Figure 1 : Relationship between linear absorption spectroscopy and I(4) 2D ES.

From: Coherent multidimensional optical spectra measured using incoherent light

Figure 1

(a) Optical apparatus for linear absorption spectroscopy uses a two-beam interferometer with interferometric delay η between arms E1 and E2. (b) The wiggly lines represent the envelope of the electric field of noisy light as a function of time, and the * indicates a common relative time point in the otherwise identical noisy beams for ease of viewing. Brackets indicate the two-point time-correlation function intrinsic to intensity-level detection. (c) Measured 1D interferogram of DTP in butanol shows coherent oscillations at optical frequencies. (d) Optical apparatus for 2D spectroscopy uses a four-beam interferometer with interferometric delays σ, τ and ζ between arms EA, EB, EC and ELO. We depict the time-integrated measurement; we used a spectrometer and CCD instead of a photodiode in the frequency-resolved measurements. (e) In I(4) 2D ES, the detected time-correlation function between Esig and ELO is recast as a four-point time-correlation function by considering the four-wave mixing process. The intractable four-point time-correlation function can then be broken down into products of two-point time-correlation functions. (f) Measured 2D interferogram of DTP in butanol for τ=0. Coherent oscillations at optical frequencies persist in all four quadrants.