Demonstration of a side-pumped cross-seeded thin-slab pre-amplifier for high-power Ti:Sa laser systems.

We demonstrate a room-temperature Ti:Sapphire (Ti:Sa) amplifier that uses a cross pump-seed geometry (cross-thin-slab) to generate 30-mJ output pulses at 0.5-kHz repetition rate, and 25 mJ at 1 kHz when pumped by 100-mJ, 515-nm pulses from a diode-pumped Yb:YAG laser. The geometry allows to maintain a crystal temperature of ∼30°C using cooling water at 10°C. The amplifier is an attractive solution for use in the first stages of amplification in high peak and high average power chirped pulse amplification laser systems.

Single-shot large field of view Fourier transform holography with a picosecond plasma-based soft X-ray laser.

It is challenging to obtain nanoscale resolution images in a single ultrafast shot because a large number of photons, greater than 1011, are required in a single pulse of the illuminating source. We demonstrate single-shot high resolution Fourier transform holography over a broad 7 µm diameter field of view with ∼ 5 ps temporal resolution. The experiment used a plasma-based soft X-ray laser operating at 18.9 nm wavelength with nearly full spatial coherence and close to diffraction-limited divergence implemented utilizing a dual-plasma amplifier scheme. A Fresnel zone plate with a central aperture is used to efficiently generate the object and reference beams. Rapid numerical reconstruction by a 2D Fourier transform allows for real-time imaging. A half-pitch spatial resolution of 62 nm was obtained. This single-shot nanoscale-resolution imaging technique will allow for real-time ultrafast imaging of dynamic phenomena in compact setups.

1.1 J Yb:YAG picosecond laser at 1 kHz repetition rate.

We demonstrate the generation of 1.1 J pulses of picosecond duration at 1 kHz repetition rate (1.1 kW average power) from a diode-pumped chirped pulse amplification Yb:YAG laser. The laser employs cryogenically cooled amplifiers to generate λ=1030nm pulses with average power of up to 1.26 kW prior to compression with excellent beam quality. Pulses are compressed to 4.5 ps duration with 90% efficiency. This compact picosecond laser will enable a variety of applications that require high energy ultrashort pulses at kilohertz repetition rates.

Demonstration of a kilowatt average power, 1 J, green laser.

A λ=515nm laser generating joule-level pulses at 1 kHz repetition rate was demonstrated by frequency doubling 1.2 J, 2 ns temporally shaped square pulses from a cryogenically cooled Yb:YAG laser in an LBO crystal. A doubling efficiency of 78% resulted in 0.94 J second-harmonic pulses at 1 kHz. The unconverted light interacted with a second LBO crystal to generate >100mJ second-harmonic pulses to reach a total green average power of 1.04 kW. A conversion efficiency of 89% was achieved for 0.58 J green pulses at 1 kHz. These results open the possibility to pump high energy femtosecond lasers at kilohertz repetition rates.

Laser induced damage in coatings for cryogenic Yb:YAG active mirror amplifiers.

We report results of a study of the laser induced damage threshold (LIDT) behavior of ion beam sputtered HfO2/SiO2 multilayer coatings on Yb:YAG using 1-on-1 and N-on-1 test protocols. The tests were conducted at ambient, vacuum, and cryogenic conditions using 280 ps pulses at λ=1030nm. The 1-on-1 LIDT of antireflection (AR) stacks is found to be only slightly reduced under vacuum and cryogenic conditions, while that of high reflectivity (HR) stacks is insensitive to environmental conditions within the uncertainty of the measurements. Cryogenic N-on-1 tests show the LIDT of the HR coating is almost the same as in the 1-on-1 tests. Conversely, the cryogenic N-on-1 test of the AR coating shows damage at ∼13J/cm2, a fluence lower than the 20.4J/cm2 of 1-on-1 tests. The AR damage behavior is found to be affected by imperfections at the Yb:YAG surface. These findings show that high surface quality is required to increase energy extraction from active mirror laser amplifiers.

Programmable pulse synthesizer for the generation of Joule-level picosecond laser pulses of arbitrary shape.

