many orders of magnitude larger
than the fifth-order 2D Raman signal. Success in measuring the off-resonant 2D
Raman spectrum of CS 2 was finally achieved in 2002 when clever experimental
geometries and detection schemes were developed to suppress the cascaded signal
intensity [53, 55]. The possibility of conducting 2DRR spectroscopy without
contributions from cascaded artifacts was first studied by our group in 2014 [22].
Cascades were more recently shown to be negligible in a related fifth-order method
[33]. These investigations suggest potential for even higher-order nonlinear
spectroscopy techniques such as the Raman echo [19, 56].
Fig. 6 Cascades of four-wave mixing signals generally dominate in off-resonant 2D Raman experiments.
In the desired ‘‘direct’’ process, all 6 field-matter interactions take place with an individual molecule. The
cascaded response is generally negligible in 2DRR spectroscopy because it involves 8 field-matter
interactions, whereas the direct process only requires 6. Both types of nonlinearities are subject to the
same selection rules under electronically resonant conditions
Top Curr Chem (Z) (2017) 375:87
123
257
Reprinted from the journal