Sub-nanometer scale depth patterning on sapphire crystal by femtosecond soft x-ray laser pulse irradiation.

Damage thresholds and structures on a metal aluminum and an aluminum oxide crystal induced by the soft x-ray free electron laser irradiations were evaluated. Distinctive differences in damage thresholds and structures were observed for these materials. On the aluminum oxide crystal surface, in particular, a novel, to the best of our knowledge, surface processing, which we suggest defining as "peeling," was recognized. Surface structures formed by peeling had extremely shallow patterning of sub-nanometer depth. For the newly observed peeling process, we proposed a scission of chemical bond, i.e., binding energy model, in the crystal.

Damage threshold of LiF crystal irradiated by femtosecond hard XFEL pulse sequence.

Here we demonstrate the results of investigating the damage threshold of a LiF crystal after irradiating it with a sequence of coherent femtosecond pulses using the European X-ray Free Electron Laser (EuXFEL). The laser fluxes on the crystal surface varied in the range ∼ 0.015-13 kJ/cm2 per pulse when irradiated with a sequence of 1-100 pulses (tpulse ∼ 20 fs, Eph = 9 keV). Analysis of the surface of the irradiated crystal using different reading systems allowed the damage areas and the topology of the craters formed to be accurately determined. It was found that the ablation threshold decreases with increasing number of X-ray pulses, while the depth of the formed craters increases non-linearly and reaches several hundred nanometers. The obtained results have been compared with data already available in the literature for nano- and picosecond pulses from lasers in the soft X-ray/VUV and optical ranges. A failure model of lithium fluoride is developed and verified with simulation of material damage under single-pulse irradiation. The obtained damage threshold is in reasonably good agreement with the experimentally measured one.

Independent contribution of optical attenuation length in ultrafast laser-induced structural change.

Although laser irradiation with femtosecond pulses is known to generate crystallization and morphological changes, the contribution of optical parameters to material changes is still in discussion. Here, we compare two structures irradiated near Si-L2,3 edges by an extreme ultraviolet femtosecond pulse. Our result implies that, despite the femtosecond irradiation regime, these values of the optical attenuation length between the wavelengths of 10.3-nm and 13.5-nm differ by one order of magnitude. From the structural comparison, the original crystalline state was maintained upon irradiation at 13.5-nm, on the other hand, transition to an amorphous state occurred at 10.3-nm. The difference in optical attenuation length directly influence to the decision of material crystallization or morphological changes, even if the irradiation condition is under the femtosecond regime and same pulse duration. Our result reveals the contribution of optical attenuation length in ultrafast laser-induced structural change.

Surface processing of PMMA and metal nano-particle resist by sub-micrometer focusing of coherent extreme ultraviolet high-order harmonics pulses.

We demonstrate sub-micrometer processing of two kinds of thin films, polymethyl methacrylate (PMMA) and metal nano-particle resist, by focusing high-order harmonics of near-IR femtosecond laser pulses in the extreme ultraviolet (XUV) wavelength region (27.2-34.3 nm) on the thin film samples using an ellipsoidal focusing mirror. The ablation threshold fluences for the PMMA sample and the metal nano-particle resist per XUV pulse obtained by the accumulation of 200 XUV pulses were determined to be 0.42mJ/cm2 and 0.17mJ/cm2, respectively. The diameters (FWHM) of a hole created by the ablation on the PMMA film at the focus were 0.67 µm and 0.44 µm along the horizontal direction and the vertical direction, respectively. The fluence dependence of the Raman microscope spectra of the processed holes on the PMMA sample showed that the chemical modification, in which C=C double bonds are formed associated with the scission of the PMMA polymer chains, is achieved by the irradiation of the XUV pulses.

Soft x-ray laser beamline for surface processing and damage studies.

We have developed a soft x-ray laser (SXRL) beamline equipped with an intensity monitor dedicated to ablation study such as surface processing and damage formation. The SXRL beam having a wavelength of 13.9 nm, pulse width of 7 ps, and pulse energy of around 200 nJ is generated from Ag plasma mediums using an oscillator-amplifier configuration. The SXRL beam is focused onto the sample surface by the Mo/Si multilayer coated spherical mirror. To get the correct irradiation energy/fluence, an intensity monitor composed of a Mo/Si multilayer beam splitter and an x-ray charge-coupled device camera has been installed in the beamline. The Mo/Si multilayer beam splitter has a large polarization dependence in the reflectivity around the incident angle of 45°. However, by evaluating the relationship between reflectivity and transmittance of the beam splitter appropriately, the irradiation energy onto the sample surface can be derived from the energy acquired by the intensity monitor. This SXRL beamline is available to not only the ablation phenomena but also the performance evaluation of soft x-ray optics and resists.

