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Irradiation of high Z elements such as iodine, gold, gadolinium with monochromatic X-rays causes photoelectric effects that include the release of Auger electrons. Decay of radioactive iodine such as I-123 and I-125 also results in multiple events and some involve the generation of Auger electrons. These electrons have low energy and travel only a short distance but have a strong effect on DNA damage including the generation of double-strand breaks. In this chapter, we focus on iodine and discuss various studies that used iodine-containing chemicals to generate Auger electrons and cause DNA double-strand breaks. First, DNA synthesis precursors containing iodine were used to place iodine on DNA. DNA binding dyes such as iodine Hoechst were investigated for Auger electron generation and DNA breaks. More recently, iodine containing nanoparticles were developed. We describe our study using tumor spheroids loaded with iodine nanoparticles and synchrotron-generated monochromatic X-rays. This study led to the demonstration that an optimum effect on DNA double-strand break formation is observed with a 33.2keV X-ray which is just above the K-edge energy of iodine.

1. Introduction Burst Intensification by Singularity Emitting Radiation (BISER) [1] is the emission of coherent traveling waves by multi-stream flow singularities. Such singularities can be produced in relativistic plasma by high-power lasers, resulting in very bright coherent x-ray emission [2],[3]. 2. Experimental We have recently performed an experiment on BISER coherent x-ray generation with the Astra Laser at the CLF RAL, UK. We extensively optimized the experimental conditions, namely the plasma density and profile, and laser spot [4] and pulse, making use of substantial beam time. This resulted in a ~2 orders of magnitude higher BISER brightness compared to earlier experiments with 10 TW lasers. 3. Results In this presentation we discuss results from two soft x-ray spectrographs: a three-channel flat field spectrograph [5] in the forward direction (channels at −0.6, 0, and 0.8°) operating in the 17-34 nm range and a normal-incidence imaging spectrograph [6] in a 10° off-axis direction operating in the 12.4-20 nm range. Under optimum conditions, both spectrographs showed μJ and higher BISER pulse energies per shot in the corresponding spectral ranges and within their acceptance angles. This implies multi-100 μJ BISER pulses in the entire spectral range and within the full angular width. Acknowledgement We acknowledge support from the Astra Laser Group and the CLF Target Fab, Mechanical, and Electrical. Financial support: JSPS Kakenhi JP 19KK0355 and 19H00669, CLF, Russian Science Foundation (20-62-46050), IAP RAS, ELI-Beamlines, MŠMT by the project "Advanced Research Using High Intensity Laser Produced Photons and Particles" (CZ.02.1.01/0.0/0.0/16_019/0000789), High Field Initiative (CZ.02.1.01/0.0/0.0/15_003/0000449) from the European Regional Development Fund, and the QST Director Fund 創成的研究 #20. References [1]A. S. Pirozhkov, T. Zh. Esirkepov et al., "Burst intensification by singularity emitting radiation in multi-stream flows," Sci. Rep. 7, 17968 (2017). [2]A. S. Pirozhkov, et al. "Soft-X-Ray Harmonic Comb from Relativistic Electron Spikes" PRL 108, 135004 (2012). [3]A. S. Pirozhkov, et al., "High order harmonics from relativistic electron spikes" NJP 16, 093003 (2014). [4]A. S. Pirozhkov, et al. "Laser Requirements for High-Order Harmonic Generation by Relativistic Plasma Singularities," Quantum Beam Sci. 2, 7 (2018). [5]D. Neely, et al., "A multi-channel soft X-ray flat-field spectrometer," AIP Conf. Proc. 426, 479 (1998). [6]A. N. Shatokhin, et al., "High-resolution stigmatic spectrograph for a wavelength range of 12.5-30 nm," Opt. Express 26, 19009 (2018)., OPIC-2022 (ALPS-2022)

