Magneto-electric antennas for directed light emission

• Published in: 2013 Conference on Lasers & Electro-Optics Europe & International Quantum Electronics Conference CLEO EUROPE/IQEC p. 1
• Main Authors: Hancu, Ion M., Curto, Alberto G., Castro-Lopez, Marta, Kuttge, Martin, van Hulst, Niek F.
• Format: Conference Proceeding
• Language: English
• Published: IEEE 01.05.2013
• Subjects: Dipole antennas; Directive antennas; Magnetic moments; Magnetic resonance imaging; Magnetoelectric effects; Perpendicular magnetic recording
• DOI: 10.1109/CLEOE-IQEC.2013.6801864

Summary form only given. By directing light, optical antennas can enhance light-matter interaction and improve the efficiency of nanophotonic devices. Though several designs have been presented 1 , most of them have been scaled-down radio frequency antennas which have narrow bandwidths and are sensitive to the position of the coupled emitters. Here we present a magneto-electric antenna: by exploiting the interference between magnetic and electric modes in a split-ring resonator (SRR) we experimentally realize a compact and robust optical antenna which outperforms larger, multi-element antennas in both bandwidth and directionality.Our magneto-electric antenna is in essence a U-shaped SRR2 with a narrow gap, optimized to enhance the coupling of electric dipole sources to the antenna modes (Fig. 1a). Thus, if a local luminescence source is coupled to this antenna then it will simultaneously drive perpendicular electric and magnetic3,4 dipole moments whose inverted symmetries produce a directed light emission5. We experimentally demonstrate this concept using a microscope objective to collect the emission of quantum dots coupled to the magneto-electric antennas. By imaging at the back focal plane, momentum-space images of single antennas are recorded and the directionality can be characterized. The collected luminescence around λ = 800 nm is observed to be highly directed over the entire quantum dot emission spectrum (Fig. 1b), in contrast to the relatively narrowband YagiUda antenna6 based on an array of dephased electric dipoles.Through FDTD simulations we find that the radiated power enhancement and unidirectional emission are both maintained over a broad bandwidth even when the emitter position is displaced within the gap, resulting in the robust performance observed experimentally when the emitters are attached to the antenna at random positions. Finally we analyze the electromagnetic near field and discuss the resonant modes which cause this directed emission. Our results bring directional optical antennas a step closer to practical applications in light emission, sensing and light harvesting thanks to the more compact (λ2/10), more broadband and more robust single-element design. Furthermore, the directional response of split-ring resonator unit cells is an intriguing property to exploit in metamaterials.