Revealing exciton energy structure and interactions in quantum dots by 25 kHz shot-to-shot phase-cycling 2D electronic spectroscopy

The use of spectroscopic techniques to resolve the energy level structure of excitonic excited states and to elucidate the interaction mechanisms between excitons in quantum dots is of vital importance for the development and application of such materials. However, various static and dynamic one-dimensional spectroscopic techniques are limited by inhomogeneous broadening effects, making it challenging to directly observe the fine-level energy structure of quantum dots. We developed a 25 kHz shot-to-shot phase-cycling two-dimensional (2D) electronic spectroscopy technique to investigate the excitonic energy structure and exciton-exciton interactions in a quantum dot aggregate. Using CdTe/CdSe/ZnS core-shell-shell quantum dots as a model system, we implemented a 36-step phase-cycling scheme to acquire rephasing 2D spectra at zero waiting time under varying excitation powers. In these spectra, both diagonal and off-diagonal features reveal spectral components that are otherwise obscured in one-dimensional spectroscopy due to inhomogeneous broadening. The presence of off-diagonal peaks in the 2D spectra indicates non-negligible interactions between excitons with different transition energies. Power-dependent 2D spectroscopy reveals that, with increasing excitation power, high-energy states exhibit greater resistance to Auger recombination. The experimental method developed in this work may contribute to advancing the theory of excited-state structures and dynamics in quantum dots and other low-dimensional materials.