Publications by Simon Hooker


Developments in laser-driven plasma accelerators

Nature Photonics 7 (2013) 775-782

SM Hooker

Laser-driven plasma accelerators provide acceleration gradients that are three orders of magnitude greater than those generated by conventional accelerators, offering the potential to shrink the length of accelerators by the same factor. To date, laser acceleration of electron beams to produce particle energies comparable to those offered by synchrotron light sources has been demonstrated with plasma acceleration stages that are only a few centimetres long. This Review describes the operation principles of laser-driven plasma accelerators, and gives an overview of their development from their proposal in 1979 to recent demonstrations. Potential applications of plasma accelerators are described, and the challenges that must be overcome before they can become practical tools are discussed. © 2013 Macmillan Publishers Limited.


Quasi-phase-matching of high harmonic generation using counter-propagating pulses

EPJ Web of Conferences 41 (2013)

K O'Keeffe, SM Hooker

We investigate quasi-phase-matching over a range of harmonic orders using trains of up to 8 uniformly-spaced counter-propagating pulses. For trains of up to 4 pulses the measured enhancement of the harmonic signal scales with the number of pulses N as (N+1) 2 , as expected. However, for trains with N > 4, no further enhancement of the harmonic signal is observed. The effect is ascribed to changes in the coherence length with the generating medium. The pressure dependence of quasi-phase-matching is also investigated and it is found that additional peaks in the harmonic intensity are consistent with higher-order QPM processes. © Owned by the authors, published by EDP Sciences, 2013.


Two-Pulse Ionization Injection into Quasilinear Laser Wakefields

Physics Review Letters 111 (2013) 155004

N Bourgeois, J Cowley, SM Hooker


Polarization-controlled quasi-phase matching for linearly and circularly polarized high harmonic generation

2013 Conference on Lasers and Electro-Optics Europe and International Quantum Electronics Conference, CLEO/Europe-IQEC 2013 (2013)

LZ Liu, K O'Keeffe, SM Hooker

High harmonic generation (HHG) is a nonlinear process in which odd multiples of a fundamental driving field are produced when an intense laser pulse is focused into a low density gas. HHG is an attractive source of coherent, tuneable light with wavelengths in the XUV and soft X-ray range and has found a broad range of applications in ultrafast physics and imaging. However, due to the phase mismatch between the driving and generated fields, the intensity of the generated harmonics oscillates as a function of propagation distance, z, between 0 and some maximum value. The period of oscillation is 2L c where L c is the coherence length. One way of overcoming the problem of phase-mismatching is quasi-phase matching (QPM), in which harmonic generation is suppressed or modified in out-of phase regions. Here we propose a new class of QPM techniques by controlling the polarization in an optical waveguide by either: (i) rotating the polarization of the driving field in a circularly birefringent system [1]; or (ii) modulating the ellipticity of the driver polarization in a linearly birefringent system [2] . The key advantage of this new class of polarization-controlled QPM is its simplicity compared to other QPM techniques. © 2013 IEEE.


Longitudinal electron bunch profile reconstruction by performing phase retrieval on coherent transition radiation spectra

PHYSICAL REVIEW SPECIAL TOPICS-ACCELERATORS AND BEAMS 16 (2013) ARTN 040701

SI Bajlekov, M Heigoldt, A Popp, J Wenz, K Khrennikov, S Karsch, SM Hooker


Quasi-phase-matching of high-order-harmonic generation using multimode polarization beating

Physical Review A - Atomic, Molecular, and Optical Physics 87 (2013)

LZ Liu, K O'Keeffe, SM Hooker

The generalization of quasi-phase-matching using polarization beating and of multimode quasi-phase-matching (MMQPM) for the generation of high-order harmonics is explored, and a method for achieving polarization beating is proposed. If two (and in principle more) modes of a waveguide are excited, modulation of the intensity, phase, and/or polarization of the guided radiation will be achieved. By appropriately matching the period of this modulation to the coherence length, quasi-phase-matching of high-order-harmonic radiation generated by the guided wave can occur. We show that it is possible to achieve efficiencies with multimode quasi-phase-matching greater than the ideal square wave modulation. We present a Fourier treatment of QPM and use this to show that phase modulation, rather than amplitude modulation, plays the dominant role in the case of MMQPM. The experimental parameters and optimal conditions for this scheme are explored. © 2013 American Physical Society.


