[show abstract][hide abstract] ABSTRACT: The two-photon-absorption coefficient of KDP, BBO, LTB, and CLBO crystals has been determined from the measurement of the
intensity dependent transmission through long samples. The intensity of the sub-picosecond KrF excimer laser pulses on the
samples was varied from 0.2–80GW/cm2. The linear absorption of the samples was determined by using a low intensity, long pulse KrF laser. The first-principle
simulations to the experimental data show a TPA value of 0.48cm/GW for KDP, 0.5cm/GW (o-ray) and 0.34cm/GW (e-ray) in BBO,
0.22cm/GW in LTB and 0.53cm/GW in CLBO.
Applied Physics B 04/2012; 81(8):1123-1126. · 1.78 Impact Factor
[show abstract][hide abstract] ABSTRACT: We demonstrate experimentally that the “coherent” contrast pedestal in compressed pulses from CPA lasers originates from the diffraction gratings in the pulse stretcher. The pedestal intensity correlates with the level of scatter from the gratings.
[show abstract][hide abstract] ABSTRACT: Electrons are accelerated up to 0.8 GeV in a self-injecting laser wakefield accelerator driven at a plasma density of 5.5 × 1018 cm-3 by a 10 J, 55 fs, 800 nm laser pulse in the blow-out regime. The electron beam stability is correlated with the laser focal spot pointing stability and depends on the target alignment. Variations of the laser pulse energy, focal spot size and energy within the full width at half maximum have little effect on the electron beam profile (stability) but impact the electron energy (stability). The peak electron energy is higher than expected for the initial vacuum intensity. This is evidence for intensity amplification which also explains the observation of polyenergetic beamlets.
Plasma Physics and Controlled Fusion 01/2011; · 2.37 Impact Factor
[show abstract][hide abstract] ABSTRACT: We report on the characterization of the specular reflection of 50 fs laser pulses in the intensity range 1017-1021 W cm-2 obliquely incident with p-polarization onto solid density plasmas. These measurements show that the absorbed energy fraction remains approximately constant and that second harmonic generation (SHG) achieves efficiencies of 22±8% for intensities approaching 1021 W cm-2. A simple model based on the relativistic oscillating mirror concept reproduces the observed intensity scaling, indicating that this is the dominant process involved for these conditions. This method may prove to be superior to SHG by sum frequency mixing in crystals as it is free from dispersion and retains high spatial coherence at high intensity.
New Journal of Physics 01/2011; · 4.06 Impact Factor
[show abstract][hide abstract] ABSTRACT: We report on an experimental study of the "coherent" contrast feature that frequently appears in petawatt(PW)-class laser pulses as an exponentially-rising pedestal within a few tens of picoseconds of the compressed pulse. We show that scattering from the diffraction gratings in the stretcher is the principal source of this feature. Replacing the gratings by new, higher-quality components resulted in an order-of-magnitude reduction in the intensity of the pedestal.
[show abstract][hide abstract] ABSTRACT: We report the first experimental observation of a long-wavelength hosing modulation of a high-intensity laser pulse. Side-view images of the scattered optical radiation at the fundamental wavelength of the laser reveal a transverse oscillation of the laser pulse during its propagation through underdense plasma. The wavelength of the oscillation λ(hosing) depends on the background plasma density n(e) and scales as λ(hosing)∼n(e)(-3/2). Comparisons with an analytical model and two-dimensional particle-in-cell simulations reveal that this laser hosing can be induced by a spatiotemporal asymmetry of the intensity distribution in the laser focus which can be caused by a misalignment of the parabolic focusing mirror or of the diffraction gratings in the pulse compressor.
[show abstract][hide abstract] ABSTRACT: The role of plasma channels as waveguides for laser-wakefield accelerators is discussed in terms of the results of experiments performed with the Astra-Gemini laser, numerical simulations using the code WAKE, and the theory of self-focusing and self-guiding of intense laser beams. It is found that at a given electron density, electron beams can be accelerated using lower laser powers in a waveguide structure than in a gas-jet or cell. The transition between relativistically self-guided and channel-assisted guiding is seen in the simulations and in the behaviour of the production of electron beams. We also show that by improving the quality of the driving laser beam the threshold laser energy required to produce electron beams can be reduced by a factor of almost 2. The use of an aperture allows the production of a quasi-monoenergetic electron beam of energy 520 MeV with an input laser power of only 30 TW.
