This thesis reports on the development of glass-based femtosecond laser sources around 2 µm wavelength. In order to be able to produce 2 µm radiation the dopants used were trivalent Thulium (Tm³⁺) and trivalent Holmium (Ho³⁺) that could be optically pumped with Ti:Sapphire radiation at 0.8 µm and semiconductor disk lasers (SDL) at 1.2 µm. The samples were produced at Leeds University and polished in-house in bulk form and deployed in free space laser cavities. Tellurite compounds doped with Tm³⁺ produced stable continuous wave 1.94 µm radiation when pumped at 800 nm with a maximum efficiency of 28.4% with respect to the absorbed power and maximum output power around 120 mW when pumped using a Ti:Sapphire operating around 0.8 µm. The radiation was broadly tunable across 130 nm. Tm³⁺-Ho³⁺ doubly doped tellurite samples lased around 2.02 µm with maximum efficiency of 25.9% and with P[subscript(OUT)]=75 mW and a smooth tunability of 125 nm. The fluorogermanate glass doped with Tm³⁺ gave an absorbed to output power efficiency of 50%. The maximum continuous wave output powers obtained were around 190 mW and limited by the available pump power at 0.8 µm. These results together with a very low threshold of 60 mW of incident power were comparable to the crystalline counterparts to this gain medium. The Tm3+ tellurite and the Tm³⁺-Ho³⁺ tellurite compounds were also pumped by an SDL operating at 1215 nm to obtain an indication of the viability of such a pump scheme. The results were a maximum internal slope efficiency of 22.4% with a highest output power of 60 mW. The comparison demonstrated that 1.2 µm pumping was competitive with using 0.8 µm wavelength. The use of semiconductor saturable absorbing mirror (SESAM) technology was used for the modelocking of these lasers. The SESAM was produced in Canada and implanted with As⁺ ions in order to reduce the relaxation time. Trains of transform-limited laser pulses at 222 MHz as short as 410 fs centred at 1.99 µm were produced for the first time with a bulk Tm³⁺:Fluorogermanate glass. The maximum average output power obtained was of 84 mW. The same SESAM deployed on the Tm³⁺-Ho³⁺ Tellurite compounds gave trains of transform-limited pulses as short as 630 fs at 2.01 µm with a repetition rate of 143 MHz and a maximum averaged output power of 43 mW. The regime of propagation obtained was soliton-like and the modelocking was self-starting. The results obtained with bulk glass were very promising and open interesting research pathways within the realm of amorphous bulk gain media.

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.

In this thesis two previously separate fields of study are brought together: optical micromanipulation and ultrashort laser research. Here, the benefits of combining the high peak powers of ultrashort pulsed lasers and conventional optical micromanipulation techniques are explored. As optical trapping has been studied extensively, the focus of this research is on optical guiding. Moreover, the emphasis is on the use of Bessel beams as these have been shown to offer greater guiding distances than comparable Gaussian beams. The studies within this thesis show that optical guiding in Bessel and Gaussian beams is governed by the average power of the laser. However, the benefits of guiding with ultrashort pulsed lasers to exploit multi-photon processes become evident as the demonstration of simultaneous optical guiding and second harmonic generation in microscopic nonlinear crystal fragments is detailed. This work is developed by using ultrashort pulses to induce two-photon excitation-induced fluorescence in the guiding medium. This allows direct visualisation of the beam-particle interaction and measurement of the reconstruction of the Bessel beam around an object. Some studies using two-photon excitation to investigate Bessel beam penetration through turbid media are discussed. Finally, the work is concluded by exploring the use of pulsed white-light lasers in optical guiding. The wavelength-dependent propagation and reconstruction properties of the white-light Bessel beam are studied before some preliminary optical guiding experiments are discussed. From this, the broad bandwidth of the supercontinuum source is found to offer extended guiding distances in Gaussian beams thereby negating the need for Bessel beams.

