Whilst optics is one of the oldest field in science, there are still aspects of electromagnetism that we are only beginning to uncover. For instance, it was demonstrated that materials with simultaneously negative permittivity and permeability exhibit certain exotic behavior; where familiar physical phenomena, such as refraction, are reversed. As such, these materials came to be known as negative refractive index materials (NRIM) and their collective properties as negative refraction. One of the most important and remarkable property of NRIM is perfect lensing - the ability to transport both supra and sub-wavelength optical information from one surface (object plane) to another (image plane), forming images with unprecedented resolution, beyond the diffraction limit. Perfect lensing itself is a consequence of deeper symmetries in electromagnetism, encapsulated in the language of transformation optics - with which we have both a descriptive tool to unify diverse electromagnetic configuarations, as well as a prescriptive tool to design media which bends light at will. While, such transformation medium and NRIM have been demonstrably realised using metamaterials, several challenges remain, of which loss is the major challenge. It is therefore under this trinity of concepts: metamaterials, negative refraction and transformation optics that this thesis is presented. In particular, this thesis illustrates the convergence of the later two concepts, giving rise to a novel class of perfect lens - the compact perfect lens. Here, we shall investigate, their unique properties, construction, challenges, and the dynamics of these lenses. In particular the formulation to study dynamics and effects of losses, is universal; applicable to lenses of any geometry. Finally we shall also propose an alternative universal, top-down approach to overcome losses in perfect lenses using phase conjugation, and demonstrate the capacity of such lenses to see through lossy, translucent obstacles.

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Bessel and Bessel-like beams in Kerr-like nonlinear materials are numerically investigated. This is conducted with a view to exploiting the behaviour of such profiles for the direct laser writing of periodic structures in highly nonlinear glasses. A highly efficient numerical model is developed for the propagation of radially symmetric profiles based on the quasi-discrete Hankel transform (QDHT), making use of a reconstruction relation to allow the field to be sampled at arbitrary positions that do not coincide with the numerical grid. This Hankel-based Adaptive Radial Propagator (HARP) is shown to be up to 1000 times faster than standard FFT-based methods. The critical self-focusing of the Gaussian beam is reproduced to confirm the accuracy of HARP. Following this the critical self-focusing behaviour of a Bessel-Gauss beam is investigated. It is observed that, for certain parameters, increasing the beam power may prevent blowup in the Bessel-Gauss beam. Below the threshold for self-focusing the Bessel-Gauss beam exhibits periodic modulation in the direction of propagation. The existing equation describing this behaviour is shown to be inaccurate and a modification is proposed based on a power dependent beat-length. This modified beat-length equation is demonstrated to be accurate in both the paraxial and quasi-nonparaxial regime. As the beam decays, the intensity modulation appears negatively chirped. It is demonstrated that this chirp may be controlled through careful shaping of the window. It is also shown that a small Gaussian seed beam may be used to control the positions of the maxima. It is demonstrated that a set of nonlinear Bessel functions exist that exhibit a similar quasi-stationary behaviour in a nonlinear medium to the linear Bessel beam in a linear medium. Furthermore it is shown for the first time that higher-order, Bessel-like, stationary solutions exist for beams with azimuthal phase, and boundary conditions for these functions are derived.

Plasmonics could bridge the gap between photonics and electronics at the nanoscale, by allowing the realization of surface-plasmon-based circuits and plasmonic chips in the future. To build up such devices, elementary components are required, such as a passive plasmonic lens to focus free-space light to nanometre area and an active plasmonic modulator or switch to control an optical response with an external signal (optical, thermal or electrical). This thesis partially focuses on designing novel passive and active plasmonic devices, with a specific emphasis on the understanding of the physical principles lying behind these nanoscale optical phenomena. Three passive plasmonic devices, designed by conformal transformation optics, are numerically studied, including nanocrescents, kissing and overlapping nanowire dimers. Contrary to conventional metal nanoparticles with just a few resonances, these devices with structural singularities are able to harvest light over a broadband spectrum and focus it into well-defined positions, with potential applications in high efficiency solar cells and nanowire-based photodetectors and nanolasers. Moreover, thermo-optical and electrooptical modulation of plasmon resonances are realized in metallic nanostructures integrated with either a temperature-controlled phase transition material (vanadium dioxide, VO2), or ferroelectric thin films. Taking advantage of the high sensitivity of particle plasmon resonances to the change of its surrounding environment, we develop a plasmon resonance nanospectroscopy technique to study the effects of sizes and defects in the metal-insulator phase transition of VO2 at the single-particle level, and even single-domain level. Finally, we propose and examine the use of two-dimensional metallic nanohole arrays as a refractive index sensing platform for future label-free biosensors with good surface sensitivity and high-throughput detection ability. The designed plasmonic devices have great potential implications for constructing nextgeneration optical computers and chip-scale biosensors. The developed plasmon resonance nanospectroscopy has the potential to probe the interfacial or domain boundary scattering in polycrystalline and epitaxial thin films.

