Polyyne molecules were produced as a result of the femtosecond laser irradiation of liquid acetone (CH3)2CO and alkane molecules hexane C6H14 and octane C8H18 using 800 nm, 100 fs duration pulses. These polyynes have been detected as a Raman band in irradiated liquid from 1800 to 2200 cm−1. Polyyne molecules generally detected as a Raman band in SERS experiment are C8H2, C10H2, C12H2 and C14H2. Two well established experimental techniques, time of flight mass spectrometry and surface enhanced Raman spectrometry were used to identify positively polyyne formation as a result of femtosecond laser irradiation of acetone and alkane liquids. Small polyynes C2H2, C4H2, and C6H2 were positively detected in the time of flight mass spectrometer TFMS, while longer polyynes from C6H2, C8H2, C10H2, C12H2 and C14H2 were detected by surface enhanced Raman spectroscopy SERS. Intensity capping occurs in a liquid due to filamentation, and the resulting intensity in a liquid is s 1013 W/cm2 during irradiation. This results in main process of ionization in the larger part of the laser focus as multiphoton ionization MPI. Focal volume increase in a liquid provides a larger volume where ions C+, C+2 and C2+are produced to initiate chemical reactions outside the laser focus. The current work established positively, that the longer polyyne formation does not occur by dehydrogenation of alkane molecules by only breaking the C-H bonds as was previously anticipated. It is proposed in this work that lengthening of polyyne chains occurs due to addition reaction of species of C+, C+2 and C2+ to double bonded species themselves produced as a result of the breaking down of the parent molecules in the laser focus. The carbon addition reactions occur outside the laser focus due to the close proximity of molecules in the liquid phase.

Polyyne molecules were produced as a result of the femtosecond laser irradiation of liquid acetone (CH3)2CO and alkane molecules hexane C6H14 and octane C8H18 using 800 nm, 100 fs duration pulses. These polyynes have been detected as a Raman band in irradiated liquid from 1800 to 2200 cm−1. Polyyne molecules generally detected as a Raman band in SERS experiment are C8H2, C10H2, C12H2 and C14H2. Two well established experimental techniques, time of flight mass spectrometry and surface enhanced Raman spectrometry were used to identify positively polyyne formation as a result of femtosecond laser irradiation of acetone and alkane liquids. Small polyynes C2H2, C4H2, and C6H2 were positively detected in the time of flight mass spectrometer TFMS, while longer polyynes from C6H2, C8H2, C10H2, C12H2 and C14H2 were detected by surface enhanced Raman spectroscopy SERS. Intensity capping occurs in a liquid due to filamentation, and the resulting intensity in a liquid is s 1013 W/cm2 during irradiation. This results in main process of ionization in the larger part of the laser focus as multiphoton ionization MPI. Focal volume increase in a liquid provides a larger volume where ions C+, C+2 and C2+are produced to initiate chemical reactions outside the laser focus. The current work established positively, that the longer polyyne formation does not occur by dehydrogenation of alkane molecules by only breaking the C-H bonds as was previously anticipated. It is proposed in this work that lengthening of polyyne chains occurs due to addition reaction of species of C+, C+2 and C2+ to double bonded species themselves produced as a result of the breaking down of the parent molecules in the laser focus. The carbon addition reactions occur outside the laser focus due to the close proximity of molecules in the liquid phase.

This thesis investigates laser micromachining using a high pulse repetition frequency (high-PRF) femtosecond fibre laser. Three different types of industrial-grade metals, Stainless steel, Copper, and Aluminium are investigated. The impact of the processing parameters on material removal is studied. Finally the feasibility of the technology in three dimensional micro structuring is explored. The thesis contributes to clarify the main interaction mechanisms occurring in high-PRF femtosecond laser processing. Heat accumulation and particle shielding are identified as main material removal influencing mechanisms. As a result of heat accumulation, lowered ablation thresholds are detected for Aluminium (0.16 J/cm² at 1.02 MHz versus 0.33 J/cm² at 20 kHz) and Stainless steel (0.088 J/cm² at 1.02 MHz versus 0.11 J/cm² at 20 kHz). For the high heat conductive Copper heat accumulation is largely ruled out. Particle shielding is investigated by ultra high speed camera imaging. It is shown that the ablation plumes enlarge at the higher pulse repetition rates. A parameter study investigates material ablation. From this study, appropriate machining parameters are derived with regard to both high ablation rate and removal efficiency, and small roughness: Aluminium: 5 μm pulse spacing / 5 μJ pulse energy, Copper: 7.5 μm pulse spacing / 7 μJ pulse energy, Stainless steel: 5 μm pulse spacing / 3 μJ pulse energy. In addition experimentally and theoretically determined volume ablation rates are compared. For this, a material removal calculation model is designed. Good agreements between theoretical and experimental values are obtained by taking into account effective penetration instead of optical penetration for energy transport. A surface temperature calculation model is designed, providing useful insights into heat accumulation. Heat accumulation observed for Aluminium and Stainless Steel is confirmed by surface temperature rise, calculated based on the remaining energy. Improvement of the model by enhanced energy coupling yields surface temperatures above the melting temperature. This is conclusive to experimental observations. Finally the feasibility of the high-PRF femtosecond laser technology in micromachining is demonstrated by micro mould fabrication. Utilising these moulds, micro-fluidic plastic demonstrators are fabricated by micro-injection moulding.

In this thesis, the development of a femtosecond Cr⁴⁺:forsterite solid-state laser is described where the mode-locking procedure was initiated using two novel saturable absorbers. One was a GaInNAs quantum-well device and the other a quantum-dot-based saturable absorber. These devices had not previously been exploited for the generation of femtosecond pulses from a solid-state laser but in the course of this project, successful mode-locked laser operation in the femtosecond domain was demonstrated for both devices. When the GaInNAs device was incorporated in the Cr⁴⁺:forsterite laser, transform-limited pulses with durations as short as 62fs were obtained. The performance of this femtosecond laser was significantly superior to that for previous quantum-well based saturable absorbers in the 1300nm spectral region. The dynamics of the device were investigated with the aim of refining subsequent devices and to explore the potential to grow future devices for use at longer wavelengths. At the outset of my research work quantum-dot based saturable absorbers had not be used for the mode locking of solid-state lasers in the femtosecond regime. The work presented in this thesis showed that quantum-dot structures could be exploited very effectively for this purpose. This was initially achieved with the quantum-dot element being inclined at an off-normal incidence within the cavity but experimental assessment together with further development of the device allowed for implementation at normal incidence. Reliable operation of the femtosecond laser was demonstrated very convincingly where transform-limited pulses of 160fs duration were generated. Having developed practical femtosecond Cr⁴⁺:forsterite lasers, the final part of the project research was directed towards exemplar applications for a laser operating in the 1300nm spectral region. These were biophotonics experiments in which assessments of both deep tissue penetration and two-photon chromosome cutting were undertaken. This work confirmed the suitability of the 1300nm laser radiation for propagation through substantial thicknesses of biological tissue (~15cm). The demonstration of highly localised two-photon cutting of Muntjac deer chromosomes also represented a novel result because single-photon absorption could be avoided effectively and the temporal broadening of the femtosecond pulses in the delivery optics arising from group velocity dispersion around 1300nm was minimal.