R. Clady

Self-limited underdense microplasmas in bulk silicon induced by ultrashort laser pulses

This research has received financial support from the French National Research Agency (ANR 2010-JCJC-913- 01), the French Carnot Star Institute (Via-LASER), and the CNRS PICS Program No. 45052. A.R. is grateful for partial support by the Australian Research Council’s (Discovery Project DP 120102980) and by the Air Force Office of Scientific Research (USA, Grant No. FA 9550-12-1-0482).

Direct measurement of ambipolar diffusion in bulk silicon by ultrafast infrared imaging of laser-induced microplasmas

Carrier kinetics in the density range of N = 10(17) - 10(20) cm(-3) is investigated inside the bulk of crystalline silicon. Most conventional experimental techniques used to study carrier mobility are indirect and lack sensitivity because of charging effects and recombination on the surface. An all optical technique is used to overcome these obstacles. By focusing 1.3-mu m femtosecond laser pulses in the volume, we inject an initial free-carrier population by two-photon absorption. Then, we use pump-and-probe infrared microscopy as a tool to obtain simultaneous measurements of the carrier diffusion and recombination dynamics in a microscale region deep inside the material. The rate equation model is used to simulate our experimental results. We report a constant ambipolar diffusion coefficient D-a of 2.5 cm(2) s(-1) and an effective carrier lifetime tau(eff) of 2.5 ns at room temperature. A discussion on our findings at these high-injection levels is presented

High repetition rate (100 Hz), high peak power, high contrast femtosecond laser chain

High intensity femtosecond laser are now routinely used to produce energetic particles and photons via interaction with solid targets. However, the relatively low conversion efficiency of such processes requires the use of high repetition rate laser to increase the average power of the laser-induced secondary source. Furthermore, for high intensity laser-matter interaction, a high temporal contrast is of primary importance as the presence of a ns ASE pedestal (Amplified Spontaneous Emission) and/or various prepulses may significantly affect the governing interaction processes by creating a pre-plasma on the target surface. We present the characterization of a laser chain based on Ti:Sa technology and CPA technique, which presents unique laser characteristics : a high repetition rate (100 Hz), a high peak power (>5 TW) and a high contrast ratio (ASE<10-10) obtained thanks to a specific design with 3 saturable absorbers inserted in the amplification chain. A deformable mirror placed before the focusing parabolic mirror should allow us to focus the beam almost at the limit of diffraction. In these conditions, peak intensity above 1019W.cm-2 on target could be achieved at 100 Hz, allowing the study of relativistic optics at a high repetition rate.

Crossing the threshold of ultrafast laser writing in bulk silicon

An important challenge in the field of three-dimensional ultrafast laser processing is to achieve permanent modifications in the bulk of silicon and narrow-gap materials. Recent attempts by increasing the energy of infrared ultrashort pulses have simply failed. Here, we establish that it is because focusing with a maximum numerical aperture of about 1.5 with conventional schemes does not allow overcoming strong nonlinear and plasma effects in the pre-focal region. We circumvent this limitation by exploiting solid-immersion focusing, in analogy to techniques applied in advanced microscopy and lithography. By creating the conditions for an interaction with an extreme numerical aperture near 3 in a perfect spherical sample, repeatable femtosecond optical breakdown and controllable refractive index modifications are achieved inside silicon. This opens the door to the direct writing of three-dimensional monolithic devices for silicon photonics. It also provides perspectives for new strong-field physics and warm-dense-matter plasma experiments.Ultrafast laser processing is a versatile three-dimensional photonic structuring method but it has been limited to wide band gap materials like glasses. Here, Chanal et al. demonstrate direct refractive-index modification in the bulk of silicon by extreme localization of the energy deposition.

Thin films characterizations to design high-reflective coatings for ultrafast high power laser systems

Dielectrics as single layers and broadband high-reflective stacks were deposited by electron beam deposition processes compatible with 1-meter class optics. After being physically and optically characterized, samples were irradiated with several ultrafast lasers (KYW:Yb 500fs, Ti:Sa 40fs and Ti:Sa 11fs) with single and multi-pulses. The setups of the test platforms, laser-induced damage threshold investigations of intrinsic materials, dielectric multilayers and hybrid metal/dielectric multilayers and electric field intensity distributions are described.