Laser-induced damage thresholds and mechanism of silica glass induced by ultra-short soft x-ray laser pulse irradiation.

Laser-induced damage thresholds (LIDTs) of silica glasses obtained by the femtosecond soft x-ray free-electron laser (SXFEL, 13.5 nm, 70 fs) and the picosecond soft x-ray laser (SXRL, 13.9 nm, 7 ps) are evaluated. The volume of the hydroxyl group in the silica glasses influenced its LIDTs. The LIDTs obtained in this research by the femtosecond SXFEL and the picosecond SXRL were nearly identical, but were different from that by the nanosecond soft x-ray pulse. The photoionization processes of silica glass in context of the laser-induced damage mechanism (LIDM) are also discussed. In the ultra-short soft x-ray pulse irradiation regime, the LIDM can be speculated to include the spallation process with a scission of bondings.

Design of a prototype split-and-delay unit for XFEL pulses, and their evaluation by synchrotron radiation X-rays.

A prototype split-and-delay unit (SDU) for X-ray free-electron laser (XFEL) pulses is proposed based on the Graeff-Bonse four-Bragg-reflection interferometer by installing 12.5° slopes. The SDU can continuously provide a delay time from approximately -20 to 40 ps with a resolution of less than 26 fs. Because the SDU was constructed from a monolithic silicon crystal, alignment is straightforward. The obtained thoroughputs of the SDU reached 0.7% at 7.46 keV and 0.02% at 14.92 keV. The tunability of the delay time using the proposed SDU was demonstrated by finding the interference effects of the split X-rays, and the time resolution of the proposed SDU was evaluated using the width of the interference pattern recorded on the X-ray charge-coupled device camera by changing the energy, i.e. longitudinal coherence length, of the incident X-rays. It is expected that the proposed SDU will be applicable to XFEL experiments using delay times from tens of femtoseconds to tens of picoseconds, e.g. intensity correlation measurements.

Evaluation of a flat-field grazing incidence spectrometer for highly charged ion plasma emission in soft x-ray spectral region from 1 to 10 nm.

A flat-field grazing incidence spectrometer operating on the spectral region from 1 to 10 nm was built for research on physics of high temperature and high energy density plasmas. It consists of a flat-field grating with 2400 lines/mm as a dispersing element and an x-ray charged coupled device (CCD) camera as the detector. The diffraction efficiency of the grating and the sensitivity of the CCD camera were directly measured by use of synchrotron radiation at the BL-11D beamline of the Photon Factory (PF). The influence of contamination to the spectrometer also was characterized. This result enables us to evaluate the absolute number of photons in a wide range wavelength between 1 and 10 nm within an acquisition. We obtained absolutely calibrated spectra from highly charged ion plasmas of Gd, from which a maximum energy conversion efficiency of 0.26% was observed at a Nd:YAG laser intensity of 3 × 1012 W/cm2.

Observation and theory of X-ray mirages.

The advent of X-ray lasers allowed the realization of compact coherent soft X-ray sources, thus opening the way to a wide range of applications. Here we report the observation of unexpected concentric rings in the far-field beam profile at the output of a two-stage plasma-based X-ray laser, which can be considered as the first manifestation of a mirage phenomenon in X-rays. We have developed a method of solving the Maxwell-Bloch equations for this problem, and find that the experimentally observed phenomenon is due to the emergence of X-ray mirages in the plasma amplifier, appearing as phase-matched coherent virtual point sources. The obtained results bring a new insight into the physical nature of amplification of X-ray radiation in laser-induced plasma amplifiers and open additional opportunities for X-ray plasma diagnostics and extreme ultraviolet lithography.

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.

γ-H2AX and phosphorylated ATM focus formation in cancer cells after laser plasma X irradiation.