Arterial diseases such as hypertension, carotid stenosis, and abdominal aortic aneurysm (AAA) may progress silently without symptoms and contribute to acute cardiovascular events such as heart attack, stroke, and aneurysm rupture, which are consistently among the leading causes of death worldwide. The arterial pulse wave, regarded as one of the fundamental vital signs of clinical medicine, originates from the heart and propagates throughout the arterial tree as a pressure, flow velocity, and wall displacement wave, giving rise to the natural pulsation of the arteries. The dynamic properties of the pulse wave are intimately related to the physical state of the cardiovascular system. Thus, the assessment of the arterial wall dynamics driven by the pulse wave may provide valuable insights into vascular mechanical properties for the early detection and characterization of arterial pathologies. The focus of this dissertation was to develop and clinically implement Pulse Wave Imaging (PWI), an ultrasound elasticity imaging-based method for the visualization and spatio-temporal mapping of the pulse wave propagation at any accessible arterial location. Motion estimation algorithms based on cross-correlation of the ultrasound radio-frequency (RF) signals were used to track the arterial walls and capture the pulse wave-induced displacements over the cardiac cycle. PWI facilitates the image-guided measurement of clinically relevant pulse wave features such as propagation speed (pulse wave velocity, or PWV), uniformity, and morphology as well as derivation of the pulse pressure waveform. A parametric study investigating the performance of PWI in two canine aortas ex vivo and 10 normal, healthy human arteries in vivo established the optimal image acquisition and signal processing parameters for reliable measurement of the PWV and wave propagation uniformity. Using this framework, three separate clinical feasibility studies were conducted in patients diagnosed with hypertension, AAA, and carotid stenosis. In a pilot study comparing hypertensive and aneurysmal abdominal aortas with normal controls, the AAA group exhibited significantly higher PWV and lower wave propagation uniformity. A “teetering” motion upon pulse wave arrival was detected in the smaller aneurysms (< 5 cm in diameter) but not in the larger aneurysms (> 5.5 cm in diameter). While no significant difference in PWV or propagation uniformity was observed between normal and hypertensive aortas, qualitative differences in the pulse wave morphology along the imaged aortic segment may be an indicator of increased wave reflection caused by elevated blood pressure and/or arterial stiffness. Pulse Wave Ultrasound Manometry (PWUM) was introduced as an extension of the PWI method for the derivation of the pulse pressure (PP) waveform in large central arteries. A feasibility study in 5 normotensive, 9 pre-hypertensive, and 5 hypertensive subjects indicated that a significantly higher PP in the hypertensive group was detected in the abdominal aorta by PWUM but not in the peripheral arteries by alternative devices (i.e. a radial applanation tonometer and the brachial sphygmomanometer cuff). A relatively strong positive correlation between aortic PP and both radial and brachial PP was observed in the hypertensive group but not in the normal and pre-hypertensive groups, confirming the notion that PP variation throughout the arterial tree may not be uniform in relatively compliant arteries. The application of PWI in 10 stenotic carotid arteries identified phenomenon such as wave convergence, elevated PWV, and decreased cumulative displacement around and/or within regions of atherosclerotic plaque. Intra-plaque mapping of the PWV and cumulative strain demonstrated the potential to quantitatively differentiate stable (i.e. calcified) and vulnerable (i.e. lipid) plaque components. The lack of correlation between quantitative measurements (PWV, modulus, displacement, and strain) and expected plaque stiffness illuminates to need to consider several physiological and imaging-related factors such as turbulent flow, wave reflection, imaging location, and the applicability of established theoretical models in vivo. PWI presents a highly translational method for visualization of the arterial pulse wave and the image-guided measurement of several clinically relevant pulse wave features. The aforementioned findings collectively demonstrated the potential of PWI to detect, diagnose, and characterize vascular disease based on qualitative and quantitative information about arterial wall dynamics under pathological conditions.

