D. R. Jones

Radiation sources based on laser-plasma interactions

Plasma waves excited by intense laser beams can be harnessed to produce femtosecond duration bunches of electrons with relativistic energies. The very large electrostatic forces of plasma density wakes trailing behind an intense laser pulse provide field potentials capable of accelerating charged particles to high energies over very short distances, as high as 1 GeV in a few millimetres. The short length scale of plasma waves provides a means of developing very compact high-energy accelerators, which could form the basis of compact next-generation light sources with unique properties. Tuneable X-ray radiation and particle pulses with durations of the order of or less than 5 fs should be possible and would be useful for probing matter on unprecedented time and spatial scales. If developed to fruition this revolutionary technology could reduce the size and cost of light sources by three orders of magnitude and, therefore, provide powerful new tools to a large scientific community. We will discuss how a laser-driven plasma wakefield accelerator can be used to produce radiation with unique characteristics over a very large spectral range.

Femtosecond laser irradiation of polymethylmethacrylate for refractive index gratings

Polymethylmethacrylate (PMMA) or Perspex is an inexpensive polymer widely used for making the cores of communications grade polymer optical fibres (POFs) and as a substrate for polymer optoelectronic devices and integrated waveguides. Periodic refractive index structures have been written in undoped PMMA using multiple pulses of 40 fs duration from a 1 kHz Ti:sapphire femtosecond laser operating at the fundamental (800 nm). A refractive index change (Δn) of 5 ± 0.5 × 10−4 was observed before the onset of striations. Optimization of writing conditions for refractive index modification of POF fibres or bulk undoped PMMA will enable structures such as Bragg gratings, long-period gratings, mode couplers, microlens arrays, and zone plates to be written.

Plasma characterization with terahertz time-domain measurements

Terahertz time–domain spectral techniques are applied to the characterization of a He discharge plasma. Electro-optically sampling of the electric field of a quasi-unipolar terahertz pulse transmitted through the plasma has allowed both the real and imaginary parts of the plasma permittivity to be simultaneously measured over a large spectral range. The plasma density and the collisional frequency are determined within a 30 ps duration measurement window. An anomalously high collisional frequency has been measured.

Pulse-duration dependency of femtosecond laser refractive index modification in poly(methyl methacrylate)

Refractive index modification of pure poly(methyl methacrylate) (PMMA) is investigated as a function of pulse duration using femtosecond lasers at 800 and 387 nm wavelength. It is observed that at 800 nm, the refractive index is modified more efficiently as the pulse duration decreases below 100 fs, whereas at 387 nm, efficient index modification is accomplished with longer, 180 fs pulses. Results suggest that three- and two-photon absorption is responsible for modification of pure PMMA at 800 nm and 387 nm, respectively. Repeated irradiation with short pulses of low laser fluence allows control of the photomodification via incubation, thus reducing bulk damage.

The Strathclyde terahertz to optical pulse source (TOPS)

We describe the newly created free-electron laser facility situated at the University of Strathclyde in Scotland, which will produce ultra-short pulses of high-power electromagnetic radiation in the terahertz frequency range. The FEL will be based on a 4 MeV photoinjector producing picosecond 1 nC electron pulses and driven by a frequency tripled Ti:sapphire laser thus ensuring synchronism with conventional laser based tuneable sources. A synchronised multi-terawatt Ti:sapphire laser amplifier will be used in the study of laser/plasma/electron beam interactions and as a plasma based X-ray source. A substantial user commitment has already been made in support of the programme.

Ionisation and fragmentation of polycyclic aromatic hydrocarbons by femtosecond laser pulses at wavelengths resonant with cation transitions

When femtosecond laser pulses irradiate hydrocarbon molecules, then many fragmentation channels evident in nanosecond irradiation are bypassed, providing a strong analytically useful parent ion. However a number of molecules show only a very small or indeed no parent ions and recent papers suggest that those that do not produce parent peaks have cation transitions in resonance with the femtosecond laser wavelength. This Letter shows that this resonance effect is not universal and some aromatic molecules not only show strong parent peaks but also doubly and triply ionised entities when their cation absorption spectrum is strongly resonant at either the 800 or 400 nm or indeed both.

Ionisation and fragmentation dynamics of laser desorbed polycyclic aromatic hydrocarbons using femtosecond and nanosecond post-ionisation

Nanosecond laser desorption/femtosecond laser mass spectrometry (LD/FLMS) incorporating a reflectron time-of-flight mass spectrometer has been used to study the ionisation/fragmentation of polycyclic aromatic hydrocarbons (PAHs) in intense laser fields (7 .0 × 10 14 to 9.3 × 10 15 Wc m −2 ). Pulses of 80 fs, 800 nm have been used to post-ionise the PAHs anthracene, tetracene and pentacene. For each molecule strong singly and doubly charged parent ions are observed accompanied by fragmentation. In addition, strong triply charged parent ions (M 3+ ) are observed for anthracene and weaker M 3+ signals for tetracene and pentacene are also observed. Nanosecond post-ionisation (266 nm, 16 ns) spectra of the molecules have been recorded and are included for comparison with the femtosecond data. Similarities in the observed fragmentation pattern of low-mass fragments of the nanosecond and low intensity femtosecond spectra are highlighted. In addition, as the laser intensity increases, it is observed that fragmentation pathways preferentially switch from C mH3 + ion yield to Cm + production for m = 2–5 at a critical intensity which is molecule dependent. (Int J Mass Spectrom 220 (2002) 69–85)

Linearly tapered discharge capillary waveguides as a medium for a laser plasma wakefield accelerator

Gas-filled capillary discharge waveguides are commonly used as media for plasma wakefield accelerators. We show that effective waveguides can be manufactured using a femtosecond laser micromachining technique to produce a linearly tapered plasma density, which enables the energy of the accelerator to be enhanced significantly. A laser guiding efficiency in excess of 82% at sub-relativistic intensities has been demonstrated in a 40 mm long capillary with a diameter tapering from 320 μm to 270 μm, which gives rise to an on-axis, time-averaged plasma density that varies from 1.0 × 1018 cm−3 to 1.6 × 1018 cm−3.

Note: femtosecond laser micromachining of straight and linearly tapered capillary discharge waveguides

Gas-filled capillary discharge waveguides are important structures in laser-plasma interaction applications, such as the laser wakefield accelerator. We present the methodology for applying femtosecond laser micromachining in the production of capillary channels (typically 200-300 μm in diameter and 30-40 mm in length), including the formalism for capillaries with a linearly tapered diameter. The latter is demonstrated to possess a smooth variation in diameter along the length of the capillary (tunable with the micromachining trajectories). This would lead to a longitudinal plasma density gradient in the waveguide that may dramatically improve the laser-plasma interaction efficiency in applications.

Straight and linearly tapered capillaries produced by femtosecond laser micromachining

Gas-filled capillary discharge waveguides are a commonly employed medium in laser–plasma interaction applications, such as the laser wakefield accelerator, because they can simultaneously guide high-power laser pulses while acting as the medium for acceleration. In this paper, the production of both straight and linearly tapered capillaries using a femtosecond laser micromachining technique is presented. A tapered capillary is shown to possess a smooth variation in diameter (from 305 μm to 183 μm) along its entire 40 mm length, which would lead to a longitudinal plasma density gradient, thereby dramatically improving the laser–plasma interaction efficiency in applications. Efficient guiding with up to 82% energy transmission of the fundamental Gaussian mode of a low intensity, 50 fs duration laser pulse is shown for both types of capillary waveguide.