We report the demonstration of a pulse synthesizer based on spatial beam splitting and pulse stacking for the generation of picosecond laser pulses of Joule-level energy with arbitrary shape. An array of liquid crystals is used to control the amplitude of ten individual sub-pulses, and sliding retroreflectors are used to adjust their temporal separations. The synthesizer was used in combination with a λ=1.03 µm diode-pumped cryogenically-cooled Yb: YAG chirped pulse amplification laser to synthesize 1.3 J pulses or pulse trains of arbitrary shapes up to 9 ns duration with a temporal resolution as short as 8 ps. This pulse synthesizer offers the opportunity to incorporate a self-learning system to search for the optimal laser pulse shapes for various applications including optimized plasma conditions in laser-plasma based soft x-ray lasers and plasma sources for extreme ultraviolet lithography.

Characterization of absorptance homogeneity in thin-film coatings for high-power lasers by thermal lensing microscopy.

A focus error method photothermal microscope was designed for the characterization of absorptance homogeneity in thin-film coatings for high-power lasers. The technique relies on the detection of the thermal lens induced by the local absorption of a light power focused laser. The detailed design of the instrument is presented. The resolution of the system is better than 0.1 ppm and allows the realization of spatial sweeps and even measurements of the evolution of absorption as a function of time with a spatial resolution of 1 µm. These capabilities allow the location of defects and their characterization.

Investigation of laser annealing mechanisms in thin film coatings by photothermal microscopy.

We study the evolution of the absorptance of amorphous metal oxide thin films when exposed to intense CW laser radiation measured using a photothermal microscope. The evolution of the absorptance is characterized by a nonexponential decay. Different models that incorporate linear and nonlinear absorption, free carrier absorption, and defect diffusion are used to fit the results, with constraints imposed on the fit parameters to scale with power and intensity. The model that best fits is that two types of interband defects are passivated independently, one by a one-photon process and the other one by a two-photon process.

Depth-Profiling Microanalysis of CoNCN Water-Oxidation Catalyst Using a λ = 46.9 nm Plasma Laser for Nano-Ionization Mass Spectrometry.

Nanoscale depth profiling analysis of a CoNCN-coated electrode for water oxidation catalysis was carried out using table-top extreme ultraviolet (XUV) laser ablation time-of-flight mass spectrometry. The self-developed laser operates at λ = 46.9 nm and represents factor of 4 reduction in wavelength with respect to the 193 nm excimer laser. The reduction of the wavelength is an alternative approach to the reduction of the pulse duration, to enhance the ablation characteristics and obtain smaller quasi-nondestructive ablation pits. Such a XUV-laser ablation method allowed distinguishing different composite components of the catalyst-Nafion blend, used to modify a screen-printed carbon electrode surface. Chemical information was extracted by fragment assignment and relative amplitude analysis of the mass spectrometry peaks. Pure Nafion and the exposed carbon substrate were compared as references. Material specific fragments were clearly identified by the detected nonoverlapping mass-to-charge peaks of Nafion and CoNCN. Three dimensional mapping of relevant mass peak amplitudes was used to determine the lateral distribution and to generate depth profiles from consecutive laser pulses. Evaluating the profiles of pristine electrodes gave insight into fragmentation behavior of the catalyst in a functional ionomer matrix and comparison of post-catalytic electrodes revealed spots of thin localized Co residues.

Micro-scale fusion in dense relativistic nanowire array plasmas.

Nuclear fusion is regularly created in spherical plasma compressions driven by multi-kilojoule pulses from the world's largest lasers. Here we demonstrate a dense fusion environment created by irradiating arrays of deuterated nanostructures with joule-level pulses from a compact ultrafast laser. The irradiation of ordered deuterated polyethylene nanowires arrays with femtosecond pulses of relativistic intensity creates ultra-high energy density plasmas in which deuterons (D) are accelerated up to MeV energies, efficiently driving D-D fusion reactions and ultrafast neutron bursts. We measure up to 2 × 106 fusion neutrons per joule, an increase of about 500 times with respect to flat solid targets, a record yield for joule-level lasers. Moreover, in accordance with simulation predictions, we observe a rapid increase in neutron yield with laser pulse energy. The results will impact nuclear science and high energy density research and can lead to bright ultrafast quasi-monoenergetic neutron point sources for imaging and materials studies.