Polarization-controlled quasi-phase matching for linearly and circularly polarized high harmonic generation

2013 CONFERENCE ON AND INTERNATIONAL QUANTUM ELECTRONICS CONFERENCE LASERS AND ELECTRO-OPTICS EUROPE (CLEO EUROPE/IQEC) (2013)

LZ Liu, K O'Keeffe, SM Hooker, IEEE


Polarization-controlled quasi-phase matching for linearly and circularly polarized high harmonic generation

Optics InfoBase Conference Papers (2013)

LZ Liu, K O'Keeffe, SM Hooker


Complete Spatial Characterization of an Optical Wavefront Using a Variable-Separation Pinhole Pair

2013 CONFERENCE ON AND INTERNATIONAL QUANTUM ELECTRONICS CONFERENCE LASERS AND ELECTRO-OPTICS EUROPE (CLEO EUROPE/IQEC) (2013)

DT Lloyd, K O'Keeffe, SM Hooker, IEEE


Complete spatial characterization of an optical wavefront using a variable-separation pinhole pair

Optics InfoBase Conference Papers (2013)

DT Lloyd, K O'Keeffe, SM Hooker


Investigation of GeV-scale electron acceleration in a gas-filled capillary discharge waveguide

New Journal of Physics 15 (2013)

PA Walker, N Bourgeois, W Rittershofer, J Cowley, N Kajumba, AR Maier, J Wenz, CM Werle, S Karsch, F Grüner, DR Symes, PP Rajeev, SJ Hawkes, O Chekhlov, CJ Hooker, B Parry, Y Tang, SM Hooker

The generation of GeV-scale electron beams in a gas-filled capillary discharge waveguide with good reproducibility is discussed. Beams of electrons with energies above 900 MeV, and with root-mean-square divergences of 3.5 mrad, are observed for a plasma density of 2.2 × 10 18 cm -3 and a peak input laser power of 55 TW. The variation of the maximum electron energy with the plasma density is measured and found to agree well with simple models. Injection and acceleration of electrons at the to date lowest plasma density of 3.2 × 10 17 cm -3 are reported. The energy spectra of the generated electron beams exhibit good shot-to-shot reproducibility, with the observed variations attributable to the measured shot-to-shot jitter of the laser parameters. Two methods for correcting the effect of beam pointing variations on the measured energy spectrum are described. © IOP Publishing and Deutsche Physikalische Gesellschaft.


Polarization-controlled quasi-phase-matching of high harmonic generation

EPJ Web of Conferences 41 (2013)

LZ Liu, K O'Keeffe, SM Hooker

A new class of quasi-phase matching high harmonic generation is proposed where the polarization of the driving field is controlled in a waveguide. The first circularly polarized quasi-phase matched source is shown to be possible. © Owned by the authors, published by EDP Sciences, 2013.


Complete spatial characterization of an optical wavefront using a variable-separation pinhole pair.

Opt Lett 38 (2013) 1173-1175

DT Lloyd, K O'Keeffe, SM Hooker

We present a technique for measuring the transverse spatial properties of an optical wavefront. Intensity and phase profiles are recovered by analysis of a series of interference patterns produced by the combination of a scanning X-shaped slit and a static horizontal slit; the spatial coherence may be found from the same data. We demonstrate the technique by characterizing high harmonic radiation generated in a gas cell, however the method could be extended to a wide variety of light sources.


Transverse beam profile measurements of laser accelerated electrons using coherent optical radiation

AIP Conference Proceedings 1507 (2012) 258-261

N Bourgeois, M Heigoldt, W Rittershofer, A Popp, K Khrennikov, SI Bajlekov, S Karsh, SM Hooker

We use coherent optical transition radiation (COTR) to measure the transverse profile of laser-accelerated electron bunches. The retrieved electron beam profiles are compared to scintillator-based beam profile measurements. © 2012 American Institute of Physics.


