Soft X-ray Laser Research Articles

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.

Modeling plasma-based soft x-ray lasers of up to 20 GW

Modeling plasma-based soft x-ray lasers of up to 20 GW

X-ray Chirped Pulse Amplification: towards GW Soft X-ray Lasers

Extensive modeling of the seeding of plasma-based soft X-ray lasers is reported in this article. Seminal experiments on amplification in plasmas created from solids have been studied in detail and explained. Using a transient collisional excitation scheme, we show that a 18 µJ, 80 fs fully coherent pulse is achievable by using plasmas pumped by a compact 10 Hz laser. We demonstrate that direct seeding of plasmas created by nanosecond lasers is not efficient. Therefore, we propose and fully study the transposition to soft X-rays of the Chirped Pulse Amplification (CPA) technique. Soft X-ray pulses with energy of 6 mJ and 200 fs duration are reachable by seeding plasmas pumped by compact 100 J, sub-ns, 1 shot/min lasers. These soft X-ray lasers would reach GW power, corresponding to an increase of 100 times as compared to the highest peak power achievable nowadays in the soft X-ray region (30 eV–1 keV). X-ray CPA is opening new horizon for soft x-ray ultra-intense sources.

Powerful high-repetition-rate tabletop soft x-ray lasers

Powerful high-repetition-rate tabletop soft x-ray lasers

Impact of wave front and coherence optimization in coherent diffractive imaging

We present single shot nanoscale imaging using a table-top femtosecond soft X-ray laser harmonic source at a wavelength of 32 nm. We show that the phase retrieval process in coherent diffractive imaging critically depends on beam quality. Coherence and image fidelity are measured from single-shot coherent diffraction patterns of isolated nano-patterned slits. Impact of flux, wave front and coherence of the soft X-ray beam on the phase retrieval process and the image quality are discussed. After beam improvements, a final image reconstruction is presented with a spatial resolution of 78 nm (half period) in a single 20 fs laser harmonic shot.

Intensity distribution of a capillary-discharge 46.9 nm soft X-ray laser

We realize a Ne-like Ar 46.9 nm soft X-ray laser pumped by a capillary discharge. The study of the laserpulse-intensity distribution is important for applications of soft X-ray lasers. The intensity distribution demonstrates the gain distribution, plasma radius, and axial plasma density that contribute to the study of the laser-pulse formation. To measure the intensity in different positions of the X-ray laser spot, we moved transversally an X-ray diode (XRD) assembled with a slit. We obtain the onedimensional intensity distribution. We find a laser divergence (FWHM) of 4.0 mrad. According to the gain-guided model, we calculate the intensity distribution. The measured divergence of 4.0 mrad roughly corresponds to a plasma radius a approximately equal to 230–250 μm and on-axis electron density ne≈ 8.0∙1018−9.0∙1018 cm−3. The results of calculations indicate that the divergence of the intensity distribution increases when the plasma radius decreases and the on-axis electron density increases.

Scintillation characteristics of LiCaAlF6-based single crystals under X-ray excitation

LiCaAlF6-based scintillators are studied under X- and soft gamma-ray excitations. Under nanosecond pulsed soft X-ray laser excitation the scintillation decay is measured with extremely high dynamical resolution and broad time scale. The undoped LiCaAlF6 shows complex temperature dependence of exciton luminescence and tunneling-driven energy transfer process in scintillation decay. In both the Ce and Eu-doped LiCaAlF6 the dominant part of measured scintillation decay is due to prompt recombination of electrons and holes at the doped emission centers. Nevertheless, the measured light yield value is considerably lower with respect to the derived upper limits. Possible origin of its deterioration is discussed.

Breakthroughs in Photonics 2012: New Advances in Soft X-Ray Lasers and Applications

This paper reviews the major accomplishments in the development of plasma-based soft X-ray lasers and their applications in 2012. Significant experimental breakthroughs include the extension of the lasing range down to 1.5 nm in an X-FEL photoionized plasma and the 100-Hz operation of a 18.9-nm laser with a record average power of 0.15 mW. New fields of applications, exploiting the high-brightness, coherence, and compactness of the source, were demonstrated. Further breakthroughs toward higher peak power are anticipated from numerical simulations of injection-seeded soft X-ray lasers.

