Coherent Two-Dimensional Nanoscopy

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Science  11 Aug 2011:
DOI: 10.1126/science.1209206


We introduce a spectroscopic method that determines nonlinear quantum-mechanical response functions beyond the optical diffraction limit and allows direct imaging of nanoscale coherence. In established coherent two-dimensional (2D) spectroscopy, four-wave-mixing responses are measured using three ingoing and one outgoing wave; thus, the method is diffraction limited in spatial resolution. In coherent 2D nanoscopy, we use four ingoing waves and detect the final state via photoemission electron microscopy, which has 50 nanometer spatial resolution. We recorded local nanospectra from a corrugated silver surface and observed subwavelength 2D line shape variations. Plasmonic phase coherence of localized excitations persisted for about 100 fs and exhibited coherent beats. This was analyzed with a model of coupled oscillators leading to Fano-like resonances in the hybridized dark- and bright-mode response.