Lightning-rod effect

Figure 15.2 The lightning rod effect When the polarization of the incident radiation is parallel to the tip axis, the electric field will be highly enhanced at the tip apex.

Optical field enhancement occurs at sharp corners or edges of metal nanostructures. This is known as the lightning rod effect [111-113]. The effect is mainly determined by the geometry of the sample and shows little dependence on the incident wavelength. In Fig. 4.15b, the lightning rod effect is dominant over the plasmon-mode resonance enhancement. Difference between Fig. 4.15a, b may originate from the difference in the resonance conditions of the nanorods or from microscopic structures created at the end-edges of the nanorod. 

From here one concludes that the smaller A (the more elongated the ellipsoid), the greater the enhancement factor. This field enhancement is similar to the well-known "lightning-rod effect". For example, when A = 0.1 b/a 1/3) the condition (3.121) gives e 2 = —9. For silver this equality holds at A 4900 A and correspondingly e -l = 0.3, which results in j/max = 9 10. ... 

As mentioned in Sect. I, there are several diiferent mechanisms that operate in a well-designed NFT for enhancing the FOM. For the purposes of this article either the peak E intensity in the medium or the dissipated power in the medium will be chosen as the FOM for studying these enhancement mechanisms for HAMR. Depending on the specific NFT design, the order of importance of these mechanisms may vary, but in general the best NFTs will combine most or all of these mechanisms. The ones we will consider in this section are localized surface plasmon resonance (LSPR), the lightning rod effect, and the dual-dipole effect. 

The triangle antenna also provides an excellent illustration of the lightning rod effect. In this case the FDTD technique is used to compute the fields at the apex of the antenna as the radius of curvature at the apex is varied. All calculations are carried out with a cell size of (2.5 nm). The results are graphed in Fig. 11. The peak field at the apex for this particular antenna design and within the accuracy of the FDTD calculation is somewhat smaller than the absolute peak field as can be seen from Fig. 7. Clearly it is beneficial to design the NFT with a sharp point(s) to both enhance the field intensity and localize it within the recording medium. 

The strong effect on field enhancement of the lightning rod effect leads directly to a remark which, although obvious, nevertheless... 

Antennas have also been proposed as NFTs for HAMR. The simplest antenna design may be the triangle, as shown in Fig. 28. The lightning rod effect was demonstrated in Sect. IV2 for a triangle antenna in free space. This antenna also exhibits a LSPR. It does not... 

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