Burst Intensification by Singularity Emitting Radiation (BISER) [1] is the phenomenon of extremely bright coherent wave emission (constructive interference, N^2 effect) by singularities of multi-stream flows. High-power femtosecond lasers can produce such flows and corresponding relativistic plasma singularities in underdense plasma; these singularities emit very bright coherent x-rays [2]-[3]. We have recently performed an experiment on BISER coherent x-ray generation with the Astra Laser in the CLF RAL, UK. We observed BISER coherent soft x-rays with two spectrographs: a three-channel flat field spectrograph [4] in the forward direction (channels at -0.6, 0, and 0.8°) operating in the 17-34 nm range and a normal-incidence imaging spectrograph [5] in a 10° off-axis direction operating in the 12.4-20 nm range. Both spectrographs, in addition, provided data on the angular distribution of the BISER radiation. We took advantage of an extensive experiment term and long daily laser operation hours to optimize the laser (in particular, focal spot, which is a necessary prerequisite for BISER [6], and pulse width) and the experimental conditions (in particular, the density profile). These resulted in ~2 orders of magnitude enhancement in BISER photon number, providing >10^11 photons and >μJ energies per pulse in each channel at 7 TW laser power, the result which could previously be obtained at >100 TW laser power only. We acknowledge support from the Astra Laser Group and the CLF Target Fab, Mechanical, and Electrical. Financial support: JSPS Kakenhi JP 19KK0355 and 19H00669, CLF, Russian Science Foundation (20-62-46050), IAP RAS, ELI-Beamlines, the Ministry of Education, Youth and Sports of the Czech Republic by the project "Advanced Research Using High Intensity Laser Produced Photons and Particles" (CZ.02.1.01/0.0/0.0/16_019/0000789), and the QST Director Fund 創成的研究 #20. [1] A. Pirozhkov, T. Esirkepov et al., "Burst intensification by singularity emitting radiation in multi-stream flows," Sci. Rep. 7, 17968 (2017). [2] A. S. Pirozhkov et al. "Soft-X-Ray Harmonic Comb from Relativistic Electron Spikes" PRL 108 135004 (2012) [3] A. S. Pirozhkov et al., "High order harmonics from relativistic electron spikes" New J. Phys. 16, 093003 (2014). [4] D. Neely et al., "A multi-channel soft X-ray flat-field spectrometer," AIP Conf. Proc. 426, 479 (1998). [5] A. Shatokhin et al., "High-resolution stigmatic spectrograph for a wavelength range of 12.5-30 nm," Opt. Express 26 19009 (2018). [6] A. S. Pirozhkov, et al. "Laser Requirements for High-Order Harmonic Generation by Relativistic Plasma Singularities," Quantum Beam Sci. 2, 7 (2018)., Annual meeting of the JSAP

Ultra-high intensities open a way towards Terawatt and potentially Petawatt gamma-ray pulses in the so called Gamma Flare regime. We performed our first experiment towards this regime with the J-KAREN-P laser operating at intensities close to 10^22 W/cm^2. In this presentation I will overview the experiment, including very bright hard x-ray generation (sub-MeV to a few-MeV spectral range), high-order harmonic generation (30 to 70 eV), and multiple additional diagnostics revealing information on preplasma (properties of reflected 1st, 2nd, and 3rd harmonics and back-reflection). I will compare these results with the integrated HD and PIC simulations., OPTO-2021 Symposium on Photon and Beam Science

We describe our experiment performed with the J-KAREN-P laser at the intensity approaching 10^22 W/cm^2. In particular, we present the results obtained with hard x-ray (0.1-3 MeV) spectrographs comprising 10 scintillators and a method to reconstruct the x-ray spectra., 3rd International Conference on Nuclear Photonics (NP2020)

At PW-class laser facilities pulse intensities of the order of 10^21 – 10^22 W/cm^2 can be reached by focusing high quality laser beams to spots of a few microns size. Under such conditions, during fs- time intervals a huge amount of optical energy is concentrated into a tiny volume and by the interaction with solid targets creates a plasma with high energy density. Such plasma can be sources of non-linear phenomena, among known and predicted by theory there is the acceleration to high energy of protons, ions, electrons and neutrons, generation of HOH and gamma-flare, creation of exotic states of hollow ions and WDM. The observed phenomena are defined by the type of target, laser energy and strongly depend on the prepulse condition. Achieving highly ionized states for elements with medium Z allows to effectively perform x-ray diagnostics of plasma parameters and preplasma conditions by means of high-resolution spectroscopy based on the analysis of spectra of H-like and He-like ions. Here we l present a review on the recent results on application of x-ray spectroscopic diagnostics of plasmas generated in the interaction of J-KAREN-P ultra-high intensity laser pulses with solid and clusters targets under conditions of different intrinsic laser contrast and under the cases of specially incorporated prepulse for controlled preplasma formation. Plasma parameters were evaluated by means of time integrated x-ray spectroscopic measurements and kinetic modeling using the code PrismSPECT. Certain line intensity relations, including the relations (Ly_a / He_b), (Ly_a / Ly_a satellites), relation between Ly_a satellites, as well as broadening and the shape of line, were used as the main tools for evaluation of plasma parameters. Each of the tools has preferred dependences on plasma electron temperature, electron density, electric and magnetic field strength. Four types of experiments are under consideration., 3rd International Conference on Nuclear Photonics NP2020

