Showing papers by "Gerard Mourou published in 2004"

Highly efficient relativistic-ion generation in the laser-piston regime.

Abstract: An intense laser-plasma interaction regime of the generation of high density ultrashort relativistic ion beams is suggested. When the radiation pressure is dominant, the laser energy is transformed efficiently into the energy of fast ions.

Generation and characterization of the highest laser intensities (10(22) W/cm2).

Abstract: We generated a record peak intensity of 0.7×1022 W/cm2 by focusing a 45-TW laser beam with an f/0.6 off-axis paraboloid. The aberrations of the paraboloid and the low-energy reference laser beam were measured and corrected, and a focal spot size of 0.8 µm was achieved. It is shown that the peak intensity can be increased to 1.0×1022 W/cm2 by correction of the wave front of a 45-TW beam relative to the reference beam. The phase and amplitude measurement provides for an efficient full characterization of the focal field.

Optics at critical intensity: Applications to nanomorphing

Abstract: Laser-induced optical breakdown by femtosecond pulses is extraordinarily precise when the energy is near threshold. Despite numerous applications, the basis for this deterministic nature has not been determined. We present experiments that shed light on the basic mechanisms of light-matter interactions in this regime, which we term "optics at critical intensity." We find that the remarkably sharp threshold for laser-induced material damage enables the structure or properties of materials to be modified with nanometer precision. Through detailed study of the minimum ablation size and the effects of polarization, we propose a fundamental framework for describing light-matter interactions in this regime. In surprising contrast to accepted damage theory, multiphoton ionization does not play a significant role. Our results also reject the use of the Keldysh parameter in predicting the role of multiphoton effects. We find that the dominant mechanism is Zener ionization followed by a combination of Zener and Zener-seeded avalanche ionization. We predict that the minimum feature size ultimately depends on the valence electron density, which is sufficiently high and uniform, to confer deterministic behavior on the damage threshold even at the nanoscale. This behavior enables nanomachining with high precision, which we demonstrate by machining highly reproducible nanometer-sized holes and grooves in dielectrics.

Relativistic generation of isolated attosecond pulses in a lambda 3 focal volume.

Abstract: Lasers that provide an energy encompassed in a focal volume of a few cubic wavelengths (lambda(3)) can create relativistic intensity with maximal gradients, using minimal energy. With particle-in-cell simulations we found, that single 200 attosecond pulses could be produced efficiently in a lambda(3) laser pulse reflection, via deflection and compression from the relativistic plasma mirror created by the pulse itself. An analytical model of coherent radiation from a charged layer confirms the pulse compression and is in good agreement with simulations. The novel technique is efficient (approximately 10%) and can produce single attosecond pulses from the millijoule to the joule level.

Attosecond electron bunches.

Abstract: Electron bunches of attosecond duration may coherently interact with laser beams. We show how $p$-polarized ultraintense laser pulses interacting with sharp boundaries of overdense plasmas can produce such bunches. Particle-in-cell simulations demonstrate attosecond bunch generation during pulse propagation through a thin channel or in the course of grazing incidence on a plasma layer. In the plasma, due to the self-intersection of electron trajectories, electron concentration is abruptly peaked. A group of counterstream electrons is pushed away from the plasma through nulls in the electromagnetic field, having inherited a peaked electron density distribution and forming relativistic ultrashort bunches in vacuum.

Study of hard x-ray emission from intense femtosecond Ti:sapphire laser–solid target interactions

Abstract: Interaction of intense Ti:sapphire laser with solid targets has been studied experimentally by measuring hard x-ray and hot electron generation. Hard x-ray (8–100 keV) emission spectrum and Kα x-ray conversion efficiency (ηK) from plasma have been studied as a function of laser intensity (1017–1019 W/cm2), pulse duration (70–400)fs, and laser pulse fluence. For intensity I>1×1017 W/cm2, the Ag ηK increases to reach a maximum value of 2×10−5 at an intensity I=4×1018 W/cm2. Hot electron temperature (KTh) and ηK scaling laws have been studied as a function of the laser parameters. A stronger dependence of KTh and ηK as a function of the laser fluence than on pulse duration or laser intensity has been observed. The contribution of another nonlinear mechanism, besides resonance absorption, to hard x-ray enhancement has been demonstrated via hot electron angular distribution and particle-in-cell simulations.

