| Abstract |
All modern photonic structures in useful applications for humanity, like illumination, computer and television screens, optical networks, photovoltaics, quantum information systems etc, are based on the control of the spontaneous emission of light. Microcavites are such photonic structures in semiconducting systems, where the bound state between an electron and hole, known as exciton is met with the spacially confined photonic modes of an optical cavity of micrometer or less thickness. The quantum effect of Strong Coupling in microcavities is addressed by considering a quasi-particle called polariton, the study of which in the GaAs system, has lead to the fabrication of novel photonic structures, like the so-called polariton LED and the polariton laser of ultra-low threshold, exploiting polaritons bosonic nature. This behaviour gives polariton unique properties like the stimulated scattering, the parametric amplification, the lasing, the condensation and superfluidity.
The semiconducting material of Gallium Nitride (GaN) which shows a robust exciton at room temperature due to its large exciton binding energy, is an ideal candidate for fabrication of optoelectronic applications like microcavities, capable to perform at ambient temperature. However, fundamental problems such as a) the broadening of exciton linewidth due to structural defects and cracks and b) the small nitrides-based DBR stopband, comparable to the Rabi splitting between the upper and the lower polariton are met in monolithic growth of nitrides microcavities, prohibiting the strong coupling between exciton and photon. In this thesis these problems are solved simultaneously by incorporating a thin GaN film in an optical microcavity made of dielectric Bragg mirrors.
Primary tool of the fabrication process is the so-called photoelectrochemical (PEC) wet etching of a sacrificial underlying layer for the fabrication of ultra-thin, free-standing GaN membranes. This process provides the required ultra-smooth surfaces of the yielded GaN films, as such as it preserves their high optical quality. The deposition of dielectric mirrors on both sides of the film, using the sputtering technique, accomplishes the all-dielectric GaN microcavity. The optical characterization of these microcavities shows for the first time an indication of strong coupling of GaN exciton and the 1st Bragg mode of one of the Bragg mirrors, introducing a novel quasi-particle called Bragg-polariotn or Braggoriton.
This encouraging result is the proof that the all-dielectric GaN microcavity fabrication is feasible using the photoelectrochemical etching method, capable to show strong exciton-photon coupling and large Q-factors. It comprises the basis for the development of novel polariton emitters, operating at room temperature, as such as for the study of new physics governed by the bosonic nature of polations.
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Strong light-matter coupling in GaN microcavities
