[en] Highlights: • Sub-wavelength periodic ripples are formed by a femtosecond laser Bessel beam on c-Si in vacuum. • The periodic pattern extends along the laser-polarization direction to reach a very low fluence zone, suggesting surface EM waves propagating from the peak fluence point. • Circular polarization ablates a hole of ∼600-nm diameter with a perfectly round opening. - Abstract: We investigated the effect of periodic trenches (ripples) formed through sub-micrometer machining by focusing linearly polarized femtosecond laser pulses with conical axicons to a ∼1 μm spot on c-Si in vacuum. The machined patterns were compared with the fluence distribution in the Bessel zone observed with a CCD camera. A fluence well below the ablation threshold successively forms a modification pattern, periodic ripples, and a crater with increasing number of laser shots. The rippled zone covers an area larger than the expected domain of laser modification and extends along the polarization direction of the laser beam. In contrast, a circularly polarized laser beam forms a round hole with a clear edge and does not leave a ripple-like corrugation. In comparison with a laser beam with circular polarization, a laser beam with linear polarization having the same fluence forms a crater with less laser shots because of pre-formed ripples but at the expense of degrading the machining quality. These results suggest that the formation of ripples results from surface plasmon waves generated at the peak fluence point. The periodic ripples can potentially be applied in the fabrication of gratings and photonic crystals.

[en] DNA repair has been shown to be associated with several hereditary diseases with a high incidence of cancer. Among them, xeroderma pigmentosum (XP) has been most extensively studied for clinical and repair characteristics and genetic heterogeneity. A new complementation group found in Japan, the F group, showed significantly different characteristics from other groups, having slower but considerable excision repair capacity of ultraviolet (UV) damage despite very low unscheduled DNA synthesis (UDS) after UV irradiation. This suggested that UDS may not represent the relative capacity of excision repair or repair responsible for cellular sensitivity to the agents. A discrepancy between UDS and cellular sensitivity in terms of cell killing was also noted in the effect of mitomycin-C on Bloom's syndrome cells, and that of antipain on XP cells. Estimates of gene frequency of the cancer-prone hereditary diseases and the frequency of the heterozygotes or carriers of these genes along with some of the chromosome-related cancer-prone diseases were made. The total frequency of the population with a high risk of cancer may be in the neighborhood of 3% at maximum. Judging from the fact that approximately 20% of the population in Japan may eventually develop cancer, the genetic factor should not be regarded as the major cause of cancer in Japan. (author)

[en] Femtosecond laser machining of crystalline Si in vacuum resulted in the formation of pillars and particles of ∼100 nm in size at the wall surfaces and the periphery of the ablated hole. These structures were created at a laser fluence below the ablation threshold. The nanopillars and nanoparticles appear to grow from the target surface. The target surface near the particles showed molten features with descending height, indicating significant mass transport from the surface layer to the particles. The nanopillars and nanoparticles likely formed as a result of successive crystal growth processes including amorphization of the laser-irradiated target surface, followed by crystalline nucleation, melting of the amorphous Si surrounding the crystalline particles, and liquid Si creeping over particle surfaces leading to an increase in particle size. By repeating these processes, the particles grow in cumulative laser shots. These particles are the major debris components distributed near micron-sized holes formed at the ablation threshold fluence in vacuum.

[en] The ablation products formed by femtosecond laser irradiation of crystalline Si are investigated in detail in air at 1 atm. Ablation products possess a cotton–wool like morphology accumulated on the target surface in the vicinity of the ablated crater. Web-like links of roundish fine particles are also found in more distant regions. The particles are either single or polycrystalline Si linked with finer particulates and amorphous material. The particles are considered to be the result of a fragmentation and coalescence process of molten Si in the air. Electron beam diffraction indicates a small fraction of crystalline quartz among the debris particles. The cotton–wool-like debris are composed of an oxygen-rich Si compound with an atomic concentration ratio resembling that of SiO₂. Ablated matter in vacuum within the peripheral zone of the ablated crater predominantly takes the form of roundish particles with a molten feature. Particles found at a distance of a few millimeters from the crater are nearly spherical and composed of crystalline Si. Particles located far from the crater are likely solidified droplets of liquid Si fragmented from the crater that have cooled at a rate favorable to crystallization during flight.

