Fundamental aspects of 2D photon echo-type spectroscopy have been discussed in detail, where the effects from chromophore-bath interactions, electronic couplings between chromophores, excitation transfer process, coherence quantum beats, and chemical exchange processes on the frequency distributions, shapes, and amplitudes of diagonal and cross-peaks. When a given pair of singly excited states are energetically close to each other so that the frequency resolution is not perfect, the crosspeaks cannot be easily identified due to the spectral congestion problems. Then, it is not always straightforward to quantitatively estimate the cross-peak amplitudes to extract information on structure and dynamics of coupled multi-chromophore systems. In this regard, the polarization-controlled measurement has been considered to be a useful technique for selectively eliminating the diagonal peaks from the 2D spectrum.1-4 Sometimes, one can measure the anisotropy of cross-peak to determine the relative angle between two different transition dipoles, which can provide information on the 3D structure of target molecules such as peptides in solution.
Fundamental aspects of 2D photon echo-type spectroscopy have been discussed in detail, where the effects from chromophore-bath interactions, electronic couplings between chromophores, excitation transfer process, coherence quantum beats, and chemical exchange processes on the frequency distributions, shapes, and amplitudes of diagonal and cross-peaks. When a given pair of singly excited states are energetically close to each other so that the frequency resolution is not perfect, the crosspeaks cannot be easily identified due to the spectral congestion problems. Then, it is not always straightforward to quantitatively estimate the cross-peak amplitudes to extract information on structure and dynamics of coupled multi-chromophore systems. In this regard, the polarization-controlled measurement has been considered to be a useful technique for selectively eliminating the diagonal peaks from the 2D spectrum.1-4 Sometimes, one can measure the anisotropy of cross-peak to determine the relative angle between two different transition dipoles, which can provide information on the 3D structure of target molecules such as peptides in solution.
A theoretical description of one- and two-color photon echo peak shifts (PEPS) and two-dimensional (2D) photon echo spectrum (PES) of a coupled chromophore system are presented. The effects of population relaxation in the one-exciton states on both the PEPS and the 2D PES are investigated. For values of time T shorter than the population relaxation time, a finite two-color peak shift magnitude and nonzero cross peaks in the 2D PES provide evidence of electronic coupling between the chromophores. These two distinct observables, i.e., PEPS and off-diagonal peaks, both originate from the electronic coupling. However, it is shown that the PEPS and 2D PES methods can provide complementary information on the structure-dependent nonlinear optical responses of coupled chromophore systems.
The two-dimensional (2D) temporal shape of the (degenerate) three-pulse photon-echo (3-PPE) polarization P(3)(t′,τ12,τ23=0) has been theoretically analyzed for a weakly interacting two-site system (TSS) in the presence of site inhomogeneous broadening. The TSS has been modeled in terms of two identical, energy-degenerate, excitonically coupled pairs of electronic states in the site-representation (i.e., two S0 and S1 states forming a 4-level system). The 2D time-domain signal S(t′,τ12,τ23=0) has been derived in a closed-form solution which allowed to formulate the analytical Fourier transform analog under sequential δ-pulse excitation. The resulting 2D PE signal in frequency–frequency space S(ωt′,ωτ12) is formed in the region of the resonant, optical carrier frequency by, generally, four peaks, arranged at the edges of a square with distances determined by the excitonic coupling. The two diagonal peaks are of one-exciton origin, while the two off-diagonal (cross) peaks involve two-exciton effects, too. The relative heights of the four peaks are in distinct relation with the mutual orientation of the site-localized transition dipoles, thus these 2D line shapes serving as a structural probe for resonantly interacting molecules. The shapes of the peaks reflect the ratio between homogeneous and inhomogeneous dephasings and can be used for structural studies of statically disordered systems of TSS. The semiclassical two-site model is intended to provide a preparatory platform for mimicking electronically coupled segmental pairs dispersed in energy and space along the strongly disordered main chain contour. The results of the model calculation are taken to discuss the potential and the limitations of 2D line shape echo-spectroscopy in the study of short-range coherences and electronic coupling in π-conjugated polymers.
An understanding of chemical reactivity begins with an understanding of the dynamics involved regarding system-bath interactions. Spectroscopic studies of these interactions in condensed-phase multichromophoric systems are intricate and contain much information. Photon-echo spectroscopy has proven to be a useful tool for probing these interactions. A description of three-pulse photon-echo peak shift spectroscopy (3PEPS)--theory, experiment, and application--for the study of solvation and dynamics of nanoscale excitons is presented. Also, we discuss how two-dimensional photon-echo spectroscopy (2DPE) relates to 3PEPS and show how 3PEPS data can be extracted from 2D photon-echo data.
