Beaming the second harmonic generation from a double layered bull's eye structure

We report on a numerical investigation of the beaming effect of second harmonic generation (SHG) from a double layered bull's eye structure where a nanobowtie is sandwiched between. Both linear and nonlinear responses were calculated by using finite difference time domain (FDTD) method. Bottom and top layer's bull's eye were optimized for the fundamental field enhancement and the far-field SHG directional beaming, respectively. Combined with bowtie, a pair of pyramid tips, the maximum intensity of the SHG in the far field is about 5 orders of magnitude higher than that of the reference structure, a bare hole aperture on metal film. The divergence angle of SHG in the far field is confined within ±4°. Furthermore, the local field asymmetry is discussed.