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I will present a proposed implementation of entangled photon-pair two-dimensional fluorescence spectroscopy (EPP-2DFS) to probe the nonlinear optical response of electronically coupled molecular dimers [1]. The method uses a technique from quantum optics–a separated two-photon ('Franson') interferometer, which generates time-frequency-entangled photon pairs. This interferometer is incorporated into the framework of a fluorescence-detected 2D optical spectroscopic experiment [2].
The continuous stream of entangled photons are temporally shaped and phase-modulated in the interferometer, and are used to excite the two-photon-absorbing system, whose excited-state population is selectively detected by simultaneously monitoring the sample fluorescence and the transmitted exciting fields. In comparison to standard 'classical' 2DFS techniques using coherent laser pulses and standard pulse-scanning sequences, advantages of this scheme are the suppression of uncorre lated background signals, the enhancement of simultaneous time-and-frequency resolution, the suppression of diagonal 2D spectral features, and the enhancement and narrowing of off-diagonal spectral cross-peaks that contain information about electronic couplings. These effects are a consequence of the built-in correlation between temporal and spectral degrees of freedom of the parametric down-conversion light source. Our numerical simulations of an electronically coupled molecular dimer show that the EPP-2DFS spectrum is greatly simplified in comparison to its standard classical 2D counterpart. Our results indicate that EPP-2DFS can be used to greatly reduce the number of equivalent coherence pathways (or Feynman path integrals) that contribute to the signal, and thereby allows for a more direct extraction of the Hamiltonian parameters of electronically coupled molecular systems.
References
[1] M.G Raymer, A. H. Marcus, J. R. Widom, D. L. P. Vitullo, "Entangled Photon-Pair Two-Dimensional Fluorescence Spectroscopy (EPP-2DFS)," J. Phys. Chem. B 117, 15559-15575 (2013)
[2] P. F. Tekavec, G. A. Lott, A. H. Marcus, "Fluorescence-Detected Two-Dimensional Electronic Coherence Spectroscopy by Acousto-Optic Phase Modulation," J. Chem. Phys. 127, 214307-1-21 (2007).
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