Masatoshi Hatayama

Generation of sub-10-fs, 5-mJ-optical pulses using a hollow fiber with a pressure gradient

We propose and demonstrate a pulse compression technique using a hollow fiber with a pressure gradient. This technique improves the spatial and spectral qualities of femtosecond laser pulses spectrally broadened by self-phase modulation, and allows an increase of the pulse energy for pulse compression. Using chirped mirrors for dispersion compensation, we have successfully compressed the pulse to less than 10 fs with an energy as high as 5 mJ.

Propagation dynamics of femtosecond laser pulses in a hollow fiber filled with argon: constant gas pressure versus differential gas pressure

We investigate the dynamics of femtosecond laser pulses propagating in a hollow fiber filled with argon, through a full numerical solution of the nonlinear Schrodinger equation. The simulation results show that, if the intensity is low and no ionization takes place, the spatial profile of the beam does not change very much so that its propagation model may be simplified to a one-dimensional model. If the intensity is high and ionization takes place, the spatial dynamics as well as temporal dynamics become very complicated because of self-focusing and defocusing. It is found that, for the same value of the B integral, self-focusing inside a hollow fiber can be substantially suspended by a differential gas pressure technique, where the gas pressure is set to be a minimum at the entrance and then increased with the propagation distance. Numerical simulations show that using such a technique, the energy transmitted during propagation inside hollow fiber is significantly enhanced, and the spatial phase is also improved.

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.

Spatiotemporal dynamics of high-intensity femtosecond laser pulses propagating in argon

The spatiotemporal dynamics of high-intensity femtosecond laser pulses propagating in argon have been investigated experimentally and the results compared with numerical calculations. After the beam expands and refocuses, the pulse splits in two. While the subcomponent expands immediately, the main component propagates as a filament. The pulse duration of the main component is less than half the initial pulse duration and the spectrum is broadened by a factor of two.

Broadband extreme ultraviolet multilayer mirror for supercontinuum light at a photon energy of 35-65 eV

We have developed a broadband multilayer mirror for photon energies between 35 and 65 eV. This extreme ultraviolet (EUV) mirror has an almost flat reflectivity profile at normal incidence, and the average reflectivity is 8%. The bandwidth of this mirror is 30 eV centered around 50 eV, 1.5 times broader than conventional multilayer mirrors in this energy region. This new mirror can be used in a variety of applications using supercontinuum EUV light sources including attosecond metrology.

Wide-range narrowband multilayer mirror for selecting a single-order harmonic in the photon energy range of 40-70 eV.

An experimental demonstration of a wide-range narrowband multilayer mirror for selecting a single-order high-harmonic (HH) beam from multiple-order harmonics in the photon energy range between 40 eV and 70 eV was carried out. This extreme ultraviolet (XUV) mirror, based on a pair of Zr and Al0.7Si0.3 multilayers, has a reflectivity of 20-35% and contrast of more than 7 with respect to neighboring HHs at angles of incidence from 10 to 56.9 degrees, assuming HHs pumped at 1.55 eV. Thus, specific single-order harmonic beams can be arbitrarily selected from multiple-order harmonics in this photo energy range. In addition, the dispersion for input pulses of the order of 1 fs is negligible. This simple-to-align optical component is useful for the many various applications in physics, chemistry and biology that use ultrafast monochromatic HH beams.

High-Transmittance Free-Standing Aluminum Extreme Ultraviolet Filter

We have designed and fabricated a high-transmittance, extremely flat, free-standing extreme ultraviolet (EUV) filter for the wavelength region between 17–50 nm. The Al thin-film filter, which is coated with a SiC film to increase its oxidation tolerance, has a transmittance of 29% at 30 nm. This value is approximately 5 times higher than that of an uncoated filter fabricated using a conventional method.

Magnetic characteristics and nanostructures of FePt granular films with GeO2 segregant

To realize a granular film composed of L10-FePt grains with high uniaxial magnetic anisotropy energy, Ku, and segregants for energy-assisted magnetic recording, a FePt-GeO2/FePt-C stacked film was investigated in the engineering process. The FePt-GeO2/FePt-C stacked film fabricated at a substrate temperature of 450 °C realized uniaxial magnetic anisotropy, Kugrain, of about 2.5 × 107 erg/cm3, which is normalized by the volume fraction of FePt grains, and a granular structure with an averaged grain size of 7.7 nm. As the thickness of the FePt-GeO2 upper layer was increased to 9 nm, the Ku values were almost constant. That result differs absolutely from the thickness dependences of the other oxide segregant materials such as SiO2 and TiO2. Such differences on the oxide segregant are attributed to their chemical bond. The strong covalent bond of GeO2 is expected to result in high Ku of the FePt-GeO2/FePt-C stacked films.

Mo/Si multilayer mirrors with 300-bilayers for EUV lithography

Mo/Si multilayer mirror with 300-bilayers for EUV lithography is developed for the purpose of long-lifetime use in LPP EUV source. The multilayer mirrors are fabricated by a magnetron sputtering method and are characterized by coherence scanning interferometry, XRR and EUV reflectometer. The results show the excellent performance of 320 layer pair multilayer being useful for EUV multilayer mirror.

Progress and Prospects of X-Ray Laser Research in QST

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.