W. Theobald

High-efficiency dielectric reflection gratings: design, fabrication, and analysis

We report on reflection gratings produced entirely of dielectric materials. This gives the opportunity to enhance the laser damage threshold over that occurring in conventional metal gratings used for chirped-pulse-amplification, high-power lasers. The design of the system combines a dielectric mirror and a well-defined corrugated top layer to obtain optimum results. The rules that have to be considered for the design optimization are described. We optimized the parameters of a dielectric grating with a binary structure and theoretically obtained 100% reflectivity for the -1 order in the Littrow mounting for a 45 degrees angle of incidence. Subsequently we fabricated gratings by structuring a low-refractive-index top layer of a multilayer stack with electron-beam lithography. The multilayer system was fabricated by conventional sputtering techniques onto a flat fused-silica substrate. The parameters of the device were measured and controlled by light scatterometer equipment. We measured 97% diffraction efficiency in the -1 order and damage thresholds of 4.4 and 0.18 J/cm(2) with 5-ns and 1-ps laser pulses, respectively, at a wavelength of 532 nm in working conditions.

Diagnostics of a laser-induced dense plasma by hydrogen-like carbon spectra

The interpretation of a measured x-ray spectrum from a carbon plasma induced by subpicosecond laser pulses (Wilhein T et al 1998 J. Opt. Soc. Am. B 15 1235) is re-examined. We calculate a synthetic Lyman spectrum emitted from a plasma layer at a given temperature and density, using a microscopic approach to the lineshape in dense, non-ideal plasmas. Special attention is given to the Stark broadening of the Ly-γ line. Self-absorption is taken into account within a simple model considering one-dimensional radiation transport in a plasma layer. Comparing the synthetic spectrum with the experimental one, we infer a temperature of T = 106 K and an electron density of ne = 3×1021 cm-3. While the temperature is in agreement with Wilhein et al, the inferred density disagrees by one order of magnitude.

Three-halves harmonic emission from femtosecond laser produced plasmas

Measurements of three-halves harmonic radiation (3ω0/2) produced by femtosecond, Ti:sapphire laser pulses (⩽2×1017 W/cm2) in long density scale length plasmas generated from solid aluminum targets are presented. The 3ω0/2 emission yield shows excellent agreement with theories of the two-plasmon decay instability in the predominantly linear regime.

Novel laser ion sources

Development in the field of high-power laser systems with repetition rates of several Hz and energies of few joules is highly active and opening, giving new possibilities for the design of laser ions sources. Preliminary investigations on the use of four different laser and target configurations are presented: (1) A small CO2 laser (100 mJ, 10.6 μm) focused onto a polyethylene target to produce C ions at 1 Hz repetition rate (CERN). (2) An excimer XeCl laser (6 J, 308 nm) focused onto solid targets (Frascati). (3) A femtosecond Ti: sapphire laser (250 mJ, 800 nm) directed onto a solid targets (Jena). (4) A picosecond Nd: yttrium–aluminum–garnet (0.3 J, 532 nm) focused into a dense medium of atomic clusters and onto solid targets (London). The preliminary experimental results and the most promising schemes will be discussed with respect to the scaling of the production of high numbers of highly charged ions. Different lasers are compared in terms of current density at 1 m distance for each charge state.

HIGH-ORDER HARMONICS FROM SOLID TARGETS AS A PROBE FOR HIGH-DENSITY PLASMAS

The interaction of 100-fs Ti:sapphire laser pulses with thin foil targets was experimentally investigated at intensities exceeding 10(18) W/cm(2). High harmonics were observed in transmission through an overdense plasma in the direction of the incident beam. From the cutoff frequency of the harmonics an electron density of 1 x 10(24) cm(-3) is inferred, indicating a compression of the plasma by the ponderomotive force of the laser pulse.

Three-halves harmonic emission from femtosecond laser produced plasmas with steep density gradients

Detailed measurements of the angular distribution of 3 ω0/2 radiation are presented in short scale length plasmas (0.8–7 μm) generated by laser radiation at intensities reaching the relativistic level (1016–6×1018 W/cm2). The experimental results are in very good agreement with theoretical predictions based on two-plasmon decay and stimulated Raman scattering instabilities. New three-halves harmonic generation mechanisms are an identified characteristic of femtosecond laser induced parametric instabilities. These are the joint interaction of incident and reflected laser beams as well as stimulated Raman scattering. It is shown both experimentally and theoretically that the three-halves harmonic radiation is a useful preplasma diagnostic tool.

Scaling laser-diode pumped solid-state amplifiers to the petawatt level

Summary form only given. Increased performance of high-power laser-diodes, decline in laser-diode-power cost, development of new lasing materials, progress on key optical components, and novel pulse-compression techniques enable the design of efficient petawatt-class amplifiers based on diode-pumped chirped-pulse amplification technology. New research in high-field plasma physics, laser-pumped X-ray sources, laser fusion, particle accelerators and generators, as well as astro-physics is envisioned. We have developed laser-diode bars for long pulse pumping that deliver more than 200 W peak-power at almost 50% electrical to optical efficiency. Progress in semiconductor technology, heat-sink design, and diode packaging ensures safe pulsed operation for the required lifetime at driving currents of more than 220 A.. The system design of POLARIS (Petawatt Optical Laser Amplifier for Radiation Intensive Experiments) and supporting data is presented, and the prospects and scaleability of diode-pumped high-power laser-technology discussed. The front-end consisting of a Ti:sapphire oscillator, a low-aberration stretcher, and a regenerative amplifier delivers 1 mJ in 2 ns pulses with 14 nm band width centered at 1035 nm.

Optical damage of sputtered gold films irradiated with femtosecond laser pulses

Multi-shot damage tests were performed of gold coated mirrors in the femtosecond and in the nanosecond laser pulse regime. Sputtered gold films from different suppliers of various thicknesses were investigated. Considerable differences in the optical quality and the damage threshold are reported. The best films withstand a maximum fluence of 0.7 J/cm2 for 50-fs Ti:sapphire laser irradiation (804 nm) and 7 J/cm2 for 8-ns Nd:YAG irradiation (1064 nm). For gold films with poor optical quality a permanent surface modification one order of magnitude below the damage threshold was observed.

Probing Black Hole Physics in the Laboratory Using High Intensity Femtosecond Lasers

Mass acceleration in femtosecond laser plasmas exceeding 1018 g is measured using the frequency resolved optical gating technique. Accelerations of this magnitude appear in the stretched horizon regime of black holes. Detection of chirped light from black hole candidates would allow for unique identification.

Relativistic accelerations in femtosecond laser-produced plasmas

The interaction of femtosecond laser pulses with solids at intensities up to 10/sup 18/ W/cm/sup 2/ leads to the formation of high-density plasmas with electron densities exceeding 10/sup 23/ cm/sup -3/. It is well known that frequency-resolved measurements of reflected laser light from a plasma surface reveal the mass motion driven by the heating laser pulse. A more complete picture of the plasma motion can be obtained by measuring the phase and amplitude of the reflected pulse. Frequency-resolved optical gating (FROG) detection is a method that provides a 2D temporally resolved spectrum from which phase and amplitude information can be retrieved. We have used this technique to measure the reflected waveform in a high-intensity laser-solid interaction experiment.