M. Fraenkel

Reduction of damage threshold in dielectric materials induced by negatively chirped laser pulses

The threshold fluence for laser induced damage in wide band gap dielectric materials, fused silica and MgF2, is observed to be lower by up to 20% for negatively (down) chirped pulses than for positively (up) chirped, at pulse durations ranging from 60 fs to 1 ps. This behavior of the threshold fluence for damage on the chirp direction was not observed in semiconductors (silicon and GaAs). Based on a model including electron generation in the conduction band and Joule heating, it is suggested that the decrease in the damage threshold for negatively chirped pulse is related to the dominant role of multiphoton ionization in wide gap materials.

Shadow monochromatic backlighting: Large-field high resolution X-ray shadowgraphy with improved spectral tunability

The shadow monochromatic backlighting (SMB) scheme, a modification of the well-known soft X-ray monochromatic backlighting scheme, is proposed. It is based on a spherical crystal as the dispersive element and extends the traditional scheme by allowing one to work with a wide range of Bragg angles and thus in a wide spectral range. The advantages of the new scheme are demonstrated experimentally and supported numerically by ray-tracing simulations. In the experiments, the X-ray backlighter source is a laser-produced plasma, created by the interaction of an ultrashort pulse, Ti:Sapphire laser (120 fs, 3–5 mJ, 10 16 W/cm 2 on target) or a short wavelength XeCl laser (10 ns, 1–2 J, 10 13 W/cm 2 on target) with various solid targets (Dy, Ni + Cr, BaF 2 ). In both experiments, the X-ray sources are well localized spatially (∼20 μm) and are spectrally tunable in a relatively wide wavelength range (λ = 8–15 A). High quality monochromatic (δλ/λ ∼ 10 −5 –10 −3 ) images with high spatial resolution (up to ∼4 μm) over a large field of view (a few square millimeters) were obtained. Utilization of spherically bent crystals to obtain high-resolution, large field, monochromatic images in a wide range of Bragg angles (35° < Θ < 90°) is demonstrated for the first time.

Intraband and interband absorption of femtosecond laser pulses in copper

We investigated the optical properties of pure copper irradiated by a femtosecond laser pulse. Self-absorption of 50-fs laser pulses at 800 nm and 400 nm wavelengths (below and above the interband absorption threshold, respectively) is studied for peak laser intensities up to 10 15 W/cm 2 . Theoretical description of laser interaction with copper target is developed, solving numerically the energy balance equations for electron and ion subsystems together with Maxwell equations for laser radiation field inside the target. The theory accounts for both intraband and interband absorption mechanisms. We treated in detail the changes in electron structure and distribution function with an increase in electron temperature, as well as the ensuing changes in thermodynamic properties, collision frequencies, optical and transport coefficients. Experimental work on self-absorption of femtosecond laser pulses in copper targets at 800 nm and 400 nm wavelengths is ongoing. Results for 800 nm wavelength are reported. Theory and experiment are in good agreement.

X-Ray Spectrum Emitted by Laser-Produced Barium Plasma in the 8 to 13.5 Å Wavelength Range

A detailed analysis of the x-ray spectrum emitted by a laser-produced plasma of barium in the 8–13.5A wavelength range is given. The plasma is produced by the irradiation of an ultra-short pulse laser on a BaF2 target. Ab-initio calculations using the RELAC relativistic computer code are performed to identify 3d–nf (n = 4 to 8), 3p–4s, and 3p–4d transitions of Ni-like Ba XXIX, and corresponding satellite transitions in neighbouring ionization states: Fe-, Co-, Cu-. Zn-, Ga- and Ge-like ions. By changing the focusing conditions of the laser pulse on the solid target, one can get different ionization charge distributions and resolve some ambiguous identifications.

Large-field high-resolution x-ray monochromatic microscope based on spherical crystals and high-repetition-rate laser-produced plasmas

The combination of a table-top laser produced plasma X-ray source and spherically bent crystals for the soft X-ray region is used in traditional X-ray microscopy schemes. The X-ray source is well localized both spatially (approximately 20 micrometer) and temporally (1 ps - 10 ns, it depends on the used laser) and it is spectrally tunable in a relatively wide range (6 - 19 angstrom). High quality monochromatic ((delta) (lambda) /(lambda) approximately 10-5 - 10-3) images with high spatial resolution (up to approximately 4 micrometer) and in a large field of view (few mm) are presented. For many applications, these low-cost compact systems can offer a simple alternative to the larger installations which are usually used. It was demonstrated that the spherically bent crystals can be efficiently used in a wide ((Theta) equals 40 - 90 degrees) range of reflection angles, thus allowing wide wavelength selection. A very efficient concentration of monochromatic X-Ray radiation into different spot shapes (line, circle spot, etc) is demonstrated.