The usefulness of laser plasma X-ray pulses for medical and radiation biological studies was investigated, and the effects of laser plasma X rays were compared with those of conventional sources such as a linear accelerator. A cell irradiation system was developed that used copper-Kα (8 keV) lines from an ultrashort high-intensity laser to produce plasma. The absorbed dose of the 8 keV laser plasma X-ray pulse was estimated accurately with Gafchromic® EBT film. When the cells were irradiated with approximately 2 Gy of laser plasma X rays, the circular regions on γ-H2AX-positive cells could be clearly identified. Moreover, the numbers of γ-H2AX and phosphorylated ataxia telangiectasia mutated (ATM) foci induced by 8 keV laser plasma X rays were comparable to those induced by 4 MV X rays. These results indicate that the laser plasma X ray source may be useful for radiation biology studies.

Note: Application of laser produced plasma K alpha x-ray probe in radiation biology.

A dedicated radiation biology x-ray generation and exposure system has been developed. 8.0 keV in energy x-ray pulses generated with a femtosecond-laser pulse was used to irradiate sample cells through a custom-made culture dish with a silicon nitride membrane. The x-ray irradiation resulted in DNA double-strand breaks in the nucleus of a culture cell that were similar to those obtained with a conventional x-ray source, thus demonstrating the feasibility of radiobiological studies utilizing a single burst of x-rays focused on single cell specimens.

Fabrication and evaluation of a wideband multilayer laminar-type holographic grating for use with a soft-x-ray flat-field spectrograph in the region of 1.7 keV.

A multilayer laminar-type holographic grating having an average groove density of 2400 lines/mm is designed and fabricated for use with a soft-x-ray flat-field spectrograph covering the 0.70-0.75 nm region. A varied-line-spaced groove pattern is generated by the use of an aspheric wavefront recording system, and laminar-type grooves are formed by a reactive ion-etching method. Mo/SiO2 multilayers optimized for the emission lines of Hf-M, Si-K, and W-M are deposited on one of the three designated areas on the grating surface in tandem. The measured first-order diffraction efficiencies at the respective centers of the areas are 18%-20%. The flat-field spectrograph equipped with the grating indicates a spectral linewidth of 8-14 eV for the emission spectra generated from electron-impact x-ray sources.

Development of multilayer laminar-type diffraction gratings to achieve high diffraction efficiencies in the 1-8 keV energy region.

W/C and Co/SiO(2) multilayer gratings have been fabricated by depositing a multilayer coating on the surface of laminar-type holographic master gratings. The diffraction efficiency was measured by reflectometers in the energy region of 0.6-8.0 keV at synchrotron radiation facilities as well as with an x-ray diffractometer at 8.05 keV. The Co/SiO(2) and W/C multilayer gratings showed peak diffraction efficiencies of 0.47 and 0.38 at 6.0 and 8.0 keV, respectively. To our knowledge, the peak efficiency of the W/C multilayer grating is the highest measured with hard x rays. The diffraction efficiency of the Co/SiO(2) multilayer gratings was higher than that of the W/C multilayer grating in the energy range of 2.5-6.0 keV. However, it decreased significantly in the energy above the K absorption edge of Co (7.71 keV). For the Co/SiO(2) multilayer grating, the measured diffraction efficiencies agreed with the calculated curves assuming a rms roughness of approximately 1 nm.

Fabrication of multilayer mirrors consisting of oxide and nitride layers for continual use across the K-absorption edge of carbon.

The development of multilayer mirrors for continual use around the K-absorption edge of carbon (4.4 nm) has been begun. Cobalt oxide (Co3O4), silicon oxide (SiO2), and boron nitride (BN) are found to be suitable for multilayer mirrors on the basis of theoretical calculations for wavelengths around the carbon K-absorption edge region. X-ray reflectivity curves with CuKalpha1 x rays of the fabricated Co3O4/SiO2 multilayers have sharp Bragg peaks, and the layer structures evaluated from transmission electron microscopy (TEM) observations are uniform. On the other hand, the Bragg peaks of Co3O4/BN multilayers split, and aggregated Co3O4 is observed. To improve the Co3O4 layer structure, chromium oxide (Cr2O3) was mixed into Co3O4. The mixed oxide layer structure in the Mix/BN multilayer (Mix = Co3O4 + Cr2O3) is relatively uniform, and the Bragg peaks do not split.