Triangle Antenna

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 

It is interesting to compare these results with calculations of the triangle anteima in ftee space. Plots of the extinction, scattering, and absorption cross sections for the 100-nm triangle antenna in free space along with the peak field intensity at the apex are shown in Fig. 32. The resonance occurs at 675 nm, significantly shifted from the resonance wavelength in the presence of the medium. The peak field intensity occurs at the apex of the antenna, and is clearly 

One way in which the problem of lack of confinement of the coupled power to the medium can be solved is to cant the antenna so that only the tip is close to the medium. Another approach is to add a small beak at the end of the antenna as shown in Fig. 33.  

If a beak with a 20-nm width, length, and height is added to the triangle antenna, then the resonance wavelength is slightly shifted to 725 nm, but the field intensity within the medium at resonance is much better confined, as shown in Fig. 34. The dissipated power in a 50-nm cylinder in the medium is 2.9%. 

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Figure 6. Peak field intensity at the tip of a gold triangle antenna embedded in free space versus wavelength for excitation an incident plane wave of unit amplitude polarized along the length of the antenna. The antenna has an apex angle of 45°, a length of 200 nm, and a thickness of 80 nm.

Figure 7. Local field intensity for a gold triangle antenna with a length of 200 nm, radius of curvature at the apex of 20 nm, apex angle of 45°, and thickness of 80 nm. The incident plane wave of unit amplitude is polarized along the x axis. The FDTD cell size is (2.5 nm).2...

Figure 8. Field intensity computed along the x axis of the triangle antenna in Fig. 6 showing the e qionential decay characteristic of the held from surface plasmons. The decay for negative x into the gold antenna is of course much faster than the decay into the surrounding dielectric.

Figure 9. Field intensity at the apex of the triangle antenna as a function of the antenna length computed for plane wave excitation at a wavelength of 775 nm.

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. 

Figure 11. Peak field intensity at the apex of a triangle antenna versus radius of curvature. The antenna is 200 nm long, 80 nm thick, with a 45° apex angle. The incident plane wave has a wavelength of 775 nm.

In this section the results of the previous two sections are combined to compare several NFT designs that have been suggested for use in data storage. In particular, the triangle antenna and the bow-tie antenna are compared with the circular aperture, the tapered rectangular aperture, the bow-tie aperture, and the C aperture. All NFTs are illuminated by a highly focused beam using a SIL with a refractive index of 1.5 to obtain an optical spot size with dimensions... 

Figure 29. Wavelength dependence of field intensity and dissipated power in the medium from a 100-nm-long triangle antenna with a 30° apex angle.

Figure 30. Field intensity in the medium from a 100-nm-long triangle antenna with a 30° apex angle at a wavelength of 750 nm. (Reprinted from Ref. [7]. Copyri t 2006 with permission from the Institute of Pure and Applied Physics.)...

Figure 32. Extinction, scattering, and absorption cross sections normalized by the area of the antenna for the 100-nm triangle antenna on a glass substrate as a function of wavelength. The peak E field intensity versus wavelength is also plotted.

Let us study the Sierpinski gasket array of Figure 8.17(a). A subarray employing three dipoles on the corners of an equilateral triangle h = 0.5X spacing on each side) and an expansion factor of 2 is utilized. Resolution is set to only 8 points/wavelength and three snapshots of the antennas electric near field are depicted in Figure 8.17(b). The simulations are conducted via the optimized operators of Section 5.4. 

When assembling the circuit in Fig. 16.2, a pair of 4.7 mfd capacitors are put in the collector and emitter positions, C and E, but none is in the optional base position labeled B. The antenna (radio aerial), indicated as a triangle, is not... 

Another very often met nanoantenna stmcture is the bowtie nanoantenna [313]. It consists of two triangles aligned along their symmetry axes. A feed gap is formed between their tips. Bowtie antennas have a broader bandwidth together and at the same time ensure large field localizations in the feed gap. A similar type of antenna is the diabolo-type nanoantenna [314], where the triangles overlap. 

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