[en] A compact femtosecond Ti:sapphire/KrF laser system which produces ultraviolet and ultrashort pulse has been developed. It consists of a mode-locked Ti:Sapphire laser, a regenerative amplifier, a frequency conversion system and a KrF excimer amplifier. The system can produce 50 mJ of pulse energy at a pulse width of 440 fs. The pulse width can be reduced down to 100 fs region using a prism pair compressor. (author)

[en] Skin fibroblasts from a patient with Bloom's syndrome (86NoKi) were assayed for various DNA repair activities after ultraviolet light (UV) irradiation. Cultured fibroblasts as well as lymphocytes obtained from this patient showed a high frequency of spontaneous sister chromatid exchanges (SCEs). There was no significant difference between 86NoKi fibroblasts and skin fibroblasts from normal donors in the sensitivity to UV as measured by inactivation of colony forming activity, the capacity of host-cell reactivation (HCR) of UV-irradiated virus, and the amount of unscheduled DNA synthesis (UDS) after UV irradiation. However, the yield of UV-induced SCEs in 86NoKi cells was significantly higher than that in normal cells. (author)

[en] Cell-cycle progression is coordinately regulated by cyclin-dependent kinases (CDKs). The inhibition of CDKs by p21 ^{wafl/Cipl/Sdil} prevents the apoptosis of cells treated with DNA-damaging agents. In this study, we found that butyrolactone I, a specific inhibitor of CDC2 family kinases, blocks the X-ray- or doxorubicin-induced apoptosis of DLD1 (p21 +/+) human colorectal carcinoma cells in a dose-dependent manner. We also found that butyrolactone I inhibits the CDK2 activity and enhances cell survival after an X-ray irradiation or doxorubicin treatment in both DLD1 (p21 -/-) and DLD1 (p21 +/+) cells. These findings suggest that butyrolactone I prevents apoptosis by the direct inhibition of CDK and also, possibly, by CDK-inhibition through p53-independent p21-induction. Our findings indicate that CDK activity is required for DNA-damaging agent-induced apoptosis. (author)

[en] Organisms have the ability to protect their genetic materials against endogenous and environmental agents such as reactive oxygen species (ROS), ionizing radiation and a variety of chemical compounds. The basis of the protection mechanisms has been evolutionarily conserved, from yeast to human, but each organism has often specialized mode of regulation by using different set of machineries, especially in lower eukaryotes. In this review, we describe novel findings about the unconventional molecular mechanisms of DNA repair, DNA damage response, and genetic and phenotypic variations involved in adaptation to environmental conditions, in the budding yeast Saccharomyces cerevisiae. These findings provide insights on how organisms adapt to the environmental stresses to survive to the next generation through the evolutionary process. (author)

[en] In the 1970s and 1980s, Planel et al. reported that the growth of paramecia was decreased by shielding them from background radiation. In the 1990s, Takizawa et al. found that mouse cells displayed a decreased growth rate under shielded conditions. The purpose of the present study was to confirm that growth is impaired in organisms that have been shielded from background radiation. Radioprotection was produced with a shielding chamber surrounded by a 15 cm thick iron wall and a 10 cm thick paraffin wall that reduced the γ ray and neutron levels in the chamber to 2% and 25% of the background levels, respectively. Although the growth of Paramecium tetraurelia was not impaired by short-term radioprotection (around 10 days), which disagreed with the findings of Planel et al., decreased growth was observed after long-term (40-50 days) radiation shielding. When mouse lymphoma L5178Y cells were incubated inside or outside of the shielding chamber for 7 days, the number of cells present on the 6th and 7th days under the shielding conditions was significantly lower than that present under the non-shielding conditions. These inhibitory effects on cell growth were abrogated by the addition of a ¹³⁷Cs γ-ray source disk to the chamber. Furthermore, no growth retardation was observed in XRCC4-deficient mouse M10 cells, which display impaired DNA double strand break repair. (author)

No abstract available