We present multi-diagnostic results from laser-plasma experiment at ~10^22 W/cm^2, including optical and high-order harmonics, back-reflection, soft x-rays, keV x-rays, and sub-MeV to MeV x-rays, simultaneously with important laser parameters. We discuss methods to find at-focus target position and compare intensity dependences., OPTICS & PHOTONICS International Congress 2021

After recent upgrade of the Petawatt-class J-KEREN-P laser system the experiments with on-target relativistic intensity up to 10^22 W/cm^2 (pulse energy ~10 J, pulse length ~40 fs, focal spot down to 1.5 um) are routinely carried out at KPSI, QST. In general, for solid targets the plasma formation threshold is of ~10^10-10^12 W/cm^2, therefore the long ASE prepulse, short and ultrashort pedestal intensity crucially impact the formation of plasma conditions and the observed phenomena. It was demonstrated that there exists a strong dependence of the degree of Fe ionization on a laser prepulse, and that an effective generation of energetic, multiply-charged moderate- and high-Z ions can be driven only with laser pulses of ultra-high contrast. Here we report on the new experiments performed at J-KAREN-P laser with the tailored plasma density profile created by specially incorporated prepulse. For the prepulse the ~1/100 of the main pulse power was picked up and delivered to the target with controlled sub-nanosecond time interval prior to the main pulse. The prepulse was focused: (1) on the front of a stainless steel foil for the formation of downstream preplasma with conditions aiming the efficient generation of x-ray flare and high-order harmonics (experiment led by A.S. Pirozhkov); and (2) on the rear-side of a 4-Chlorosterene foil mimicking upstream preplasma, which has the necessary density profile for collisionless shock ion acceleration, when the main pulse is irradiated (experiment led by Y. Fukuda and Y. Sakawa). For both experiments, we present the detailed analysis of plasma parameters provided by means of high-resolution x-ray spectroscopic methods, based on emission characteristics of plasma in the spectral range of Ne-like Fe and H-like Cl ionization states, respectively., International Conference on High Energy Density Sciences (HEDS2021)

Recently it was shown that injection of gas preliminary cooled down to cryogenic temperatures is a promising way to produce micron-scale clusters. For example, in hydrogen clusters with the size up to ≈2 μm were produced via a conical nozzle at 25 K, 6 MPa. Micron scale clusters can be also created with another pre-cooled gases, for example Ar or Kr, but for other temperature values. So large clusters diameter value becomes comparable with a waist width of a laser beam produced by the J-KAREN-P laser facility. As a result, an experimental case can occur, when most of the laser energy is absorbed in a single cluster. Such situation can be considered as an interaction of a laser pulse with matter, state of which is close to solid-state. In the presentation plasma X-ray spectra features indicating the described case of high-intensity laser and large-scale-cluster interaction are discussed. They were observed experimentally during irradiation of cryogenic (T = 140 K – 220 K) Ar flows by ultra-intensive (I = 10^22 W/cm^2) femtosecond laser pulses generated by the J-KAREN-P laser. Registered X-ray spectra of produced plasma contains Ar XVIII Ly-α and Ar XVII He-α of comparable intensities. It should be noted that the Ar XVIII Ly-α line has been for the first time observed in laser-matter interaction experiments. It indicates that the density of irradiated matter is high enough to provide a rate of impact ionization process, which is sufficient to create a significant amount of H-like Ar ions. It is a challenge to reach such condition for a pure gas or small clusters. Thus, the observed spectra are considered as an evidence of a large micron cluster existing in the beam waist region., International Conference on High Energy Density Sciences (HEDS2021)

