Theory and Simulations of Atomic Force Microscopy-Based Two-dimensional Electronic Spectroscopy.

Probing excited states and ultrafast dynamics in heterogeneous and interfacial systems requires enhanced spatial and temporal resolution. Coherent two-dimensional electronic spectroscopy (2DES) offers a subpicosecond temporal resolution but its spatial resolution is restricted by the diffraction limit. Atomic force microscopy (AFM) overcomes the diffraction limit and routinely attains a sub-10 nm spatial resolution. In this Letter we establish a theoretical framework for the AFM-detected 2DES spectroscopy─a technique that is yet to be experimentally realized but promises to deliver high temporal and spatial resolutions simultaneously. AFM-2DES would measure thermal expansion of the sample in response to interactions with multiple time-delayed laser pulses which interrogate electronic states of the system. We illustrate that the information contained in AFM-2DES spectra is complementary to other far-field action 2D spectroscopies, thus establishing AFM-2DES spectroscopy as a valuable addition to the arsenal of ultrafast spectroscopy techniques.