CAREER: Nonlinear Dynamics of Exciton-Polarons in Two-Dimensional Metal Halides Probed by Quantum-Optical Methods

With support from the Chemical Structure, Dynamics, and Mechanisms A (CSDM-A) program in the Division of Chemistry, Ajay Ram Srimath Kandada of Wake Forest University is incorporating quantum-optical principles into ultrafast nonlinear spectroscopy to investigate photo-excitation dynamics in materials close to the single-photon excitation limit. Nonlinear optical spectroscopy, which is widely used to explore light-matter interactions, requires high light intensities due to fundamental limitations in classical optical detection. Unfortunately, this often introduces significant ambiguity in the derived photophysical models. This research project seeks to overcome this limitation by harnessing the superior signal-to-noise ratio provided by quantum-optical methods. Dr. Srimath Kandada and his students will use this enhanced experimental capability to investigate the unique exciton physics of two-dimensional derivatives of metal halide perovskites. Improved comprehension of perovskite photophysics is expected to drive the optimization of their optoelectronic properties, with potential benefits for a variety of applications. The education and outreach program, centered on the optics and spectroscopy components of the research program, will build a synergistic partnership between the students and faculty at Wake Forest University and the local high schools to motivate greater participation of students from diverse backgrounds in STEM (science, technology, engineering and mathematics) education and research. This research project aims to measure the dynamics of exciton-lattice correlations and the fluctuations that drive them, to accurately estimate the intrinsic dephasing rate of excitons and inter-exciton interactions and to measure the dynamics of carrier thermalization and exciton formation without many-body contributions. The first part of the project will focus on the nature of polaronic coupling of photo-excitations in two-dimensional metal halides and the role of dynamic lattice fluctuations. The experiment involves the measurement of noise in the amplitude and phase of the transient optical response in the presence of an impulsively generated population of coherent phonons. This will enable the estimation of contributions from stochastic lattice fluctuations to the phonon anharmonicity and polaronic dressing of excitons. The second part of the project will employ quantum-entangled photons as a probe of many-body exciton dynamics. The energy correlations between a pair of spectrally entangled photons, transmitted through the resonant medium, is to be used to accurately estimate exciton dephasing rate and biexciton interactions, bereft of multi-order expansion issues. The photon entanglement is also to be exploited to measure the population dynamics of excitons at extremely low excitation densities in a (classical) pump - (quantum) probe experiment. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.