New Journal of Physics 04/2010; 12:045008. · 4.06 Impact Factor
[show abstract][hide abstract] ABSTRACT: Modern intense ultrafast pulsed lasers generate an electric field of sufficient strength to permit tunnel ionization of the valence electrons in atoms. This process is usually treated as a rapid succession of isolated events, in which the states of the remaining electrons are neglected. Such electronic interactions are predicted to be weak, the exception being recollision excitation and ionization caused by linearly-polarized radiation. In contrast, it has recently been suggested that intense field ionization may be accompanied by a two-stage `shake-up' reaction. Here we report a unique combination of experimental techniques that enables us to accurately measure the tunnel ionization probability for argon exposed to 50 femtosecond laser pulses. Most significantly for the current study, this measurement is independent of the optical focal geometry, equivalent to a homogenous electric field. Furthermore, circularly-polarized radiation negates recollision. The present measurements indicate that tunnel ionization results in simultaneous excitation of one or more remaining electrons through shake-up. From an atomic physics standpoint, it may be possible to induce ionization from specific states, and will influence the development of coherent attosecond XUV radiation sources. Such pulses have vital scientific and economic potential in areas such as high-resolution imaging of in-vivo cells and nanoscale XUV lithography. Comment: 17 pages, 4 figures, original format as accepted by Nature Physics
[show abstract][hide abstract] ABSTRACT: Laser-plasma wakefield-based electron accelerators are expected to deliver ultrashort electron bunches with unprecedented peak currents. However, their actual pulse duration has never been directly measured in a single-shot experiment. We present measurements of the ultrashort duration of such electron bunches by means of THz time-domain interferometry. With data obtained using a 0.5 J, 45 fs, 800 nm laser and a ZnTe-based electro-optical setup, we demonstrate the duration of laser-accelerated, quasimonoenergetic electron bunches [best fit of 32 fs (FWHM) with a 90% upper confidence level of 38 fs] to be shorter than the drive laser pulse, but similar to the plasma period.
[show abstract][hide abstract] ABSTRACT: Optical imaging is a versatile diagnostic for investigations of plasmas generated under intense laser irradiation. Electro-optic gating techniques operating on the >100 ps timescale are commonly used to reduce the amount of light detected from self-emission of hot plasma or improve the temporal resolution of the detector. The use of an optical Kerr gate enables a superior dynamic range and temporal resolution compared to electronically gated devices. The application of this method for enhanced imaging of laser produced plasmas with gate time approx100 fs is demonstrated, and the possibility to produce a sub-10 fs, high dynamic range 'all optical' streak camera is discussed.
[show abstract][hide abstract] ABSTRACT: The generation of quasimonoenergetic electron beams, with energies greater than 500 MeV, in a laser-plasma accelerator driven by 2.5 J, 80 fs laser pulses guided in a low density plasma channel, is investigated. The laser energy required to achieve electron injection is found to depend strongly on the quality of the input laser focal spot. Simulations show that, although the matched spot size of the plasma channel is greater than the self-focused spot size, the channel assists relativistic self-focusing and enables electron injection to occur at lower plasma densities and laser powers than would be possible without a waveguide.
Review of Modern Physics 01/2010; · 44.98 Impact Factor
[show abstract][hide abstract] ABSTRACT: Astra Gemini is a dual beam Ti:Sapphire laser capable of delivering up to 0.5 PetaWatt in each of its two beams . The system can fire once every 20 seconds (producing over 1000 shots per day) and has over 200 diagnostic channels, including spectra, pulse length, traces, near and far-field images. This combination of multiple diagnostics and high shot rate leads to an unprecedented amount of performance and diagnostic data to save and analyse. To cope with this demand a system has been developed to automatically capture and analysis laser data on every shot, store it in an Oracle database and retrieve it on demand. A graphical user interface has been written to extract, sort and display the data in a tabular form. Powerful functions have been implemented to allow any parameters to be selected and plotted against one another to analyse performance trends and fluctuations. Metadata about each diagnostic can also be input to build a holistic picture of the laser system and help with future analysis. To increase the value it is planned to incorporate the target area experimental diagnostics into the system and make the data available to participating experimenters anywhere around the world .
