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Exciton-Polaritons in BODIPY-filled Microcavities

Georgiou, Kyriacos (2019) Exciton-Polaritons in BODIPY-filled Microcavities. PhD thesis, University of Sheffield.

K Georgiou PhD Thesis 2018.pdf
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This thesis concerns the fabrication and study of strongly coupled organic microcavities containing a series of different boron-dipyrromethene (BODIPY) fluorescent dyes dispersed in an optically inert polystyrene matrix. The photophysics of the different BODIPY dyes are first studied and it is shown that they are promising materials for polariton condensation. DBR-DBR microcavities containing thin films of dye/polystyrene blends are then investigated under angular white-light reflectivity and CW laser excitation; measurements that show that they can enter the strong coupling regime. Polaritons in such high quality factor structures are shown to undergo a phase transition when excited with a high density pulsed excitation, forming a polariton condensate. Power dependent and interferometry measurements are used to identify the condensation threshold and the spatial coherence length of the polariton condensate. Lower Q-factor microcavities, comprised of two silver mirrors are fabricated, containing two different BODIPY dyes. Energy transfer between the molecules is engineered using two different processes; (1) direct short-range dipole-dipole coupling between the molecules, and (2) polariton-mediated energy transfer. We assess the efficiency of the energy transfer by quantifying the polariton population density along each polariton branch following laser excitation. It is concluded that short-range (<3 nm) energy transfer induced by dipole-dipole coupling is more efficient compared to long-range (60 nm) polariton-mediated energy transfer, although the long-range process is estimated to transfer up to 87% of states to the lower-polariton branch. The generation of anti-Stokes polariton fluorescence is studied in low Q-factor metallic cavities following resonant excitation at the bottom of the lower polariton branch. Here, it is concluded that thermal energy in the system provides the excess of energy needed for emission of photons having higher energy than that of the initial laser excitation. Using temperature dependent and time resolved measurements it is concluded that polaritons return to the exciton reservoir by optically pumping a molecule in a vibrationally excited ground state. The exciton created then emits fluorescence that populates polariton states with an energy higher than the laser energy resulting in anti-Stokes polariton fluorescence. We believe such systems will be of significant interest in exploring laser-cooling phenomena in solid-state systems.

Item Type: Thesis (PhD)
Keywords: organic semiconductors, exciton-polaritons, microcavities, photonics
Academic Units: The University of Sheffield > Faculty of Science (Sheffield) > Physics and Astronomy (Sheffield)
Identification Number/EthosID: uk.bl.ethos.766567
Depositing User: Mr Kyriacos Georgiou
Date Deposited: 21 Jan 2019 09:42
Last Modified: 25 Sep 2019 20:06
URI: http://etheses.whiterose.ac.uk/id/eprint/22677

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