Exciton Polaron Dynamics in 2D Metal Halide Perovskite Derivatives: Two-dimensional hybrid organic-inorganic perovskites (2D-HOIPs) are an emerging class of semiconductors composed of quasi-2D layers of metal-halide octahedra (e.g., PbI4, SnI4) separated by layers of long (> 1nm) organic cations (e.g., PEA= phenylethylammonium). The latter facilitate electronic and dielectric confinement within the metal-halide layers resulting in quantum-well like structures. As a consequence, there is a substantial enhancement in the Coulomb interactions between photo-generated carriers resulting in excitons (bound electron-hole states) with remarkably high binding energies. On the other hand, the ionic character of the perovskite lattice leads to strong coupling of its vibrational degrees of freedom with the electronic states. Charge carriers (electrons and holes) are thus dressed by phonons and are bound to local lattice deformations to create polaronic states. Given the charge neutrality of excitons and their large binding energy, one would expect that such polaranic effects will have no consequence on them. However, contrary to such an expectation, extensive experimental investigations performed by our group in collaboration with the group of Prof. Carlos Silva (Georgia Tech) have established a deterministic role of phonon coupling in the coherent and recombination dynamics of excitons. Based on these observations, we proposed that the excitons in 2D HOIPs are in fact exciton-polarons: quasi particles with Coulomb correlations that are re-normalized by lattice dynamics via polaronic effects.

These observations are of fundamental importance not only in the context of the development of the semiconductor physics of 2D-HOIPs, but generally for a much broader class of materials in which multi-particle coupling and many-body effects are mediated by interactions with a fluctuating noisy lattice. By providing an experimental access into these characteristics via their optical properties, HOIPs offer an ideal test-bed to develop this broadly important materials science. We are now exploring the material parameter space in HOIPs, involving dimensionality and chemical composition to further advance the comprehension of many-body effects in these material systems and substantiate our “exciton polaron” hypothesis.

Selected Publications:

• A. R. Srimath Kandada and C. Silva, Exciton polarons in two-dimensional hybrid metal-halide perovskites, J. Phys. Chem. Lett., 11, 3173-3184 (2020).
• A.R. Srimath Kandada, H. Li, F. Thouin, E. R. Bittner and C. Silva, Stochastic scattering theory for excitation-induced dephasing: Time dependent nonlinear coherent exciton lineshapes, J. Chem. Phys., 153, 164706 (2021).
• F. Thouin, D. Valverde-Chavez, C. Quarti, D. Cortecchia, I. Bargigia, D. Beljonne, A. Petrozza, C. Silva and A. R. Srimath Kandada, Phonon coherences reveal the polaronic character of the excitons in two-dimensional lead-halide perovskites, Nature Materials, 18, 349-356 (2019).

Material Spectroscopy with Entangled Photons: Optical spectroscopies used to assess the many-body interactions rely on the measurement of a perturbative on-linear response induced in materials by a sequence of femtosecond laser pulses. However, there are two important limitations to such methodologies. Firstly, spectral congestion due to energetic overlap of various photo-excitations and the presence of cascaded lower order contributions atop the higher-order response imposes ambiguity over the observed dynamics. Secondly, an unavoidable limitation is set by the signal to-noise ratio of the employed techniques, where the experiments are often conducted at relatively high laser powers, which may lead to material degradation during the measurement.

We are developing an alternative strategy based on the quantum measurement of photon entanglement after the interaction with matter as a means to estimate the multi-body interactions. The entangled photon pairs that compose a pure quantum state are the “system”, and the material-sample is the “bath”. The system-bath interactions are light-matter interactions. The “bath” fluctuations induced by nonlinear correlations such as exciton-exciton or exciton-lattice interactions, drive the change in the photon quantum state. By characterizing the quantum state of the biphoton field transmitted through the sample, we propose to exclusively probe the multi-particle correlations in excitonic materials, and at single-photon intensities.

Selected Publications:

• E. R. Bittner, H. Li, A. Piryatinski, A. R. Srimath Kandada and C. Silva, Probing exciton/exciton interactions with entangled photons: Theory, J. Chem. Phys., 152, 071101 (2020).
• A. R. Srimath Kandada, I. Bargigia, E. R. Bittner and C. Silva, Quantum process tomography of entangled photons as a probe of intermediates of singlet fission in a tetracene derivative, Arxiv:1909. 12869 [physics.chem-ph].

For an up-to-date list of publications, see the Google Scholar page.