Gold elliptical nanoantennas as probes for near field optical microscopy

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Abstract
We investigate the light scattering by individual nanometer-sized gold particles attached at the apex of fiber-based probes for near field optical microscopy. The dependence of the light scattering by the gold nanoparticle on the wavelength, the shape, and the surrounding medium dielectric profile are theoretically described and experimentally investigated, demonstrating that the tuning of the particle’s size and shape plays a crucial role in the light scattering process. In the case of gold spherical nanostructures, the plasmon resonance occurs at 540 nm in air, and 600 nm in water. A higher surrounding medium refraction index leads to a redshift of the plasmon resonance in the gold particle. Moreover, for elliptical structures, the orientation of the polarization of the incident field, as well as the relative ratio of the ellipse dimensions along its main axis, govern the position of the plasmon resonances. The light transmission spectrum for several probes where a single elliptical gold particle has been grafted exhibits at least one resonance in the range 600–650 nm. Surface plasmon resonances are of importance for the field enhancement at the end of the modified tip, thus leading to improvement of the achievable resolution for near field optical microscopy. An optimized design of the metallic nanostructure deposited at the apex of the near field probe is needed to maximize the resonance and would therefore lead to a high-performance probe for scanning near field optical microscopy.