High-energy ion generation by short laser pulses

Abstract: This paper reviews the many recent advances at the Center for Ultrafast Optical Science (CUOS) at the University of Michigan in multi-MeV ion beam generation from the interaction of short laser pulses focused onto thin foil targets at intensities ranging from 10 17 to 10 19 W/cm 2 . Ion beam characteristics were studied by changing the laser intensity, laser wavelength, target material, and by depositing a well-absorbed coating. We manipulated the proton beam divergence using shaped targets and observed nuclear transformation induced by high-energy protons and deuterons. Qualitative theoretical approaches and fully relativistic two-dimensional particle-in-cell simulations modeled energetic ion generation. Comparison with experiments sheds light on ion energy spectra for multi-species plasma, the dependences of ion-energy on preplasma scale length and solid density plasma thickness, and laser-triggered isotope yield. Theoretical predictions are also made with the aim of studying ion generation for high-power lasers with the energies expected in the near future, and for the rel- ativistic intensity table-top laser, a prototype of which is already in operation at CUOS in the limits of several- cycle pulse duration and a single-wavelength spot size. © 2004 MAIK "Nauk a/ Interperiodica".

Dependence of hard x-ray yield on laser pulse parameters in the wavelength-cubed regime

Abstract: Conversion efficiency and electron temperature scaling laws are experimentally studied in the wavelength-cubed (λ3) regime, where a single-wavelength focus allows low energy pulses incident on a Mo target to produce x rays with excellent efficiency and improved spatial coherence. Focused intensity is varied from 2×1016 to 2×1018 W/cm2. Conversion efficiency and electron temperature are best described by a power law for energy scaling while an exponential law best describes the scaling of these parameters with pulse duration.

Method for forming nanoscale features

Abstract: Here is presented a versatile technique for machining of nanometer-scale features using tightly-focused ultrashort laser pulses. By the invention, the size of features can be reduced far below the wavelength of light, thus enabling nanomachining of a wide range of materials. The features may be extremely small (<20 nm) and are highly reproducible.

Phase-matched enhancements of high-harmonic soft X-rays by adaptive wave-front control with a genetic algorithm

Abstract: We demonstrate the enhancement of high-harmonic soft-X-ray generation by adaptive wave-front control of a 745-nm Ti:sapphire laser by use of a 59-channel membrane deformable mirror, combined with a genetic algorithm, for the first time to our knowledge. The harmonics ranging from 17 nm to 28 nm were enhanced by factors up to 13. The numerical calculations illustrate that the enhancements of the plateau harmonics are due to macroscopic phase-matching effect, whereas those of the cutoff harmonics are due to the increase in focal intensity.

Generation and characterization of the highest laser intensities

Abstract: 2by focusing a 45-TW laser beam with an f0.6 off-axis paraboloid. The aberrations of the paraboloid and the low-energy reference laser beam were measured and corrected, and a focal spot size of 0.8 mm was achieved. It is shown that the peak intensity can be increased to 1.0 3 10 22 Wcm 2 by correction of the wave front of a 45-TW beam relative to the reference beam. The phase and amplitude measurement provides for an efficient full characterization of the focal field. © 2004 Optical Society of America OCIS codes: 010.1080, 140.3590, 320.7090.

Isolated attosecond pulses generated by relativistic effects in a wavelength-cubed focal volume

Abstract: Lasers that provide an energy encompassed in a focal volume of a few cubic wavelengths (λ3) can create relativistic intensity with maximal gradients using minimal energy. With particle-in-cell simulations we found that single 200-as pulses could be produced efficiently in a λ3 laser pulse reflection by means of deflection and phase compression caused by the coherent motion of the plasma electrons that emit these pulses. This novel technique is efficient (∼10%) and can produce single attosecond pulses from the millijoule to the joule level.

Fiber laser-based euv-lithography

Abstract: A method and apparatus is disclosed for performing lithography operation. A fiber laser (18) is provided that generates laser light that is used by adaptive optics (20) to focus the laser light onto a plasma target (30) to generate plasma as a source of EUV radiation.

Effects of wavelength and doping concentration on silicon damage threshold

Abstract: Our damage experiment shows threshold fluence does not decrease with increasing doping concentration and its dependence on wavelength is weak, with photon energies above or below bandgap. Our results indicate impact ionization is the dominating mechanism.

Relativistic optics: A gateway to attosecond physics

Abstract: Compact lasers can now produce intensities so large that laser-matter interactions are dominated by the relativistic character of the electrons. The new field of relativistic optics offers a gateway to attosecond physics and nonlinear quantum electrodynamics.

