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Quantum Light Spectroscopy of Polariton Lasers

Periodic Reporting for period 2 - QUANTUM LOOP (Quantum Light Spectroscopy of Polariton Lasers)

Reporting period: 2018-10-01 to 2019-09-30

Coherent sources of light are crucial and integral components of many of current day technologies with their applications in communications, sensing and biomedical fields. Lasers based on polaritons (light-matter quasi-particles) are envisaged to be energy efficient alternatives for such sources. However, devices which operate at room temperature based on this principle have remained elusive to the scientific community. The context of this project is to establish the essential photo-physical knowledge about emerging semiconducting materials which are potential candidates for such applications, via advanced optical spectroscopic techniques. One of the major objectives of this project was to investigate the multi-particle interactions in novel hybrid organic-inorganic perovskites in the context of their application in polariton based devices. In parallel, the project also attempted to develop new spectroscopic tools based on entangled-photon pairs which will provide unambiguous probe of the multi-body correlations in materials.

Coherent nonlinear optical spectroscopies were employed on a variety of hybrid organic inorganic perovskites to obtain a comprehensive perspective on the nature and dynamics of excitons particularly in lower dimensional derivatives that host two-dimensional excitations. We have provided evidence for the ubiquitous role of the crystal vibrations in the excitonic characteristics and proposed exciton polaron model in these material systems. We also identified 2D perovskite architectures to create polariton lasers and embedded them within high quality microcavities. We have also developed theoretical and experimental tools based on entangled photon pairs to estimate many-body correlations in excitonic systems, which will be further developed in the future as alternative material probes.
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.
The activities within this projec thave produced impactful and comprehensive results on the photo-physics of hybrid perovskites. These observations will not only enable successful implementation of these materials in quantum optoelectronic technologies, but also assist in developing a general theoretical framework to treat photo-excitation dynamics in polar semiconductors. This in turn will enable design of novel material architectures for next generation technologies. The preliminary work on the strongly coupled microcavities with 2D perovskites performed within this project has provided the platform for their further optimization to realistically achieve both photon and polariton lasing and also non-classical sources of light.

The experimental and theoretical tools for quantum spectroscopy developed within this project can potentially lead to a completely novel perspective in material spectroscopy and in the treatment of light-matter interactions. Apart from offering unambiguous probes of matter correlations, these investigations may inspire methodologies to use matter to manipulate photon-entanglement, which is at the heart of a quantum-optical logic gate.
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