Attosecond Hard X-ray Free Electron Laser

In this paper, several schemes of soft X-ray and hard X-ray free electron lasers (XFEL) and their progress are reviewed. Self-amplified spontaneous emission (SASE) schemes, the high gain harmonic generation (HGHG) scheme and various enhancement schemes through seeding and beam manipulations are discussed, especially in view of the generation of attosecond X-ray pulses. Our recent work on the generation of attosecond hard X-ray pulses is also discussed. In our study, the enhanced SASE scheme is utilized, using electron beam parameters of an XFEL under construction at Pohang Accelerator Laboratory (PAL). Laser, chicane and electron beam parameters are optimized to generate an isolated attosecond hard X-ray pulse at 0.1 nm (12.4 keV). The simulations show that the manipulation of electron energy beam profile may lead to the generation of an isolated attosecond hard X-ray of 150 attosecond pulse at 0.1 nm.

Oversampling smoothness: an effective algorithm for phase retrieval of noisy diffraction intensities.

Coherent diffraction imaging (CDI) is high-resolution lensless microscopy that has been applied to image a wide range of specimens using synchrotron radiation, X-ray free-electron lasers, high harmonic generation, soft X-ray lasers and electrons. Despite recent rapid advances, it remains a challenge to reconstruct fine features in weakly scattering objects such as biological specimens from noisy data. Here an effective iterative algorithm, termed oversampling smoothness (OSS), for phase retrieval of noisy diffraction intensities is presented. OSS exploits the correlation information among the pixels or voxels in the region outside of a support in real space. By properly applying spatial frequency filters to the pixels or voxels outside the support at different stages of the iterative process (i.e. a smoothness constraint), OSS finds a balance between the hybrid input-output (HIO) and error reduction (ER) algorithms to search for a global minimum in solution space, while reducing the oscillations in the reconstruction. Both numerical simulations with Poisson noise and experimental data from a biological cell indicate that OSS consistently outperforms the HIO, ER-HIO and noise robust (NR)-HIO algorithms at all noise levels in terms of accuracy and consistency of the reconstructions. It is expected that OSS will find application in the rapidly growing CDI field, as well as other disciplines where phase retrieval from noisy Fourier magnitudes is needed. The MATLAB (The MathWorks Inc., Natick, MA, USA) source code of the OSS algorithm is freely available from http://www.physics.ucla.edu/research/imaging.

An experimental attempt to produce Thomson scattering with soft X-ray lasers

Thomson scattering is one of the important methods of diagnosing parameters of the hot and dense plasma state that receive widespread attention. But the probe light wavelength for Thomson scattering is more limited to the visible or ultraviolet light, so the electron density that is copable to be diagnosed is much lower than the critical density of the driving laser. In comparison, soft X-ray laser as a probe is hopeful for diagnosing higher density of plasma. The experimental attempt of soft X-ray laser Thomson scattering has been carried out using Ne-like Ge soft X-ray laser generated on the SG-II high-power laser device. According to the scattering case, incoherent scattering and coherent scattering experiments were carried out, but it is unable to obtain the significant scattering spectrum. Theoretical analysis shows that the main reason may be the focusing power density of Ne-like Ge soft X-ray laser being not enough, in the experiments. By optimizing the experimental conditions, it is expected to obtain coherent Thomson scattering in the future.