Laser femtoseconde : système de micro-usinage pour chirurgie de la cornée

Abstract: Introduction Les auteurs presentent un systeme experimental de chirurgie automatisee par laser femtoseconde Nd:verre. Materiel et methodes Nous utilisons un laser infrarouge a 1 065 nm. Ce laser comporte un oscillateur laser et un amplificateur constitue d’une matrice de verre dopee au Neodyme. Le systeme d’etirement et de compression des impulsions est constitue d’un systeme de reseaux pouvant produire des impulsions de quelques centaines de femtosecondes. Le taux de repetition des impulsions est ajustable de 1 a 10 kHz avec une energie maximale par impulsion voisine de 60 ∝J. La delivrance des impulsions lasers s’effectue a travers un objectif adapte installe sur une table optique. Les echantillons sont fixes sur une chambre anterieure mobilisee dans les trois axes X-Y-Z par des moteurs de precision micrometrique. Afin d’evaluer les effets biologiques des impacts lasers sur le tissu corneen, nous avons dans un premier temps caracterise les degâts collateraux sur des echantillons de dioxyde de silicone en microscopie electronique a balayage. Les cornees humaines utilisees sont delivrees par la Banque Francaise des yeux. Resultats Le systeme experimental permet la realisation de decoupes entierement automatisees dans un milieu transparent. La grande souplesse de reglage des parametres de traitement nous permet de determiner le seuil de claquage optique, de caracteriser les degâts collateraux et d’apprecier la regularite de ces decoupes. Conclusion Le developpement de la chirurgie automatisee passe par l’utilisation de lasers, le laser femtoseconde etant tout a fait adapte a la chirurgie corneenne. Cependant, il est necessaire de disposer de systemes d’experimentation tels que celui presente dans cette etude, afin d’etudier precisement les interactions de ce laser sur la matiere pour optimiser les parametres de traitement.

Femtosecond laser micromachining of single-crystal superalloys

Abstract: Investigations on femtosecond laser micromachining of single crystal superalloys with and without plasma-sprayed thermal barrier coatings were conducted under laser fluences ranging from 0.1 J/cm 2 up to 160 J/cm 2 . Micromachining was carried out in air using a titanium:sapphire laser system (O = 780 nm) operating at a repetition rate of 1 kHz and delivering individual pulses of ~150 fs duration. The ablation threshold of the single crystal superalloy was determined as 203 ± 20 mJ/cm 2 . Laser-induced damage was examined by means of scanning electron microscopy and transmission electron microscopy. These studies indicate a complete absence of any melting, recast layers, heat-affected zones or microcracks in the vicinity of the machining area. The only form of damage observed in the single crystal superalloy machined near or above the ablation threshold was a laser-induced plastically deformed layer with a maximum extent of ~5 Pm. Machining through ceramic thermal barrier coatings on a superalloy produced no delamination along the superalloy/coating interfaces or cracks within the TBC or bond coat. The residual roughness of the machined surface was in the sub-micron range. The present study suggests that femtosecond laser micromachining is a very promising technique for production of finescale features in multi-layer material systems for aerospace and power generation components.

Ultrafast laser-based micro-CT system for small animal imaging

Abstract: We investigated ultrafast laser-based x-ray (ULX) source as an attractive alternative to a microfocal x-ray tube used in micro-CT systems. The laser pulse duration was in the 30 fs-200 fs range, the repetition rate in the 10 Hz - 1 kHz range. A number of solid targets including Ge, Mo, Rh, Ag, Sn, Ba, La, Nd with matching filters was used. We optimized conditions for x-rays generation and measured: x-ray spectra, conversion efficiency (from laser light to x-rays), x-ray fluence, effective x-ray focal spot size and spatial resolution, contrast resolution and radiation dose. Good quality projection images of small animals in single-and dual-energy mode were obtained. ULX generates narrow x-ray spectra that consist mainly of characteristic lines that can be easily tailored (by changing laser beam target) to the imaging task, (e.g. to maximize contrast while minimizing radiation dose). X-ray fluence can exceed fluence produced by conventional microfocal tube with 10 μm focal-spot hence allowing for faster scans with very high spatial resolution. Changing the laser target, and thus matching the characteristic emission lines with the investigated animal's thickness and composition, can be done quickly and easily. Using narrow emission lines for imaging, instead of broad bremsstrahlung, offers superior dose utilization and limits beam-hardening effects. Employing two narrow emission lines-above and below the absorption edge of a contrast agent-in quick succession allows dual-energy-subtraction micro-CT for imaging with a contrast medium. Dual-energy-subtraction is not practical with a microfocal tube. Compact, robust, ultrafast lasers are commercially available, and their characteristics are rapidly improving. We plan to construct a prototype in vivo ultrafast laser-based micro-CT system.

