Second-harmonic generation in (111) gallium arsenide nanoantennas

Dielectric nanoantennas have emerged in recent years as a promising platform for nanoscale second-harmonic generation (SHG) light sources and as building blocks for SHG metasurfaces. The group of III-V semiconductor materials with zincblende (ZB) crystal structure has played a key role in this development since it contains materials that feature high refractive indices and low losses in the near infrared (NIR), and strong second-order nonlinearities owing to the broken inversion symmetry in these crystals. However, one drawback of these materials is the peculiar nature of the second-order nonlinear susceptibility χ_ijk⁽²⁾ where i, j and k relate to the major crystalline axes [100], [010] and [001]. Its components are only nonzero for i ≠ j ≠ k ≠ i. This commonly leads to 'doughnut-shaped' radiation patterns with zero power radiated along the optical axis for SHG nanocylinders fabricated from (100) wafers, where the crystal axes align with the laboratory frame defined by the nanocylinder orientation [1,2]. In order to attain higher directivity along the optical axis and hence improving collection efficiency, the system's symmetry has to be reduced [2,3].