On the phenomenon and potential applications of pulsed laser-reshaped silver nanoparticles embedded in soda-lime glass
This thesis presents studies on a novel ‘meta-material’ as a potential candidate to replace the traditional Electro-Optic crystals (GaP, ZnTe) used in ultrashort bunch monitors for electron/positron accelerators. This study is aimed at showing the linear and non-linear optical properties of such materials, and creating a toolbox for both optical characterisation and manipulation of their properties using an ultra-short pulsed laser. The material studied throughout this thesis is a composite of silver nanoparticles (a “nanocomposite”) embedded within soda-lime glass. The Surface Plasmon Resonance (SPR) is a feature of these particles that is responsible for its unique optical properties. It is shown in this work how SPR is utilised for shape modification of silver nanoparticles with the use of a ps- pulsed laser with various laser beam polarisations. The impact of linear polarisation irradiation is investigated. It is found that multipulse irradiation has the effect of elongating nanoparticles to form prolate spheroids, which results in a dichroic effect on the composite as a whole, caused by the anisotropic SPR band shift. It is also shown that changing the laser polarisation from linear to radial and/or azimuthal changes the character of the reshaped nanoparticles. It was observed that a localised change of ellipsoid orientation is achieved, resulting in a non-directionally-dependent SPR band shift. Second Harmonic Generation (SHG) has been observed from reshaped nanoparticles embedded in soda-lime glass. A comparison of the effect was made between ps-pulsed reshaped, fs-pulsed reshaped and mechanically stretched samples containing silver nanoparticles. Multiphoton Absorption Induced Luminescence (MAIL) was observed along with the SHG and characterised for the various laser polarisation components. The dependence of the aforementioned effects on the elongated nanoparticle aspect ratio was shown to have a great impact. A novel method for reshaped nanoparticles characterisation is presented. It is based on the laser-induced SHG and MAIL signal and is proved to give a precise measurement of the nanoparticle shape and orientation. Frequency Resolved Optical Gating (FROG) measurement of a fs-pulse is measured with great accuracy, in the case where the BBO nonlinear crystal is replaced by the reshaped nanoparticle composite. This was demonstrated to be caused by the anisotropic SHG of the ellipsoidal nanoparticles. Preliminary THz based measurements were performed as a part of a feasibility study of the application of these composites in the EO-based detection of ultrashort electron bunches. Future work is suggested in order to achieve more efficient EO detection.
Physics, Nanoparticles, CERN CLIC, Nonlinear optics, Metamaterials, Laser materials processing, Electro optics, Laser, Optics, Nanotechnology
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