Abstract

In the recent years, optical analogs of the Yagi-Uda antennas, widely used in the microwave regime, have been proposed and experimentally studied. The electromagnetic properties of these systems are dominated by multipolar resonances arising from interactions and coupling between different elements. Both metallic and dielectric nanostructures, supporting electric and magnetic resonances, have been studied in order to tailor the far field emissivity of a point source (i.e. a quantum dot) as a function of the wavelength and of the position of the feeder with respect to the antenna ensemble [1-3]. Here we show that a similar concept can be applied if thermal radiation from heated coupled nanoantennas is considered. The main difference in our case is the lack of a deterministic feeder. There is not a point source that can be suitably placed in order to optimize coupling with the antenna, indeed all the structure contributes to the total field radiated through chaotic thermal fluctuations. We developed a numerical model based on the fluctuational electrodynamics approach and on the discretization of the resulting volume integral equation to calculate relative emissivity and spatial emission pattern of nanoparticle ensembles smaller than the thermal wavelength λ_T=hc/k_BT [4]. In figure 1 we show as an example the relative emissivity for a system composed by three Au rods: two rods are of the same size (410nm x 73nm x 40 nm) while the rod on the left is (410nm x 73nm x 65 nm) as sketched in the inset. The distance between the rods is 20 nm. We note that the relative emissivity spectrum exhibits both electric (λ₁=1.5 µm) and magnetic and electric (λ₂=2.0 µm) dipole resonances. The emission pattern at λ₁ has a typical dipole like shape, while at λ₂ most of the energy is emitted along the y- axis in the negative direction with a solid angle of 0.4 sr. The emission patterns have been calculated at a temperature of 1200 K and are plotted in the same color scale to show that the power emitted at λ₂ along the y-axis is an order of magnitude higher than the power emitted at λ₁. Finally we show that the integration of designed emitters with photonic elements such as waveguides could be considered in order to manipulate the evanescent components of the emitted radiation for the creation of a new class of integrated sources.

© 2015 IEEE