Classical transport theory predicts strong coupling between thermal transport and magnetic field dynamics in laser produced plasmas; for example, fields are carried with the thermal flux, via the Nernst effect, while simultaneously deflecting it, giving rise to a Righi-Leduc heat-flow. Coupling between these effects is shown here to drive a new kind of instability-the magnetothermal instability-which is described in detail for the first time. A linear perturbation theory is derived in the absence of density gradients and hydrodynamical effects, and yields unstable solutions which propagate as magnetothermal waves. The theory is compared with full non-linear simulation in the context of a recent nanosecond gas-jet experiment and found to be in good agreement; exhibiting typical growth-rates and characteristic wavelengths of order 10ns-1 and 50µ m respectively. Further incorporation of density gradients and hydrodynamics into the magnetothermal stability analysis is shown to introduce the well-known field generating thermal instability source term, which can either complement or counteract the magnetothermal mechanism. Inequalities for predicting the dominance of each process are given: of the two, the magnetothermal mechanism is found to represent the main-and sometimes only-source of unstable feedback for the conditions considered here. Using super-Gaussian transport theory, the implications of inverse-bremmstrahlung heating for transport in laser-plasmas are also explored. Super-Gaussian modifications are shown to suppress a number of classical processes, by up to ~90% in some cases, while simultaneously introducing new effects, such as advection of magnetic field up density gradients. The combined consequences of these modifications are considered for the field generating thermal instability, and super-Gaussian effects are found to reduce growthrates by [greater than or similar to] 80% compared to predictions from classical transport theory under inertial confinement fusion conditions. The development of a unique code CTC, written to assist the exploration of both classical and super-Gaussian transport phenomena, and the new magnetothermal instability, is also described.

In this thesis I describe investigations of the interaction of strong laser fields with nanoscale targets, particularly with atomic clusters. I have explored laser-irradiated clusters in a new regime of interaction where the pulse duration approaches the few-cycle regime and the cluster ions essentially do not move during the laser pulse. A key result of this thesis is the observation of a new anisotropy in the ion emission from the explosion of xenon and argon clusters subjected to ultra-short (∼ 30 fs) near-infrared high intensity (∼ 1015 Wcm−2) laser pulses. In this regime more energetic ions are emitted in the direction perpendicular to the laser polarisation axis, which is the reverse of the well-known anisotropy previously observed in experiments with longer (∼ 100 fs) pulses. I show that the new anisotropy is a transient phenomenon present for a limited range of pulse durations, that is specific to the cluster size and atom species. As the pulse duration is increased the new anisotropy diminishes and a standard anisotropy appears. To explain the observed anisotropy, I have developed an electrostatic model, showing that the intracluster electric field can have a maximum in the direction perpendicular to the laser polarisation axis, leading to anisotropic ion acceleration consistent with experimental observations. These measurements and modeling give access to the initial dynamics, present early in the interaction of an intense laser field with a nanoscale dielectric. In addition to investigations of gas phase clusters I have also studied nanostructures on surfaces. An experiment to study time-dependent plasmonic fields with attosecond streaking is being designed and built. Here I present numerical simulations of nanoplasmonic streaking and address the issues that have to be considered for the ongoing experiment. I show how the plasmonic field can be retrieved from the simulated streaked spectra.

Laser Surface Authentication (LSA) has emerged in recent years as a potentially disruptive tracking and authentication technology. A strong need for such a solution in a variety of industries drove the implementation of the technology faster than the scientific understanding could keep up. The drive to miniaturise and simplify, the need to be robust against real-world problems like damage and misuse, and not least, intellectual curiosity, make it clear that a firmer scientific footing is important as the technology matures. Existing scattering and biometric work are reviewed, and LSA is introduced as a technology. The results of field-work highlight the restrictions which are encountered when the technology is applied. Analysis of the datasets collected in the trial provide, first, an indication of the performance of LSA under real-world conditions and, second, insight into the potential shortcomings of the technique. Using the particulars of the current sensor’s geometry, the LSA signal is characterised. Measurements are made of the decorrelation of the signature with linear and rotational offsets, and it is concluded that while surface microstructure has a strong impact on the rate of decorrelation, this dependency is not driven by the surface’s feature size. A new series of experiments examine that same decorrelation for interference effects with different illumination conditions, and conclude that laser speckle is not an adequate explanation for the phenomenon. The results of this experimental work inform a mathematical description of LSA based on a combination of existing bi-static scattering models used in physics and ray-tracing, which is implemented numerically. The results of the model are found to be a good fit to experimental work, and new predictions are made about LSA.

