Plasmonic nanosystems are typically used in optical experiments to concentrate electromagnetic energy into sub-wavelength volumes. In this context, metal-dielectric-metal (MDM) nanosystems provide an attractive geometry for achieving strong field enhancement and mode confinement on a few-nanometer scale. Unfortunately, the disadvantage of the MDM configuration can be that the enhanced near fields are confined to the dielectric spacer and are not externally accessible. This limits the suitability of such platforms for applications, where the interaction of light with external nano-objects is desirable, such as sensing, detection, and quantum optics. To expose the enhanced near fields to the environment, we selectively and isotropically etch the gap dielectric layer, which results in patch antennas on thin silicon oxide pedestals. We perform optical sensing experiments and simulations and confirm the sensitivity of the optical near fields to the refraction index change in a small volume.

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