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Title: Polarisation controlled quasi-phase matching of high harmonic generation
Author: Liu, Lewis
ISNI:       0000 0004 5920 9562
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2014
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This thesis focuses on the development of high harmonic generation (HHG) by using polarisation controlled quasi-phase matching QPM as well as related topics. A new class of QPM techniques called polarisation-controlled QPM is introduced where linear or circlar birefringence enables the modulation of the driving field's polarisation state called polarisation-beating QPM (PBQPM) for linear birefringence and optical rotation QPM (ORQPM) for circular birefringence respectively. PBQPM uses a linear birefringence to modulate periodically the driving pulse between linear and circular/elliptical polarisation states. Because elliptical or circular polarisation of the driving pulse suppresses harmonic generation, by appropriately matching the beat length of the driving field's polarisation state to the coherence length of the harmonic generation, QPM can be achieved. In the second technique, ORQPM, propagation of the driving radiation in a system exhibiting circular birefringence causes its plane of polarisation to rotate; by appropriately matching the period of rotation to the coherence length, it is possible to avoid destructive interference of the generated radiation. Not only does ORQPM have similar enhancements as true-phase matching, it is also the first proposed QPM source for circularly polarised high harmonics. The importance of phase modulation in QPM, especially relating to modebeating in hollow-core waveguides where harmonics is being generated are also explored theoretically. Based on this, a novel technique for analyzing random phase matching using a continuous phase-diffusion treatment has been developed; theoretical analytical models are shown to produce excellent agreement with simulations. It is further shown that random phase matching may be responsible for additional broadening of the high harmonic spectrum, especially at higher harmonic orders. Because mode and polarisation control is central to polarisation-controlled QPM, four waveguide mode decomposition techniques from single shot CCD data have been developed. The extraction of phase and coupling coefficients are demonstrated experimentally. A novel analytical general solution for the phase introduced by a phase-only spatial light modulator to generate a given far-field phase and amplitude was developed. The solution was demonstrated experimentally and shown to enable excellent control of the far-field amplitude and phase. Finally, circular and linear birefringent waveguides were explored. Analytic solutions to rectangular birefringent hollow-core waveguides were developed and some initial demonstration experiments were performed.
Supervisor: Hooker, Simon Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Atomic and laser physics ; high harmonic generation ; waveguides