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Title: Femtosecond laser irradiation of Poly (methyl methacrylate) for refractive index modification and photochemical analysis
Author: Taranu, Anca
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2013
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This thesis explores a new technique for investigating the photochemical mechanisms of femtosecond laser inscription of permanent photonic structures in Poly(methylmethacrylate) (PMMA). The refractive index (RI) structures were fabricated with a direct writing method without ablation, and analysed using a non-invasive method - namely: Raman mapping spectrometry. The writing conditions for the photonic structures under investigation are mainly represented by 800nm and 400nm wavelength with 44fs and 100fs pulse length and a low repetition rate in the kHz domain. The mass percentage of the induced monomer and end groups modification (MMA) as a measure of the modification of the ratio of C=C and C=O Raman transition varies linearly with the total fluence (total). The mass percentage of the induced monomer and end groups change is defined by the modification of normalised ratio of the Raman intensity of C=C bond (I(C=C)) and the Raman intensity of C=O bond (I(C=O)) which is denoted by I(C=C/C=O)n. The modification of this ratio is denoted by I (C=C/C=O)n and also by MMA. MMA varies linearly with total with a positive slope for both writing conditions due to the induced main chain scission and unzipping. If total increases by 1J/cm2, it is predicted an increase in MMA, by (1.550±0.11)x10-2 (cntsxcm2)/J, for the near infrared (NIR) irradiated samples that is higher than the increase of MMA for the ultraviolet (UV) irradiated sample that show a value of (1.9200.274)x10-3 ( (cntsxcm2)/J). The same trend was found for the variation of MMA with diffraction efficiency () for NIR irradiated structures and also for UV irradiated structures. If  increases by 1cnt, it is predicted that there will be an increase in MMA, by (4.233±0.383) cnts for NIR irradiated samples that is lower than the increase of MMA for the UV irradiated sample that shows a value of (14.3922.477) cnts. The variation of MMA with  is higher for UV irradiated samples than for NIR irradiated samples, and this indicates that the nonlinear absorption of two photons produces a larger percentage of the monomer and end groups than the nonlinear absorption of three photons. Gel Permeation Chromatography (GPC), which is a destructive analytical method, was applied only for the investigation of the time dependent behaviour of the molecular weight of the photonics structures which were written with the parallel writing technique using 775nm wavelength and 160fs pulse length that shows an increase of 66 in  after seven days from the laser irradiation. Twenty-four hours after laser irradiation, the GPC results show that the weighted average molecular weight (Mw) of the exposed sample of 28,610,000 Daltons is about thirty times higher than the MW of the unexposed sample of 963,425 Daltons. This is an indication of the photo-cross-linking reaction. As a result of this reaction, the polymer chains link together through intermolecular forces to form a 3D network which produces an increase of molecular weight. It was also observed that there was a further decrease of molecular weight after three days to 437,441 Daltons due to main chain scission and unzipping. The main chain scission is actually the breaking of C-C bonds between structural units and the formation of radicals which further produce the monomer and end groups (MMA) through the unzipping reaction which leads to a decrease of the molecular weight. The main chain scission occurred with the greatest efficiency after three days following the end of irradiation, when the number of the main chain scissions (Ns) reached the maximum value of 1.193. An increase of molecular weight signifies an increase of the refractive index since the optical density has increased. The mechanical properties of PMMA optical fibres (e.g., Young's modulus) and of bulk PMMA (e.g., glass transition temperature) were investigated using Dynamical Mechanical Analysis (DMA) tests (e.g., stress-strain test and temperature ramp/frequency sweep test). These measurements were performed to study the effect of the manufacturing process that involves stretching and heating or cooling on the mechanical properties of PMMA optical fibres and unmodified PMMA material. T he ultimate aim of this section was to see the effect of the laser irradiation on the strain properties of an optical fibre sensor with gratings. The stress strain results show an increase of Young's modulus of the PMMA optical fibre of 5%, and this is an indication of decreased elasticity which is induced during the fabrication process. For a femtosecond laser irradiated region with UV wavelength, it is expected that there will be an increase of Young's modulus to 65%. This variation was obtained inthe research group from The Photon Science Institute by measuring Young's modulus for a diffraction grating which was written in PMMA with 180fs pulse length and 387nm wavelength and which was subjected to a strain. The elasticity was measured using the displacement of the first order diffracted beams as a result of a modification due to the applied strain [ ]. The temperature ramp/frequency sweep test shows an increase of glass transition temperature of the bulk PMMA of 54.12% which is also an indication of decreased elasticity induced during the fabrication process. A further increase in this temperature is expected for UV irradiated samples.
Supervisor: Scully, Patricia Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: femtosecond irradiated PMMA ; subusurface refractive index structures ; confocal Raman spectroscopy ; refractive index structures