毛细管放电氩等离子体的电离价研究

毛细管放电软X射线激光或极紫外光刻光源研究中等离子体的电离价是一个重要的参量，它决定光源的波长。依据毛细管放电的特点以及参考文献中提供的数据推导出毛细管放电氩等离子体电离价的经验公式。利用国内首台毛细管放电极紫外光刻光源演示装置研究了氩等离子体的光谱，实验表明峰值~28 kA的脉冲电流对30 Pa氩气放电或峰值~34 kA的脉冲电流对40 Pa氩气放电都产生了Ar7+。而公式计算出30 Pa和40 Pa时分别需要24.4~39 kA和32.7~52.3 kA的峰值电流。这说明经验公式具有一定的实用意义。 Ionized degree of Ar plasma in capillary discharge soft X-ray laser or Extreme Ultraviolet Lithographic (EUVL) source research was an important parameter which determined the wavelength of the plasma source. According to the characteristics of capillary discharge and the data from literature, we deduced an experiential expression of ion-ized degree of Ar plasma in capillary discharge by analogy method. With the first set of capillary discharge EUVL source demonstrative device in China, the spectra of Ar plasma was studied. It was found from experiment that either 30 Pa Ar gas discharged by peak pulse current of -28 kA or 40 Pa Ar gas discharged by peak pulse current of -34 kA could produce Ar7+ ion. While the simulating results showed that it was necessary for the 30 Pa and 40 Pa Ar gas to be ionized to Ar7+ ion by peak pulse current of 24.4 - 39 kA and 32.7 - 52.3 kA respectively. These results proved that the experien- tial expression had some definite practical significance.

Ar气气压对毛细管放电软X光激光输出的影响

Ar气气压是毛细管放电软X光激光研究中的关键参量，其对激光的产生机理和输出等发挥重要的作用。本文通过系统地研究Ar气气压对激光输出的影响，发现当Ar气气压从38 Pa升高到65 Pa时，激光信号的最大输出值确实从~17 V提高到~18.4 V。但前提条件是适当地增大预脉冲电流，并适当地延长预–主脉冲延时，使主脉冲电流能够更充分地利用良好的预电离等离子体状态，获得增益系数较大的激光介质。这些研究工作对进一步优化放电参量，提高激光的输出功率等具有重要的意义。 Ar pressure was such a key parameter in capillary discharge soft X-ray laser research that it played important role in generating mechanism and output and so on of laser. Through systemic investigation of the impact of Ar pressure on laser output in this article, it was found that the maximum output of laser signal had really been increased from ~17 V to ~18.4 V when Ar pressure went up from 38 Pa to 65 Pa. But the prerequisite was to increase the pre-current pulse and to prolong the delay time between pre- and main-current pulse properly which could make the main-current pulse utilize the better state of pre-ionized plasma adequately and obtain the laser medium with larger gain coefficient. These research works had important significance in optimizing the discharge parameter further and improving the output power of laser and so on.

Experimental verification of femtosecond laser ablation schemes by time-resolved soft x-ray reflective imaging

Pump and probe reflective imaging using a soft x-ray laser probe was applied to the observation of the early stage of femtosecond laser ablation process on platinum. In strongly excited area, drastic and fast reflectivity drop was observed. In moderately excited area, the decay of the reflectivity is slower than that in the strongly excited area, and the reflectivity reaches its minimum at t = 160 ps. In weakly excited area, laser-induced reflectivity change was not observed. In addition, the point where the reflectivity dip was observed at t = 10 ps and t = 40 ps, coincides with the position of the edge of reflectivity drop at t = 160 ps. These results give the critical information about the femtosecond laser ablation.

Three-dimensional Maxwell-Bloch calculation of the temporal profile of a seeded soft x-ray laser pulse

We present three-dimensional modeling of amplification of a high-order harmonic seed by a soft x-ray laser plasma. The time-dependent evolution of the x-ray signal is determined from a fully dynamic Maxwell-Bloch calculation. At high seed intensities, a simplified one-dimensional calculation leads to strong Rabi-like temporal oscillations of the output signal. However, such oscillations have not been observed experimentally. Our calculations demonstrate that this is due to spatial non-uniformities in the plasma gain that cause the Rabi oscillations to dampen dramatically. Large amplitude Rabi-like oscillations are expected to appear only in long and uniform plasma. Such targets require optimized guiding techniques.

