Shape Analysis of Laser Deformed Metallic Nanoparticles H. Graener, A. Podlipensky * , G.Seifert, M. Leitner ** , B. Sepiol ** Martin-Luther-University Halle, Institute of Physics, 06099 Halle, Germany *present address: Institute of Optics, Information and Photonics (Max Planck Research Group), University of Erlangen-Nuremberg **University Vienna, Institute of Materials Physics, Vienna, Austria heinrich.graener@physik.uni-halle.de Abstract: High-intensity fs-laser pulses permanently change the form of silver nanoparticles in glass and result in optical dichroism. Optical and small angle x-ray scattering experiments are shown which clarify the 3-dimensional shape of the resulting particles. 2007 Optical Society of America Ocis codes: 160.2750; 260.3910; 320.7130 Glass samples containing spherical silver nanoparticles show a distinct, isotropic extinction band at about 400 nm caused by the surface plasmon resonance, with a dependence of the band characteristic on size and size distribution of the nanoparticles and on the glass matrix[1]. Illuminating this material with intense, linear polarized, ultrashort laser pulses (frequency near the resonance) induces a permanent optical dichroism[2], which is always related to the polarization axis of the laser. The dichroism is caused by a deformation of the nanoparticles[3]. Transmission electron microscopy (TEM) pictures show an example of an original particle (Fig. 1a), a particle illuminated with a high intensity pulse of I ≈ 2 TW/cm² (Fig. 1b), and one with a low intensity of 0.5 TW/cm² multi shot illumination (Fig. 1c)[4]. The thick arrow indicates the laser polarisation direction. The striking feature is the apparent 90° rotation of the central particle. As the TEM pictures are necessarily two- dimensional, obviously further analysis of the three-dimensional shapes of the resulting particles is needed. To overcome this problem a sample was irradiated (low intensity regime, p-polarized laser pulses) with an angle of incidence of 50°. Then extinction spectra were measured with varying angles of incidence and parallel polarizations of laser light and probing light (p-pol, Fig: 2a). One observes an extinction maximum around 500 nm if the observation angle is the same as the irradiation angle (θ int =0). By changing the observation angle, Fig. 1: TEM Pictures of silver nanoparticles; a) before ultrashort pulse irradiation; b) after irradiation with a single pulse of I ≈ 2TW/cm²; c) after multi pulse irradiation with I = 0.5 TW/cm² a b c E r 300 400 500 600 700 800 0.0 0.2 0.4 0.6 0.8 1.0 Extinction Wavelength λ / nm θ int = 0° θ int = 30° θ int = 60° (a) 300 400 500 600 700 800 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Extinction Wavelength λ λ λ λ / nm θ int = 5° θ int = 30° θ int = 55° (b) Fig. 2: angle dependent extinction spectra after p-polarized laser irradiation; a) p-polarized probe; b) s-polarized probe a2533_1.pdf JTuA109.pdf