Spectral Isolation and Measurement of Surface-Trapped State Multidimensional Nonlinear Susceptibility in Colloidal Quantum Dots

Multiresonant coherent multidimensional spectroscopy (CMDS) is a powerful new method for probing the coupling between vibrational modes and their dynamics. The line narrowing that occurs because of the multidimensional nature of CMDS allows the separation of homogeneous and inhomogeneous broadening and enhances spectral resolution. Recent work has extended multiresonant CMDS to electronic resonances in quantum confined nanostructures. Vibrational modes of the solvent also appear in the CMDS spectra. The phase oscillations of the vibrational and electronic coherences interfere and change the line shapes. Since the form of the vibrational third-order susceptibility and hyperpolarizability are well-known and since they can be measured against known standards, it becomes possible to use the interference effects as a probe of the absolute magnitude and phase of the electronic resonances. This approach is demonstrated using PbSe quantum dots where incomplete capping causes ultrafast relaxation to a new electronic state that appears directly in the CMDS spectra. The new state is believed to be a mixed core/surface exciton. Closed-form expressions for the electronic nonlinearities are used to analyze the frequency dependence of the fully resonant complex hyperpolarizability of the 1S exciton and the surface-trapped state. The ability of mixed frequency/time domain multiresonant CMDS methods to spectrally resolve surface states promises to be an important new way to characterize the interface states in complex heterostructures and the surface states that define the stability of nanostructures resulting from different synthetic strategies.