Dr. Kyriacos Georgiou Profile

Dr. Kyriacos Georgiou

at Univ of Sheffield

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Publications (4)

Proceedings Article | 5 October 2023 Presentation

Polariton-assisted ultrafast energy delocalization in a donor-acceptor organic microcavity probed by two-dimensional electronic spectroscopy

Margherita Maiuri, Mattia Russo, Kyriakos Georgiou, Armando Genco, Simone De Liberato, Giulio Cerullo, David Lidzey, Andreas Othonos, Tersilla Virgili

Proceedings Volume PC12650, PC126500F (2023) https://doi.org/10.1117/12.2681450

KEYWORDS: Optical microcavities, Polaritons, Spectroscopy, Resonance energy transfer, Molecules, Ultrafast phenomena, Ultrafast laser spectroscopy, Multilayers, Signal detection, Energy transfer

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Singlet fission, in which two triplet excitons are formed from one photon, has the potential to increase the power conversion efficiency of solar cells. While singlet fission has been studied extensively over the past decade, the role of vibrational coupling remain unclear [1]. Femtosecond stimulated Raman spectroscopy (FSRS) is an ultrafast nonlinear optical technique that investigates vibrational structures of molecules providing both high temporal resolution (∼100 fs) and reasonable spectral resolution (10cm−1). In this three−pulse technique, excited and ground state vibrational spectra are obtained to gain structural information about singlet and triplet kinetics in a fast singlet fission process. We have investigated the excited-state dynamics of 6,13-bis triiso-propylsilylethynyl (TIPS) Pentacene in concentrated solution to determine the role of excimer and aggregate formation in singlet fission solutions. Using transient absorption, we find that the mechanism of singlet fission remains dominated by diffusive encounters in highly-concentration solutions (Dvořák et al. [2]). Our research extends to track vibrations during the singlet fission process using FSRS. Here we present the time-resolved vibrational spectrum of highly-concentrated TIPS-Pentacene solutions. The molecular structural evolution is probed by monitoring vibrational changes in Raman transitions after a variable time delay. The data is compared with DFT computational methods to identify the excited state vibrational species. These results will be used to gain further insights into the singlet fission process. Kim, W., & Musser, A. J. (2021). Tracking ultrafast reactions in organic materials through vibrational coherence: vibronic coupling mechanisms in singlet fission. Advances in Physics: X, 6(1), 1918022. Dvořák, M., Prasad, S. K., Dover, C. B., Forest, C. R., Kaleem, A., MacQueen, R. W., ... & Schmidt, T. W. (2021). Singlet fission in concentrated TIPS-pentacene solutions: The role of excimers and aggregates. Journal of the American Chemical Society, 143(34), 13749-13758.

Proceedings Article | 15 March 2023 Presentation

Along for the ride: the influence of dark states in organic polariton transport

Raj Pandya, Arjun Ashoka, Kyriacos Georgiou, Scott Renken, Rahul Jayaprakash, Akshay Rao, Andrew Musser

Proceedings Volume PC12419, (2023) https://doi.org/10.1117/12.2649379

KEYWORDS: Polaritons, Ultrafast phenomena, Polymers, Photon polarization, Organic semiconductors, Organic materials, Optical microcavities, Microscopy, Light wave propagation, Femtosecond phenomena

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Proceedings Article | 19 April 2021 Presentation

Nonlinear dynamics of the energy landscape in Frenkel exciton-polariton condensates

Anton Zasedatelev, Denis Sannikov, Timur Yagafarov, Anton Putintsev, Kyriacos Georgiou, David Lidzey, Pavlos Lagoudakis

Proceedings Volume 11770, 117700H (2021) https://doi.org/10.1117/12.2593507

KEYWORDS: Polaritons, Nonlinear dynamics, Excitons, Transient nonlinear optics, Energy transfer, Scattering, Polarization, Optical microcavities, Ultrafast phenomena, Quantitative analysis

