Showing papers by "Gerard Mourou published in 2000"

Smart microscope: an adaptive optics learning system for aberration correction in multiphoton confocal microscopy.

Abstract: Off-axis aberrations in a beam-scanning multiphoton confocal microscope are corrected with a deformable mirror. The optimal mirror shape for each pixel is determined by a genetic learning algorithm, in which the second-harmonic or two-photon fluorescence signal from a reference sample is maximized. The speed of the convergence is improved by use of a Zernike polynomial basis for the deformable mirror shape. This adaptive optical correction scheme is implemented in an all-reflective system by use of extremely short (10-fs) optical pulses, and it is shown that the scanning area of an f:1 off-axis parabola can be increased by nine times with this technique.

Generation of relativistic intensity pulses at a kilohertz repetition rate.

Abstract: By using adaptive optics to correct the wave-front distortion of a 21-fs, 0.7-mJ, 1-kHz laser, we are able to focus the pulses to a 1‐µm spot with an f/1 off-axis parabolic mirror. The peak intensity at the focal position is 1.5×1018 W/cm2, which is to the authors’ knowledge the first demonstration of generating relativistic intensity at a kilohertz repetition rate.

Applications of femtosecond lasers in corneal surgery

Abstract: We investigated potential applications of ultrashort (femtosecond) pulsed laser technology in corneal refractive surgery. When compared with longer pulsewidth nanosecond or picosecond laser radiation, femtosecond laser-tissue interactions are characterized by significantly smaller and more deterministic photodisruptive energy thresholds, as well as reduced shock waves and smaller cavitation bubbles. Femtosecond laser technology may be able to perform a variety of corneal refractive procedures with high precision, offering advantages over current mechanical and laser devices and techniques, enabling entirely new approaches for refractive surgery. An analytically solvable shell model has been developed to predict the results of completely intrastromal incisionless surgery for myopic and hyperopic refractive corrections.

Second-harmonic generation and wave-front correction of a terawatt laser system

Abstract: By combining second-harmonic generation and wave-front correction of a hybrid Ti:sapphire–Nd:glass terawatt laser chain, we were able to generate a focused intensity above 1019 W/cm2, with an estimated 109:1 intensity contrast ratio. The frequency of the laser is doubled by use of a type I KDP crystal, and wave-front correction is achieved with a deformable mirror coupled to a wave-front sensor.

Spectroscopy of solid density plasmas generated by irradiation of thin foils by a fs laser

Abstract: We present time-resolved spectroscopy of laser-produced plasmas achieved with a 500 fs resolution. Time history of Al K-shell emission from hot solid density plasmas produced by the interaction of an intense subpicosecond laser pulse (150–400 fs, 0.53 μm, 5×1018 W/cm2) with a thin free-standing foil target (500–1000 A) appears to change drastically with laser pulse duration, the X-ray emission duration decreasing with the heating pulse duration. We report here what we believe to be the first observations of subpicosecond thermal X-ray emissions (Al Heα, 500–700 fs) in laser-produced plasmas. Such results are of interest for applications to the observation of ultrafast phenomena in matter.

Dense and Relativistic Plasmas Produced by Compact High-Intensity Lasers

Abstract: High-intensity lasers interacting with plasmas are used to study processes in the laboratory that would otherwise only occur in astrophysics. These include relativistic plasmas, electron acceleration in ultrahigh field-gradient wake fields, pressure ionization and continuum lowering in strongly coupled plasmas, and X-ray line emission via Raman scattering.

Adaptive compensation of aberrations in ultrafast 3D microscopy using a deformable mirror

Abstract: 3D imaging using a multiphoton scanning confocal microscope is ultimately limited by aberrations of the system We describe a system to adaptively compensate the aberrations with a deformable mirror We have increased the transverse scanning range of the microscope by three with compensation of off-axis aberrationsWe have also significantly increased the longitudinal scanning depth with compensation of spherical aberrations from the penetration into the sample Our correction is based on a genetic algorithm that uses second harmonic or two-photon fluorescence signal excited by femtosecond pulses from the sample as the enhancement parameter This allows us to globally optimize the wavefront without a wavefront measurement To improve the speed of the optimization we use Zernike polynomials as the basis for correction Corrections can be stored in a database for look-up with future samples

High contrast 150-terawatt laser for high field laser-plasma interaction studies

Abstract: Summary form only given. High power chirped pulse amplification (CPA) lasers are capable of providing enough power to study relativistic regime-of laser-plasma interaction, including effects such as high-energy particle acceleration, relativistic self-focusing and nonlinear Thomson scattering. To achieve this regime focused laser intensity must be over 10/sup 18/ W/cm/sup 2/ (typically-10/sup 19/-10/sup 20/ W/cm/sup 2/). If solid metal target is irradiated the laser intensity contrast on the nanosecond timescale must be better than 12 orders of magnitude to ensure that the target does not disintegrate before the pulse arrives. To improve the pulse contrast in the 25 fs CPA laser system, we are currently building for high field laser-plasma studies at CUOS, we pursue an approach, originally developed at CUOS for longer pulses (/spl sim/100 fs). We pre-amplify the oscillator output pulse to submicrojoule energy level without stretching the pulse, improve the pulse contrast by using a saturable absorber and stretch the pulse thereafter for further amplification.

