H. Sakagami

M. Hashida

M. Shimizu

S. Sakabe

S. Tokita

T. Ogata

Y. Miyasaka

National Institute for Fusion Science (NIFS-Repository)

Amphiphilic molecules such as lipids and surfactantsare composed of both a hydrophilic part and a hydropho-bic part. In aqueous solutions, amphiphilic moleculesoften self-assemble into various structures such as mi-celles, mesophases, and bicontinuous structures1). Suchself-assembly of amphiphilic molecules plays an impor-tant role in many biological and industrial processes.Although numerous computer simulation studies haveso far been done on micelle formation of amphiphilicmolecules, each of which consists of a hydrophilic headgroup and a hydrophobic tail group, there have beenfew theoretical and simulation studies on micelle forma-tion of bolaamphiphilic molecules, each of which con-tains a hydrophobic stalk and two hydrophilic ends. Thepurpose of this study is to clarify the molecular mech-anism of micelle formation in bolaamphiphilic solution.With a view to investigating the micelle formation pro-cess in bolaamphiphilic solution at the molecular level,we perform the molecular dynamics (MD) simulations ofcoarse-grained bolaamphiphilic molecules with explicitsolvent molecules and analyze the micelle formation pro-cess.The computational model is similar to that used inour previous work2),3). A bolaamphiphilic molecule ismodeled as a semiflexible chain which is composed ofa hydrophobic stalk with three particles and two hy-drophilic ends (H1 and H2), each of which consists ofone particle. A solvent molecule is modeled as a hy-drophilic particle. As bonded potentials, we considera bond-stretching potential and a bond-bending poten-tial. The interaction between a hydrophilic particle anda hydrophobic particle is modeled by a repulsive softcore potential and all other interactions are modeled bya Lennard-Jones (LJ) potential. Note that the LJ inter-action parameter ε∗hs2 between a hydrophilic end particle(H2) and a solvent particle can be varied. The numericalintegrations of the equations of motion for all particlesare performed using the velocity Verlet algorithm at con-stant temperature with a time step of 0.0005. We applythe periodic boundary conditions and the number den-sity is set to 0.75. The total number of particles is 5832.Initially, we provide homogeneous bolaamphiphilic solu-tions with the amphiphilic concentration of cs = 0.1 athigh temperature (T ∗ = 10) for various values of the in-teraction parameter ε∗hs2 (0.5 ≤ ε∗hs2 ≤ 5.0). The systemis then quenched to T ∗ = 1.3 and MD simulations of5.0 × 107 time steps are carried out for each simulationrun.We show, in Fig. 1, snapshots of micelles formed bybolaamphiphilic molecules for ε∗hs=1.0, 2.0 and 4.0 atcs = 0.1. Isosurfaces of the density of the hydropho-bic particles, which are calculated by Gaussian splattingtechniques, are depicted in this figure. This figure tellsus that the platelike micelle changes into the wormlikemicelles, and then into the spherical micelles at cs = 0.1as the interaction parameter ε∗hs2 increases.(a)(b)(c)Fig. 1. Snapshots of micelles formed by bolaamphiphilicmolecules at cs = 0.1: (a) the platelike micelle (ε∗hs2 =1.0), (b) the wormlike micelles (ε∗hs2 = 3.0) and (c) thespherical micelles (ε∗hs2 = 5.0). Isosurfaces of the den-sity of the hydrophobic particles, which are calculatedby Gaussian splatting techniques, are depicted to showthe micellar shape clearly. Note that solvent moleculesare not displayed for clarity.1) Hamley, I.W., Introduction to Soft Matter (J. Wiley,Chichester, 2007) Rev. ed.2) Fujiwara, S., Itoh, T., Hashimoto, M., Nakamura, H.and Tamura, Y.: Plasma Fusion Res. 5 (2010) S2114.3) Fujiwara, S., Itoh, T., Hashimoto, M., Tamura, Y.,Nakamura, H. and Horiuchi, R.: Plasma Fusion Res.6 (2011) 2401040.The periodic grating structures structures havebeen found in insulators, semiconductors, and metals.For metals irradiated with linearly polarized femtosec-ond laser, the self-organized grating structures on themetal surface are oriented perpendicular to the laser po-larization. The interspaces of the grating structures areshorter than the laser wavelength and depend on laserfluence. However, the self-organization mechanism ofthe grating structures remains under investigation. Thestructures depend on material properties and laser pa-rameters (wavelength, fluence, pulse duration, numberof pulses, etc.). Even for the same metal, not all experi-mental results reported in the literature can be directlycompared because of