X-ray irradiation of high Z elements causes photoelectric effects that include the release of Auger electrons that can induce localized DNA breaks. We have previously established a tumor spheroid-based assay that used gadolinium containing mesoporous silica nanoparticles and synchrotron-generated monochromatic X-rays. In this work, we focused on iodine and synthesized iodine-containing porous organosilica (IPO) nanoparticles. IPO were loaded onto tumor spheroids and the spheroids were irradiated with 33.2 keV monochromatic X-ray. After incubation in CO₂ incubator, destruction of tumor spheroids was observed which was accompanied by apoptosis induction, as determined by the TUNEL assay. By employing the γH2AX assay, we detected double strand DNA cleavages immediately after the irradiation. These results suggest that IPO first generate double strand DNA breaks upon X-ray irradiation followed by apoptosis induction of cancer cells. Use of three different monochromatic X-rays having energy levels of 33.0, 33.2 and 33.4 keV as well as X-rays with 0.1 keV energy intervals showed that the optimum effect of all three events (spheroid destruction, apoptosis induction and generation of double strand DNA breaks) occurred with a 33.2 keV monochromatic X-ray. These results uncover the preferential effect of K-edge energy X-ray for tumor spheroid destruction mediated by iodine containing nanoparticles., アインシュタインの光電効果をがん細胞の中で再現 放射線治療への新展開. 京都大学プレスリリース. 2021-07-14., Quantum physics helps destroy cancer cells. 京都大学プレスリリース. 2021-07-14.

Coherent x-rays via the Burst Intensification by Singularity Emitting Radiation (BISER) mechanism are generated from relativistic plasma in helium gas target. A broad modulation of the BISER spectrum, which is significantly wider than the harmonic order, is observed and characterized. In particular, we found that the modulation period can be as large as 41 eV.

Using QED PIC and hydrodynamic simulations, we optimize the -flare generation scheme from the high power laser interaction with tailored target showing the efficient conversion of the laser pulse energy to -photons.

Relativistic mirrors in laser plasmas offer a promising way for development of compact and tunable sources of coherent short-wavelength radiation. Here we investigate recoil effects on reflection of high-intensity laser pulses from such mirrors.

We have developed an extreme ultraviolet (EUV) laser irradiation system at QST. EUV laser has a wavelength of 13.9 nm, and this laser wavelength is close to the wavelength of EUV lithography (λ = 13.5 nm). The irradiation system has an intensity monitor based on the Mo/Si multilayer beam splitter. This intensity monitor provides the irradiation energy onto the sample surface. So it is possible to examine and confirm damage/ablation thresholds of optical elements and doses for the sensitivity of resist materials, which have the same specifications of those in the EUV lithography.

In order to realize a compact and high repetition rate plasma X-ray laser, we have developed a recombination scheme plasma X-ray laser. In particular, we focused on a laser oscillation of Li-like aluminum 3d-4f transition (15.5 nm), which was already demonstrated in a high-density and low-temperature Al laser plasma with a low gain coefficient. In this study, by using an Nd:YAG laser (500 ps, 3 J, 16 pulse-trains) we created the lasing media and succeeded in laser oscillation at 15.5 nm. The optimization of the pulse-train in terms of the pulse duration and its interval enhanced the gain coefficient to g = 4.3 cm−1.

Relativistic mirrors can be realized with strongly nonlinear Langmuir waves excited by intense laser pulses in underdense plasma. On reflection from the relativistic mirror the incident light affects the mirror motion. The corresponding recoil effects are investigated analytically and with particle-in-cell simulations. It is found that if the fluence of the incident electromagnetic wave exceeds a certain threshold, the relativistic mirror undergoes a significant back reaction and splits into multiple electron layers. The reflection coefficient of the relativistic mirror as well as the factors of electric field amplification and frequency upshift of the electromagnetic wave are obtained., 8 pages, 5 figures, submitted to Physics of Plasmas

高Z元素にX線を照射すると、局所的なDNA切断を誘発する可能性のあるオージェ電子の放出を含む光電効果を引き起こす。我々はこれまで、ガドリニウムを含有したメソポーラスシリカナノ粒子と放射光単色X線によるがんスフェロイドの殺傷効果について研究を進めてきた。本報ではヨウ素に焦点を当て、ヨウ素を含有した多孔質有機シリカ（IPO）ナノ粒子を合成した。IPOをがんスフェロイドに取り込ませ、スフェロイドに33.2keVの単色X線を照射した。インキュベーション後、TUNEL assay法で評価をしたところ、アポトーシス誘導を伴うがんスフェロイドの破壊が観察された。さらに、γH2AX assay法によって、照射直後にDNAの二重鎖切断が起きていることを確認した。これらの結果は、IPO がX線照射時にまずDNAの二重鎖切断を引き起こし、続いて、がん細胞をアポトーシスに誘導することを示唆している。33.0～33.4keVの単色X線で、エネルギーを0.1keVずつ変化させながら実験を行ったところ、がんの形状破壊、アポトーシス誘導、DNA二重鎖切断の3つのイベントは、すべて33.2keVの単色X線のときに発生していた。これらの結果は、ヨウ素含有ナノ粒子によって媒介されたがんスフェロイド破壊における、K吸収端エネルギーX線の優先的効果を示している。

