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Title: Tuning light-matter interactions inside organic microcavities using semiconductor polymer chain geometry
Author: Le Roux, Florian
ISNI:       0000 0004 9355 2153
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2020
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Strong light-matter interaction inside planar microcavities has been the object of intense scrutiny thanks to a wide variety of technological and theoretical achievements for which future applicability relies on understanding and controlling both photonic and excitonic environments. Recently, demonstrations of polariton lasing, non-equilibrium Bose Einstein Condensation (BEC) and superfluidity at room temperature have been reported thanks to the large oscillator strengths and exciton binding energies of Frenkel excitons in organic semiconductors. Among this class of material, semiconducting conjugated polymers are promising thanks to their versatility, remarkable optical properties and tunability. In this thesis, we explore different methods to manipulate the intra- and inter-chain properties of a subgroup of semiconducting polymers, the polyfluorenes, to shape the interactions of their excitons with light. We begin our work by examining the essential properties of semiconducting conjugated polymers, which we then embed inside microcavities to observe strong light-matter interactions. We present a brief overview of exciton-polaritons in organic microcavities, highlight recent developments in their understanding and introduce the measurement and fabrication techniques that we use throughout the thesis. In our first experimental investigation, we perform conformational changes to a polymer's backbone inside metallic microcavities thereby inducing two sub-populations of excitons with a pre-determined fraction. We present the mode characteristics of these structures with ultrastrong coupling (USC) of the first disordered population of excitons and a splitting of the lower polariton branch induced via gradual introduction of the second population of excitons. We measure the photoluminescence (PL) only emanating from the lowermost polariton branch, which allows us to exert conformational control over the emission energy and its angular variation to create dispersion-free cavities with highly saturated blue-violet emission. In our second experimental realization, we report an inter-chain tunability via the fabrication and optical characterization of organic microcavities containing liquid-crystalline conjugated polymers (LCCP)s aligned on top of a thin transparent Sulfuric Dye 1 (SD1) photoalignment layer. Transition dipole moment alignment enables a systematic increase in the interaction strength, with unprecedented solid-state Rabi splitting energies ~Ω0 of up to 1.80 eV, the first to reach energies comparable to those in the visible spectrum, accompanied by the highest-to-date organic microcavity coupling ratio: 65%. We also demonstrate that the coupling strength is polarization-dependent with bright polariton PL for TE polarization parallel to the polymer chains and either no emission or weakly coupled emission from the corresponding TM polarization. We finally examine the reflectivity and transmissivity of TE-polarized waves incident on microcavities containing excitons with in-plane uniaxially oriented transition dipole moments both theoretically and experimentally. We discuss the propagation of the electric field through the cavity and compare our results with previous reports. We confirm that in all cases, the reflected and transmitted electric fields derive from photons leaking parallel and perpendicular to the transition dipole moment orientation. We conclude our work by examining the opportunities that are enabled from our two experimental techniques for the future of polaritonics.
Supervisor: Riordan, Donal ; Taylor, Robert Sponsor: Wolfson Harrison Scholarship
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: microcavities ; organic semiconductors ; exciton-polaritons ; semiconducting polymers ; condensed matter physics