experimental differences, for exam-ple, in the laser parameters. For discussing the mecha-nism, a greater amount of systematic experimental datais now needed. One approach to gather systematic ex-perimental data is to investigate the relation betweenthe metal type and the interspaces of grating structures.Another approach is to investigate the relation betweenthe ablation rate and the interspaces for typical metals.We have reported that the grating structure inter-spaces depend on laser fluence for Cu1) irradiated with≤100-fs laser pulses and for several metals2) irradiatedwith 160-fs laser pulses. In the laser fluence ranges whereself-organized grating structures are formed, the inter-spaces of the grating structures are shorter than the laserwavelength of 800 nm. The interspaces increase up to680 nm as laser fluence is increased. We have reportedthat the laser fluence dependence of the grating structureinterspaces is the same for Ti, Pt, Mo, and W metalson which the self-organized structures are formed2),3).We found that, for these metals, the ratio of the grat-ing structure interspaces to the wavelength of the laserlight is 0.85 at FM (the upper limit of the laser fluencerange in which the self-organized grating structures areformed). We explained this dependence of the inter-spaces on laser fluence on the basis of a parametric decay(stimulated Raman scattering) model4). In this model,surface plasma waves (SP waves) are induced at the in-terface between free space (air) and either laser-producedplasma or metal plasma by parametric decay, resultingin the decay of the incident laser light into an SP waveand a scattered electromagnetic wave. The wavelengthof the plasma wave induced on the surface depends ononly the plasma frequency (electron density) of the sur-face plasma. The ratio of the wavelength of the SP waveto that of the laser light changes from 0.5 to 0.85 forplasma frequencies in the range of 0 < ωp/√2 < ωL,where ωp and ωL are the frequencies of the plasma andlaser light, respectively. When the plasma electron den-sity is dependent on laser fluence, the wavelength of theSP wave in turn depends on laser fluence. This modelsatisfactorily explains the results reported for Cu4) andTi, Pt, Mo, and W2). These results indicate that theparametric decay model is valid in regard to only the de-pendence of the interspaces on laser fluence. At present,however, the body of experimental data is insufficientfor verifying this model. The dependence of the inter-spaces on the wavelength of the incident laser can alsobe explained in this model.In the experiment, we investigated the states ofmetal surfaces (Ti and Mo) irradiated with femtosec-ond laser pulses at wavelengths of 800 and 400 nm5) .We evaluated the dependence of periodic structure in-terspaces on laser fluence at the different wavelengths.We also examined the dependence of ablation rate onlaser fluence at the different wavelengths. We found arelation between the ablation rate and laser fluence atwhich self-organized periodic structures were formed. Inaddition, we evaluated the threshold of plasma genera-tion to discuss the extent of the parametric decay model’svalidity.In order to visualize the surface plasma wave in-duced by femtosecond laser, two-dimensional particle incell simulation by using the code FISCOF6) has beendemonstrated for initially pre-formed plasma on a tar-get. The dependence of grating interspace on the laserwavelength was investigated7). It is found that the grat-ing interspace is found to be almost half of the laserwavelength in spite of different wavelengths. The depen-dences of the grating interspace on the laser wavelengthin simulations are similar to those in experiments.1) Hashida, M. et al.: Proc. SPIE Vol.4830 (2003)452.2) Okamuro, K. et al. : Phys. Rev. B 82(2010)165417.3) Hashida, M. et al.:J. Laser Micro/Nanoeng.7(2012)194.4) Sakabe, S. et al: Phys. Rev. B 79, 033409 (2009).5) Hashida, M. et al.:Appl. Phys. Lett. B 102, 174106(2013).6) Sakagami, H. et al.:Proc. 2nd Int. Conf. Inertial Fu-sion Sci. Appl.( 2001), pp380-383.7) Ogata, T. et al: Proc. LPM2012, No.12-15(2012)pp.1-4.421§18. Mechanism of Nanostructure Formation on Metal Surface Induced by Intense Short Pulse LaserHashida, M. (Kyoto Univ.), Ogata, T. (Nagoya Univ.),  Sakagami, H.,  Shimizu, M., Miyasaka, Y., Tokita, S., Sakabe, S. (Kyoto Univ.)

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

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

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