Room temperature polariton lasing in organic semiconductors

Abstract

Exciton-polaritons in strongly coupled microcavities give rise to coherent emission, termed polariton lasing. In principle, the threshold can be extremely low as population inversion is unnecessary in contrast to photon lasing. Organic semiconductors are in particular of interest because of their combination of high exciton binding energy and high oscillator strength, sustaining polaritons at room temperature.

This thesis concerns an experimental study aiming to realise ultra-low threshold polariton lasing in organic semiconductors. First, room temperature polariton lasing is observed in a highly disordered prototypical conjugated polymer, poly(9,9dioctylfluorene) (PFO), at an incident threshold of 27.7 µJ/cm², which is only the second report of polariton lasing in a conjugated polymer. Interferometry measurements show stable fringes with maximum visibility of 72%. Second, polariton lasing is demonstrated in two ladder-type oligofluorenes with the same functional groups but different conjugation lengths. The respective absorbed thresholds are 12 µJ/cm² and 17 µJ/cm² which are comparable to the lowest reported value in organic microcavities. Due to different degrees of disorder in the two materials, a different number of excitonic transitions couple with the cavity photon mode, resulting in a variation in Rabi splitting and a 27 nm shift in polariton lasing wavelength. Finally, polariton lasing at the lowest incident threshold (13.5 µJ/cm² and 9.7 µJ/cm²) reported to date is realised in dihexylfluorene-vinylphenylcarbazole based, highly soluble small molecules. The coherence time is estimated to be ~10 ps which is two orders of magnitude longer than the polariton lifetime. The step-like nonlinearity (blueshift) observed versus pump fluence indicates an interplay between different exciton depletion channels.

Date of Award	28 Jun 2021
Original language	English
Awarding Institution	University of St Andrews
Supervisor	Ifor David William Samuel (Supervisor) & Graham Turnbull (Supervisor)