Showing papers by "Gerard Mourou published in 2007"

Relativistic laser-matter interaction: from attosecond pulse generation to fast ignition

Abstract: The field of laser?matter interaction has branched out in two main directions. The first, motivated by laser inertial confinement fusion, warm-dense-matter, fast ignition and astrophysics in laboratory, and the second driven by ultra-high intensity, exotic physics, high-energy particle, photon beam generation and time-resolved attosecond (zeptosecond) science. The degree of maturity from both experimental and theoretical stand-points is such that a large European infrastructure for each branch is contemplated as part of the European Roadmap. The first one, HiPER-PETAL will be dedicated to fast ignition with the aim of obtaining a thermonuclear gain of 100, whereas the second, Extreme Light Infrastructure (ELI) could go beyond the relativistic regime to foray into the ultra-relativistic domain >1024?W?cm?2. In this paper we highlight the intriguing perspectives that these two projects will offer.

In situ monitoring of second-harmonic generation in human corneas to compensate for femtosecond laser pulse attenuation in keratoplasty

Abstract: The application of femtosecond lasers in corneal transplant surgery requires high pulse energies to compensate for the strong optical scattering in pathological corneas. However, excessive energies deteriorate the quality of the incisions. The aim of this study is to demonstrate the dependence of side effects on local radiant exposure, numerical aperture, and tissue properties, to quantify the penetration depth of the laser for individual corneas, and to provide a method for optimizing the energy in the volume of the cornea. We examine histological and ultrastructural sections of clear and edematous corneas with perforating and lamellar incisions performed at different pulse energies. We demonstrate that the augmented energies in edematous corneas may result in unwanted side effects even when using high numerical apertures. The dependence of the laser beam penetration depth on pulse energy is evaluated by histology and an exponential decrease is observed. We show that the penetration length can be determined by evaluating the backscattered second-harmonic emission associated with the nonlinear optical properties of the tissue. This approach represents a noninvasive method for the in situ quantification of the laser beam attenuation, enabling us to adapt the pulse energy accordingly. Experiments using adapted energies show that the side effects are minimized.

Nanochannels fabricated by high-intensity femtosecond laser pulses on dielectric surfaces

Abstract: Direct scanning electron microscopy examination reveals a complex structure of narrow, micron-deep, internal nanochannels within shallow, nanoscale, external craters fabricated on glass and sapphire surfaces by single high-intensity femtosecond laser pulses, with nearly the same intensity thresholds for both features. Formation of the channels is accompanied by extensive expulsion of molten material produced via surface spallation and phase explosion mechanisms, and redeposited around the corresponding external craters. Potential mechanisms underlying fabrication of the unexpectedly deep channels in dielectrics are considered.

Energy scaling of quasi-monoenergetic electron beams from laser wakefields driven by 40-TW ultra-short pulses

Abstract: By focusing 40-TW, 30-fs laser pulses to the peak intensity of 1019 W/cm2 onto a supersonic He gas jet, we generate quasi-monoenergetic electron beams for plasma density in the specific range 1.5×1019 cm-3≤ne≤3.5×1019 cm-3. We show that the energy, charge, divergence and pointing stability of the beam can be controlled by changing ne, and that higher electron energies and more stable beams are produced for lower densities. The observed variations are explained physically by the interplay among pump depletion and dephasing between accelerated electrons and plasma wave. Two-dimensional particle-in-cell simulations support the explanation by showing the evolution of the laser pulse in plasma and the specifics of electron injection and acceleration. An optimized quasi-monoenergetic beam of over 300 MeV and 10 mrad angular divergence is demonstrated at a plasma density of ne≃1.5×1019 cm-3.

Generation of hard X-rays using an ultrafast fiber laser system.

Abstract: We report the first hard X-ray source driven by a femtosecond fiber laser. The high energy fiber CPA system incorporated a 65μm LMA fiber amplifying stage which provided 300-fs recompressed pulses and diffraction limited beam quality with M2 < 1.07. A deformable mirror was used to optimize the wavefront and the spot size was focused down to 2.3 μm with an f/1.2 paraboloidal mirror. 50μJ was deposited on the nickel target with 2×1015-W/cm2 focal intensity and a distinctive Ni Kα-line (7.48 keV) emission was measured with 5×10-8 energy conversion efficiency.