Comment on “New probing techniques of radiative shocks”

In this comment, we discuss the possibility of imaging the radiative precursor of a strong shock with a 21.2nm soft x-ray laser probe and we analyze the data presented in C.Stehlé et al “New probing techniques of radiative shocks”, (Optics Communications 285, 64, 2012) in order to derive some estimation of the achieved resolution. We show that the presented results are inconclusive for the existence of a radiative precursor. Furthermore, our best estimation of cold and warm Xenon VUV opacities tells that 21.2nm backlighting would not be able to probe this radiative precursor.

Gain lifetime measurement of a Ni-like Ag soft X-ray laser

Experimental results of a two-stage Ni-like Ag soft X-ray laser operated in a seed-amplifier configuration are presented. Both targets were pumped applying the double-pulse grazing incidence technique with intrinsic travelling wave excitation. The injection of the seed X-ray laser into the amplifier target was realized by a spherical mirror. The results show amplification of the seed X-ray laser and allow for a direct measurement of the gain lifetime. The experimental configuration is suitable for providing valuable input for computational simulations.

A proposal for multi-tens of GW fully coherent femtosecond soft X-ray lasers

X-ray free-electron lasers1,2 delivering up to 1 × 1013 coherent photons in femtosecond pulses are bringing about a revolution in X-ray science3,4,5. However, some plasma-based soft X-ray lasers6 are attractive because they spontaneously emit an even higher number of photons (1 × 1015), but these are emitted in incoherent and long (hundreds of picoseconds) pulses7 as a consequence of the amplification of stochastic incoherent self-emission. Previous experimental attempts to seed such amplifiers with coherent femtosecond soft X-rays resulted in as yet unexplained weak amplification of the seed and strong amplification of incoherent spontaneous emission8. Using a time-dependent Maxwell–Bloch model describing the amplification of both coherent and incoherent soft X-rays in plasma, we explain the observed inefficiency and propose a new amplification scheme based on the seeding of stretched high harmonics using a transposition of chirped pulse amplification to soft X-rays. This scheme is able to deliver 5 × 1014 fully coherent soft X-ray photons in 200 fs pulses and with a peak power of 20 GW. Researchers describe a path towards 5 × 1014 fully coherent soft X-ray photons in 200 fs pulses reaching 20 GW peak power. The proposed amplification scheme is based on seeding stretched high harmonics using a transposition of Chirped Pulse Amplification to soft x-rays.

Observations of surface modifications induced by the multiple pulse irradiation using a soft picosecond x-ray laser beam

To study the interactions between picosecond soft x-ray laser (SXRL) beams and material surfaces, gold (Au), copper (Cu), and silicon (Si) surfaces were irradiated with SXRL pulses having a wavelength of 13.9 nm and a duration of ∼7 ps. Following irradiation, the surfaces of the substrates were observed using a scanning electron microscope and an atomic force microscope. With single pulse irradiation, ripple-like structures were formed on the Au and Cu surfaces. These structures were different from previously investigated conical structures formed on an Al surface. In addition, it was confirmed that the development of modified structures, i.e., growth of hillocks on the Au and Cu surfaces, was observed after multiple SXRL pulse exposures. However, on the Si surface, deep holes that seemed to be melted structures induced by the accumulation of multiple pulses of irradiations were found. Therefore, it was concluded that SXRL beam irradiation of various material surfaces causes different types of surface modifications, and the changes in the surface behaviors are attributed to the differences in the elemental properties, such as the attenuation length of x-ray photons.

Soft x-ray plasma-based seeded multistage amplification chain

To date, plasma-based soft x-ray lasers have demonstrated experimentally 1 μJ, 1 ps (1 MW) pulses. This Letter reports extensive study using time-dependant Maxwell-Bloch code of seeding millimeter scale plasmas that store more than 100 mJ in population inversion. Direct seeding of these plasmas has to overcome very strong amplified spontaneous emission (ASE) as well as prevent wake-field amplification. Below 100 nJ injected energy, seed produces pulses with picosecond duration. To overcome this limitation, a new scheme has been studied, taking advantage of a plasma preamplifier that dramatically increases the seed energy prior to entering the main plasma amplifier leading to ASE and wake-free, fully coherent 21.6 μJ, 80 fs pulses (0.27 GW).