Light Enhancement and Direction Control Using Bowtie Antenna Arrays
The focus of this work is to determine the potential for arrays of nanoscale bowtie antennas for light enhancement and direction control in nano-optics devices. This is achieved by studying the effects of illumination on the backscattered and total fields, along with the intensity of the propagated light for different array sizes on different substrates. The Wave Optics Module in COMSOL Multiphysics® was used to simulate the bowtie nanoantenna arrays and semiconductor materials using material parameter values. A 3D model was used to properly model the polarization effect of the reflected/ backscattered fields. The bowtie nano-antennas were placed at an equal distance from each other with a distance of 500 nm and the arrays were compared to a single bowtie antenna of the same geometry. The simple arrays consisted of 3-6 antennas to allow for faster computation time and ease of data analysis. A 3D Gaussian beam was applied to simulate the excitation beam and the Frequency Domain stationary solutions were obtained for a series of simulation parameter sets. A range of wavelengths between 650 nm and 1500 nm were used to illuminate the antennas and the polarization was also altered to observe the changes in propagation for polarization shifts. The intensity output was measured by integrating the electric field over the surface area of a partial circle placed a set distance from the array. In addition gold strips were added allowing for the study of propagation changes, and a partial semicircle was used to replicate a lens of suitable NA that would be used in the experimental setup.
Once the wavelength reached 1150 nm and above; an array of three bowtie antennas proved capable of shifting the polarization and direction of the light if polarized perpendicular to the long axis of the bowtie antenna due to the interference produced between the tops of the triangular structures. This focused light is on average twice as intense as that produced by the strong coupling enhancement in the gap between a single bowtie antenna pair orientated perpendicular to the array direction. In addition the array can produce strong enhancement in light polarized parallel to the long axis of the antenna providing directed light which an enhancement of between 3.5 and 8.2 that produced by a single antenna in the same orientation. Studying the propagation along the gold strips also indicated that this interference has some control of the directionality and polarization of the input light allowing for the realization of switch type behavior. Further increases in the enhancement could be obtained by increasing the number of antennas in the array regardless of substrate changes carried out in the simulations. These results could prove useful both for light direction or more efficient semiconductor quantum dot excitation in nano-optics devices. This work was funded by the Irish Research Council (GOIPG/2018/10)
