Abstract

Electric and magnetic dipolar and quadrupolar Mie resonances in high index dielectric particles exhibit high quality factor that compensate their low field confinement compared to the plasmon resonances of metallic particles [1]. In a host matrix of refractive index n = 1.45, and when a dipolar electric or magnetic emitter is placed at the center of a dimer of Si spheres, we obtain enhancement factors of the magnetic versus electric decay rates of 4 around λ=1.54 μm [2], that could be interesting for manipulating decay rates of trivalent erbium ions (see Fig. 1 left). We derived the analytical expressions of the decay rates of an electric/magnetic dipole transition when coupled to an electric or magnetic dipolar or quadrupolar mode. We show that in parallel coupling, an electric mode (resp. magnetic) can increase the decay rates of both magnetic and electric dipoles Optical antennas are used to control the decay rates and the desexcitation nature of an emitter, but they can also be used to shape the emission pattern in order to emit light preferentially towards a given direction [3]. We carried out this study in the context of the Kerker's conditions which induce a minimal front or back scattered power when a magneto-dielectric sphere is illuminated by a plane wave. These conditions were recently generalised to particles which present both magnetic and electric dipole resonances, for instance high index particles about 200 to 500 nm in diameter in the visible to infrared regime [4-5]. We demonstrated that those particles can also be used in order to enhance the directivity of the radiation from a magnetic dipole transition dipole. An analytical model accurately predicts the conditions which induce minimal front or back radiated power when the particle is illuminated in the near field. We thus derived an extension of the Kerker’s conditions to near-field optics, which can help in designing compact, highly directive antennas (see Fig. 1, right) [3].

© 2013 IEEE