This thesis describes the phenomenon of high harmonic generation from atoms irradiated by intense, ultrashort laser pulses. Particular attention is paid to the benefits achieved by using multicolour driving fields. A theoretical description of few-cycle laser pulses is presented, together with their interaction with free-electrons and the role they play in various nonlinear optical processes. A number of numerical models are presented to simulate high harmonic generation from atomic systems. These are used to analyse and explain the temporal structure of the emitted high-frequency dipole radiation. Propagation of the macroscopic harmonic response through a gaseous volume is modelled and the role of phase-matching explained in detail. We consider focussing geometry in optimising the yield of particular harmonics, together with the effects of free-electrons within the interaction region. We discuss means by which multicolour fields may overcome some of the constraints of single-colour high harmonic generation. Using two delayed pulses of the same frequency and parallel polarisation we demonstrate significant cut-off extension without increasing total ionisation throughout the pulse, crucial for maintaining harmonic yield close to the saturation limit. We also explain the significant yield enhancements observed in recent experiments using two parallel colours of incommensurate frequency. Finally, we describe the use of a second, perpendicularly polarised colour in trajectory selection, allowing for a temporal filtering of harmonic emission. Using an ω + 1.5ω frequency ratio also allows for a reduction in the periodicity of emitted attosecond pulse trains, permitting the production of isolated attosecond pulses with longer driving fields. Furthermore, by controlling the relative phase between the two colours, the ellipticity of these attosecond pulses may also be controlled.

Light has provided the means to learn and gather information about the physical world throughout history. In a world where science moves to smaller scales and more specialised problems however, the boundaries of current technology are continually challenged, motivating the search for more sophisticated systems providing greater information content, sensitivity and increased dimensionality. Utilising the vectorial nature of light presents a promising avenue by which to meet these growing requirements. Polarisation can, for example, be used to transmit information, or alternatively, changes in polarisation induced by an object allow study of previously neglected material properties, such as birefringence and diattenuation. Central to this thesis is thus the characterisation and exploitation of the opportunities afforded by the electromagnetic (i.e. vectorial) nature of light. To this end the work follows three running themes: quantification of polarisation information; formulation of simple propagation tools for electromagnetic waves; and development of specific polarisation based optical systems. Characterising the informational limits inherent to polarisation based systems reduces to considering the uncertainty present in any observations. Uncertainty can, for example, arise from stochastic variation in the polarisation state being measured, or from random noise perturbations upon detection. Both factors are considered and quantified here. Development of vectorial optical systems does, however, pose significant difficulties in modelling, due to mathematical complexity and computational requirements. A number of new tools are hence developed, as prove applicable to a wide variety of applications. Examples are naturally given. To illustrate the potential of polarisation based systems, specific current topics are discussed; namely the growing demand for data storage, and single molecule studies. It will be shown that polarisation, can not only be used to multiplex information in data pits on optical media, but also to allow full 3D study of single molecules. Factors pertinent to such studies are studied in detail.

Semiconducting polymers have attracted considerable attention as novel gain materials for laser devices. An important future target in this context is the realisation of a thin- film polymer laser diode. Since inorganic semiconductors are amongst the most important devices in modern optoelectronic technology, there is a lot of interest in achieving electrically pumped laser action in organic semiconductors as a way to broadly tunable lasers covering the whole visible spectrum and producing low-cost laser sources for optical networks. This thesis reports the results of a study on the design and characterisation of optically pumped blue and violet emitting polymer lasers. The laser devices are based on a range of materials belonging to the polyfluorene family of conjugated polymers which generally show efficient, low threshold stimulated emission. For future electrically pumped polymer lasers, a further reduction of the threshold is crucial since a low threshold fluence directly translates into low current densities. The optical properties of in total three polyfluorene copolymers are investigated. Lasers based on one of these copolymers are optically-pumped and emission wavelength tuning is demonstrated by altering both grating period and gain polymer thickness, allowing us to cover a part of the spectral region between the blue and ultra-violet that has not been addressed yet by organic semiconductor lasers. Furthermore, a systematic numerical study of the optical environment on the performance of blue emitting lasers on conducting DFB resonators is presented, which is followed by a demonstration of optically-pumped polymer lasers based on ITO gratings. Finally, the results of a systematic study into optically pumped blue emitting polymer lasers based on circular Bragg (CBR) resonators is reported. An optimised design strategy is implemented and involves matching the grating pro files with the nulls and maxima from the Bessel functions that represent the radial distribution of the fi eld in a circular resonator.