Direct metrology of coherent short-wavelength beamlines is important for obtaining operational beam characteristics at the experimental site. However, since beam-time limitation imposes fast metrology procedures, a multi-parametric metrology from as low as a single shot is desirable. Here a two-dimensional (2D) procedure based on high-resolution Fresnel diffraction analysis is discussed and applied, which allowed an efficient and detailed beamline characterization at the SACLA XFEL. So far, the potential of Fresnel diffraction for beamline metrology has not been fully exploited because its high-frequency fringes could be only partly resolved with ordinary pixel-limited detectors. Using the high-spatial-frequency imaging capability of an irradiated LiF crystal, 2D information of the coherence degree, beam divergence and beam quality factor M 2 were retrieved from simple diffraction patterns. The developed beam metrology was validated with a laboratory reference laser, and then successfully applied at a beamline facility, in agreement with the source specifications.

Synchrotron generated monochromatic X-rays can be precisely tuned to the K-shell energy of high Z materials resulting in the release of the Auger electrons. In this work, we have employed this mechanism to destruct tumor spheroids. We first loaded gadolinium onto the surface of mesoporous silica nanoparticles (MSNs) producing gadolinium-loaded MSN (Gd-MSN). When Gd-MSN was added to the tumor spheroids, we observed efficient uptake and uniform distribution of Gd-MSN. Gd-MSN also can be taken up into cancer cells and localize to a site just outside of the cell nucleus. Exposure of the Gd-MSN containing tumor spheroids to monochromatic X-ray beams resulted in almost complete destruction. Importantly, this effect was observed at an energy level of 50.25 keV, but not with 50.0 keV. These results suggest that it is possible to use precisely tuned monochromatic X-rays to destruct tumor mass loaded with high Z materials, while sparing other cells. Our experiments point to the importance of nanoparticles to facilitate loading of gadolinium to tumor spheroids and to localize at a site close to the nucleus. Because the nanoparticles can target to tumor, our study opens up the possibility of developing a new type of radiation therapy for cancer., 単色Ｘ線とナノ粒子により、がん患部でX線エネルギー効果を増幅する方法の開発に成功 --オージェ電子発見から100年、放射線がん治療の新境地を開く--. 京都大学プレスリリース. 2019-10-02.

The overview of the recent results for discovery and investigations of a very exotic phenomenon – optical mirage in the X-ray spectral range – is presented. It was found that the mirage could be created in the form of coherent virtual point source, emerging in the vicinity of the second plasma in two-stage oscillator-amplifier X-ray laser. The X-ray source-mirage, rigidly phased with the initial radiation of generator, occurs only when amplification takes place in the amplifier plasma and leads to the appearance of the interference pattern in the form of concentric rings in the spatial profile of the output X-ray laser beam. The equation describing the emergence of X-ray mirage was found, numerical solution of which shows that its formation is similar to that of the optical mirages observed at propagation of light rays through an inhomogeneously heated air. Obtained results have already demonstrated novel comprehension into the physical nature of amplification of X-ray radiation, opening additional opportunities for X-ray interferometry, holography, and other applications, which require multiple rigidly phased sources of coherent radiation.

Relativistic mirrors can be realized with strongly nonlinear Langmuir waves excited by intense laser pulses in underdense plasma. On reflection from the relativistic mirror, the incident light affects the mirror motion. The corresponding recoil effects are investigated analytically and using particle-in-cell simulations. It is found that if the fluence of the incident electromagnetic wave exceeds a certain threshold, the relativistic mirror undergoes a significant back reaction and splits into multiple electron layers. The reflection coefficient of the relativistic mirror and the factors of electric field amplification and frequency upshifty of the electromagnetic wave are obtained.

Coherent x-rays via the Burst Intensification by Singularity Emitting Radiation (BISER) mechanism are generated from relativistic plasma in helium gas target. A broad modulation of the BISER spectrum, which is significantly wider than the harmonic order, is observed and characterized. In particular, we found that the modulation period can be as large as 41 eV.

