Fluorescence-Detected\nTwo-Dimensional Electronic Spectroscopy\nof a Single Molecule

Single-molecule fluorescence\nspectroscopy is a powerful method\nthat avoids ensemble averaging, but its temporal resolution is limited\nby the fluorescence lifetime to nanoseconds at most. At the ensemble\nlevel, two-dimensional spectroscopy provides insight into ultrafast\nfemtosecond processes, such as energy transfer and line broadening,\neven beyond the Fourier limit, by correlating pump and probe spectra.\nHere, we combine these two techniques and demonstrate coherent 2D\nspectroscopy of individual dibenzoterrylene (DBT) molecules at room\ntemperature. We excite the molecule in a confocal microscope with\na phase-modulated train of femtosecond pulses and detect the emitted\nfluorescence with single-photon counting detectors. Using a phase-sensitive\ndetection scheme, we were able to measure the nonlinear 2D spectra\nof most of the DBT molecules that we studied. Our method is applicable\nto a wide range of single emitters and opens new avenues for understanding\nenergy transfer in single quantum objects on ultrafast time scales.