Pour des applications technologiques d’enregistrement magnétique de l’information à haute densité et vitesse d’écriture et de lecture ultra-rapide, les chercheurs se sont penchés vers des méthodes de manipulation de l’aimantation sans application de champ magnétique externe. Il a été découvert qu’il était possible de renverser de manière déterministe l’aimantation de plusieurs matériaux ferri- et ferro-magnétiques à l’aide uniquement d’impulsions laser ultracourtes polarisées circulairement. Ce retournement tout-optique s’est avéré être un processus cumulatif nécessitant plusieurs impulsions ultracourtes dans les matériaux ferromagnétiques. Notamment dans les multicouches (Co/Pt), le retournement tout-optique se fait en deux étapes : une désaimantation indépendamment de l’hélicité suivie d’une ré-aimantation dans une direction ou l’autre selon l’hélicité. Pour autant, le mécanisme à l’origine du rétablissement de l’ordre magnétique n’a pas été étudié jusqu’à présent. Dans cette thèse, nous avons étudié le mécanisme de renversement de l’aimantation dans les couches ferromagnétiques résultant de l’excitation par impulsions laser ultracourtes polarisées circulairement. Pour cela, nous étions intéressé par la réponse d’une paroi de domaine dans les couches minces de Pt/Co/Pt à la suite d’une excitation laser et en fonction de la polarisation de la lumière. Nous avons démontré la possibilité d’induire un déplacement tout-optique et déterministe d’une paroi de domaine. Nous montrons que la propagation de la paroi résulte de la compétition entre trois contributions : le gradient de température dû aux effets de chauffage par le laser, l’effet de l’hélicité de la lumière et les effets de piégeages de la paroi. Par ailleurs, par mesures expérimentales du dichroïsme circulaire, nous excluons un mécanisme purement thermique du déplacement de paroi. Ainsi nous confirmons que le retournement tout-optique des couches ferromagnétiques se fait par une nucléation suivie d’une ré-aimantation par propagation déterministe des parois de domaines selon l’hélicité. De plus, nous avons exploré la possibilité d’utiliser le retournement tout-optique dans des dispositifs spintroniques pour l’enregistrement de l’information à haute densité. Pour se faire, il est nécessaire d’étudier les effets de tailles du retournement lorsque le matériau est structuré en îlots à l’échelle du micro- ou nanomètre. Nous avons montré qu’un plus grand nombre d’impulsions laser est nécessaire afin de renverser l’aimantation de micro-disques comparés à la couche continue ferromagnétique. Il en résulte que le champ dipolaire aide le renversement de l’aimantation dans les couches continues rendant ainsi le retournement tout-optique énergétiquement plus favorable / Over the past decade, the demand for an even higher capacity to store data has been gradually increasing. To achieve ultrafast and ultrahigh density magnetic data storage, low-power methods to manipulate the magnetization without applying an external magnetic field has attracted growing attention. The possibility to deterministically reverse the magnetization with only circularly polarized light was evidenced in multiple ferri- and ferro-magnetic materials. This phenomenon was called helicity-dependent all-optical switching (HD-AOS). In ferromagnets, it was demonstrated that HD-AOS was a cumulative and multishot process with a helicity-independent demagnetization followed by a helicity-dependent magnetization recovery. Yet, the microscopic mechanism of this helicity-dependent remagnetization remained highly debated. In this thesis, we investigated the magnetization reversal mechanism of all-optical switching in ferromagnetic materials. To explore a potential switching process through domain nucleation and domain wall (DW) propagation, we studied the response of a DW upon femto- or pico-second laser irradiation in Co/Pt thin films that exhibit HD-AOS. We reported helicity-dependent all-optical domain wall motion. We demonstrated that it results from the balance of three contributions: the temperature gradient due to the laser heating, the helicity effect and the pinning effects. By measuring the magnetic circular dichroism, a purely thermal mechanism of the laser-induced DW motion appears to be excluded. Furthermore, we examined the size effects in AOS in Co/Pt films patterned into microdots with a diameter between 10 and 3 μm. This allowed us to explore the role of the dipolar field in the switching mechanism. We discovered that a larger number of laser pulses was required to reverse the magnetization of a microdot compared to the continuous film. This indicated that the dipolar field actually eases the magnetization reversal in the full film. Thus, AOS is less energy-efficient in patterned films, hence making Pt/Co/Pt multilayers not an ideal candidate for integrating AOS in spintronic devices

By means of N-body simulations we study the ion and electron dynamics in molecular first-row hydride clusters when exposed to intense and short X-ray pulses. We find that, for a particular range of X-ray intensities, fast protons are ejected from the system on a considerably shorter time scale than that of the screened core. As a consequence, the structure of heavy atoms is kept intact", which may be relevant in the context of X-ray based molecular imaging. Moreover the final charge states of the heavy ions are considerably lower than those of the ions in pristine atomic clusters exposed to the same laser pulses, which is in agreement with recent measurement of methane cluster at the LCLS in Stanford.

