Demonstration of a 100 Hz repetition rate gain-saturated diode-pumped table-top soft x-ray laser

Recent Development of High-Energy Short-Pulse Lasers with Cryogenically Cooled Yb:YAG

High-power solid-state lasers are among the hot research directions at the forefront of laser research and have major applications in industrial processing, laser-confined nuclear fusion, and high-energy particle sources. In this paper, the properties of Yb:YAG and Nd:YAG crystals as gain media for high-power solid-state lasers were briefly compared, according to the results of which Yb:YAG crystals are more suitable for high-power applications. Then, the effects of the thermodynamic and spectral properties of Yb:YAG crystals with temperature were analyzed in detail, and it was shown that the laser beams amplified by the cryogenically cooled Yb:YAG crystals could have higher beam quality, higher pump absorption efficiency, lower pump threshold, and higher gain. The change in properties of Yb:YAG crystal at low temperature makes it more suitable as a gain medium for high-power lasers. Subsequently, two types of kilowatt-class lasers using cryogenically cooled Yb:YAG crystals as gain media are introduced—100 J, 10 Hz nanosecond lasers and 1 J, 1 kHz picosecond lasers. Their configuration, main parameters, and typical output results were analyzed. Finally, future directions in the development of cryogenically cooled Yb:YAG lasers are discussed.

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.

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.

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.

Close to transform-limited, few-cycle 12 µJ pulses at 400 kHz for applications in ultrafast spectroscopy

Non-collinear optical parametric amplification has become the leading technology for amplifying few-cycle carrier-envelope phase (CEP) stable pulses to high energy at extreme repetition rates. In this work, a parametric amplifier system devoted to ultrafast photoionization experiments with coincidence detection is reported. The amplifier delivers CEP-stable few-cycle pulses with an average power of 5 W, and operates at repetition rates between 400 and 800 kHz. Close to transform-limited compression of the few-cycle pulses is achieved with minimized spatio-temporal distortions. Potential limitations introduced by spatio-temporal couplings to applications in attosecond science are analyzed. In particular, it is shown that pulse front tilt resulting from non-collinear amplification can considerably reduce the asymmetry in stereo above threshold ionization (stereo-ATI) experiments.

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.

2D IR spectroscopy at 100 kHz utilizing a Mid-IR OPCPA laser source

We present a 100 kHz 2D IR spectrometer. The system utilizes a ytterbium all normal dispersion fiber oscillator as a common source for the pump and seed beams of a MgO:PPLN OPCPA. The 1030 nm OPCPA pump is generated by amplification of the oscillator in cryocooled Yb:YAG amplifiers, while the 1.68 μm seed is generated in a OPO pumped by the oscillator. The OPCPA outputs are used in a ZGP DFG stage to generate 4.65 μm pulses. A mid-IR pulse shaper delivers pulse pairs to a 2D IR spectrometer allowing for data collection at 100 kHz.

Terahertz-driven linear electron acceleration

The cost, size and availability of electron accelerators are dominated by the achievable accelerating gradient. Conventional high-brightness radio-frequency accelerating structures operate with 30–50 MeV m⁻¹ gradients. Electron accelerators driven with optical or infrared sources have demonstrated accelerating gradients orders of magnitude above that achievable with conventional radio-frequency structures. However, laser-driven wakefield accelerators require intense femtosecond sources and direct laser-driven accelerators suffer from low bunch charge, sub-micron tolerances and sub-femtosecond timing requirements due to the short wavelength of operation. Here we demonstrate linear acceleration of electrons with keV energy gain using optically generated terahertz pulses. Terahertz-driven accelerating structures enable high-gradient electron/proton accelerators with simple accelerating structures, high repetition rates and significant charge per bunch. These ultra-compact terahertz accelerators with extremely short electron bunches hold great potential to have a transformative impact for free electron lasers, linear colliders, ultrafast electron diffraction, X-ray science and medical therapy with X-rays and electron beams.

Pulses of light offer a way to create particle accelerators that are a fraction of the size of conventional approaches. Here, the authors demonstrate the linear acceleration of electrons with kiloelectronvolt energy gain and in extremely short bunches using optically-generated terahertz pulses.