We irradiated soft X-ray laser pulses onto aluminum film targets, which were evaporated on silicon and lithium fluoride substrates. After laser irradiation, we observed irradiated surfaces and confirmed damage with nanometer scale-modified structures. These damage structures were the same as those formed on the aluminum bulk surface. In this article, we discuss a possibility of the growth process of damage structures on the Al surface based on the experimental results.

A simplified flash-lamp pumped high-average-power Nd:YAG Q-switched laser system based on a master oscillator power amplifier platform was developed toward outside laser remote sensing. The performance of the laser system was demonstrated, obtaining 4.7 J output pulse energy with a 50 Hz operating frequency on the optical breadboard of 1.8 m x 0.7 m size. The pulse energy from master oscillator was approximately 250 mJ with 14 ns pulse duration that was amplified by first Nd:YAG rod crystals with double pass amplification. Then, output laser pulse from first YAG rod was amplified by second and third Nd:YAG rod crystals. The beam pattern was image relayed using lens pair between all Nd:YAG rods to maintain the good beam spatial profile in rod amplifiers to avoid the optical damages induced by non-uniform beam profile. The focal lengths of thermal lens effect in each Nd:YAG rod crystal was about 2 m that were compensated by an adjustment of lens pairs. The amplified pulse laser was focused using focusing lens pair on the concrete surface to generate panel vibrations by laser ablation and/or thermal stress, acting thus as a hammer. The focal length of lens pair was approximately 7 m that is assumed the typical a tunnel roof in Japan. The energy transfer efficiency from final amplifier to concrete surface was approximately 87%, its main reason of reduction of efficiency was beam quality of master oscillator. That efficiency was 89% with only oscillator beam.

We discuss the new regime of high-order harmonic generation by relativistic -irradiance multi-terawatt femtosecond lasers focused onto gas jet targets [PRL, 108, 135004, 2012; NJP, 16, 093003, 2014]. The laser induces multi-stream relativistic plasma flow resulting in the formation of density singularities: structurally stable, oscillating electron spikes coherently emitting high-frequency radiation. Here we analyse the dependence of the harmonic yield on the focal spot quality and derive the required laser parameters for efficient harmonics generation. We show the status of the J-KAREN-P laser [IEEE J. Sel. Topics Quantum Electron., 21, 1601118, 2015] and report on the progress towards satisfying these requirements.

High repetition rate and high pulse energy laser system as a pump source for Ti:sapphire laser used in laser-driven plasma soft X-ray laser system are required. We developed a new Nd:YAG laser system using master oscillator power amplifier platform. The maximum output energy as over 4 J was obtained at an operating frequency of 50 Hz with a negligible amplified spontaneous emission.

In the present report, we discussed our recent results concerning the investigations of high energy density matter. By using of high-resolution X-ray spectroscopic measurements and kinetic simulations, we demonstrate that the energy of femtosecond laser pulses is efficiently converted to X-ray radiation and produces exotic states in solid density plasma regions. It is shown that exotic hollow multicharged ions states can be obtained and studied not only by ultra-intense X-ray radiation of XFEL lasers, but also upon using optical laser technology. We have concluded that observable features of the hollow-ion spectra are sensitive to such plasma parameters as density, temperature, hot-electron fraction, and intensity of the X-ray pumping radiation and could be used for effective diagnostics of warm dense matter parameters.

Laser-induced damage thresholds (LIDTs) on silica glass was evaluated with soft X-ray laser (SXRL) pulses. Silica glass is one of the most useful materials for near-infrared and visible laser systems. The high-photon energy of the SXRL would have great potential to study the laser-induced damage mechanisms. The LIDTs on silica glass measured by 13.9-nm, 7-ps SXRL pulses were several dozen of mJ/cm2 as the experimental results. The damage morphologies are estimated by a simple 1D calculation.

This paper reviews recent progress made in the development of coherent x-ray sources and their applications in material science and laser processing in the National Institutes for Quantum and Radiological Science and Technology (QST). The upgrade of Ti:Sapphire laser for a grazing incidence pumping (GRIP) scheme was started, to further develop a coherent x-ray source. Investigating the applications of x-ray lasers, we observed temporal evolution of metal surface ablation using an x-ray laser probe, and the damage to an EUV mirror structure by the x-ray pulse irradiation.