In this thesis we present the realization of a source of 1.5-cycle carrier-envelope phase stable laser pulses in the mid-infrared spectral region. We used ytterbium laser system generating 1 µm pulses as a pump of setup, where the beam is split into several parts and interact in nonlinear optical media. 2 µJ pulses with duration 18 fs at 50 kHz repe- tition rate are produced. By spectral broadening in crystal GGG, 9,9 fs pulses can be achieved. The mid-IR pulses was characterized by third harmonics generation frequency resolved optical gating in the interferometric configuration. Fourier filtering of the mea- sured interferogram allows for the complete reconstruction of amplitude and phase of the ultrashort pulses generated by our setup. The pulses will in future serve for experimental investigation of ultrafast strong-field phenomena in solids. 1

A series of picosecond excite-probe experiments was performed on various concentrations of aqueous and ethanolic solutions of rhodamine B in order to determine the existence of dimerization in those solutions. The goals of the research presented in this dissertation were twofold. Initially, various techniques of time-resolved spectroscopy were to be employed to further characterize the ground and excited-state molecular properties of the aqueous RB dimer. The information obtained, and the techniques developed in that study would then be utilized in an effort to secure evidence which would support or refute the claims of rhodamine B dimerization in an ethanolic solution.

Photo-thermo-refractive (PTR) glass is an ideal photosensitive material for recording phase volume holograms. It is a homogeneous multi-component silicate glass that demonstrates all the advantages of optical glass: thermal stability, high laser damage threshold, and a wide transparency range. Moreover the ability to record phase patterns (i.e. spatial refractive index variations) into PTR glass has resulted in the fabrication of volume holograms with diffraction efficiency greater than 99%. The conventional method of recording a hologram in PTR glass relies on exposure to continuous-wave ultraviolet laser radiation. In this dissertation the interaction between infrared ultrashort laser pulses and PTR glass is studied. It is shown that photosensitivity in PTR glass can be extended from the UV region to longer wavelengths (near-infrared) by exposure to ultrashort laser pulses. It is found that there exists a focusing geometry and laser pulse intensity interval for which photoionization and refractive index change in PTR glass after thermal development occur without laser-induced optical damage. Photoionization of PTR glass by IR ultrashort laser pulses is explained in terms of strong electric field ionization. This phenomenon is used to fabricate phase optical elements in PTR glass. The interaction between ultrashort laser pulses and volume holograms in PTR glass is studied in two laser intensity regimes. At intensities below ~10^12 W/cm^2 properties such as diffraction efficiency, angular divergence, selectivity, and pulse front tilt are shown to agree with the theory of linear diffraction for broad spectral width lasers. A volume grating pair arrangement is shown to correct the laser pulse distortions arising from pulse front tilt and angular divergence. At higher intensities of irradiation, nonlinear generation and diffraction of third harmonic is observed for three types of interactions: sum-frequency generation, front-surface THG generation, and THG due to phase-matching with a grating formed by modulation of the nonlinear refractive index of PTR glass.

In this thesis the control of ultrafast (10⁻¹³ s) optical pulses used for metrological applications has been investigated. Two different measurement set-ups have been considered, both based around the `pump-probe' technique, where an optical pulse is divided into two parts, one to `pump' or excite a physical system of interest, the other to `probe' or measure the outcome. In both cases the measurement uses electro-optic sampling (EOS), where an electric field is measured by detecting changes in the optical probe pulse polarisation after interaction with the field. In the first study, a method for wavelength metrology in the terahertz (THz) region has been demonstrated by producing an optical pulse shaper and genetic algorithm to control pump pulses and so indirectly influence the THz spectra they generate. In the second study an OPO (optical parametric oscillator) has been developed to provide ultrafast optical pulses for the generation of < 100 fs electrical pulses for metrology using quantum interference control (QUIC). QUIC electrical signals have been demonstrated successfully by charge accumulation measurements and the QUIC electrical pulse temporally measured using EOS, though the low signal levels due to power restrictions mean the QUIC electrical pulse is unsuitable for metrology at this time. Finally, a portable optical pulse measurement device based around frequency-resolved optical gating (FROG) has been designed, built and tested. This has been shown to be capable of retrieving amplitude and phase information in both the temporal and spectral domains for optical pulses as short as 20 fs duration. The ability to characterise shaped pulses also has been demonstrated successfully, with the requirements for full automation identified.