Collimation of plasma-produced x-rays by spherical crystals: Ray-tracing simulations and experimental results

Ray-tracing simulations, validated by experimental results, demonstrate that high intensity collimated x-ray beams can be produced from an isotropic x-ray source. A spherically bent mica crystal was used to collimate and monochromatize x rays emitted by a femtosecond laser-produced plasma. The result is a short pulse x-ray beam with a high degree of collimation (less than 1 mrad divergence), good spectral resolution (10−2<Δλ/λ<10−4), and tunability over a wide spectral range. The role of the experimental parameters in the resulting beam divergence is thoroughly analyzed by ray-tracing modeling. These simulations are validated by test experiments. The ray-tracing calculations define a set of boundaries in the experimental parameters, which will guarantee the achievement of collimated beams better than 1 mrad in further experiments.

Increase of multilayer x‐ray reflectivity induced by pulsed laser heating

The reflectivity of tungsten‐carbon (W‐C) multilayers, heated by 7 ns Nd‐YAG laser pulses was measured using soft x‐ray radiation at 4.4 nm. The reflectivity permanently increased and the Bragg angle decreased after heating the multilayer at fluences of 0.3 J/cm2. The reflectivity increase was achieved in a narrow domain of heating fluences. A quantitative model based on laser absorption and heat transport, that explains the melting phenomenon of tungsten and the heating to a temperature where graphitization is possible in the carbon layers, is presented. The two phenomena: (a) increase in x‐ray reflectivity and (b) expansion of the multilayer mirror can be explained by the laser heating. The improvement of reflectivity is caused by the smoothening of the tungsten layers due to melting, while the change in the spacing is attributed to the carbon phase transition (graphitization).

High power solid state single-frequency pulsed laser for Doppler velocimeters

A high power solid state single-frequency pulsed oscillator/amplifier laser system developed for Doppler velocimeters is described. The oscillator is based on a continuous wave diode pumped Nd:YAG single frequency laser, operating at a wavelength of 1064 nm. The output of the laser is chopped into pulses of 100 ns to 10 μs duration and 10 Hz repetition rate by a Pockels cell and polarizers. The pulses are amplified by a double pass flash-pumped Nd:YAG amplifier to kW power levels. The amplified laser pulses are frequency doubled with high efficiency to 532 nm in a periodically poled KTiOPO4 crystal. This ladder system is useful for many velocimeter applications and its utilization in an optically recording velocity interferometer system is described.

INTERACTION OF INTENSE SHORT LASER PULSES WITH AIR AND DIELECTRIC MATERIALS

A study of the propagation of intense short laser pulses in air and the interaction of these pulses with distant targets is described. It is shown that the beam filamentation pattern can be controlled by introducing beam astigmatism. In addition, it is demonstrated that the collapse distance of intense femtosecond laser beams scales as P-1/2 for input powers that are moderately above the critical power for self focusing, and that at higher powers the collapse distance scales as P-1. Related to the interaction of intense short pulses with distant targets, it is measured that the threshold fluence for optical damage in wide gap materials is lower by up to 20% for negatively chirped pulses than for positively chirped, at pulse durations ranging from 60 fs to 1 ps.

GUIDING OF HIGH LASER INTENSITIES IN LONG PLASMA CHANNELS

Plasma channels have been widely used to guide intense laser pulses over many Rayleigh lengths. Using optimized segmented capillary discharges, we demonstrated guided propagation of ultra short (100 fs) high intensity (1016 W/cm-2, limited by the laser system) pulses over distances up to 12.6 cm and intensities above 1018W/cm2 for 1.5cm boron nitride capillary. Both radial and longitudinal density profiles of plasma channels were studied under various discharge conditions. A new diagnostic technique is presented in which the transport of a guided laser pulse at different delay times from the initiation of the discharge is sampled on a single discharge shot. Using external, 10 nsec Nd YAG laser of several tenths of milijoules to ignite polyethylene capillaries we have demonstrated channels of various length in density range of 1017 - 1019cm-3 and up to 25% deep. The longitudinal profiles were found to be remarkably uniform in both short and long capillaries. The Boron Nitride capillary has provided a guiding medium that can withstand more than 1000 shots. Using these capillaries we have guided laser intensities above 1018W/cm2. The laser ignition of capillary discharge provided reliable almost jitter free approach. The concerns related to influence of relatively high current density flow through capillary on the injected electrons were studied extensively by us both theoretically and experimentally using a simple injection method. The method is based on the interaction of a high intensity laser pulse with a thin wire placed near capillary entrance. The influence of magnetic fields was found to be insignificant. Using this method we have studied transport of electrons though capillary discharge.