We modified a soft X-ray laser (SXRL) interferometer synchronized with a Ti:sapphire laser to observe a single-shot image of the nanoscale structure dynamics of materials induced by an optical laser pulse. The lateral resolution on the sample surface was improved to 0.7 μm using precise imaging optics. Using this system, we succeeded in observing thin film structures above the solid (or liquid) surface in the femtosecond laser ablation process of metals (Au). The thin film worked as soft X-ray beam splitter. This result shows a thin film was smooth and dense (with a roughness of a few nanometers and near sold density). Furthermore, it gave rise to the possibility of generating novel transient soft X-ray optics .

The irradiation damage for Mo/Si, Ru/Si, and Nb/Si multilayers are investigated using the soft X-ray laser system at QST. Scanning electronic microscopy observations reveal that the size of the damage on the Nb/Si multilayer is smaller than the other multilayers. This suggests that the damage fluence by the EUV irradiation of the Nb/Si multilayers is greater than the other multilayers.

While it has been expected that X-ray laser will be widely applied to biomedical studies, this has not been achieved to date and its biological effects such as DNA damage have not been evaluated. As a first step for its biological application, we developed a culture cell irradiation system, particularly designed for a plasma-driven soft X-ray laser pulse, to investigate whether the soft X-ray laser is able to induce DNA double strand breaks (DSBs) in living cells or not. The human adenocarcimona cell line A549 was irradiated with the soft X-ray laser at a photon energy of 89 eV and the repair focus formation of the DSBs was assessed by immunofluorescence staining with antiphosphorylated DNA-PKcs (p-DNA-PKcs), ATM (p-ATM) and γ-H2AX antibody. The p-DNA-PKcs, ATM, and γ-H2AX foci were clearly identified after soft X-ray laser irradiation. Furthermore, the increase in the X-ray laser shot number, even from a single shot, results in the increase in p-DNA-PKcs foci. These results are the first evidence that the 89 eV soft X-ray laser is able to induce DSB in living cells. Our study demonstrated that this irradiation system is a useful tool for investigating the radiobiological effect of soft X-ray laser.

Spatial structures of ablative mass flow produced by femtosecond laser pulses are studied. In experiments with a gold film, the Ti:sapp laser pulse having a focal size of 100 microns on a target was used, while a soft X-ray probe pulse was utilized for diagnostics. The experimental data are compared with simulated mass flows obtained by two-temperature hydrodynamics and molecular dynamics methods. Simulation shows evolution of a thin surface layer pressurized after electron–ion thermalization, which leads to melting, cavitation and formation of spallation liquid layer. The calculated asymptotic surface velocity of this layer as a function of fluence is in reasonably good agreement with experimental data.

Ultrashort laser pulse is a unique nanometallurgical tool which operates at extreme conditions: ultimate strength of material, the smallest spatiotemporal scales, and nonequlibrium crystallization. Approaches to ultimate strength and to highly nonequlibrium crystallization are tightly coupled with the smallness of spatiotemporal scales. Usage of the tool opens new opportunities to create materials with enhanced surface hardness, anticorrosion properties, and diverted optical constants. To use these advantages we have first of all to develop clear and reliable physical model. The paper presents new results concerning interactions of optical or X-ray lasers with metals. It is shown that ultrashort laser pulse melts surface layer, sends shock into bulk, and foams molten metal. Dense dislocation bilayer is created thanks to fast recrystallization (the first sublayer) and plastic transformations behind strong shock (the second sublayer). Plastic shock generated at moderate laser intensities attenuates sharply during its propagation into metal. During this attenuation, a plastic shock regenerates into a powerful elastic shock. This process defines boundary between the second dislocation sublayer and undamaged solid. Mechanical breaking of foam after its strong stretching and fly away of a part of melt together with fast freezing are responsible for appearance of chaotic frozen nanostructures at an irradiated surface.

We present an overview of our systematic studies of the surface modifications resulting from the interactions of both single and multiple picosecond soft x-ray laser (SXRL) pulses with materials, such as gold (Au), copper (Cu), aluminum (Al), and lithium fluoride (LiF). We show experimentally the possibility of the precise nanometer size structures (~10–40 nm) formation on their surfaces by ultra-low (~10–30 mJ/cm2 ) fluencies of single picosecond SXRL pulse. Comparison experimental results with the atomistic model of ablation, which was developed for the single SXRL shot interaction with dielectrics and metals, is provided. Theoretical description of surface nanostructures is considered and is shown that such structures are formed after laser illumination in a process of mechanical spallation of ultrathin surface layer of molten metal. Spallation is accompanied by a strong foaming of melt, breaking of foam, and freezing of foam remnants. Those remnants form chaotic nanostructures, which are observed in experiments. Our measurements show that electron temperature of matter under irradiation of SXRL was lower than 1 eV. The model calculation also predicts that the ablation induced by the SXRL can create the significant low electron temperature. Our results demonstrate that tensile stress created in LiF and metals by short SXRL pulse can produce spallative ablation of target even for drastically small fluencies, which open new opportunities for material nano processing.

