High-energy laser-pulse self-compression in short gas-filled fibers

We examine the spatiotemporal compression of energetic femtosecond laser pulses within short gas-filled fibers. The study is undertaken using an advanced nonlinear pulse propagation model based on a multimode generalized nonlinear Schrödinger equation that has been modified to include plasma effects. Plasma defocusing and linear propagation effects are shown to be the dominant processes within a highly dynamical mechanism that enables 100-fs pulses to be compressed into the few-cycle regime after <50 mm of propagation. Once the mechanism has been introduced, parameter spaces are explored and compressor designs suitable for performing high-field experiments in situ are presented. We finish by showing how these designs may be extended to novel wavelengths and driving pulses delivered by state-of-the-art high-repetition-rate lasers

Quantum efficiency measurements in oxygen-containing gallium lanthanum sulphide glasses and fibers doped with Pr³⁺

The quantum efficiency of Pr3+ emission at 1.3µm from the 1G4 - 3H5 transition is measured in Gallium Lanthanum Sulphide (GLS) glass containing varying quantities of lanthanum oxide. The variation of quantum efficiency with host composition is described, and the variation of quantum efficiency with pump wavelength in oxide-containing hosts is compared to a model of the effect of the addition of oxygen on the spectroscopy of the Pr ion. Oxide-containing GLS glasses can show quantum efficiencies of up to 84% of that of pure GLS, while retaining considerably better thermal and glass-forming properties. No degradation of quantum efficiency is seen when GLS glass is pulled into fiber form

Experimental demonstration of a high-flux capillary based XUV source in the high ionisation regime

High harmonic generation (HHG) has proven to be a fascinating and incredibly useful nonlinear optical phenomenon and has led to the realisation of tabletop sources of coherent extreme ultraviolet (XUV) radiation. Capillary based geometries in particular have attracted a great deal of attention due to their lengthy interaction regions and the potential to phase-match the HHG process leading to a large increase in XUV flux. Unfortunately due to plasma induced nonlinear and dispersive effects the simple phase-matching mechanism detailed in [1] cannot be scaled to high energy pump pulses and high gas pressures. In this work we have used a computational model [2] to design a capillary that can support a broad interaction region well-suited to quasi-phase-matching (QPM) while simultaneously reducing the effect that XUV reabsorption has on the output flux of the source. This modelling work has involved adjusting both the capillary length and gas density profile (figure 1a) in order to produce rapid oscillations in the radially integrated ionization fraction (figure 1b) coupled with a rapid decrease in gas pressure at the capillary exit. Our theory suggests that these oscillations are driven by a nonlinear self-compression process modulating the intensity of the pump pulse as it propagates through the plasma-filled waveguide [3]. Subsequent experimental work has shown an increase in XUV flux of almost 50 times over our previous capillary-based source (see figure 1c), and preliminary estimates suggest a photon flux of 1012 photons s-1 harmonic-1 in the 45 eV spectral region

Statistical description of capillary-based high-harmonic generation

High-harmonic generation (HHG), where the interaction of high-intensity laser light with matter generates ultrashort XUV pulses, is an attractive option for a table-top source of coherent light at nanometre wavelengths. Its efficiency can be improved by performing the HHG in a gas-filled capillary instead of the more common gas jet or cell due to improved interaction length and phase matching. However, because of the highly nonlinear interaction between pump light, neutral atoms, generated plasma, and XUV radiation in this regime, accurate computer simulations and predictions are highly complex and time consuming

Full characterisation of a focussed extreme ultraviolet beam using a non-redundant array of apertures

