S. Payeur

Single shot phase contrast imaging using laser-produced Betatron x-ray beams.

Development of x-ray phase contrast imaging applications with a laboratory scale source have been limited by the long exposure time needed to obtain one image. We demonstrate, using the Betatron x-ray radiation produced when electrons are accelerated and wiggled in the laser-wakefield cavity, that a high-quality phase contrast image of a complex object (here, a bee), located in air, can be obtained with a single laser shot. The Betatron x-ray source used in this proof of principle experiment has a source diameter of 1.7 μm and produces a synchrotron spectrum with critical energy E(c)=12.3±2.5 keV and 10⁹ photons per shot in the whole spectrum.

Investigation of laser-driven proton acceleration using ultra-short, ultra-intense laser pulses

We report optimization of laser-driven proton acceleration, for a range of experimental parameters available from a single ultrafast Ti:sapphire laser system. We have characterized laser-generated protons produced at the rear and front target surfaces of thin solid targets (15 nm to 90 μm thicknesses) irradiated with an ultra-intense laser pulse (up to 1020 W⋅cm−2, pulse duration 30 to 500 fs, and pulse energy 0.1 to 1.8 J). We find an almost symmetric behaviour for protons accelerated from rear and front sides, and a linear scaling of proton energy cut-off with increasing pulse energy. At constant laser intensity, we observe that the proton cut-off energy increases with increasing laser pulse duration, then roughly constant for pulses longer than 300 fs. Finally, we demonstrate that there is an optimum target thickness and pulse duration.

Infrared generation by filamentation in air of a spectrally shaped laser beam

We demonstrated the generation of infrared radiation by filamentation of a spectrally shaped femtosecond laser beam. The spectrum is divided in two distinctive parts using an acousto-optic programmable dispersive filter (AOPDF) as a pulse shaper, resulting in two pulses of different colors. One pulse is frequency doubled and the beams are then focused to produce an optical filament. Efficient infrared generation occurred in the filament zone through the four-wave mixing interaction. This in-line setup allowed perfect spatial overlap of the pulses, fine control of the relative delay and the remote control of the infrared spectral distribution through spectral shaping of the initial femtosecond laser beam via the AOPDF.

The interaction of polarized microwaves with planar arrays of femtosecond laser-produced plasma filaments in air

The interaction of polarized microwaves with subwavelength arrays of parallel plasma filaments, such as those produced by the propagation of high-power femtosecond laser pulses in ambient air, was investigated by calculating the reflection and transmission coefficients as a function of the incidence angles using the finite-difference time-domain (FDTD) method. The time evolution of these coefficients was calculated and compared with experiments. It is found that the plasma filaments array becomes transparent when the polarization of the microwave radiation is perpendicular to the filaments axis, regardless the incidence angle of the microwave with respect to the filaments, except near grazing incidence. Increasing the filaments electron density or diameter, or decreasing the electron collision frequency or filaments spacing, decreases the transmission and increases the reflection. Transmission decreases when increasing the number of filament layers while reflection remains unchanged as the number of filam...

High energy femtosecond pulse compression

An original method for retrieving the Kerr nonlinear index was proposed and implemented for TF12 heavy flint glass. Then, a defocusing lens made of this highly nonlinear glass was used to generate an almost constant spectral broadening across a Gaussian beam profile. The lens was designed with spherical curvatures chosen in order to match the laser beam profile, such that the product of the thickness with intensity is constant. This solid-state optics in combination with chirped mirrors was used to decrease the pulse duration at the output of a terawatt-class femtosecond laser. We demonstrated compression of a 33 fs pulse to 16 fs with 170 mJ energy.

Efficient spectral-step expansion of a filamenting laser pulse.

We report an efficient transfer of 800 nm energy into both the ultraviolet and the far infrared (IR) during the filamentation in air of an appropriately shaped laser pulse. The multiorder enhancement of the IR supercontinuum in the 3-5 μm atmospheric transmission windows was achieved thanks to spectral-step cascaded four-wave mixing occurring within the spectrum of the shaped femtosecond laser pulse. These results also point out the limit of the self-phase modulation model to explain the spectral broadening of a filamenting laser pulse.

Characterization of the laser beam distortion due to the thermal load on high average power femtosecond laser systems

We report observation of laser beam distortion due to the thermal load associated with high energy (110 mJ) and high average power (11 Watts) femtosecond laser system with vacuum compressor. To improve laser-based light source brightness, it is crucial to develop laser systems with higher energy and higher average power. Managing the high thermal loading on vacuum optical components and demonstration of brightness stability are key issues in the implementation of this approach. We characterize such thermally induced distortions using beam wavefront measurements and propose compensation methods to attain long term stability.

Longitudinal field acceleration of electrons by a tightly focused TM01 laser mode

Energetic electrons generation by longitudinal field acceleration from a laser pulse was demonstrated. The longitudinal field was generated by focusing a radially polarised TM01 ultrashort laser pulse (1,8 microns, 550 uJ, 15 fs) with a high numerical aperture parabola. The created longitudinal field was intense enough to ionised and accelerated electrons with a few tens of keV from a low density oxygen gaz. The energy, spectrum, number of charges per shot and divergence of the generated electron bunches have been measured and will be presented. Electron bunch pulse duration, space charge effects and energy tunability will also be discussed.

Laser-based proton acceleration experiments at the ALLS facility using a 200 TW high intensity laser system

Collimated beams of energetic protons are produced by the interaction of short duration high intensity laser pulses with solid foils. This field has been the subject of many studies in the last decade. This interest is motivated by the wide range of application of such beams: ion based fast ignitor schemes, probing of electromagnetic fields in plasma, isotope production or hadron therapy. The recently commissioned 200 TW laser system (5 J, 25 fs, 1010 laser pulse contrast, 10 Hz repetition rate at 800 nm) at the Advanced Laser Light Source (ALLS) facility has been used to study proton acceleration with femtosecond laser pulses. The proton spectrum was characterized using a time of flight detector. Due to the high contrast of the laser pulse, foil targets as thin as 30 nm could be studied.

High intensity research at the Advanced Laser Light Source (ALLS) facility: from 200 TW to 500 TW

We present an overview of the research developed with the ALLS 200 TW laser system: high field physics, electron acceleration, and X-ray sources. We will pursue them with the laser upgrade up to 500 TW.