Near and FarField Properties of Nanoprisms with R

Authors: Bartłomiej Grześkiewicz Krzysztof Ptaszyński Michał Kotkowiak

Publish Date: 2014/03/07

Volume: 9, Issue: 3, Pages: 607-614

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Abstract

Photonic devices can be developed and their working principle can be understood only by considering the phenomena taking place at the nanoscale level Optical properties of plasmonic structures depend on their geometric parameters and are sensitive to them Recently many advanced methods for the preparation of nanostructures have been proposed however still the geometric parameters are inaccurate Numerical simulations provide a powerful tool for the analysis of plasmonic nanostructures To the best of our knowledge there are not many papers on nearfield and farfield properties of single nanoprism and nanoprism dimer the socalled bowtie with rounded edges For this purpose Finite Integration Technique implemented to the CST Microwave Studio was used Besides the edge rounding an additional modification of the resonance modes was investigated achieved by placement of a spherical nanoparticle in the gap between the prisms Results of numerical simulations indicate that the radius of the curvature edges strongly affects the plasmon peak localization and this effect cannot be neglected in plasmonic device design Increase in the radius of edge curvature causes main extinction crosssection peak blueshift in all cases analyzed Moreover our calculations imply that the nanoparticle in the gap between prisms strongly influences the dependence of spectral properties on the radius curvatureNoble metal nanostructures can strongly enhance an electric field upon incident light illumination due to the phenomenon called localized surface plasmon resonance Collective oscillations of quasifree electrons on the surface of metallic structures and their interactions with molecules lead to the observed surface enhancement of Raman and fluorescence signals 1 2 A lot of structures with different shapes have been hitherto theoretically and experimentally investigated while plasmonic properties strongly depend on the morphology of the nanostructures 3 4 5 6 7 8 One of the uptodate and the most interesting structures is nanoprisms 4 5 8 Nowadays many advanced methods are used for nanostructure fabrication including chemical synthesis of nanoparticles and the bottomup approach 9 10 11 12 Nevertheless it is not possible to obtain perfect geometric parameters eg sharp edges of nanoprisms or welldefined distance between nanostructures 13 Theoretical simulations can predict how morphological changes of nanostructures influence their near and farfield properties The most common methods are the following Finite difference time domain method FDTD discrete dipole approximation DDA and Finite Integration Technique FIT 5 7 14The strong local field enhancement in a single nanoprism seems to be suitable for various applications Intensity of electric field can be increased using nanoprism dimers with perfectly sharpened edges 13 It has been noted that enhancement of nearfield optical intensity at the corner with nonzero radius can be correctly predicted but the influence of the curvature of the corners of the mono and dimer nanoprisms on farfield properties has only been little studied 13 15 16The influence of rounded edges on the optical properties has been experimentally detected or theoretically simulated for different structures Qian et al 17 have observed the blueshift of extinction peaks for nanoboxes with modified inner and outer edge rounding McMahon et al 18 for silver nanocubes found a correlation between the changes in geometrical dimensions ie corner rounding and their optical response Raziman and Martin 19 have considered the rounding of nanorod antennae and observed significant changes in the scattering farfield properties Goldys et al 20 have pointed out that only by taking into account real geometry it is possible to produce results similar to the experimental onesThe process of fabrication of metal nanoobjects always introduces some rounding effect on the structures This effect must be incorporated into the simulation process in optical response prediction 13 18 19 As follows from the points mentioned above the shape of the fabricated structure is always different from the ideal one If the real geometry of nanostructures is taken into account during numerical calculations a good agreement between the experimental and theoretical predictions could be obtained The relationship between the optical response structure and dielectric environment of nanostructures is important to effectively design the devices employing their plasmonic properties 18In this paper the extinction spectra and electric nearfield enhancement/distribution of single and dimer Au nanoprism with rounded edges were investigated by the FIT method this calculation method is implemented into CST Microwave Studio software CST MWS wwwcstcom What is more the modification of resonance modes of nanoprism dimer by a single nanoparticle NP in the gap between them was also examined In order to modify plasmonic modes NP made of different materials was used including metallic and insulating phasesAll simulations presented in this paper use the CST MWS’s frequency domain solver with a tetrahedral mesh the method used before by Dyck et al 23 Tetrahedral grid provides flexibility in approximating arbitrary ie rounded geometries while hexahedral mesh gives poor approximation unless a very fine mesh is used 24In this paper the FIT method is used to calculate the optical extinction radar and absorption cross sections ie ECS RCS ACS respectively and the electromagnetic field enhancement distribution leftoverrightarrowEright/leftoverrightarrowE 0right where leftoverrightarrowE 0right is the magnitude of the incident field and leftoverrightarrowEright is the magnitude of the local electric field

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