Directional Scattering in a Germanium Nanosphere in the Visible Light Region

Previous designs of photonic nanoantennas are based on noble metal plasmonic structures, suffering from large ohmic loss and only possessing dipolar plasmon modes. This has driven the intense search for all-dielectric materials (ADMs) beyond noble metals. Here, for the first time, a strong scattering anisotropy in a Ge nanosphere is demonstrated in the visible and near-infrared regions. The forward-to-backward scattering ratio for an individual Ge nanosphere (150 nm) can reach a maximum value of approximate to 20 theoretically and approximate to 8 experimentally. This scattering behavior derives from the special real part and imaginary part of refractive index of Ge. Differing form other high-index ADMs such as Si and GaAs, the electric dipole and magnetic dipole resonances of Ge nanospheres are closer to each other in the spectrum due to the non-negligible imaginary part of refractive index. The spectral overlap between electric dipole and magnetic dipole resonances endows Ge nanospheres with efficient directional scattering near the scattering peak. Fano resonances with strong directivity are observed in Ge nanosphere dimers, which is the result of a broad electric gap mode coupled with a hybrid magnetic mode. These findings make Ge nanospheres a promising candidate for nanoantennas, directional sources, and optical switches.