The initial stages of femtosecond laser ablation of gold were observed by single-shot soft X-ray laser interferometer and reflectometer. The ablation front surface and the spallation shell dome structure were observed from the results of the soft X-ray interferogram, reflective image, and shadowgraph. The formation and evolution of soft X-ray Newton’s rings (NRs) were found by reflective imaging at the early stages of the ablation dynamics. The soft X-ray NRs are caused by the interference between the bulk ablated surface and nanometer-scale thin spallation layer. The spallation layer was kept at the late timing of the ablation dynamics, and the height of that reached over 100 μm. The temporal evolution of the bulk ablated surface was observed in the ablation dynamics. From these results, we have succeeded in obtaining the temporal evolution of the ablation front exfoliated from the gold surface.

Understanding the physics of laser-matter interactions in ultrashort pulses is important for many well-acknowledged applications from material modifications to biology. We numerically and experimentally consider the effect of sub-picosecond Ti:sapp laser actions on 60-100 nm gold films mounted onto a fused silica substrate. The pulse energy is sufficient to ablate the films. For the first time, we show that there are different regimes of ablation, and the formation of the 3D structures depends on the value of the absorbed fluence Fabs and the adhesion strength padh between the film and the substrate. Namely, a delamination threshold Fdelam and an ablation threshold Fabl (Fdelam < Fabl) exist if adhesion is weak. Above the lower threshold Fdelam, the whole film delaminates from the substrate. Above the higher threshold Fabl, the thin film ruptures near its middle plane. The external half of the film flies away, while the internal half remains on the glass substrate. There are two thresholds Fdelam and Fabl...

Femtosecond laser ablation processes on platinum, gold, and tungsten were observed by the single shot pump and probe reflective imaging using a soft x-ray laser probe. To avoid the timing error due to the jitter, we adopted a posteriori correction technique by simultaneous measurement of timing between the pump and probe pulses for every single shot, using a soft x-ray streak camera. A clear difference was found in the temporal behavior of the dynamical response of the soft x-ray reflectivity depending on the irradiated laser fluence in these three materials. On the other hand, the narrow dark rings were found in Pt and W, while an additional bright ring was found outside the dark disk in Au. Our result gives the experimental data comparable with various numerical simulations.

Experimental and theoretical investigations of aluminum (Al) and gold (Au) surface modification by soft X-ray laser pulse are presented. Well-polished samples of Al and Au are irradiated by ps-duration pulse with wavelength of 13.9 nm at the energy range of 24–72 nJ. Differences in the melting and the ablation processes for those materials are observed. It is shown that at low laser pulse energy, the nanoscale ripples on the surface may be induced by melting without following ablation. In that case, the nanoscale changes in the surface are caused by splash of molten metal under gradient of fluence. At higher laser pulse energy, the ablation process occurs and craters are formed on the surface. However, the melting determines the size of the modified surface at all ranges of the laser energies. For interpretation of experimental results, the atomistic simulations of melting and ablation processes in Al and Au are provided. The calculated threshold fluencies for melting and ablation are well consistent with measured ones.

We captured single shot images of ZnO emission patterns originating from excitation with the EUV laser at Japan Atomic Energy Agency (JAEA). The EUV laser was focused to a spot size of 1 μm on the ZnO crystal by a Fresnel Zone Plate (FZP). The FZP was shifted along the propagation direction of the EUV beam, essentially changing the spot size at the ZnO surface. The emission pattern was detected by a magnifier. The waist radii were determined from the fitting curve as 5.0 μm along the horizontal axis and 4.7 μm along the vertical axis. We also evaluated the spatial resolution of the magnifier, which consists of a Schwarzschild mirror, two lenses and a camera lens to be about 4 μm. These results suggest that the ZnO crystal has sub-micron spatial resolution when used as an in-situ imaging device for the diagnostics of EUV/X-ray sources.

Synchrotron generated monochromatic X-rays can be precisely tuned to the K-shell energy of high Z materials resulting in the release of the Auger electrons. It has been clearly shown that precisely tuned monochomatic X-rays enable us to destruct tumor mass loaded with high Z materials.