Extremely large near-field enhancements in the vicinity of plasmonic nanoantennas on top of bull’s eye structures exhibiting the extraordinary transmission of light

Abstract

In this paper, we demonstrate that the optical electromagnetic fields in the vicinity of plasmonic nanoantennas are substantially enhanced on combining the plasmonic nanoantennas with bull’s eye structures (BESs) as compared to those of isolated plasmonic nanoantennas or of isolated bull’s eye structures. The optical electromagnetic fields are transmitted with high field enhancement through the subwavelength apertures of the BESs at the resonance wavelength, owing to the extraordinary transmission (EOT) of light. By hybridization of the two plasmonic nanoresonators (nanoantennas and BESs), optical energy is coupled to the nanoantennas from the light transmitted through the BESs. This coupling of light energy to the nanoantennas in the transmission mode using EOT and strong near-field enhancement in the vicinity of the gap between the nanoantennas integrated with the BESs has been analyzed numerically using the finite difference time domain (FDTD) method. We optimized the geometrical parameters of both BESs and nanoantenna structures for achieving the highest possible near-field enhancement. The resonance wavelength of this combined plasmonic structure can easily be tuned from visible to the near-infrared range by varying the periodicity of the BESs. Employing the proposed hybrid nanostructure consisting of a BES coupled to a plasmonic nanoantenna, an electric field enhancement of more than 1000 (i.e. an intensity enhancement greater than 1 × 106) is achieved for a 10 nm gap between the two arms of the bowtie nanoantenna, of more than 4000 (i.e. an intensity enhancement greater than 1.6 × 107) for a 5 nm gap, and of more than 9000 (i.e. an intensity enhancement greater than 8.1 × 107) for a 2.5 nm gap. These values of electric field enhancement are substantially higher than what have been reported previously. In this paper, we also present a comparison of the near-field enhancements in the vicinity of plasmonic bowtie nanoantennas coupled to different types of plasmonic nanogratings—two-dimensional nanohole arrays in plasmonic thin films, nanoline arrays in plasmonic thin films, or subwavelength apertures in plasmonic thin films surrounded by concentric periodic grooves. Moreover, we provide a comparison of the electric field enhancements in the vicinity of different types of plasmonic nanoantennas coupled to plasmonic nanogratings. The proposed hybrid nanostructure can open new possibilities in different fields such as surface-enhanced Raman scattering (SERS), plasmon-enhanced fluorescence (PEF), optical trapping, and nonlinear optics.