Superresolved femtosecond laser ablation.

Abstract: The determinist behavior of the femtosecond ablation process allows morphing features well under the diffraction limit by utilizing the thresholding effect, down to the nanometer scale. Because there are a vast range of applications where scaling down the size of the features is a major concern, we investigate the use of superresolving pupil plane filters. As is well known, these filters redistribute the focused optical intensity for a narrower bright spot and, as a trade-off, increase the sidelobes. However, this drawback can be rendered insignificant if all the outer optical power is kept under the determinist threshold value. Two types of pure absorbing binary filter have been tried, giving credence to a size reduction of the ablations in fused silica.

Methods of forming microchannels by ultrafast pulsed laser direct-write processing

Abstract: Microscale and/or nanoscale structural features are formed in material by using an ultrashort pulsed laser (e.g., a femtosecond pulsed laser). Methods of forming such structures comprise applying laser energy generated by an ultrashort pulsed laser to a surface of a substrate having a first layer and a distinct second layer. The first layer has a first ablation threshold that is less than the applied laser energy and the second layer has a second ablation threshold that is greater than the applied laser energy. The laser energy penetrates through the second layer to the first layer. The applied laser energy results in damage (or an ablation event) at the first layer that exerts force sufficient to delaminate the second layer, thereby forming a void space that is a channel having a major elongate axis or alternately forming an open groove.

ILE 25PW single laser beamline: The French step for the European Extreme Light Infrastructure (ELI)

Abstract: We present the design of a single ultra intense laser beamline delivering 25PW pulses at one shot per minute as a first step of an ultra intense high field science European project (extreme light infrastructure).

Femtosecond laser corneal surgery with in situ determination of the laser attenuation and ablation threshold by second harmonic generation

Abstract: Femtosecond lasers start to be routinely used in refractive eye surgery. Current research focuses on their application to glaucoma and cataract surgery as well as cornea transplant procedures. To avoid unwanted tissue damage during the surgical intervention it is of utmost importance to maintain a working energy just above the ablation threshold and maintain the laser energy at this working point independently of the local and global tissue properties. To quantify the attenuation of the laser power density in the tissue by absorption, scattering and modification of the point spread function we monitor the second harmonic radiation generated in the collagen matrix of the cornea when exposed to ultrashort laser pulses. We use a CPA system with a regenerative amplifier delivering pulses at a wavelength of 1.06 mm, pulse durations of 400 fs and a maximum energy of 60 mJ. The repetition rate is adjustable from single shot up to 10 kHz. The experiments are performed on human corneas provided by the French Eye bank. To capture the SHG radiation we use a photomultiplier tube connected to a lockin amplifier tuned to the laser repetition rate. The measured data indicates an exponential decay of the laser beam intensity in the volume of the sample and allows for the quantification of the attenuation coefficient and its correlation with the optical properties of the cornea. Complementary analyses were performed on the samples by ultrastructural histology.

Femtosecond laser surface ablation of transparent solids: understanding the bulk filamentation damage

Abstract: Direct SEM examination reveals a comple x nanoscale structure of deep narrow central channels within shallow wide external craters produced by single-shot high-intensity femtosecond laser radiation on Corning 0211 glass and sapphire surfaces. These internal narrow channels are not expected from ordinary surf ace melt spallation and expulsion processes characteristic of the external surface nanocraters, but exhibit nearly the same appearance threshold. Surprisingly, the nanochannel radiuses rapidly saturate vers us incident laser intensity indicating bulk rather than surface character of laser energy deposition, in contrast to the external craters extending versus laser intensity in a regular manner. These facts may be explained by channeling of electromagnetic radiation by near-surface ablative filamentary propagation of in-tense femtosecond laser pulses in the highly electronically excited dielectrics, by spherical aberrations in the surface layer, or deep drilling of the samples by short-wavelength Bremsstrahlung radi ation of relatively hot surface electron-hole or electron-ion plasma. The double st ructure of ablated surface na no-features is consistent with similar structures observed for bulk damage features fabricated by femtosecond laser pulses at supercritical laser powers, but much lower laser intensities. Keywords: ultrashort (femtosecond) laser pulses; high NA focusing; dielectric surfaces; dense electron-hole plasma; ultrafast electronic bandgap renormalization; high-order susceptibilities; damped nanofilaments; spherical aberra-tions; short-wavelength thermal emission of dense hot surface electron-ion plasma; surface and bulk damage fea-tures

