Optical near-field enhancement around lithographic metallic nanostructures using an azo-dye polymer: direct observation
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Optical near-field enhancement around lithographic metallic nanostructures using an azodye polymer: direct observation and realization of sub-wavelength complex structures Christophe Hubert1, Anna Rumyantseva1, Gilles Lérondel1, Johan Grand1, Sergeï Kostcheev1, Laurent Billot1, Alexandre Vial1, Renaud Bachelot1, and Pascal Royer1 Gilbert Chang2,3, Stephen K. Gray2, and Gary P. Wiederrecht2, George C. Schatz3 1 Laboratoire de nanotechnologie et d’Instrumentation Optique, Université de Technologie de Troyes, 12 rue marie curie, B.P. 2060, 10000 Troyes, France 2 Chemistry Division and Center for Nanoscale Materials, Argonne National Laboratory, Argonne, IL 60439 3 Department of Chemistry and Institute for Nanotechnology, Northwestern University, Evanston IL 60208 ABSTRACT We report on the direct observation of optical near-field enhancement around metallic nanoparticles. We used an easy to set up approach which consists in irradiating a photosensitive azo-dye polymer film spin-coated on metallic nanostructures. Photoinduced topographical modifications of the polymer film surface are characterized after irradiation by atomic force microscopy (AFM). Comparisons between AFM images and numerical simulations show that these photo-induced topography agrees with the near-field intensity distribution around the nanostructures. The possibility of generating complex structures is demonstrated. INTRODUCTION Near-field optics has attracted much attention during the past two decades because it overcomes the diffraction limit of conventional microscopes. At present, most widely used methods for optical near-field imaging of nanostructures are photon scanning tunneling microscopy (PSTM), aperture scanning near-field optical microscopy (SNOM) and apertureless scanning near field optical microscopy (ASNOM)1-3. The main difficulties related to these techniques are signal discrimination as well as image interpretation. Indeed, images can be affected by artifacts and, the interaction between the probe and the structures studied can lead to perturbations in the nearfield distribution. In order to avoid these problems, other alternative techniques have recently been proposed, using a photosensitive polymer to map the optical near field of different kinds of particles.4-6 While the different results obtained have successfully shown topographic features induced by the particles, they have not been able to spatially resolve the optical fields with the precision of scanning probe methods. We report in this work an original approach based on the exposure of lithographically designed metallic nanostructures coated with a photosensitive azo-dye polymer. The dipolar response of excited metal nanoparticles is clearly observed. Experimental results are compared with numerical simulations performed with the FDTD method. The effect of light polarization direction as well as the generation of sub-micrometer complex structures are emphasized.
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EXPERIMENTAL METHOD Sample preparation process can be divided into three steps, as illustrated
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