Subsurface photodisruption in scattering biological tissue

Abstract: Approximately five million people worldwide are blind due to complications from glaucoma. Current surgical techniques often fail due to infection and scarring. Both failure routes are associated with damaging surface tissues. Femtosecond lasers allow a method to create a highly precise incision beneath the surface of the tissue without damaging any of the overlying layers. However, subsurface surgery can only be performed where the beam can be focused tightly enough to cause optical breakdown. Under normal conditions, subsurface surgery is not possible since sclera is highly scattering. Using two independent methods, we show completely subsurface surgery in human sclera using a femtosecond laser. The first method is to make the sclera transparent by injecting a dehydrating agent. The second method is to choose a wavelength that is highly focusable in the sclera. Both methods may be applied in other tissues, such as skin. We show highly precise incisions in in vitro tissues. Subsurface femtosecond photodisruption may be useful for in vivo surgical technique to perform a completely subsurface surgery.

Isotope enrichment in highly-charged ionic species of ultrafast laser ablation plumes

Abstract: Summary form only given. As reported recently for boron and gallium, we have measured factors greater than 2 for isotopic enrichment in the multiple-charge state ions from ultrafast laser ablation plasmas. The earlier work has now been extended to the elements Zn, Ti, and Cu. These results confirm the enrichment of the lighter isotope on the target-normal axis of the expanding ablation plume. Angular scans of these plumes demonstrate the persistence of the enrichment effect to wide angle, being present as far out as 45 degrees from the normal direction. Systematic variations in the dominant ion species and their energy distributions, as a function of charge state, are observed in these angular scans.

Correction of the residual spectral phase in a CPA laser system using a deformable mirror in the stretcher

Abstract: Summary form only given. As the pulse duration is decreasing, it becomes mandatory to take into account the residual spectral phase that can't be corrected in CPA systems. Up to date CPA systems are well corrected for /spl phi//sub 2/, (chirp) and /spl phi//sub 3/. The remaining spectral phase defect reduces the contrast of the amplified pulse. We propose the use of a deformable mirror (DM) in a CPA setup to control the spectral phase. The DM is included in the stretcher instead of being part of an added zero dispersion line usually used for pulse shaping. The spectral phase is measured using a Spectral Phase Interferometry for Direct Electric Field Reconstruction setup (SPIDER). This spectral phase is used to control the DM shape in a closed loop configuration.

Wave front correction and second harmonic generation of a CPA terawatt laser chain

Abstract: Summary form only given. High peak-power lasers, using the chirped pulse amplification (CPA) technique, now routinely achieve the terawatt level and generate focused intensities in the 10/sup 18/-10/sup 19/ W/cm/sup 2/ range. For many laser-matter interaction experiments, especially those involving solid targets, a high peak-to-background contrast, a high intensity and a well-characterized focal spot are necessary. During the amplification process, wave front distortions appear due mainly to thermal effects in the amplification media and imperfections of large optics. These distortions lead to a broader focus spot and a loss in terms of focused intensity. Amplified Spontaneous Emission (ASE) limits the pulse contrast in CPA lasers. By combining wave front correction, high aperture parabola and second harmonic generation we obtain a high-contrast beam with a high focused intensity.

Spectral amplitude and phase control in a CPA laser system using adaptive optics in the stretcher

Abstract: Summary form only given. As pulse duration is decreasing, it becomes mandatory to take into account the residual spectral phase that can't be corrected in CPA laser systems. Up to date CPA systems are well corrected for chirp (/spl phi//sub 2/) and /spl phi//sub 3/. The remaining spectral phase defect reduces the amplified pulse contrast. Furthermore, gain narrowing in a Ti:sapphire amplifier limits the pulse duration to 30 fs. By controlling both spectral amplitude and phase we can reach pulses as short as 16 fs.

MeV proton beam driven by a high-intensity laser

Abstract: Summary form only given. The interaction of compact high-intensity subpicosecond lasers with matter has been studied for several years, having numerous applications such as table-top electron accelerators. However, only recently an interest has developed in ion acceleration, with potential applications for the initiation of nuclear reactions on a tabletop. Critical for ion acceleration is the efficiency of laser-energy conversion into a high-energy electron component, since the latter through charge separation can produce the requisite strong electrostatic fields. Thermal expansion of a laser-driven plasma and ponderomotive electron expulsion constitute the most well-known examples of electrostatic field production. While the former mechanism has been observed for many years, the latter one has only recently been observed in experiments with gas targets. For the gas targets, when the laser pulse duration /spl tau/ is long, /spl tau/>r/sub 0//c, where r/sub 0/ is the laser focal spot radius and c is the speed of light, the radial component of the pondermotive force dominates, and ions are accelerated radially.