Line pattern in plasma polymer films with embedded gold and silver nanoparticles by irradiation with ultrashort laser pu
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Line pattern in plasma polymer films with embedded gold and silver nanoparticles by irradiation with ultrashort laser pulses Andreas Kiesow, Sven Strohkark, Katrin Löschner, Andreas Heilmann Fraunhofer-Institut für Werkstoffmechanik Heideallee 19, 06120 Halle, Germany Alexander Podlipensky, Amin Abdolvand, Gerhard Seifert, Heinrich Graener Fachgruppe Optik, Fachbereich Physik Martin-Luther-Universität Halle-Wittenberg, 06099 Halle, Germany ABSTRACT A method to generate line pattern structures in thin organic films with embedded gold and silver nanoparticles is introduced. The films are irradiated with ultrashort, linearly polarized laser pulses. As we find out from electron microscopy (TEM, SEM), periodically arranged line-like structures are formed after the irradiation. We have used so far three different laser wavelengths (266 nm, 400 nm, 800 nm). The resulting structure periods in the range of 170 nm to 600 nm indicate a linear correlation of the period length of the line structures and the wavelength of the incident laser light. The direction of the particle lines corresponds to the linear polarization of the laser pulses. The anisotropic structure modification is mirrored in corresponding anisotropic optical film properties. INTRODUCTION The laser-induced photomodification of metal nanoparticles currently attracts considerable technological interest as a relatively uncomplicated method to manipulate size, shape, and spatial arrangement of metal clusters in nanostructured composite materials. Laser-induced modifications of implanted metal nanoparticles in dielectrics are reviewed by [1]. Recently, femtosecond laser pulse assisted shape modification was demonstrated for metal nanoparticles embedded in silica glass [2]; after irradiation, the originally spherical particles change to prolate ellipsoids surrounded by a halo of small particles. Here, in continuation of [3], we introduce a method where in addition to shape and size changes of the particles, a periodic superstructure with periodically arranged line-like areas is generated within the thin film. EXPERIMENTAL DETAILS The nanoparticle-containing films were deposited by alternating plasma polymerization of hexamethyldisilazane and metal evaporation on a quartz glass substrate. The films thickness is about 100 nm (see Fig. 1). The films are characterized by a multilayer structure, i.e. all particles are arranged in one plane within the plasma polymer matrix. Figure 1 shows the vertical film structure. Film preparation and properties are described in more detail in [4, 5]. For the work presented here, gold and silver were used as embedded metal. The samples were irradiated by series of linearly polarized pulses at 800 nm, 400 nm (second harmonic) and 266 nm (third harmonic), which were obtained from a mode locked Ti:sapphire laser with regenerative amplification. The parameters for the irradiation were: pulse duration of 140 fs, pulse energies in
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the range from 10 to 700 µJ per pulse, nearly Gaussian intensity profile with typical spot
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