Multiple pulse resonantly enhanced laser plasma wakefield acceleration

AIP Conference Proceedings 1507 (2012) 872-873

L Corner, R Walczak, LJ Nevay, S Dann, SM Hooker, N Bourgeois, J Cowley

We present an outline of experiments being conducted at Oxford University on multiple-pulse, resonantly-enhanced laser plasma wakefield acceleration. This method of laser plasma acceleration uses trains of optimally spaced low energy short pulses to drive plasma oscillations and may enable laser plasma accelerators to be driven by compact and efficient fibre laser sources operating at high repetition rates. © 2012 American Institute of Physics.


Electron acceleration driven in plasma channels at the Astra-Gemini laser facility

AIP Conference Proceedings 1507 (2012) 193-198

PA Walker, N Bourgeois, W Rittershofer, J Cowley, N Kajumba, AR Maier, J Wenz, CM Werle, DR Symes, PP Rajeev, SJ Hawkes, O Chekhlov, CJ Hooker, B Parry, Y Tang, VA Marshall, S Karsch, F Grüner, SM Hooker

The generation of GeV-scale electron beams in the plasma channel formed in a gas-filled capillary discharge waveguide is investigated. Electron beams with energies above 900 MeV and with root-mean-square divergence of 3.5 mrad are observed for plasma densities of 2.15 × 10 18 cm -3 and a peak input laser power of only 55 TW. The variation of the electron energy with the plasma density is measured and found to exhibit a maximum at plasma densities for which the dephasing length approximately matches the length of the plasma channel. Injection and acceleration of electrons at the relatively low plasma density of 3.2 × 10 17 cm -3 is observed. The energy spectra of the generated electron beams are shown to exhibit good shot-to-shot reproducibility, with the observed variations attributable to the measured shot-to-shot jitter of the laser parameters. Two methods for correcting for the effects on the measured energy spectrum of off-axis electron beam propagation are investigated. © 2012 American Institute of Physics.


Quasi-phase-matching high harmonic generation using trains of pulses produced using an array of birefringent plates.

Opt Express 20 (2012) 6236-6247

K O'Keeffe, T Robinson, SM Hooker

Quasi-phase-matched high harmonic generation using trains of up to 8 counter-propagating pulses is explored. For trains of up to 4 pulses the measured enhancement of the harmonic signal scales with the number of pulses N as (N + 1)², as expected. However, for trains with N > 4, no further enhancement of the harmonic signal is observed. This effect is ascribed to changes of the coherence length Lc within the generating medium. Techniques for overcoming the variation of Lc are discussed. The pressure dependence of quasi-phase-matching is investigated and the switch from true-phase-matching to quasi-phase-matching is observed.


High Harmonic Optical Generator (Polarization Beating 2/2)

(2012) UK Patent Application GB1207963.8

LZ Liu, K O'Keeffe, SM Hooker

A high harmonic optical generator comprising a laser arrangement and an optical waveguide, wherein the laser arrangement is configured to couple a beam of linearly polarized radiation at a fundamental frequency into the optical waveguide to provide a beam of optical driving radiation that propagates along a propagation axis of the optical waveguide, the optical waveguide has a hollow core for a gaseous harmonic generation medium for generation of high harmonics of the fundamental frequency, and the generator is configured for the driving radiation to have a polarization ellipticity that periodically varies along the propagation axis of the optical waveguide.


Quasi-phase-matched high harmonic generation using trains of uniformly-spaced ultrafast pulses

High Intensity Lasers and High Field Phenomena, HILAS 2012 (2012)

K O'Keeffe, T Robinson, SM Hooker

We investigate quasi-phase-matching of high harmonic generation over a range of harmonic orders using trains of up to 8 uniformly-spaced counter-propagating pulses, produced using an array of birefringent crystals. © 2012 OSA.


Optical rotation quasi-phase-matching for circularly polarized high harmonic generation

Optics Letters 37 (2012) 167066

LZ Liu, K O'Keeffe, SM Hooker

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