Generation, amplitude and phase characterization of 10/sup 21/ W/cm/sup 2/ intensity

Abstract: The intensity of 10/sup 21 /W/cm/sup 2/ was generated by correcting F/l off-axis paraboloid aberration in 30 TW diffraction limited laser system. The importance of using diffraction calculation over spot size measurement in determining intensity is discussed.

“Cascade-Raman” soliton compression with 30-fs, Terawatt pulses

Abstract: Theoretically, Raman-like soliton compression of mJ, 30-fs pulses is predicted. Initial experiments demonstrate 2 times compression of 1.3 TW/cm2 pulses under non-optimal conditions. The results agree closely with numerical simulations.

Femtosecond laser: incident energy thresholds and corneal hydration

Abstract: Purpose: The interacting phenomena that happen between femtosecond lasers in edematous or pathological corneas are still not clearly known. Their characterization is fundamental to develop femtosecond laser use for penetrating and lamellar keratoplasty. The plasma threshold corresponds to the minimal energy that must be focalised into the cornea in order to induce ionization of matter. The possibility of making one corneal cut depends on plasma creation. We determined incident energy thresholds to obtain plasma in function of different corneal depths and hydrations. Methods: We use a Nd:Glass femtosecond laser, with a CPA system, and 1065 nm wavelength. The repetition rate is adjustable from 1 to 10 KHz, with maximum pulse energy of 60 µJ. The pulse delivery is done through an objective adapted on an optic table. The samples are fixed to an anterior chamber system and moved 3–dimensionally by micrometric precision motors. Human corneas are obtained from the French Eye Bank. Three corneal groups with 10 corneas each are organized depending on corneal pachymetry: Group 1: 100 µm +/– 75 µm, Group 2: 700 µm +/– 45 µm, Group 3: 500 µm +/– 25 µm. The control group is composed of silicon dioxide samples. The samples are analyzed in confocal microscopy of the experimental set up. Results: The incident energy threshold is similar in the control group and in the group 3; it ranges from 5 J/cm² on surface to 10 J/cm² at 500 µm depth. In cornea Groups 2 and 1, the incident energy threshold increases, respectively, of 5 J/cm² and of 10 J/cm² for each 100 µm of corneal depth that the laser beam travels through. Conclusions:In each given group, the incident energy threshold increases proportionally to the corneal thickness the laser beam traveled through. To the same corneal thickness, the threshold is higher in the most edematous corneas. The knowledge of these results is fundamental in order to determinate cut parameters and side effects in corneas with hydration above the normal.

Hard x-ray emission from intense femtosecond laser-solid target interactions

Abstract: Hard x-ray (8-100 keV) spectrum emission from plasma produced by femtosecond laser solid target interactions and K 2 x-ray conversion efficiency have been studied as a function of laser intensity (10 17 W/cm ~ 10 19 W/cm 2 ), pulse duration (70 fs ~ 400 fs), laser pulse fluence and laser wavelength (800 nm and 400 nm). The Ag K 18 x-ray conversion efficiency produced by a laser pulse at 800 nm with an intensity I = 4x10 W/cm 2 can reach 2x10 -5 . We discuss the behaviour of K conversion efficiency scaling laws as a function of the laser parameters. We found that the K x-ray conversion efficiency is more dependent on laser fluence than on pulse duration or laser pulse intensity. The conversion efficiency exhibits a similar value at I ~ 1x10 18 W/cm 2 when we work with a high contrast laser pulse at 400 nm or with a low contrast laser pulse at 800 nm, but in the first case it presents a higher scaling law. Consequently, the use of 400 nm laser pulses could be an effective method to optimize the K x-ray emission via vacuum heating mechanisms.