Dose-Rate Effects in Breaking DNA Strands by Short Pulses of Extreme Ultraviolet Radiation.

In this study, we examined dose-rate effects on strand break formation in plasmid DNA induced by pulsed extreme ultraviolet (XUV) radiation. Dose delivered to the target molecule was controlled by attenuating the incident photon flux using aluminum filters as well as by changing the DNA/buffer-salt ratio in the irradiated sample. Irradiated samples were examined using agarose gel electrophoresis. Yields of single- and double-strand breaks (SSBs and DSBs) were determined as a function of the incident photon fluence. In addition, electrophoresis also revealed DNA cross-linking. Damaged DNA was inspected by means of atomic force microscopy (AFM). Both SSB and DSB yields decreased with dose rate increase. Quantum yields of SSBs at the highest photon fluence were comparable to yields of DSBs found after synchrotron irradiation. The average SSB/DSB ratio decreased only slightly at elevated dose rates. In conclusion, complex and/or clustered damages other than cross-links do not appear to be induced under the radiation conditions applied in this study.

In situ 3-D temperature mapping of high average power cryogenic laser amplifiers.

Heat generation is a key obstacle to scaling high energy solid-state lasers to the multi-kilowatt average powers required for several key applications. We demonstrate an accurate, in situ, noninvasive optical technique to that makes three-dimensional (3-D) temperature maps within cryogenic amplifiers operating at high average power. The temperature is determined by analyzing the fluorescence spectra with a neural network function. The accuracy of the technique relies on a calibration that does not depend on simulations. Results are presented for a cryogenic Yb:YAG active mirror laser amplifier operating at different pump conditions. The technique is applicable to other solid-state lasers materials.

0.85 PW laser operation at 3.3 Hz and high-contrast ultrahigh-intensity λ = 400 nm second-harmonic beamline.

We demonstrate the generation of 0.85 PW, 30 fs laser pulses at a repetition rate of 3.3 Hz with a record average power of 85 W from a Ti:sapphire laser. The system is pumped by high-energy Nd:glass slab amplifiers frequency doubled in LiB3O5 (LBO). Ultrahigh-contrast λ=400 nm femtosecond pulses were generated in KH2PO4 (KDP) with >40% efficiency. An intensity of 6.5×1021 W/cm2 was obtained by frequency doubling 80% of the available Ti:sapphire energy and focusing the doubled light with an f/2 parabola. This laser will enable highly relativistic plasma experiments to be conducted at high repetition rate.

Strategies to increase laser damage performance of Ta2O5/SiO2 mirrors by modifications of the top layer design.

Ta2O5/SiO2 high reflection (HR) interference coatings for λ∼1 µm offer superior performance at high irradiance conditions. However, these coatings are not good candidates for high peak power conditions in comparison to HfO2/SiO2 multilayer stacks. Here we show that the modification of the top layers design of a quarter wave Ta2O5/SiO2 high reflector leads to 4-5 fold increase in the laser damage fluence compared to a quarter wave (Ta2O5/SiO2)15 when tested at λ=1.03 µm using pulse durations of 0.19 and 4 ns and peak power densities of 43.5 and 216 GW/cm2. One of the designs achieved a laser damage threshold fluence of 174 J/cm2 at 4 ns, which is 10% higher than that of a HfO2/SiO2 quarter wave design.

Energy penetration into arrays of aligned nanowires irradiated with relativistic intensities: Scaling to terabar pressures.

Ultrahigh-energy density (UHED) matter, characterized by energy densities >1 × 108 J cm-3 and pressures greater than a gigabar, is encountered in the center of stars and inertial confinement fusion capsules driven by the world's largest lasers. Similar conditions can be obtained with compact, ultrahigh contrast, femtosecond lasers focused to relativistic intensities onto targets composed of aligned nanowire arrays. We report the measurement of the key physical process in determining the energy density deposited in high-aspect-ratio nanowire array plasmas: the energy penetration. By monitoring the x-ray emission from buried Co tracer segments in Ni nanowire arrays irradiated at an intensity of 4 × 1019 W cm-2, we demonstrate energy penetration depths of several micrometers, leading to UHED plasmas of that size. Relativistic three-dimensional particle-in-cell simulations, validated by these measurements, predict that irradiation of nanostructures at intensities of >1 × 1022 W cm-2 will lead to a virtually unexplored extreme UHED plasma regime characterized by energy densities in excess of 8 × 1010 J cm-3, equivalent to a pressure of 0.35 Tbar.

