Unidirectional nanoantenna for second-harmonic generation

Abstract

Due to symmetry breaking at surface and plasmonic enhancement, second-harmonic generation (SHG) from centro symmetric metal nanoparticles has a bright outlook for nanoscale sensing and detection applications. While SHG were greatly enhanced by well-engineered nanostructures, a directional (or angular) control of second harmonic (SH) radiation, which is highly important to far-field detection of nonlinear signals, has not been explored yet. In this work, a unidirectional nanoantenna is designed tore direct radiated SH wave and separate it from incident wave spectrally and spatially. To solve challenging issues in the modeling of the nanoantenna system, SH nonlinearity is described by equivalent nonlinear current sheets on the surface of metal nanoparticles. Then, the nanoantenna is simulated and optimized by the surface integral equation (or boundary element) method based on equivalent principles. A modified PMCHWT formulation is solved iteratively to capture the mutual coupling between fundamental and SH fields. As a result, the modeling of SHG is not restricted by the undepleted-pump approximation. Compared to other simulation approaches, the proposed method is efficient with a surface discretization; and could employ experimentally tabulated linear susceptibility and nonlinear surface susceptibility tensor of metals directly. The SH radiation from a plasmonic small sphere with a plane wave excitation can be modeled by the emission from an induced electric dipole and an induced electric quadruple. According to the selection rule, the SH radiation is strictly zero along the incident direction. However, SH radiation always shows a double-side scattering feature due to mirror symmetry along the radiation directions. In the designed nanoantenna, two passive dielectric nanospheres functioned respectively as a director and reflector are employed to break the mirror symmetry. By tuning the size of dielectric spheres and sphere-sphere spacing, interference and retardation effects between multiple spheres could be fully controlled, which leads to the unidirectional scattering of SH waves at a desired wavelength. The work paves a new way to far-field detection and sensing of nonlinear signals.