H. Sakagami

M. Hashida

M. Hata

Y. Miyasaka

National Institute for Fusion Science (NIFS-Repository)

Recently, the PIC simulation results have been re-ported 1), 2) to discuss the dynamics of periodic nano-structuring on solid surface induced by surface plasmawave. However generation mechanism of surface plasmawave is still under investigation. To visualize the surfaceplasma wave induced by femtosecond laser, 2D PIC sim-ulation by using the code FISCOF3),4) has been demon-strated for initially preformed plasma on a target. Forthe simulation, the surface of the preformed plasma waslocated at z = 2 µm (as shown as black dotted line inFig. 1(a)) and its thickness of 2 µm in the z directionof the (z, y) simulation plane. And the laser beam wasirradiated onto the surface from left hand side as shownin black arrow. The electron density of the pre-plasmawas varied in the range of 0 - 1.0 ncr by 0.1 ncr step,where ncr was the critical density for laser wavelengthof 800nm. The plasma was initially characterized by aMaxwellian distribution with electron temperature Te=1keV and ion temperature Ti= 0.1Te. Hydrogen plasmami/ me =1836/16 was used to speed up the simulationtime, where mi and me are the ion and electron mass.The charge of the ions was Z = 1. Even though theformation mechanism of the surface wave is essentiallysame for the plasma involving the heavy elements ZHigh,the time to develop the surface wave is sifted to longerthan that for light elements. However, we have used theelectron-hydrogen plasma to reduce the calculation time.The target of 10 ncr was located behind the pre-plasmaand its dimension of 10 µm thick and 20 µm wide. In-tense laser (I=1.0×1016 W/cm2, λL=800 nm, rise time=15 fs) was irradiated continuously onto the preformedplasma target with normal incidence. The laser was lin-early polarized with the direction parallel to y axis. Fig-ure 1 shows the electron density distribution at the t =650 fs for 0.7 ncr of pre-plasma. The simulation resultsshow that the surface wave is produced on the surface ofexpanding preformed plasma at z = 1.2 µm. The periodof the surface wave was analyzed by Fourier transformfor the electron density distribution in the area of z =1.0 - 1.5 µm and y = -5 - 5 µm. The obtained FFTspectrum is shown in Fig. 1(b). The period of surfacewave is 720 nm at 0.7 ncr and depend on preformedplasma density ncr. Figure 1(c) shows the interspace ofthe surface wave depend on preformed plasma density.Lower the preformed plasma density, interspace of thesurface wave tends to be shorter. The obtained simula-tion result is helpful to discuss the dynamics of the sur-face plasma wave generation. However, the irradiatedlaser intensity is set three orders of magnitude higherthan that obtained by the experiment since the multipulse irradiation effect could not take it into account forthis 2D-simulation in realistic calculation time. The peri-odic grating structure was self-organized experimentallyas a result of multi pulse irradiation in the range of 25- 1000 pulses5),6). After several pulses irradiation, thesurface of metal was not flat. Thus the structures withnanometer size have been distributed and reported in 7).The structures might be contributed to enhance the laserintensity locally through the near field effect. To reducethis discrepancy, we need further investigation to expressas a cumulation effect for multi pulse irradiation.Fig. 1: (a) Electron density distribution in the z-y planeat irradiation time of t =650 fs for 0.7ncr. (b) Fourier-transform spectrum for the surface electron density of(a) in the area of z = 1.0 - 1.5 µm and y = -5 - 5 µm.(c) Dependence of the normalized surface-wave on pre-formed plasma density.1) Djouder, M. et al.: Appl. Surf. Sci. 258(2012)2580.2) Djouder, M. et al.: Appl. Surf. Sci. 258(2013)711.3) Sakagami, H. and Mima, K. : Proc. 2nd Int. Conf.Iner-tial Fusion Sci. Appl. (2001)pp.380-383.4) Ogata, T., Sakagami, H., Hashida, M., and Sakabe,S. : Meeting Abstracts Phys. Soc. Jpn. 67 , Issue 1,Part 4, (2012)947.5) Bonse, J. et al.: J. Laser Appl. 24(2012)042006 .6) Hashida, M., Ikuta, Y., Miyasaka, Y. et al.: Appl.Phys. Lett. 102(2013)174106.7) Shimizu, M., Hashida, M., Miyasaka, Y. et al.: Appl.Phys. Lett. 103(2013)174106.424§22. Mechanism of Nanostructure Formation on Metal Surface Induced by Intense Short Pulse LaserHashida, M. (Kyoto Univ.),  Sakagami, H.,  Miyasaka, Y. (Kyoto Univ.),  Hata, M. (Kyoto Univ.)

Mechanism of Nanostructure Formation on Metal Surface Induced by Intense Short Pulse Laser

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