Master thesis defense by Adam Søsted Knorr

Title: Spectroscopy of Advanced Integrated Nanophotonics and Quantum Dots

Abstract: A quantum mechanical based internet requires efficient generation, collection and manipulation of photonic quantum bits (qubit). Semiconducting quantum dots have shown great promise in fulfilling the required role of a single-photon emitter in generating qubits. Polarization entangled photonic qubits can also be generated through cascaded emission from a "twice-excited" quantum dot namely the biexciton. Efficient collection of linearly polarized quantum dot emission can be done at near unity efficiency when coupled to the highly symmetric photonic crystal waveguide.

A low strain GaAs semiconducting quantum dot features small fine structure splitting compared to the more wide spread InAs type of quantum dot. Low fine structure splitting is desirable for stable entangled photon generation, which can be produced with the biexciton cascade. The GaAs quantum dot can also be operated as a single photon source and so its single photon properties are measured and compared to the state-of-the-art InAs single photon source. The biexciton is also excited in the quantum dot and its decay rate is measured.

With low fine structure splitting the otherwise linearly polarized photon states from the biexciton cascade degenerates, producing circularly polarized photons instead. A new type of photonic crystal the "glide-plane waveguide" has parity broken symmetry, leading to circular polarization solutions to the Bloch equations, which from time reversal symmetry induces chiral coupling of circular polarized light from emitters in the glide-plane waveguide. This also opens the possibility of transforming polarization entangled sources e.g. cascaded emission from a GaAs biexciton, to path entangled sources. The coupling capabilities of a quantum dot to the glide-plane waveguide is measured with resonant transmission spectroscopy. The quantum dot resonance is controlled with induced Stark shift and resonant fluorescence is performed across multiple voltages, displaying great control of quantum dots in the glide-plane environment.

Topological photonics promise reduced backscattering for edge-mode transmission between two topologically distinct insulators. This has the possibility of greatly increasing on chip propagation efficiency. Experimentally the topological edge mode is found in topologically interfaced devices, and the transmission through the devices is characterized. A method of characterizing the group index of the nanobeam waveguides and topological devices is proposed and performed. Results are compared with simulations seeming to agree besides for some systematic errors.

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