This paper presents a novel technique for characterising wavefront curvature and M2, by utilising a non-redundant array (NRA) of apertures to define the plane of investigation through an experimental extreme ultraviolet (EUV) focus. Appropriately sampled, far-field EUV scattering from this NRA is captured on a CCD as the NRA is scanned along the beam axis through the focus. By taking the inverse Fourier transform (IFT), it is possible obtain the spatial autocorrelation functions, via the Wiener-Khinchin theorem, of the exit wave field. By observing the position of the first-order peaks in the autocorrelation as a function of grid translation, both the real and imaginary parts of the complex beam parameter can be determined and the M2 calculated, yielding full characterisation of the embedded Gaussian. Since the periodicity of the grid is known, the planar pixel resolution can be calculated, also allowing the translations movement to be confirmed due to the change in angular acceptance of the fixed CCD. This makes the technique self-calibrating. A high impact, easy to use, cross field technique for full profiling of the embedded Gaussian of probe beams using a non-redundant array of apertures is presented. The technique is experimentally verified in the highly absorbing EUV spectral regime, and is expected to play a significant role in other regimes, where experimental issues prevent the use of existing techniques

Statistical analysis of pump-pulse propagation in gas-filled capillaries for high-harmonic generation

Driving high-harmonic generation (HHG) with ultrashort pulses confined to gas-filled capillaries is an efficient method of generating extreme ultraviolet and x-ray radiation. In-situ pulse compression can significantly enhance HHG efficiency [1] but requires operation in the high-ionisation limit, leading to high sensitivity to initial conditions and causing the Gaussian driving pulse to break up into a train of subpulses as it propagates. Our previous studies [1,2] have focused on the most intense subpulse, which can be very short (<10 fs). Here, we perform statistical analysis of all pulse components predicted by numerical simulation, including the contribution of the weaker subpulses, with the aim of predicting generated HHG profiles

Enhanced optical properties of Tm³⁺ in f co-doped lead germanate glasses for fibre device applications

The effect on structure and property of adding fluoride into Tm3+ doped lead-germanate glass was established and verified experimentally. It was found that up to 10 mol% of fluoride could be introduced into our original lead-germanate composition while retaining the high thermal stability ideal for fibre fabrication. Much improved spectroscopic features, namely increased fluorescent lifetimes from 3H4 and 3F4 levels in Tm3+ with increasing fluorine content, were observed. At the same time it was found that the radiative properties of Tm3+ were left unchanged by fluoride addition, indicating that reduced multiphonon relaxation was responsible for the increased fluorescent lifetimes. This was well explained and foreseen by our established structure-property relation in terms of adding fluorine to the glass. In conclusion, fluoro-germanate glass shows advantages over germanate glass in optical properties and over fluoride glass in chemical and mechanical properties for practical fiber device applications. [Presentation slides

Formula funding and special educational needs

This thesis is based on a six year research study and is set against the implementation of Local Management of Schools and the formula funding arrangements for pupils with special educational needs (SEN). The main aim of the research is to investigate the principles and practice for allocating additional resources to provide for pupils with SEN but without statements. Two theoretical perspectives are used: the 'special needs pupil' discourse and the 'school and teacher effectiveness' discourse (Galloway, Armstrong and Tomlinson, 1994). The study is in two main parts. First the theoretical component whereby a critical examination is given to the conceptualisation of special educational needs, to the principles or criteria for evaluating a funding formula and to the historical arrangements for funding pupils with SEN. The second part of the thesis is the empirical component consisting of two national surveys, a case study carried out in two LEAS Mercia and Whiteshire, and computer budget modelling for different funding formulae for all primary and secondary schools in Whiteshire (n=690 schools). Evidence is obtained throughout the study relating to the design of an 'improved' SEN formula which is evaluated according to the principles or criteria of simplicity, equity, effectiveness, responsiveness to needs, efficiency, stability of funding, cost containment and accountabilit

Nanomaterial structure determination using XUV diffraction

Diffraction using coherent XUV radiation is used to study the structure of nanophotonic materials, in this case an ordered array of 196nm spheres. Crystal structure and defects are visible, and the nanomaterial dielectric constant determined

Single exposure wavefront curvature estimation of high harmonic radiation by diffraction from a regular array

We present a novel technique for estimating the radius of curvature from a single exposure of EUV light from a high harmonic source diffracted by a grating of square apertures