Ultrafast relaxation through a dense vibrational-electronic manifold of states often requires multidimensional electroscopy spectroscopy (MES) methods to spectrally decongest the dyanmics along excitation, detection and coherence frequency axes. However exisitng methods rely on interferometric detection, do not provide spatial resolution and prone to sample scatter. The latter limitation also prevents their aplicability to scattering samples, for instance, to probe energy transfer in connected dense protein networks within photosynthetic cells. Fluorescence-detected MES (fMES) has made recent strides towards combining MES with the spatial resolution of a microscope, with promise of high-sensitivity of fluorescence detection prevailing over the disadvantaged slow nature of the experiment arising from the requirement of scanning an additional time delay. We report an alternative fMES approach which addresses the above challenges regarding experimental throughput and systematically establishes the sensitivity of fMES compared to existing MES approaches. A visible white-light continnum based fMES spectrometer with 15 fs time resolution, system limited spectral resolution and microscope objective limited spatial resolution is presented. All existing fMES approaches have relied on stepwise or continuous scaning of t1 and t3 time delays as a (t1,t3) grid (corresponding to excitation ω1 and detection ω3 axes) for each pump-probe waiting time T (correspoding to coherence axes ωT). Our approach departs from the existing approaches in that T is scanned continuously for each grid point, enabling free optimization of scan speed as well as the choice of grid-points to sample, while substantially minimizing the continuous sample exposure window, a vital advantage geared towards spatially resolved measurements. Together, these innovations allow us to systematically establish the enhanced sensitivity of fMES compared to conventional MES with measurements of room temperature vibrational coherence along with quantum beat phase possible even at the lowest sample ODs. By rapid T scanning, we also demonstrate substantially faster 1D experiments with equivalent signal-to-noise ratio, which go beyond pump-probe in their ability to isolate signals similar to a MES experiment. Our approach enables spatially-resolved experiments at lower repetition rates with minimized sample exposure window, as well as experiments with low photon count rate sources such as entangled photon pairs.

High-sensitivity fluorescence-detected multidimensional electronic spectroscopy through continuous pump-probe delay scan

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