A number of biomedical studies have shown that mid-infrared spectroscopic images can provide both morphological and biochemical information that can be used for the diagnosis of cancer. Whilst this technique has shown great potential it has yet to be employed by the medical profession. By replacing the conventional broadband thermal source employed in modern FTIR spectrometers with high-brightness, broadly tuneable laser based sources (QCLs and OPGs) we aim to solve one of the main obstacles to the transfer of this technology to the medical arena; namely poor signal to noise ratios at high spatial resolutions and short image acquisition times. In this thesis we take the first steps towards developing the optimum experimental configuration, the data processing algorithms and the spectroscopic image contrast and enhancement methods needed to utilise these high intensity laser based sources. We show that a QCL system is better suited to providing numerical absorbance values (biochemical information) than an OPG system primarily due to the QCL pulse stability. We also discuss practical protocols for the application of spectroscopic imaging to cancer diagnosis and present our spectroscopic imaging results from our laser based spectroscopic imaging experiments of oesophageal cancer tissue.

A laser-based system to allow the temporal evolution of plasmonic fields on surfaces with attosecond resolution is presented. Sub 7fs carrier envelope phase stabilised infra-red (IR) pulses with 450μJ have been generated using a hollow fiber compression system utilising self phase modulation to produce a 400nm FWHM bandwidth centred at 790nm and subsequent compression with chirped mirrors. The isolated attosecond pulse was produced using spectra selection of a continuum of extreme ultraviolet (XUV) photons generated from a single half cycle emission from the IR driving field. The isolated XUV was characterised using the atomic streak camera technique and a pulse duration of 270as was retrieved using a frequency resolved optical gating algorithm. An ultra high vacuum (10⁻¹⁰mbar) surface science system for attosecond pump-probe studies of surfaces was designed and built, connecting directly to the attosecond beamline. A sample manipulator was developed to precisely position surface samples in the IR and XUV foci. Novel vibrational decoupling mechanisms were developed, achieving only 10nm of motion measured of the sample head. An electron spectrometer with a resolution of 0.05eV for 30eV was used to measure localised nanoplasmonic intensity enhancement of 10³ from a rough silver surface (4mm RMS roughness) by collecting 35eV photoelectrons emitted by a few-cycle IR field with an intensity of ~10¹⁰W/cm². A 2-photon photoemission XUV-IR cross correlation measurement probing hot electron dynamics in a gold surface is reported, revealing femtosecond dynamics of electron thermalisation.

This thesis presents work on the high average power operation of pulsed diode-pumped solid-state lasers by using a laser configuration known as the bounce geometry. The bounce geometry has previously produced efficient, high power and high spatial quality laser outputs in continuous-wave, Q-switched and modelocked regimes. This thesis explores the capabilities of the bounce geometry for power scaling, shown using Nd:YVO4 and Nd:GdVO4 in both a passively Q-switched laser system and a variety of nonlinear mirror modelocked systems. The high gain experienced by Nd-doped gain media pumped at 808 nm has traditionally posed difficulties in producing stable passive Q-switching with Cr4+:YAG. By using a novel stigmatic design of the bounce geometry that experiences lower gain, but highly circular output, passive Q-switching with > 11 W of average power is produced, at a pulse repetition rate of 190 kHz. This is the highest output power ever achieved from a passively Q-switched Nd-doped vanadate laser to date. Nonlinear mirror modelocking is a passive modelocking technique that employs a χ(2) nonlinear medium in combination with a dichroic output coupler. The first nonlinear mirror modelocking of a bounce geometry laser is presented, obtaining 11.3 W of average power and 57 ps pulse duration using a type-II phase-matched KTP nonlinear crystal. Using type-I phase-matched BiBO, shorter pulses of 5.7 ps in duration are obtained at an average power of 6.1 W. The nonlinear mirror modelocking technique is then applied to the stigmatic bounce geometry laser, obtaining a highly stable train of modelocked pulses with pulse duration 14 ps and an average power of 12 W, with high spatial quality output. Mixed vanadate lasers offer customisation of the laser fluorescence spectrum, but tend to experience lower gain than single vanadates. Using the mixed vanadate combination Nd:Gd0.6Y0.4YVO4 in the bounce geometry, 27.5 W of average power in continuous-wave operation is shown. This is the highest power of any mixed vanadate laser ever reported. By then applying the nonlinear mirror modelocking technique to the mixed vanadate system, 16.8 W of average modelocked output power and a pulse duration of 12.7 ps is obtained. This is simultaneously the first time that the nonlinear mirror technique has been applied to mixed vanadate gain media and the highest power of any modelocked mixed vanadate laser to date. Finally, power scaling of a nonlinear mirror modelocked Nd:GdVO4 laser in the bounce geometry is achieved through use of the double bounce geometry design and through use of a high power pump diode. The system employing the high power pumping produced > 30 W of average power — world record power using the nonlinear mirror technique.

