For Thermally-Assisted Recording (TAR) to be realized, it will be necessary to confine heat to a single data track approximately 40nm wide or smaller with high efficiency. Candidate near-field optical sources typically use a low-loss metal (Au, Ag, Al, Cu) shaped in such a way to concentrate surface charge motion at a tip apex located at the slider ABS when light is incident. Oscillating tip charge creates an intense near-field pattern, heating the disk. Sometimes, the metal structure can create resonant charge motion (surface plasmons) to further increase intensity and disk heating. For example, when polarized light is aligned with the corner of a triangular-shaped gold plate, an intense near field pattern is created at that corner [1]. Resonant charge motion can occur by adjusting the triangle size to match a surface plasmon frequency to the incident light frequency. Another near-field transducer is the ridge slot waveguide from microwave circuits applied to optical frequencies (also known as the "c-aperture") [2]. This shape (Figure 1) is characterized by five parameters including the metal screen thickness. Light polarization is aligned with the ridge. Previously, a ridge waveguide in silver has been optimized at a wavelength of 516nm and a metal-to-metal fly-height of 8nm [3]. Also, far field measurements obtained for various c-aperture sizes indicate a spectral shift [4] while narrow resonant behavior has been observed when a pattern of ridges is used to excite surface plasmons around a long slot waveguide and enhance far field transmission [5].
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