Tapered Rectangular Aperture

The efficiency of the circular aperture can be improved significantly by tapering the side walls. Moreover, because the circular aperture produces an oblong dissipated power spot along the direction of the incident polarization as shown in Fig. 18, it makes sense to widen 

The peak field intensity within the recording medium as a function of wavelength is shown in Fig. 20 for several different aperture 

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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 19. Tapered gold rectangular aperture. The aperture is filled with the glass of the solid immersion lens with refractive index 1.5.

Figure 20. Field intensity in the medium versus wavelength for the tapered gold rectangular aperture.

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