Enhanced third harmonic generation in nanospheres at the anapole mode under tightly focused radially polarized beams.

The third harmonic (TH) signals in subwavelength scale devices have a wide range of applications, including nano-laser, microscopic imaging, sensing, and so on. However, the limited TH signal intensity still restricts practical applications due to the inherently small nonlinear coefficient in material and relatively weak confinement of the pump electromagnetic field. Here, we present the enhancement of TH signals in the isotropic Si nanosphere and the Au core/Si shell nanosphere exhibiting anapole mode excited by tightly focused radially polarized beams. The TH signal intensity at anapole mode in the Au core/Si shell nanosphere was enhanced by a factor of 649 compared to the Si nanosphere at magnetic dipole mode. The anapole mode and the enhanced TH signal in nanospheres are investigated numerically using full-wave simulations. These results open possibilities for exploiting low radiation anapole nanospheres for potential high THG applications.