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Bose-Einstein condensates of exciton-polaritons in inorganic semiconductor microcavities are known to possess strong interparticle interactions attributed to their excitonic component. The interactions play a crucial role in the nonlinear dynamics of such systems and can be witnessed as the high energy shift of polariton states. However, the localised nature of Frenkel excitons in strongly coupled organic microcavities precludes interparticle Coulomb exchange-interactions that affect nonlinear dynamics and change mechanisms of polariton energy shifts accordingly. We have scrutinized the origins of energy shifts in connection with nonlinear dynamics in Frenkel exciton-polariton condensates and examined the possible contributions: intracavity optical Kerr-effect, gain-induced frequency pulling, polariton interactions and effects related to saturation of optical transitions for "dark"- and "bright" molecules [1]. Unlike the conventional strongly coupled semiconductor microcavities, we have shown that nonlinear interactions within the condensate do not rely on polariton interactions but instead originated from Pauli-blocking principle forbidding double excitation of the same molecular states. We have developed a theoretical model explaining the omnipresent energy shift of the condensate wavefunction together with its spectral and polarization features at the BEC transition in a consistent way. The crucial role of intermolecular energy transfer and “dark” exciton reservoir has been demonstrated for the first time. We believe the principles explored in this work are relevant to other systems exhibiting strong coupling of Frenkel excitons with a cavity mode, regardless of a cavity type, whether one dealing with Fabry-Perot cavities or plasmonic nanocavities etc and; therefore, provide general insight on nonlinear phenomena in composite light-matter condensates. [1] T. Yagafarov, D. Sannikov, A. Zasedatelev, K. Georgiou, A. Baranikov, O. Kyriienko, I. Shelykh, L. Gai, Z. Shen, D. G. Lidzey, P. Lagoudakis, Mechanisms of blueshifts in organic polariton condensates, Commun Phys 3, 18 (2020). https://doi.org/10.1038/s42005-019-0278-6

Proceedings Article | 18 April 2021 Presentation

Quasi-steady state exciton-polariton condensation in organic microcavities

Anton Putintsev, Anton Zasedatelev, Kirsty McGhee, Tamsin Cookson, Kyriacos Georgiou, Denis Sannikov, David Lidzey, Pavlos Lagoudakis

Proceedings Volume 11770, 1177011 (2021) https://doi.org/10.1117/12.2593446

KEYWORDS: Polaritons, Optical microcavities, Navigation systems, Molecules, Luminescence, Polarons, Photons, Organic semiconductors, Optical properties, Glasses

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Strongly coupled organic microcavities are up-and-coming material systems for ambient polaritonics. A broad range of suitable materials made possible the experimental observation of polariton lasing across the whole visible range, as well as device-concepts ranging from ultra-fast transistors and all-optical logic gates to single-photon switching, all at room temperature under ambient conditions. Unlike the case of inorganic semiconductor microcavities, where continuous-wave excitation allows for the replenishment of particle losses, leading to the realization of steady-state polariton condensates, in organic semiconductors photobleaching and polaron formation prevent CW operation. BODIPY dyes have been the subject of thorough studies for their applications in the strong coupling regime. Strongly coupled BODIPY microcavities and polariton lasing in these structures allow for highly monochromatic tunable coherent emission of duration up to ~two picoseconds. Here, we use a single-mode lambda/2 strongly coupled microcavity of a BODIPY dye molecule, employ a single-shot dispersion imaging technique to study polariton lasing in a planar organic microcavity, and achieve a quasi-steady state exciton-polariton condensation under single-shot excitation in such systems. Temporal dynamics of a single-shot exciton-polariton lasing is of particular interest and importance for understanding rates of depletion and replenishment of the exciton reservoir and polariton states, respectively, under pulsed excitation. Moreover, the direct measurement of the condensate lifetime provides valuable insight into the transient processes of nonequilibrium polariton condensation. Long-lasting condensates exceeding polariton lifetime for several orders of magnitude push the system one step closer towards the regime of dynamic equilibrium and could be a missing puzzle towards polariton applications such as connected polariton devices and condensate lattices implemented at ambient conditions, opening the possibility for all-optical polariton circuitry on a chip.

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