Generation and Application of Ultrashort Laser Pulses in Attosecond Science

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

On recovery guarantees for angular synchronization

The angular synchronization problem of estimating a set of unknown angles from their known noisy pairwise differences arises in various applications. It can be reformulated as a optimization problem on graphs involving the graph Laplacian matrix. We consider a general, weighted version of this problem, where the impact of the noise differs between different pairs of entries and some of the differences are erased completely; this version arises for example in ptychography. We study two common approaches for solving this problem, namely eigenvector relaxation and semidefinite convex relaxation. Although some recovery guarantees are available for both methods, their performance is either unsatisfying or restricted to the unweighted graphs. We close this gap, deriving recovery guarantees for the weighted problem that are completely analogous to the unweighted version.Comment: 20 pages, 5 figure

Supporting decisions on conflicting land-uses: An integrated ecological-economic approach

An integrated ecological-economic decision-making approach is developed to help local stakeholders decide on land use in rural areas where the conflict between natural resource protection and economic development is pressing. It consists of four methodological steps. In the first step the political options and alternatives for action regarding changes in the land-use pattern are specified in order to derive politically relevant land-use strategies (scenarios). In the second step economic, ecological and social indicators are derived. The third step includes economic modelling (economic input-output model), environmental modelling (modelling of landscape water balance) and the qualitative and quantitative estimation of ecological and environmental effects. These efforts result in the production of a multi-indicator matrix. Finally, the fourth step deals with a combined monetary and multi-criteria evaluation resulting in a ranking of the land-use strategies. The discussion of the decision-making approach concentrates on the necessity of preliminary decisions and the possibility and necessity of stakeholders participation in the decisionmaking process. --evaluation,decision-making,multi-criteria analysis,land-use management,scenarios,benefit-cost analysis

Non-thermal radiation from molecular clouds illuminated by cosmic rays from nearby supernova remnants

Molecular clouds are expected to emit non-thermal radiation due to cosmic ray interactions in the dense magnetized gas. Such emission is amplified if a cloud is located close to an accelerator of cosmic rays and if cosmic rays can leave the accelerator and diffusively reach the cloud. We consider the situation in which a molecular cloud is located in the proximity of a supernova remnant which is accelerating cosmic rays and gradually releasing them into the interstellar medium. We calculate the multiwavelength spectrum from radio to gamma rays which emerges from the cloud as the result of cosmic ray interactions. The total energy output is dominated by the gamma ray emission, which can exceed the emission from other bands by an order of magnitude or more. This suggests that some of the unidentified TeV sources detected so far, with no obvious or very weak counterpart in other wavelengths, might be associated with clouds illuminated by cosmic rays coming from a nearby source.Comment: 4 pages, 3 figures, proceedings of the "4th Heidelberg International Symposium on High Energy Gamma-Ray Astronomy" July 7-11, 2008, Heidelberg, German

Implications of the VHE Gamma-Ray Detection of the Quasar 3C279

The MAGIC collaboration recently reported the detection of the quasar 3C279 at > 100 GeV gamma-ray energies. Here we present simultaneous optical (BVRI) and X-ray (RXTE PCA) data from the day of the VHE detection and discuss the implications of the snap-shot spectral energy distribution for jet models of blazars. A one-zone synchrotron-self-Compton origin of the entire SED, including the VHE gamma-ray emission can be ruled out. The VHE emission could, in principle, be interpreted as Compton upscattering of external radiation (e.g., from the broad-line regions). However, such an interpretation would require either an unusually low magnetic field of B ~ 0.03 G or an unrealistically high Doppler factor of Gamma ~ 140. In addition, such a model fails to reproduce the observed X-ray flux. This as well as the lack of correlated variability in the optical with the VHE gamma-ray emission and the substantial gamma-gamma opacity of the BLR radiation field to VHE gamma-rays suggests a multi-zone model. In particular, an SSC model with an emission region far outside the BLR reproduces the simultaneous X-ray -- VHE gamma-ray spectrum of 3C279. Alternatively, a hadronic model is capable of reproducing the observed SED of 3C279 reasonably well. However, the hadronic model requires a rather extreme jet power of L_j ~ 10^{49} erg s^{-1}, compared to a requirement of L_j ~ 2 X 10^{47} erg s^{-1} for a multi-zone leptonic model.Comment: Accepted for pulication. Several clarifications and additions to the manuscript to match the accepted versio