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Title: Room-temperature polariton condensates in all-dielectric microcavities
Author: Daskalakis, Konstantinos
ISNI:       0000 0004 5349 3304
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2014
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Cavity polaritons are quasiparticles formed when a photon con ned within a cavity interacts with an elementary excitation in a semiconductor that is called exciton. Under the right conditions, cavity polaritons form a macroscopic condensate in the ground state. This condensate decays through the cavity mirrors, thus providing coherent light-emission: a phenomenon termed polariton lasing. The threshold for polariton lasing can be signi cantly lower than that required for conventional lasing. Large exciton binding energies are an essential requirement to obtain polariton lasing at room temperature. Group III nitrides and ZnO are the only inorganic semiconductors possessing Wannier-Mott exciton binding energies above 25 meV, the room-temperature thermal energy. In contrast, Frenkel excitons in organic semiconductors possess binding energies of 1 eV and are thus highly stable at room temperature. This thesis consists of two parts. The first part concerns the fabrication and optical characterisation of samples consisting of an ultra-smooth GaN membrane encapsulated in an all-dielectric (SiO2/Ta2O5) distributed Bragg reflector (DBR) microcavity. By utilising the selective photo-electro-chemical (PEC) etching of an InGaN sacri cial layer, GaN membranes 200 nm thick are produced and introduced between DBRs. The second part is devoted to the demonstration of a room-temperature organic polariton condensate. The studied samples consist of a thermally evaporated 2,7-bis[9,9-di(4-methylphenyl)-fluoren-2-yl]-9,9-di(4-methylphenyl) fluorene (TDAF) thin film enclosed within an all-dielectric microcavity, consisting of SiO2 and Ta2O5 pairs. In both GaN and organic systems, the strong coupling for various detunings is demonstrated by performing angle-resolved reflectivity and photoluminescence (PL) measurements. On reaching threshold, the nonlinear increase in the PL is blueshifted with respect to low power emission, and is accompanied by a simultaneous reduction in the linewidth, marking the onset of polariton lasing at room-temperature. In the organic microcavities particularly, the condensate formed above threshold is linearly polarised and exhibits o -diagonal long-range order with a spatial coherence that is dependent on the pump shape. Moreover, the ambipolar electrical characteristics of this organic semiconductor and the high electron mobility of GaN suggest both materials as promising candidates for direct electrical injection.
Supervisor: Murray, Ray Sponsor: European Commission ; Leverhulme Trust
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral