The classical electromagnetic simulations to the Au NPs are performed using the FDTD++ package. The Maxwell's equations are solved using Yee's algorithm. Considering that Au 20 is a fragment of the face-centered cube lattice of bulk gold, or a part of the whole large-sized tetrahedron, we choose the shape of Au NP to be tetrahedral. The other reason for our choice is that we know that a weak laser field can induce an intensive near field near the apex of Au tetrahedron, even when the size of NP is small. The dielectric function of Au tetrahedron is describe by Drude-Lorentz model in the form of