Beam steering of second harmonic radiation by a compact nonlinear Yagi-Uda nanoantenna

Nanoantennas have demonstrated unprecedented capabilities for manipulating the intensity and direction of light emission over a broad frequency range. The directional beam steering offered by nanoantennas has important applications in areas including microscopy, spectroscopy, quantum computing, and on-chip optical communication. Although the Kerr effect has been explored to control the far-field radiation of nonlinear antennas, the design of the nonlinear antennas is similar to that of linear counterparts due to the dominating local-bulk dipolar mode in small metallic nanoparticles. Different from the Kerr effect, the second harmonic radiation from the metallic nanoparticle shows a complex surface-induced multipolar interaction without the local-bulk dipolar mode. Moreover, the far-field second harmonic radiation obeys a number of selection rules strongly relying on the symmetry and surface characteristics of the nanoparticle. In the work, we propose a novel nonlinear Yagi-Uda nanoantenna to direct second harmonic radiation from a metallic nanosphere. The Love's equivalence principle and self-consistent boundary element method, which capture the pump depletion and cross coupling between the fundamental and second harmonic fields, are employed to model the dielectric-metal composite nanoantenna. By carefully tuning the spacing and dimensions of two lossless dielectric elements, which function respectively as a compact director and reflector, the second harmonic radiation is deflected 90 degrees with reference to the incident light (pump) direction. This abnormal light-bending phenomenon is due to the constructive and destructive interference between the second harmonic radiation governed by a special selection rule and the induced electric dipolar and magnetic quadrupolar radiation from the two dielectric antenna elements. Simultaneous spectral and spatial isolation of scattered second harmonic waves from incident fundamental waves pave a new way towards nonlinear signal detection and sensing.