Optimisation of writing conditions for fabricating refractive index change in bulk Poly(methyl methacrylate) using femtosecond laser irradiation

Abstract

This thesis, Optimisation of writing conditions for fabricating refractive index change in bulk Poly(methyl methacrylate) using femtosecond laser irradiation, is submitted by Bo Pang in March 2014 to The University of Manchester, for the degree of Doctor of Philosophy in the faculty of Engineering and Physical Science.This thesis concentrates on the fabrication of refractive index change (Deltan) structures inside clinical grade bulk poly(methyl methacrylate) (PMMA) by femtosecond (fs) laser (1 kHz, 800/400 nm, 120 fs) irradiation. The diffraction efficiency measurement was used to determine the value of Deltan.It was observed that there was a light emission with the same frequency as the laser source, during the writing process of the surface ablation for PMMA. This observation proved the existence of the plasma, indicating that avalanche ionisation occurred during the writing process. In addition, it was observed that Deltan structures inside bulk PMMA appeared tens of seconds after the irradiation, suggesting that Deltan formation in PMMA was not caused by melting and rapid cooling, as in glass.The effect of writing speed, as an individual parameter, was investigated for the first time. For a single laser scan, when the writing speed was slow, the spatial overlapping of focal volume by each pulse occurred. It was found that in the overlapping regime, the ablation threshold increased with increasing writing speed and then decreased. The turning point occurred at the writing speed vopt. With laser fluence just below the ablation threshold, the value of Deltan increased then decreased with increasing writing speed. The maximum value of Deltan occurred at vopt; the speed at which the unit focal volume was exposed to precisely three consecutive laser pulses. vopt was defined as the optimised writing speed for creating the largest Deltan value for the 1 kHz fs laser.A spatial light modulator (SLM) was used to generate multiple focusing points with the same power for parallel processing, which reduced fabrication time of optical structures, like transmission phase gratings. For example, 4 identical foci were successfully generated using the SLM, resulting in fabricated structures exhibiting the same diameters (2.0 ± 0.1 micro metre).The depth dependent elongation of the cross-section shape of the structures was corrected using the SLM. The ratio of thickness to width of the cross-section remained the same (about 2.80 ± 0.6 micro metre), independent of the focusing depth.The effect of annealing after the laser irradiation was investigated. Annealing (70 °C, 130 hours) did not change the feature size of the structures. However, it increased the Deltan value by 58.3% and increased the normalised peak intensity of the C=C bond, (IC=C) in the Raman spectrum by between 9.6% up to 24.3%, depending on the time after fs laser irradiation. The increase in IC=C in the Raman spectrum indicated that annealing increased the speed of photon-initiated main chain breaking. So that there was a possibility that it would take shorter time for Deltan to reach its stable state.

Date of Award	31 Dec 2014
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Patricia Scully (Supervisor)