Electromagnetic held intensity

The above-mentioned nonlinear optical effects can be described by the perturbation of the electromagnetic held intensity under the electric dipole approximation. Actually, this approximation is broken in optical near-helds. Hence, a perturbation effect of multipole such as electric quadrupole or magnetic dipole should also be considered, although such a higher-order effect is normally negligible. Indeed, electric quadrupole contributions can be comparable with electric dipole contributions... 

Figure 15.10 Simulated intensity images, Ix (top) and ly (bottom) of the electromagnetic field distribution for 2.45 GHz microwave frequencies incident upon (2) 2-D equilateral triangles with 12.3 mm length and oriented with the sample geometry shown (middle). The incident field is held constant and the gap size is varied in subsequent simulations, 1 mm (left) and 12 mm (right) gap size examples are shown. Bottom Maximum I (lx (Ey2) + ly (Ey2)) pixel intensity versus gap (Top, Inset), expanded view of maximum pixel intensity versus gap size.

---