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Title: An investigation of stimulated emission processes in thin-film molecular materials and structures
Author: Xia, Yiren
ISNI:       0000 0004 9347 6963
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2020
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Molecular semiconductor materials are highly attractive as gain media for solid state lasers and amplifiers. Devices based on thin-film structures show great potential in miniaturised, tunable and flexible lasing applications at a low cost. The ultimate aim in this context is the realisation of electrically pumped organic semiconductor lasers. One of the main challenges is understanding the relationship between material properties and stimulated-device performance, i.e. identifying generally applicable molecular structures that facilitate amplified spontaneous emission (ASE) or lasing. This thesis comprehensively investigates stimulated emission processes in thin-film molecular materials and structures by numerical modelling and experimental characterisation on amorphous and glassy spin-coated samples. Absorption and emission spectroscopies are used to obtain material properties in photo-physics, and excited state lifetime is extracted from transient photoluminescence measurement. A 2-level spatial-temporal model reveals some of the key material and structure parameters for ASE in planar thin-films, which is guided by nonuniformly distributed modal gain. Meanwhile, a semi-automated ASE measurement set-up is introduced, followed by a reliable gain characterisation based on a modified single-pass travelling wave approach with homogeneous saturation. New metrics, ‘gain versus pump’ and ‘gain length product’, are proposed to evaluate gain media independent of measurement conditions. Furthermore, resonance energy transfer in molecular blends is studied systematically. Results from a set of samples with varying donor-acceptor concentration ratios unambiguously link photoluminescence quantum efficiency and excited state lifetime to ASE performance, which opens the door to tailor-made materials for organic semiconductor lasers.
Supervisor: Stavrinou, Paul N. ; Bradley, Donal D. C. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Solid state physics ; Organic semiconductors