Quantitative X-ray phase-contrast microtomography from a compact laser-driven betatron source

X-ray phase-contrast imaging has recently led to a revolution in resolving power and tissue contrast in biomedical imaging, microscopy and materials science. The necessary high spatial coherence is currently provided by either large-scale synchrotron facilities with limited beamtime access or by microfocus X-ray tubes with rather limited flux. X-rays radiated by relativistic electrons driven by well-controlled high-power lasers offer a promising route to a proliferation of this powerful imaging technology. A laser-driven plasma wave accelerates and wiggles electrons, giving rise to a brilliant keV X-ray emission. This so-called betatron radiation is emitted in a collimated beam with excellent spatial coherence and remarkable spectral stability. Here we present a phase-contrast microtomogram of a biological sample using betatron X-rays. Comprehensive source characterization enables the reconstruction of absolute electron densities. Our results suggest that laser-based X-ray technology offers the potential for filling the large performance gap between synchrotron- and current X-ray tube-based sources.

With excellent resolving power and tissue contrast, X-ray phase-contrast imaging holds great promise but the source requirements have limited its use. Here, Wenz et al. show a phase-contrast microtomogram of a biological sample using X-ray radiation driven by a high-power laser.

Electro-optical system for scanning microscopy of extreme ultraviolet masks with a high harmonic generation source

A self-contained electro-optical module for scanning extreme ultraviolet (EUV) reflection microscopy at 13.5 nm wavelength has been developed. The system has been designed to work with stand-alone commercially available EUV high harmonic generation (HHG) sources through the implementation of narrowband harmonic selecting multilayers and off-axis elliptical short focal length zoneplates. The module has been successfully integrated into an EUV mask scanning microscope achieving diffraction limited imaging performance (84 nm point spread function).

Analytic study of traveling-wave velocity variation in line-focusing schemes for plasma x-ray lasers

Efficient amplification of coherent short-wavelength pulses along a plasma gain column requires traveling-wave excitation with the sweep velocity matched to the signal group velocity. Through simulations incorporating the gain dynamics of the system, we show that the group velocity is not constant but increases monotonically along the line focus due to strong saturation. We demonstrate a line-focusing configuration that results in traveling wave excitation with the sweep velocity well matched to the spatially varying group velocity. Moreover, we show through numerical simulations that the improved velocity matching yields a significant improvement in signal amplification.

Thin film beam splitter multiple short pulse generation for enhanced Ni-like Ag x-ray laser emission

An alternative, novel multiple pulse generation scheme was implemented directly after the optical compressor output of an x-ray pump laser. The new method uses a polarization sensitive thin film beam splitter and a half-wavelength wave plate for tuning the energy ratio in the multiple short pulses. Based on this method, an extensive study was made of the running parameters for a grazing incidence pumped silver x-ray laser (XRL) pumped with a long pulse of 145 mJ in 6 ns at 532 nm and up to 1.45 J in few picoseconds at 810 nm. Fivefold enhancement in the emission of the silver XRL was demonstrated using the new pump method.

Hour-long continuous operation of a tabletop soft x-ray laser at 50-100 Hz repetition rate

We report the uninterrupted operation of an 18.9 nm wavelength tabletop soft x-ray laser at 100 Hz repetition rate for extended periods of time. An average power of about 0.1 mW was obtained by irradiating a Mo target with pulses from a compact diode-pumped chirped pulse amplification Yb:YAG laser. Series of up to 1.8 x 10(5) consecutive laser pulses of ~1 µJ energy were generated by displacing the surface of a high shot-capacity rotating molybdenum target by ~2 µm between laser shots. As a proof-of-principle demonstration of the use of this compact ultrashort wavelength laser in applications requiring a high average power coherent beam, we lithographically printed an array of nanometer-scale features using coherent Talbot self-imaging.

Wavefront improvement in an injection-seeded soft x-ray laser based on a solid-target plasma amplifier

The wavefront of an injection-seeded soft x-ray laser beam generated by amplification of high-harmonic pulses in a λ=18.9 nm molybdenum plasma amplifier was measured by a Hartmann wavefront sensor with an accuracy of λ/32 root mean square (rms). A significant improvement in wavefront aberrations of 0.51±0.03λ rms to 0.23±0.01λ rms was observed as a function of plasma column length. The variation of wavefront characteristic as a function time delay between the injection of the seed and peak of soft x-ray amplifier pump was studied. The measurements were used to reconstruct the soft x-ray source and confirm its high peak brightness.