Laser-plasma interactions have many potential applications, such as medical treatments,x-ray generation, particle acceleration and inertial confinement fusion (ICF).In all of these applications, understanding how laser energy is absorbed by the materialand converted into energetic electrons is very important. Therefore it is vitalto enhance the understanding of how these energetic electrons are created and whatmechanisms influence them. This Thesis comprises experimental studies of electron acceleration mechanismsin laser-plasma interactions, as well as simulations relevant to these experiments. The experiments described were conducted at the Rutherford Appleton Laboratoryutilising the VULCAN laser facility, and investigate laser interactions with both underdenseand overdense plasmas. In the underdense regime, the intensity dependence of the accelerated electronshas been studied experimentally, as well as the impact of the focusing geometry onthe generation of hot electrons. For high intensities, experimental measurementsshow a scaling of the temperature of the electrons with a0. Density and f-numberdependencies of the accelerated electrons are also observed. The effect of laser polarisation and target thickness on the escaping electronsis studied for laser interactions with solid targets, or overdense plasmas. It wasfound that the effective temperature of the electrons depends on both the laserpolarisation and the target thickness. The electron production from ultra-thin foils,and the effect of laser pre-pulse are also investigated.

The long term aim of this work is to study the suitability of using laser cooled Ca+ ions in Penning traps as the basic components of a quantum computer. A great deal of progress in the field of quantum computing has been made in recent years with laser cooled ions stored in radio frequency ion traps. Building a useful quantum computer with trapped ions is however extremely challenging. Penning traps offer some possible benefits over radio frequency traps. They also create some additional difficulties. The potential advantages and disadvantages of Penning traps are discussed throughout the thesis. We show that we are able to overcome the problems associated with laser cooling in Penning traps, and have trapped single ions for extended periods of time. Pairs of Ca+ ions have been aligned along the axis of a Penning trap, and have been optically resolved. A novel Penning trap array based on PCB boards has been developed. A prototype was built and tested, along with the electronics required to shuttle ions between different sub-traps. Ions have been shuttled a distance of 10 mm in 2.5 μs. A return trip efficiency of up to 75% was seen. A quantum effect – J-state mixing caused by large magnetic fields – has been observed for the first time in single atomic ions. The magnetic field causes a forbidden [Delta]J = 2 transition to become weakly allowed. This effect is of general interest in atomic physics, and is also very relevant for quantum computation studies. A quantitative prediction of the magnitude of the J-mixing effect has been derived theoretically. This is compared to experimental data, and is found to be in excellent qualitative and good quantitative agreement.

This thesis reports on the production and characterisation of the first slow and cold beam of YbF molecules using buffer gas cooling. These molecules are being used to measure the electron’s electric dipole moment, and an intense source of slow-moving molecules is desirable for this experiment. The molecules are loaded into a buffer gas cell via laser ablation where they thermalise with cold helium buffer gas. They are then detected inside the cell using laser absorption imaging and spectroscopy on the X2Σ+→A2II1/2 transition. The formation, diffusion and thermalisation dynamics of the molecules inside the cell are studied. Measurements of laser absorption versus intensity reveal that saturation of the absorption is due to a competition between optical pumping into dark states and repopulation of the addressed level by inelastic and velocity-changing collisions. A beam of YbF molecules is extracted through an aperture in the buffer gas cell and characterised using laser induced fluorescence detection. Peak fluxes of 1010 molecules per steradian per pulse, in the rotational and vibrational ground state, are obtained. The translational and rotational temperatures are in equilibrium with the cell temperature of 4 K. The forward velocity of the pulses can be varied between 130 m/s and 200 m/s by changing the buffer gas pressure. This source is an order of magnitude brighter and more than three times slower than a supersonic source of YbF molecules and provides an excellent starting point for improving the measurement of the electron’s electric dipole moment and for deceleration and trapping experiments. In order to reduce the helium load on the vacuum system and to shorten the molecular pulses, a second set-up, delivering the buffer gas into an open copper cylinder in pulses rather than in a continuous flow,is characterised and shows promising first results.

This thesis explores the existence and properties of self-induced transparency (SIT) solitons in nanophotonic waveguides. SIT solitons are shape-preserving solutions of the semi-classical Maxwell-Bloch equations, a system of nonlinearly coupled differential equations. In a first investigation, collisions of counterpropagating simultons (SIT solitons in absorbing three-level systems) are studied numerically in the plane-wave approximation and a polarisation- and group-velocity dependent soliton birth is uncovered. Apart from their fundamental interest, such light-light interaction effects may be of use for optical computing applications if they can be transferred to tightly confined light pulses. Confining light is usually achieved by using dielectric waveguides that exhibit group velocity dispersion leading to chirped pulses, which experience absorption when entering an absorbing medium. If the chirp is strong enough and the pulse intense enough, they can even completely invert an absorber. When investigating chirped pulse propagation through a dense ensemble of two-level system it is found that the chirped pulses dynamically reshape into unchirped pulses experiencing transparency. Furthermore, the conditions on the waveguide geometry to enable SIT are analysed, identifying a nanophotonic slot waveguide with a low-index gap, exhibiting high electric field enhancement and a homogeneous field profile, as the ideal candidate system for guided SIT solitons. This analysis is supported by two-dimensional numerical calculations that show the solitary character is maintained during propagation if the absorber density is high enough to ensure a slow-down of the pulse and to thus counteract the waveguide dispersion. Finally, the soliton birth due to simulton collisions and optical memory schemes proposed for plane-wave SIT are investigated in the two-dimensional slot waveguide and found to also be possible in this geometry.