Since the invention of the laser and its first application, there has been an almost continuous stream of new applications - many of which require specific laser sources. These applications often require a laser source with a specific power, pulse duration, energy and wavelength. In some cases these demands are easily met, whilst in others they have proven rather more difficult to achieve. Traditionally, wavelength versatility has been limited to the regions for which rare earth or gas gain media are available. These lasers themselves can be used to generate other wavelengths through the nonlinear processes of second and third harmonic generation, as well as sum frequency generation. Despite all of this, there still exists a significant section of the visible and infrared spectrum for which no convenient sources exist. This thesis is concerned with the development of sources in these regions along with broadband sources covering significant portions of the spectrum by themselves. These new wavelengths are generated in a variety of speciality fibres using either nonlinear processes or new gain media doped into standard silica fibres. Three types of speciality fibre are used: low concentration bismuth doped fibre which provides gain in the 1.0-1.4 μm region; photonic crystal fibres; and very high (75%) concentration germanium fibres to generate a laser source at 2.1 μm based upon stimulated Raman scattering. Photonic crystal fibres provide high nonlinearities and controllable dispersion which enables the generation of broadband supercontinuum sources based upon the interaction of many nonlinear effects. Each source will be described in depth, with particular attention given to the underlying physics that gives rise to the source. Previous and current limitations will be examined and an outlook of the future development of such sources will be discussed.

When intense and few-cycle laser pulses interact with an atomic/molecular beam highorder Above Threshold Ionisation (HATI) can take place. The resulting electrons can be re-scattered from the parent atoms/molecules and gain kinetic energy. The HATI electrons contain information about the atomic/molecular structure thus providing a method to probe atomic and molecular dynamics with sub-fs temporal and sub-angström spatial resolution. In this thesis the development of a Velocity Map Imaging apparatus capable of measuring the two-dimensional (2D) projection of the velocity distribution of electrons with energies up to 400 eV is described. This device was implemented with a molecular beam apparatus to study the electron re-scattering process that occurs when atoms/molecules are subjected to strong laser fields. Time-of-fight measurements were carried out to find the molecular beam. To perform the experiments a method to generate intense and few-cycle pulses based on hollow fibre pulse compression has been implemented. Pulses of 14 fs with energies of 500 μJ have been measured in a differentially pumped fibre set-up, with input pulses of 42 fs and 700 μJ using a home-made Frequency-Resolved Optical Gating device. The performance of the VMI apparatus was investigated by first studying the ATI rings formed by low energy electrons. Then, a study of the high energy electrons was carried out in different gases and re-scattered electrons with energies up to 100 eV were measured. The photoelectron spectra recorded with linearly polarised laser exhibit a plateau with a cut-off at 10 UP that is a characteristic of the re-scattering process. The observation of rescattered electrons was confirmed by two techniques: comparison of the data obtained with vertical polarisation (re-scattering) and circular polarisation (no re-scattering) and analysis of the structure in the angular distribution obtained in Xenon.

In this thesis, I describe the development of a sub-4 fs few-cycle laser system at Imperial College London used to generate and characterise the first single attosecond (1 as = 10-18s) pulses in the UK. Phase-stabilised few-cycle laser pulses were generated using a hollow fibre system with a chirped mirror compression setup. The pulse was fully characterised using frequency-resolved optical gating (FROG) and spectral phase interferometry for direct electric field reconstruction in a spatially encoded filter arrangement (SEA-F-SPIDER). A pulse duration of 3.5 fs was measured with an argon filled hollow fibre. These phase stabilised Infra-Red (IR) pulses were used to generate a continuous spectrum of high harmonics in the Extreme Ultraviolet (XUV) originating from a single half-cycle of the driving field. Using subsequent spectral filtering, a single attosecond pulse was generated. The isolated XUV pulse was characterised using an atomic streaking camera and a pulse duration of ~260 as was retrieved using FROG for complete reconstruction of attosecond bursts (FROG-CRAB). In an experiment conducted at the Rutherford Appleton Laboratory, high harmonics were generated using a two-colour field with an energetic beam at 1300nm and a weak second harmonic orthogonally polarized to the fundamental. By changing the phase between the two fields, a deep modulation of the harmonic yield is seen and an enhancement of one order of magnitude compared to the single colour field with the same energy is observed.