Optical micromanipulation using dispersion-compensated and phase-shaped ultrashort pulsed lasers
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Ultrashort pulsed lasers offer high peak powers at low average powers, making them ideal for maximising the efficiency of nonlinear excitation. Their broad bandwidths make it possible to tailor the pulse's temporal profile for advanced control of multiphoton excitation, techniques known as pulse shaping. This thesis represents the first combination of ultrashort pulse shaping with optical trapping and axicon dispersion compensation. We construct an optical trapping system which incorporates a 12fs duration pulsed laser, the shortest duration used to date in optical trapping. To achieve 12fs pulse durations at the sample, we must first eliminate temporal dispersion, which stretches and distorts pulses as they travel through microscope systems. We use the Multiphoton Intrapulse Interference Phase Scan (MIIPS) method to measure and compensate all orders of dispersion in our optical trapping system, verifying 12-13fs pulses at the sample. We use the dispersion-compensated optical trapping system to investigate the effects of pulse duration on optical trapping. Our theoretical arguments show that trap stiffness is independent of pulse duration. For experimental verification, we measure the trap stiffness of trapped 780nm silica spheres with back focal plane interferometry as we change pulse duration by more than an order of magnitude using quadratic pulse shaping. We find the trap stiffness unchanged within 9%. We also use quadratic pulse shaping to control two-photon fluorescence in optically trapped fluorescent polymer spheres. Next, we demonstrate two methods for producing selective two-photon excitation in trapped particles: amplitude shaping and 3rd order pulse shaping. Finally, we compensate dispersion in an axicon system, producing a non-diffracting ultrashort Bessel beam with controllable dispersion. This forms the basis for ongoing experiments exploring ultrashort Bessel beams in cellular transfection (photoporation), and examining the spatial profile of the Bessel beam as a function of the pulse's temporal profile.
Thesis, MPhil Master of Philosophy
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