Figure 4
From: 3D vertical nanostructures for enhanced infrared plasmonics
(a) color plot summarizing reflectance as a function of wavelength and interantenna distance, for arrays of 5 μm long nanoantennas (dataset from Fig. 3c). Lower order modes (1st and 2nd) exhibit a strong dependence of resonance wavelength with pitch, due to a convenient energy flow parallel to the substrate (b) and a near field coupling into the substrate (c); In detail, (b) shows far field coupling, mediated by scattered EM fields, for the first three order modes (1st monopolar mode parallel to substrate, 2nd dipolar mode mostly directed outwards with an angle of 50°, 3rd order mode mostly with an angle >60°), while (c) shows near field coupling, mediated by charge density oscillation (white arrows) into the metallic substrate (simulation: COMSOL multiphysics). As the pitch is decreased and structures are placed in a tight spatial arrangement, repulsive forces of in-phase oscillation contrast external excitation, damping the optical resonse (see 1st order resonance in (a), pitch < 3 μm).
