Designing an Experiment to Investigate Slow Light Effects in Whispering Gallery Mode Resonators
Abstract: Slow light and Whispering Gallery Mode Resonators are two interesting and useful fields with applications in optical sensing and laser stabilisation among others. Slow light can lead to long interaction times which are useful in light-matter interactions such as sensitive non-linear optics. Whispering Gallery Mode Resonators have small mode volumes and strong beam confinement leading to high Quality Factors and intense trapped light. Whispering Gallery Mode Resonators exhibit very narrow resonance linewidths which are beneficial in frequency sensitive measurements. This thesis uses a Praseodymium REI doped Yttrium orthosilicate crystal, formed into a disc shaped Whispering Gallery Mode Resonator. The aim is to design a setup which can be used to combine the two fields, slow light and Whispering Gallery Mode Resonators, in an attempt to achieve ultra-high Quality Factors and ultra-narrow resonance linewidths. The thesis details the experimental mode coupling procedure learnt from a group at Max Planck Institute for the Science of Light in Erlangen, Germany. The thesis provides a foundation for Whispering Gallery Mode Resonator experimentation in the Quantum Information group in Lund, where the thesis work was undertaken. The resulting knowledge gained is applied to design a setup holder for use in a cryostat, which provides the necessary temperatures to create slow light structures. Two beam alignment setups are constructed and used to couple into the Whispering Gallery Mode Resonator at room temperature. The first of the two setups (free beam) was difficult to operate and control over the beam alignment and focal spot shape was poor. The resulting mode coupling had a low efficiency. The second setup (fibre coupled) provided a more user friendly setup with good control over the beam alignment and focal spot shape. Results using this setup were taken for a variety of wavelengths between 593.1 nm and 610.4 nm to investigate the effect of the ions wavelength dependent absorbance, on the losses. Due to the low efficiency of the coupling in both regimes it was not possible to isolate the cause. A mismatch of focal spot size and shape is suggested as a potential area that introduces loss and therefore requires more extensive investigation. With improved beam alignment, in addition to the focal spot geometry, it should be possible to perform the absorbance measurements again in order to determine the contribution. Suggestions for further measurements including the surface roughness and mode simulations are made in an attempt to optimise the mode coupling regime to be able to accurately estimate the Quality Factor of the Whispering Gallery Mode Resonator. This is critical in order to observe changes in the Quality Factor value at cryogenic temperatures, which can be attributed to slow light effects alone.
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