[show abstract][hide abstract] ABSTRACT: The acceleration of electrons to approximately 0.8 GeV has been observed in a self-injecting laser wakefield accelerator driven at a plasma density of 5.5x10(18) cm(-3) by a 10 J, 55 fs, 800 nm laser pulse in the blowout regime. The laser pulse is found to be self-guided for 1 cm (>10zR), by measurement of a single filament containing >30% of the initial laser energy at this distance. Three-dimensional particle in cell simulations show that the intensity within the guided filament is amplified beyond its initial focused value to a normalized vector potential of a0>6, thus driving a highly nonlinear plasma wave.
[show abstract][hide abstract] ABSTRACT: We report the latest developments at the Gemini laser system at the Central Laser Facility of the STFC Rutherford Appleton Laboratory. This dual beam facility can deliver 15 J, 30 fs pulses per beam with a shot in every 20 seconds. Gemini recently completed a series of electron acceleration experiments using gas jets and gas-filled capillaries with the first beamline. The second beamline is expected to be online in 2009. Developments are being made in the target area to incorporate complex solid targets at the maximum repetition rate. With a F/2 focusing geometry, Gemini will have access to a range of intensities up to 1022 W cm-2 at high repetition rates. With a 107-108 contrast level, the ASE and pedestal preceding the main pulse can cause a preplasma and damage the targets, especially the complex ones, before the main pulse arrives. At these levels, the routine use of a minimum of two plasma mirrors is required to prevent this. We have designed a fully motorized plasma mirror system. The geometry requires a synchronised raster scan of both substrates and the reflectivity of the first plasma mirror defines the distance to the second. The raster routine has been set to incorporate this so that the plasma mirror system takes the maximum number of shots between optic changes.
[show abstract][hide abstract] ABSTRACT: Astra Gemini is a dual-beam petawatt Ti:sapphire laser at the Rutherford Appleton Laboratory in the U.K. We report measurements characterising the laser beam quality, pulse energy, and pulse duration of the first beam line.
Lasers and Electro-Optics, 2008 and 2008 Conference on Quantum Electronics and Laser Science. CLEO/QELS 2008. Conference on; 06/2008
[show abstract][hide abstract] ABSTRACT: Electron-ion recombination in a laser-induced electron recollision is of fundamental importance as the underlying mechanism responsible for the generation of high-harmonic radiation and hence for the production of attosecond pulse trains in the extreme ultraviolet and soft x-ray spectral regions. By using an ion beam target, remotely prepared to be partially in long-lived excited states, the recombination process has for the first time been directly observed and studied.
[show abstract][hide abstract] ABSTRACT: The Astra Gemini laser system is a dual-beam petawatt class CPA Ti: sapphire laser system which is being built at the Central Laser Facility as an upgrade to the existing Astra laser. Each beam will deliver 15 J in 30 femtoseconds on target (i.e. 0.5 PW), but the dual beam concept allows us not only to achieve petawatt power level on target with relatively small sizes of optics and compressor gratings, but also to access a wider range of science applications. The system is based on two multipass Ti:Sapphire amplifiers pumped by 4 beams at the second harmonic of a Nd:glass laser with an energy 25 J in each beam. The input for the amplifiers is the 1.5 J output of the existing Astra laser system stretched twice in the existing stretcher to give a 1 ns pulse length. 25 J amplified output will be compressed in vacuum grating compressors to give 0.5 PW power in each beam.
[show abstract][hide abstract] ABSTRACT: Few-cycle laser pulses are used to “pump and probe” image the vibrational wavepacket dynamics of a HD molecular ion. The quantum dephasing and revival structure of the wavepacket are mapped experimentally with time-resolved photodissociation imaging. The motion of the molecule is simulated using a quantum-mechanical model predicting the observed structure. The coherence of the wavepacket is controlled by varying the duration of the intense laser pulses. By means of a Fourier transform analysis both the periodicity and relative population of the vibrational states of the excited molecular ion have been characterized.
Journal of Modern Optics 01/2007; · 1.16 Impact Factor
[show abstract][hide abstract] ABSTRACT: The vibrational wavepacket revival of a basic quantum system is demonstrated experimentally. Using few-cycle laser pulse technology, pump and probe imaging of the vibrational motion of D+2 molecules is conducted, and together with a quantum-mechanical simulation of the excited wavepacket motion, the vibrational revival phenomenon has been characterised. The simulation shows good correlation with the temporal motion and structural features obtained from the data, relaying fundamental information on this diatomic system.