Douglass Schumacher

Above-threshold ionisation with a two-colour laser field

The authors report above-threshold ionisation in krypton irradiated by a two-colour radiation field ( lambda 1=1064 nm, lambda 2=532 nm), where the phase difference between the two colours can be precisely controlled. The ionisation rate depends not only on intensity but also on the relative phase between the two lasers, as do the relative intensities and widths of the electron spectral peaks.

Fast electron generation in cones with ultraintense laser pulses

Experimental results from copper cones irradiated with ultra-intense laser light are presented. Spatial images and total yields of Cu K{sub {alpha}} fluorescence were measured as a function of the laser focusing properties. The fluorescence emission extends into the cone approximately 300 {micro}m from the cone tip and cannot be explained by ray tracing including cone wall absorption. In addition the total fluorescence yield from cones is an order of magnitude higher than for equivalent mass foil targets. Indications are that the physics of the laser cone interaction is dominated by preplasma created from the long duration, low energy pre-pulse from the laser.

Microengineering Laser Plasma Interactions at Relativistic Intensities

We report on the first successful proof-of-principle experiment to manipulate laser-matter interactions on microscales using highly ordered Si microwire arrays. The interaction of a high-contrast short-pulse laser with a flat target via periodic Si microwires yields a substantial enhancement in both the total and cutoff energies of the produced electron beam. The self-generated electric and magnetic fields behave as an electromagnetic lens that confines and guides electrons between the microwires as they acquire relativistic energies via direct laser acceleration.

Effects of front-surface target structures on properties of relativistic laser-plasma electrons.

We report the results of a study of the role of prescribed geometrical structures on the front of a target in determining the energy and spatial distribution of relativistic laser-plasma electrons. Our three-dimensional particle-in-cell simulation studies apply to short-pulse, high-intensity laser pulses, and indicate that a judicious choice of target front-surface geometry provides the realistic possibility of greatly enhancing the yield of high-energy electrons while simultaneously confining the emission to narrow (<5°) angular cones.

Liquid crystal films as on-demand, variable thickness (50–5000 nm) targets for intense lasers

We have developed a new type of target for intense laser-matter experiments that offers significant advantages over those currently in use. The targets consist of a liquid crystal film freely suspended within a metal frame. They can be formed rapidly on-demand with thicknesses ranging from nanometers to micrometers, where the particular value is determined by the liquid crystal temperature and initial volume as well as by the frame geometry. The liquid crystal used for this work, 8CB (4′-octyl-4-cyanobiphenyl), has a vapor pressure below 10−6 Torr, so films made at atmospheric pressure maintain their initial thickness after pumping to high vacuum. Additionally, the volume per film is such that each target costs significantly less than one cent to produce. The mechanism of film formation and relevant physics of liquid crystals are described, as well as ion acceleration data from the first shots on liquid crystal film targets at the Ohio State University Scarlet laser facility.

Controlling continuum generation

The control of white-light, supercontinuum generation by shaped laser pulses is demonstrated. In particular, energy efficiency has been improved by more than an order of magnitude within specified bandwidths. The improvement is larger than model calculations predict.

Temporal resolution criterion for correctly simulating relativistic electron motion in a high-intensity laser field

Particle-in-cell codes are now standard tools for studying ultra-intense laser-plasma interactions. Motivated by direct laser acceleration of electrons in sub-critical plasmas, we examine temporal resolution requirements that must be satisfied to accurately calculate electron dynamics in strong laser fields. Using the motion of a single electron in a perfect plane electromagnetic wave as a test problem, we show surprising deterioration of the numerical accuracy with increasing wave amplitude a0 for a given time-step. We go on to show analytically that the time-step must be significantly less than λ/ca0 to achieve good accuracy. We thus propose adaptive electron sub-cycling as an efficient remedy.

The shaped critical surface in high intensity laser plasma interactions

This paper describes an investigation of the properties of the relativistic critical surface in a high intensity laser-plasma interaction, specifically the spatial morphology of the surface and its effect upon the divergence of the reflected light. The particle-in-cell code LSP running in two dimensions (2d3v) was used to model the formation of the critical surface and to show that it resides at a varying depth into the material that is dependent on both the intensity radial dependence of the laser focus as well as the shape of the longitudinal vacuum-material interface. The result is a shaped “mirror” surface that creates a reflected beam with phase and amplitude information informed by the extent of the preplasma present before the intense laser pulse arrived. A robust, highly effective means of experimentally determining the preplasma conditions for any high intensity laser-matter interaction is proposed using this effect. The important physics is elucidated with a simplified model that, within reasona...

Modeling crater formation in femtosecond-pulse laser damage from basic principles

We present the first fundamental simulation method for the determination of crater morphology due to femtosecond-pulse laser damage. To this end we have adapted the particle-in-cell (PIC) method commonly used in plasma physics for use in the study of laser damage and developed the first implementation of a pair potential for PIC codes. We find that the PIC method is a complementary approach to modeling laser damage, bridging the gap between fully ab-initio molecular dynamics approaches and empirical models. We demonstrate our method by modeling a femtosecond-pulse laser incident on a flat copper slab for a range of intensities.

Enhancing Bremsstrahlung production from ultraintense laser-solid interactions with front surface structures

We report the results of a combined study of particle-in-cell and Monte Carlo modeling that investigates the production of Bremsstrahlung radiation produced when an ultraintense laser interacts with a tower-structured target. These targets are found to significantly narrow the electron angular distribution as well as produce significantly higher energies. These features combine to create a significant enhancement in directionality and energy of the Bremstrahlung radiation produced by a high-Z converter target. These studies employ short-pulse, high intensity laser pulses, and indicate that novel target design has potential to greatly enhance the yield and narrow the directionality of high energy electrons and γ-rays. We find that the peak γ-ray brightness for this source is 6.0 × 1019 s−1 mm−2 mrad−2 at 10 MeV and 1.4 × 1019 s−1 mm−2 mrad-2 at 100 MeV (0.1% bandwidth).