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Title: Nonlinear optical processes in whispering gallery micro-disc resonators
Author: Evagorou, Michalis
ISNI:       0000 0004 6056 9418
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2016
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The field of nonlinear optics is now more than fifty years old, since the beginnings with the observation of second harmonic generation by Franken and co-workers in 1961. Since then a lot of progress has been made in this field which has many applications in frequency conversion devices and optical networks. My original contribution to knowledge in this field is the proposition of a natural quasi phase matching (NQPM) scheme, for efficient optical frequency conversion using second order nonlinearity in thin crystalline film waveguides and the development of the theory behind this scheme as an extension of the well-known quasi phase matching theory. The scheme is based on the periodic polarization inversion of the light field as it circulates in a whispering gallery (WG) micro-disc resonator. By placing the optical axis of the crystalline film material in the disc plane, this arrangement naturally produces the necessary 1t phase shift between the nonlinear polarization and the second order wave to compensate for the phase shift between the pump and harmonic waves, which is realised in conventional quasi phase matching schemes by periodically inverting the crystal domains in ferroelectric crystals such as Lithium Niobate (LN). I derive the coupled wave equations for second harmonic generation (SHG) in circular resonator structures and use these to describe the evolution of the second harmonic (SH) wave in the resonator and the efficiency of the frequency conversion. Lithium Niobate is used as an example material, for which a coefficient for the second harmonic generation in an x-cut resonator is calculated for the in-plane (TE) polarized guided mode. Optimization parameters are presented for enhancing the efficiency of the frequency conversion. A general model is derived with the use of temporal coupled mode theory which describes the second harmonic generation in any doubly resonant (at the fundamental and SH frequencies) resonator coupled with a pair of waveguides. Finally the procedure for designing such devices is presented where the conditions for NQPM are satisfied by choosing specific whispering gallery modes (WGM) for the fields and subsequently applying temperature tuning to ensure double resonance at the fundamental and SH frequencies is achieved. LN micro-disc resonators in which NQPM is feasible are designed.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available