Over the past 15 years, researchers have extended the multidimensional techniques which originated with NMR in the 1970s to infrared and visible coherent spectroscopy. These advances have dramatically enhanced the temporal resolution from the microsecond to the femtosecond regime. NMR spectroscopists have developed principles for the design of pulse sequences that enhance selected spectral features and reveal desired dynamical events. Extending these principles to the optical regime offers numerous opportunities for narrowing the line shapes in specific directions, unraveling weak cross-peaks from otherwise congested spectra, and controlling the interferences between quantum pathways. We can achieve these enhancements by shaping the spectral and temporal profiles of the pulses. Pulse polarization shaping may lead to unique probes of time-dependent chirality. In this Account, we compare two types of signals. The first, the photon echo, is generated in the direction -k(1) + k(2) + k(3), and the second, double quantum coherence, is detected at +k(1) + k(2) - k(3). Here k(1), k(2), and k(3) are the wave vectors of the three incoming pulses in chronological order. We illustrate the novel information extracted from these signals by simulations of three physical systems. In the first system, spectra of GaAs semiconductor quantum wells provide a direct look at many-body electron correlation effects. We directly observe specific projections of the many-electron wave function, which we can use to test the quality of various levels of computational techniques for electronic structure. Secondly, the spectra of photosynthetic aggregates reveal couplings between chromophores, quantum coherence signatures of chromophore entanglement, and energy-transfer pathways. Using some fundamental symmetries of pulse polarization configurations of nonlinear signals, we can construct superpositions of signals designed to better distinguish among various coherent and incoherent exciton transport pathways and amplify subtle variations among different species of the Fenna-Matthews-Olson (FMO) antenna complex. Both of the first two applications require femtosecond pulses of light in the visible range. The third application demonstrates how resonant core spectroscopy may be used to generate core excitations that are highly localized at selected atoms. Such signals can monitor the motions of valence electron wavepackets in real space with atomic spatial resolution. These future X-ray applications will require attosecond bright X-ray sources, which are currently being developed in several labs. Common principles underlie these techniques in coherent spectroscopy for spins, valence electrons, and core electronic excitations, spanning frequencies from radiowaves to hard X-rays.
Optical two-dimensional photon-echo spectroscopy is realized with shaped excitation pulses, allowing coherent control of twodimensional spectra. This development enables probing of state-selective quantum decoherence and phase/time sensitive couplings between states. The coherently-controlled two-dimensional photon-echo spectrometer with two pulse shapers is based on a passively stabilized four-beam interferometer with diffractive optic, and allows heterodyne detection of signals with a long-term phase stability of approximately Lambda/100. The two-dimensional spectra of Rhodamine 101 in a methanol solution, measured with unshaped and shaped pulses, exhibit significant differences. We observe in particular, the appearance of fine structure in the spectra obtained using shaped excitation pulses.
Heteronuclear multi-quantum spectroscopy is a powerful part of the NMR toolbox, commonly used to identify specific sequences of atoms in complex pulse sequences designed to determine the structure of complex molecules, including proteins. Optical coherent multidimensional spectroscopy (CMDS) is analogous to multidimensional NMR and many of the techniques of NMR have been adapted for application in the optical regime. This has been highly successful, with CMDS being used to understand energy transfer in photosynthesis and many body effects in semiconductor nanostructures amongst many other scientific breakthroughs. Experimental challenges have, however, prevented the translation of heteronuclear multi-quantum NMR to the optical regime, where capabilities to isolate signals in otherwise congested spectra, reduce acquisition times and enable more incisive probes of multi-particle correlations and complex electronic systems would have great benefit. Here we utilise a diffraction based pulseshaper to impose the two key avenues of selectivity required for the analogue of heteronuclear multi-quantum spectrocopy, - control of the individual pulse spectra, and control of the order and timing of interactions - while maintaining phase-locked excitation pulses. We utilise this approach to isolate the 2-quantum signal from a mixed two exciton state in a semiconductor coupled quantum well. The removal of other overlapping peaks allows analysis of the relevant peak shape and thus details of interactions with the environment and other carriers. An alternative pulse ordering also identifies a 2-quantum state associated only with a GaAs defect, the signature of which would otherwise be confused with other interaction induced effects. The experimental approach described here provides access to crucial details of excitonic interactions, which are important in a range of material systems