| Abstract |
Over the last decades, there has been an intensive research in the field of strong light-matter coupling in III-V nitride semiconductor microcavities. The reason is that they allow performing fundamental studies of blue light-matter interaction when electronic states and electromagnetic field are confined in one dimension. As their name reveals, microcavities are wavelength-sized optical cavities that can trap light of a specific wavelength depending on the cavity length. If a layer, such as a quantum well (QW), emitting/absorbing at the same wavelength of the cavity mode that is embedded in, and strong coupling conditions are satisfied, then energy “oscillates” between the cavity photon and the known exciton. The distinction between light and matter is lost if this interaction rate is engineered to be faster than other processes and new quasiparticles are generated. These new quasiparticles, the so-called “polaritons”, which are half-light and half-matter, describe the interaction between excitons and photons with the same energy and momentum.
Similar to microcavities, microdisk optical resonators were found also to exhibit low losses and high Q-factors even at room temperature due to small modal volumes without the use of any DBR mirror fabrication. In circular cavities, such as microdisks “whispering gallery modes” can be excited, originally observed for the first time by Lord Rayleigh in 1878. These excitation modes offer the possibility for exploring the strong coupling regime and hence, they are possible candidates for low threshold lasing. The main drawback of these structures is the difficulty in fabrication, since deviations from circularity and sidewall roughness can lead to optical losses.
The aim of this project was the design and fabrication of 3λ_c/2 thick high-quality AlGaN membranes by etching a sacrificial InGaN layer, with the use of Photo-electrochemical (PEC) wet etching method, and their use as active material of micro-sized cavities. AlGaN layer had embedded two sets of 4 GaN quantum wells (QWs) at the antinodes of the electric field for investigating the strong coupling regime when the fabricated membranes are sandwiched by two distributed Bragg reflectors (DBRs). The concentration of Al in the AlGaN layer was kept at low levels to reduce any lattice mismatch and strain effects. Moreover, an initial attempt was done towards making various sized microdisks in one of the samples for exploring the well-known whispering gallery modes (WGMs) by the same fabrication techniques. The growth of our samples was performed by Molecular Beam Epitaxy (MBE) on c-GaN/c-Sapphire substrates by the CEA group in France.
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Nitride polariton structures with improved characteristics
