As part of the secondment at University of Montreal and Georgia Institute of Technology, extensive investigations on the photo-excitation dynamics in three-dimensional and two-dimensional hybrid perovskites were performed. Functional material architectures were procured via collaborations with IIT Milano and investigations based on ultrafast coherent techniques were carried out. The effect of polar lattice fluctuations and coupling between electronic and vibrational degrees of freedom in these materials were comprehended. The important conclusions include:
(a) A quantitative analysis of the optical absorption, supported by coherent non-linear dynamics that established the presence of multiple excitons in a variety of 2D perovskites. (Physical Review Materials 2, 064605 (2018));
(b) Resonant impulsive stimulated Raman scattering that demonstrated that each of these excitons are dressed by distinct vibrations of the lattice. (Nature Materials 18, 349 (2019));
(c) Photon-echo measurements that identified and quantified the interaction of the excitons with each other and more importantly highlighted the presence of a protection mechanism from the crystal vibrations that reduces the probability of loss of photo-generated excitons ( Physical Review Research, 1, 032032);
(d) When two excitons overcome such a protection barrier, they form a new particle called biexciton, where they lose their individual behavior and behave as one particle. Their presence in 2D perovskites was demonstrated in Physical Review Materials 2, 034001 (2018).
(e) The relaxation process of the excitons which leads to production of light is controlled by the crystal vibrations and thus the design of appropriate crystal structure is crucial in increasing the material efficiency for light emission. (Chemistry of Materials 31, 7085 (2019)).
Specific two-dimensional perovskite architectures were identified to have ideal characteristics to be incorporated in polariton devices. Strongly coupled microcavties with 2D perovskites were fabricated and characterized to demonstrate polariton formation.
In order to address the second set of objectives, sources of entangled-photon pairs were developed and characterized by quantum-optical tomographic techniques. Experimental schemes to employ such methods for material spectroscopy were also implemented. Independently, theoretical framework to deal with quantum spectroscopies was developed in close collaboration with Prof. Bittner (University of Houston). These theoretical investigations also enabled optimization of the experimental methodologies. As a proof of concept, intermediates of singlet fission process in molecular aggregates have been probed with entangled photons to demonstrate their efficacy in probing excitonic correlations.
