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

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

Multiresonant Coherent Multidimensional Electronic Spectroscopy of Colloidal PbSe Quantum Dots

We demonstrate the use of multiresonant coherent multidimensional spectroscopy (CMDS) for obtaining 2D spectra of the diagonal and cross-peaks of the 1S and 1P excitons and biexcitons in PbSe quantum dots and their coherent and incoherent dynamics. We show that multiresonant CMDS line narrows the inhomogeneous broadening and resolves the excitonic peaks from the background that often obscures peaks. We develop theoretical methods that extract details of the homogeneous and inhomogeneous broadening, the Coulombic coupling within the biexciton, and the relative exciton and biexciton transition moments. The population dynamics are measured by scanning the excitation pulse time delays over all time orderings. Phase modulations of individual coherences are observed because of heterodyning between scattered light and the four-wave mixing signal. The experiments demonstrate that CMDS can be used to obtain quantum state resolved dynamics of the electronic states in complex nanostructures and other important materials