Femtosecond and Two-Dimensional Spectroscopy of Lead Chalcogenide Quantum Dots

摘要

Quantum dots have applications in lighting displays and in biomedical imaging, and potential applications in next generation photovoltaics. The fine structure, optical inhomogeneity, coupling between excitons, and exciton dephasing rates are important to quantify because they affect the optical properties of quantum dots. These observables are encrypted in linear absorption lineshapes but can be measured with nonlinear spectroscopy.;The first two-dimensional (2D) Fourier transform spectra of lead chalcogenide quantum dots in the short-wave infrared are presented. With the additional dimension afforded by 2D spectra, linewidths from homogeneous and inhomogeneous broadening can be separated. Simulations to model the experimental 2D spectra are performed at long relaxation times to extract the optical inhomogeneity of a PbSe quantum dot ensemble. Additional information can be determined more accurately from 2D spectra than from transient absorption measurements. Specifically, the simulation of experimental 2D spectra shows that the excited state absorption redshift, which has been attributed to the bi-exciton binding energy, is larger than previously reported results, suggesting new interpretations.;Pump-probe polarization anisotropy measurements with 15 fs pulse durations are employed to investigate the electronic structure of chlorine-passivated PbS quantum dots at the bandgap. The measurement of the initial anisotropy required the removal of unwanted signal contribution from cross-phase modulation. The presence of cross-phase modulation motivated the calculation, simulation, and measurement of this signal for spectrally resolved pump-probe transients and two-dimensional Fourier transform spectroscopy. The initial anisotropy contains information about both bi-exciton states and some hot single exciton states, and the time dependence quantifies the timescale of intervalley scattering. Our results for PbS quantum dots indicate a lower initial anisotropy than expected from the effective mass approximations for PbS quantum dots with a rapid loss of any anisotropy in less than 20 fs.