![]() Sensitivity of detection of the radiated electric field from macroscopic polarization further constrains the possibility of sub-ensemble measurements. However, non-collinear phase-matching relies on having the volume of the amplitude and phase grating 14 imprinted by the pump pulses on the sample being much larger than the diffraction-limited volume imposed by the wavelength of light, thereby limiting the available spatial resolution. The above limitations of multidimensional spectroscopy, which arise due to ensemble averaging of dynamics along T, are connected to the fact that traditional experimental implementations 13 have relied on non-collinear or partly collinear geometries with the advantage of signal detection being completely or partially background-free, respectively. These could be the timescales of homogeneous dephasing of coherences between excited states in an inhomogeneous distribution of energy gaps 10 or morphology dependence of singlet exciton fission 11 and exciton diffusion 12 rates. However, multidimensional spectroscopies with single waiting times 9 do not rephase the energetic or morphological inhomogeneities in the sample along the pump–probe waiting time such that the underlying relaxation mechanisms between overlapping vibronic bands may still be obscured. The 2D contour maps evolve with the pump–probe waiting time T and show the correlations between chromophores both within and across the vibronic bands excited by femtosecond pulses. Similar to spin echoes in NMR, Fourier transform multidimensional spectroscopy 8 can also rephase the ensemble dephasing of a collection of oscillating dipoles to reveal the homogenous dephasing timescale of an individual dipole in the ensemble. In analogy with multidimensional nuclear magnetic resonance (NMR), 6,7 the advent of optical multidimensional spectroscopic techniques has partly addressed the above issue by spectrally decongesting relaxation processes in terms of a two-dimensional (2D) contour map, which correlates the excitation and detection frequencies of a system. The aim of this Perspective is to discuss the technological developments that have eventually enabled spatially resolved multidimensional electronic spectroscopies and highlight some of the very recent findings already made possible by introducing spatial resolution in a powerful spectroscopic tool. Recent extension of these spectroscopies to provide diffraction-limited spatial resolution, while maintaining temporal and spectral information, has been exciting and has paved a way to address several challenging questions by going beyond ensemble averaging. However, measurements on ensembles have implied signal averaging over relevant details, such as morphological and energetic inhomogeneity, which are not rephased by the Fourier transform. A combination of coherent excitation of several resonances with few-cycle pulses, and spectral decongestion along multiple spectral dimensions, has enabled new insights into wide ranging molecular scale phenomena, such as energy and charge delocalization in natural and artificial light-harvesting systems, hydrogen bonding dynamics in monolayers, and strong light–matter couplings in Fabry–Pérot cavities. ![]() Over the past two decades, coherent multidimensional spectroscopies have been implemented across the terahertz, infrared, visible, and ultraviolet regions of the electromagnetic spectrum.
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