Abstract
Nonlinear optical signals in the condensed phase are often accompanied by sequences of lower-order processes, known as cascades, which share the same phase matching and power dependence on the incoming fields and are thus hard to distinguish. The suppression of cascading in order to reveal the desired nonlinear signal has been a major challenge in multidimensional Raman spectroscopy, that is, the χ(5) signal being masked by cascading signals given by a product of two χ(3) processes. Because cascading originates from the exchange of a virtual photon between molecules, it can be manipulated by performing the experiment in an optical microcavity which modifies the density of radiation field modes. Using a quantum electrodynamical (QED) treatment, we demonstrate that the χ(3) cascading contributions can be greatly suppressed. By optimizing the cavity size and the incoming pulse directions, we show that up to ∼99.5% suppression of the cascading signal is possible.
Highlights
Raman spectroscopy is an effective tool for studying molecular vibrations and offers a fingerprint by which molecules can be identified
A microscopic quantum electrodynamical (QED) treatment of cascading was developed which connects it to virtual photon exchange between molecules and was applied to various sample geometries
In one type of six-wave mixing process, light with wavevectors k1, k2 and k3 interact with one molecule via a χ(3) process to produce a field with kv = k3 − k2 + k1 and the kv-field together with externally-applied fields k4, k5 interact with another molecule via a second χ(3) event to produce the signal along the detecting direction ks = k5 − k4 + kv
Summary
We demonstrate how cascading processes in fifth-order Raman signals can be manipulated by placing the molecules in an optical microcavity. The 2D fifth-order Raman signal takes the form of Aχ(5) + Bχ(3)χ(3) where the first term originates from the direct Raman process since it takes place at the single molecule, and the second term is attributed to cascading. The sequential and parallel cascades in the cavity can be obtained by substituting the external pulses Eq (4) into the cascading signals in Eq (7) and taking the time-ordering into account. Which reduces to the free-space case without a avity
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