Thin film absorption characterization by focus error thermal lensing.

A simple, highly sensitive technique for measuring absorbed power in thin film dielectrics based on thermal lensing is demonstrated. Absorption of an amplitude modulated or pulsed incident pump beam by a thin film acts as a heat source that induces thermal lensing in the substrate. A second continuous wave collimated probe beam defocuses after passing through the sample. Determination of absorption is achieved by quantifying the change of the probe beam profile at the focal plane using a four-quadrant detector and cylindrical lenses to generate a focus error signal. This signal is inherently insensitive to deflection, which removes noise contribution from point beam stability. A linear dependence of the focus error signal on the absorbed power is shown for a dynamic range of over 105. This technique was used to measure absorption loss in dielectric thin films deposited on fused silica substrates. In pulsed configuration, a single shot sensitivity of about 20 ppm is demonstrated, providing a unique technique for the characterization of moving targets as found in thin film growth instrumentation.

Nanoscale Ultradense Z-Pinch Formation from Laser-Irradiated Nanowire Arrays.

We show that ultradense Z pinches with nanoscale dimensions can be generated by irradiating aligned nanowires with femtosecond laser pulses of relativistic intensity. Using fully three-dimensional relativistic particle-in-cell simulations, we demonstrate that the laser pulse drives a forward electron current in the area around the wires. This forward current induces return current densities of ∼0.1 GA per µm^{2} through the wires. The resulting strong, quasistatic, self-generated azimuthal magnetic field pinches the nanowires into hot plasmas with a peak electron density of >9×10^{24} cm^{-3}, exceeding 1000 times the critical density. Arrays of these new ultradense nanopinches can be expected to lead to efficient microfusion and other applications.

1 J, 0.5 kHz repetition rate picosecond laser.

We report the demonstration of a diode-pumped chirped pulse amplification Yb:YAG laser that produces λ=1.03 µm pulses of up to 1.5 J energy compressible to sub-5 ps duration at a repetition rate of 500 Hz (750 W average power). Amplification to high energy takes place in cryogenically cooled Yb:YAG active mirrors designed for kilowatt average power laser operation. This compact laser system will enable new advances in high-average-power ultrashort-pulse lasers and high-repetition-rate tabletop soft x-ray lasers. As a first application, the laser was used to pump a 400 Hz λ=18.9 nm laser.

Breaking DNA strands by extreme-ultraviolet laser pulses in vacuum.

Ionizing radiation induces a variety of DNA damages including single-strand breaks (SSBs), double-strand breaks (DSBs), abasic sites, modified sugars, and bases. Most theoretical and experimental studies have been focused on DNA strand scissions, in particular production of DNA double-strand breaks. DSBs have been proven to be a key damage at a molecular level responsible for the formation of chromosomal aberrations, leading often to cell death. We have studied the nature of DNA damage induced directly by the pulsed 46.9-nm (26.5 eV) radiation provided by an extreme ultraviolet (XUV) capillary-discharge Ne-like Ar laser (CDL). Doses up to 45 kGy were delivered with a repetition rate of 3 Hz. We studied the dependence of the yield of SSBs and DSBs of a simple model of DNA molecule (pBR322) on the CDL pulse fluence. Agarose gel electrophoresis method was used for determination of both SSB and DSB yields. The action cross sections of the single- and double-strand breaks of pBR322 plasmid DNA in solid state were determined. We observed an increase in the efficiency of strand-break induction in the supercoiled DNA as a function of laser pulse fluence. Results are compared to those acquired at synchrotron radiation facilities and other sources of extreme-ultraviolet and soft x-ray radiation.