Method and device for machining a target using a femtosecond laser beam

Abstract: The invention relates to a method and device for machining a target using a femtosecond laser beam. The invention consists in taking advantage of the deterministic nature of the ablation threshold and the nonlinear dependence thereof through the use of amplitude or phase pupillary filtering using the polarising effect or any other technique in order to reduce significantly the machining dimensions obtained by focusing a laser beam in nanotechnologies. One such filtering process modifies the distribution of the intensity in the focal plane such as to reduce the maximum of the central component of the spectrum of laser pulses while maintaining the bright rings below the deterministic ablation threshold. The invention associates the femtosecond ablation technique with deterministic threshold and the apodisation technique.

The Exawatt Laser: From Relativistic to Ultra Relativistic Optics

Abstract: We will describe the European Extreme Light Infrastructure project (ELI) dedicated to the fundamental study of laser-matter interaction in a new and unsurpassed regime of laser intensity: the ultra-relativistic regime. These investigations will rely on the development of an exawatt-class laser ~100-1000 times more powerful than either the Laser Megajoule in France or the National Ignition Facility (NIF) in the US.

Procédé et dispositif d'usinage d'une cible par faisceau laser femtoseconde

Abstract: La presente invention consiste a tirer avantage du la nature deterministe du seuil d'ablation et de sa dependance non lineaire par l'utilisation des techniques de filtrage pupillaire, de phase ou d'amplitude, par effet polarisant ou toute autre technique afin de reduire significativement les dimensions des usinages obtenus par focalisation d'un faisceau laser dans les nanotechnologies Un tel filtrage modifie la repartition de l'intensite dans le plan focal de maniere a amincir le maximum de la composante centrale du spectre des impulsions laser tout en maintenant les anneaux brillants inferieurs au seuil d'ablation deterministe La presente invention associe notamment la technique d'ablation femtoseconde a seuil deterministe et la technique d'apodisation

Mechanisms of femtosecond laser nanomachining of dielectric surfaces

Abstract: Optical damage produced by femtosecond pulsed lasers on dielectric surfaces is extremely precise, allowing the damage mechanisms to be inferred from reproducible damage characteristics. Here nanoscale femtosecond laser ablation is applied to probe the ultrafast dynamics of laser energy deposition including the generation and transport of surface electron-hole or electron-ion plasmas. For shallow surface nano-craters fabricated on quartz and glass surfaces by single 0.53 mm or 1.05 mm laser shots, their corresponding well-defined laser intensity thresholds demonstrate pronounced effects of laser wavelength, crystalline state of the dielectric and laser waist radius, indicating equal importance of laser energy deposition and transport phenomena during ablation. Simultaneously, unusually deep surface nanoholes emerge drilled by self-focusing laser beam or forward-scattered highly penetrating short-wavelength radiation from the warm, dense surface plasma.

An efficient Ni K α X-ray source driven by a high energy fiber CPA system

Abstract: In this paper we report the first demonstration of efficient hard X-ray generation using a femtosecond fiber laser. The hard X-ray source has yield efficiencies similar to those obtained with solid state laser drivers with comparable pulse energies and intensities. Further experiments on increasing X-ray yield efficiency with higher pulse energies from the current system are in progress. Since this fiber laser technology is scalable to >100-W of average power this demonstration opens a path towards compact high-brightness hard X-ray sources for imaging and time resolved diffraction experiments.

High-energy few-cycle pulse generation in a filament for relativistic applications at kHz repetition rate

Abstract: We demonstrate efficient generation of 9.5-fs 1.8 mJ pulses by filamentation. The pulse wave front, the low energy fluctuations and the good temporal contrast make this source suited for relativistic laser-solid experiments at kHz repetition rate.