Femtosecond laser: cavitation bubbles analysed by confocal microscopy

Abstract: Purpose: When a femtosecond laser beam causes photodisruption, a cavitation bubble is produced. Many laser shots may be placed close together, and the induced cavitation bubbles coalesce to create a cut. Reduction of the bubble diameter is fundamental to improving the cut quality. Methods: A home–built Nd:Glass femtosecond laser is used with a CPA system and 1065 µm wavelength. The pulse repetition rate ranges from 1 to 10 KHz, with a maximum pulse energy of about 60 µJ. The laser delivery system works through an objective lens on an optical table. The samples are fixed to a 3–dimensional moving anterior chamber system, which is connected to a micrometric stepper motor. Human corneas are obtained from the French Eye Bank and are divided into 2 groups, according to their pachymetry: Group 1 (n=10): 1000 µm +/– 75 µm, and Group 2 (n=10): 700 µm +/– 45 µm. The control group is composed of silicon dioxide samples. The treated corneas are analyzed through confocal microscopy. Results: At plasma threshold, the average cavitation bubble diameter is: 6 µm +/– 7µm in the Group 1 corneas, 25 µm +/– 1.5 µm in the Group 2 corneas and 2 +/– 1 µm in the silicon dioxide samples. In group 2 corneas and silicon dioxide samples, the majority of laser pulses generated cavitation bubbles. In group 1 corneas, far fewer of the laser pulses induced cavitation bubbles. The time it takes for one cavitation bubble to disappear is: 90 +/– 10 seconds for a 25 µm cavitation bubble, 25 +/– 5 seconds for a 6 µm cavitation bubble, and 10 minutes in the silicon samples. Conclusions: In more edematous corneas, the cavitation bubble diameter is heterogeneous. This heterogeneity is due to laser beam attenuation by the corneal edema. This information suggests that, in the absence of laser parameter adaptation, the cut quality in edematous and pathological corneas will be less than in clear corneas.

Setup for femtosecond laser corneal surgery

Abstract: Purpose: The authors present a microsurgery experimental system using Nd: Glass femtosecond laser. It is coupled to a confocal microscopy observation system to investigate intrastromal effects of the femtosecond laser. Methods: The femtosecond laser is a CPA system with a regenerative amplifier delivering pulses at a wavelength of 1.065 µm, with a pulse duration of 500 fs, an energy of 60 µJ for a repetition rate of 10 KHz. The amplified pulses power fluctuations are lower than 1%. Pulses repetition rate is adjustable up to 10kHz and can even be reduced to single shot. The laser beam has a Gaussian distribution shape TEM00. The obtainable peak power density is on the order of 10^15W/cm^2. The laser beam travels to the samples guided by one six–lens set and one magnifier. The last lens can be changed to adjust the focal distance from 2 to 5 cm and the focal spot from 1 to 4 µm. The samples are fixed to an anterior chamber system, which is driven by a micrometric precision motor system. The 3 dimensional movements are computer controlled. The laser energy is continuously monitor for measurements consistency. The experimental set up is also composed of a confocal microscope, which permits corneal sample analysis without removing them from the artificial chamber system. Results: The experimental set up permits complex geometry to be done. Penetrating and lamellar cuts could be performed in silicon dioxide and human corneal samples. The confocal microscopy observation system permits precise parameter control of the cutting system (cut shape, shot scanning and treatment depth). Conclusions: Applications for femtosecond laser are the refractive surgery and corneal keratoplasty. The development of concepts as sutureless keratoplasty needs an experimental system with large flexibility in order to work out useful cut parameters.

Isolated Attosecond Pulse Generation in the Relativistic λ 3 Regime by Reflection/Deflection/Compression

Abstract: Lasers that provide a focal energy encompassed by a volume of a few cubic wavelengths (λ3) can produce relativistic intensity with maximal gradients, using minimal energy. With particle-in-cell simulations we found, that single 200 attosecond pulses could be efficiently generated in a λ3 laser pulse reflection, via deflection and compression from the relativistic plasma mirror driven by the pulse itself. An analytical model of coherent radiation from a charged layer confirms the pulse compression and is in good agreement with simulations. This novel technique is efficient (~10%) and can produce single attosecond pulses from the millijoule to the joule level.

Regenerative Amplification of 55-Femtosecond Pulses at a 1 kHz Repetition Rate: Model and Experiment

Abstract: Chirped-pulse amplification is used to generate 0.35 mJ, 55 fs pulses in Ti:Al2O3. We discuss the modeling done to produce this result and the limitations of the methods used. The use of a kHz post-amplifier is also discussed.

Confining collateral damage to less than 10 nm during femtosecond laser machining

Abstract: We have ablated holes in pre-thinned Si foils using an ultrafast laser. High resolution imaging of the edges of these holes reveal collateral damage limited to within 10 nanometers.