Impact of the pulse contrast ratio on molybdenum K α generation by ultrahigh intensity femtosecond laser solid interaction

We present an extended experimental study of the absolute yield of Kα x-ray source (17.48 keV) produced by interaction of an ultrahigh intensity femtosecond laser with solid Mo target for temporal contrast ratios in the range of 1.7 × 107–3.3 × 109 and on three decades of intensity 1016–1019  W/cm². We demonstrate that for intensity I ≥ 2 × 1018  W/cm² Kα x-ray emission is independent of the value of contrast ratio. In addition, no saturation of the Kα photon number is measured and a value of ~2 × 1010 photons/sr/s is obtained at 10 Hz and I ~1019  W/cm². Furthermore, Kα energy conversion efficiency reaches the same high plateau equal to ~2 × 10−4 at I = 1019  W/cm² for all the studied contrast ratios. This original result suggests that relativistic J × B heating becomes dominant in these operating conditions which is supposed to be insensitive to the electron density gradient scale length L/λ. Finally, an additional experimental study performed by changing the angle of incidence of the laser beam onto the solid target highlights a clear signature of the interplay between collisionless absorption mechanisms depending on the contrast ratio and intensity.

Exceeding the bulk modification threshold of silicon with hyper-focused infrared femtosecond laser pulses

Ultra-short breakdown in the bulk of transparent materials has been intensively investigated in the last years, especially in dielectrics [1]. Whereas three-dimensional (3D) femtosecond laser micromachining in dielectrics is highly advanced, it remains extremely challenging in narrow bandgap materials such as silicon (Si). Recent numerical and experimental investigations show that only an underdense microplasma can be generated inside the bulk of crystalline Si by two-photon absorption [2]. The energy deposition inside the material is drastically limited by significant losses in the prefocal region and strong plasma effects. We provide in this study an experimental evidence of this optical limitation by 3D-imaging of the beam propagation with 60-fs, 1300-nm pulses focused 1 mm under the Si-surface. Even for high Numerical Aperture (NA) (up to 0.65), we observe a strict clamping of the delivered energy far below the level needed for material breakdown. For comparison, the horizontal line in Fig.1.a is the measured fluence threshold for surface breakdown, which can be taken as the minimal target for breakdown in the bulk.

Impact of high contrast ratio on 17.4 keV laser-based K α source for intensities 10 16 –10 19 W/cm 2 : A foot print of interaction mechanisms

Laser-driven ultrafast hard x-ray sources based on K<inf>α</inf> emission from solid targets are nowadays a powerful tool to monitor ultrafast lattice dynamics in condensed matter [1]. Such point-like sources are of great interest in biomedical imaging where spatial resolution and image quality can be highly improved [2]. Therefore, the development of high-flux K<inf>a</inf> sources with a small size remains important for the ultrafast x-ray community. Especially, with the routinally obtained now femtosecond laser performances (intensities I > 10¹⁸ W/cm² with high contrast pulses >10⁸), it is possible to explore interaction regimes where K<inf>α</inf> emission can be strongly enhanced.

High repetition rate (100Hz), high peak intensity (10 19 W.cm −2 ), high contrast femtosecond laser chain

High intensity femtosecond lasers are now routinely used to produce energetic particles and photons via interaction with solid targets [1-2]. However, the relatively low conversion efficiency of such processes requires the use of high repetition rate laser to increase the average power of the laser-induced secondary sources [3]. Furthermore, for high intensity laser-matter interaction, a high temporal contrast is of primary importance as the presence of an ASE pedestal (Amplified Spontaneous Emission) and/or various prepulses may significantly affect the laser-matter interaction processes by creating a pre-plasma on the target surface.

High Damage Threshold Meter-scale Optical Components for Multi-PW Lasers

Dielectric mirrors, hybrid metal-dielectric and a new generation of hybrid metal-dielectric gratings with high damage threshold suitable for 10 PW laser systems are presented.