Two-dimensional spectroscopy has long been used to study couplings and dynamics in material and chemical systems. In its simplest form, a 2D spectrum measures the change in a sample’s response to a probe frequency ω b that results from excitation at a pump frequency ωa. If, for example, ωa and ω b are both absorption lines of the same molecule into two different excited states, then absorption at ωa depletes the ground state and reduces the sample’s ability to absorb at ωb.

The off-diagonal peaks in a 2D spectrum can therefore aid in interpreting a complicated 1D spectrum that contains many overlapping peaks from different components. One can also use 2D spectroscopy to study changes in the sample that occur during the time between the pump and the probe. For instance, 2D optical spectra can provide information about the flow of energy through the...

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