Page 479 - Introduction to Information Optics

464 8. Information Storage with Optics

spatial frequencies higher than I/A decay in a very short distance, and never
reach the image plane.
To collect the information carried by spatial frequencies higher than I/A, the
detector must be placed very close to the object, before the evanescent wave
disappears. To transfer the high-frequency information over a significant
distance, the evanescent wave must be converted into a propagating wave. This
is done by placing a small aperture in the near field, where evanescent waves
are present. The small aperture, which is smaller than A, will convert the
evanescent wave to a propagating wave by scattering. In practice, the small
aperture is formed by a tapered fiber, and the light passing the aperture is
propagating in the fiber to the detector. Furthermore, the aperture can be
scanned to form a two-dimensional image. The image resolution now depends
on the size of the aperture and the distance from the aperture to object.
Although the concept of near field optics can be traced back to 1928 [60],
it was not demonstrated until the mid-1980s [6.1,62], when the technologies
for fabricating small apertures and for regulating the distance between the
aperture and the object, and the scanning process became mature enough.
According to the Babinet principle [3], the diffraction effect generated by a
very small aperture is the same as that of a needle tip (a needle tip is the
negative image of a small aperture). Thus, instead of a very small aperture, a
needle tip can also be brought in close proximity to the object. The light
scattered by the tip is the same as the light collected through the aperture.
Near field optical storage typically uses an aperture probe, which is a
tapered fiber [63]. The fiber is first placed into tension and then heated by a
pulsed CO 2 laser, causing the fiber to stretch in the heated region and cleave,
leaving a conically shaped fiber with a 20-nm aperture at the end. Because the
fiber is narrowed at the end, it no longer works as a waveguide, and to keep
the light inside the fiber, it is necessary to aluminize the outside of the conically
shaped region. This is done by obliquely depositing aluminum on the side walls
of the fiber, while avoiding covering the 20-nm aperture at the end. Betzig et
al. [64] first demonstrated near field magneto-optical recording in a Co/Pt
multilayer film using 20-nm-aperture tapered fiber. A 20 x 20 array of 60-nm
domains (magnetic spots) written on 120-nm center-to-center spacing was
2
demonstrated. This corresponds to a storage density of 45 Gbit/in . The
wavelengths of writing and reading light were 515 nm and 488 nm, respectively.
Consequently, the storage density of near field optical storage can go beyond
the upper limit of storage density dictated by diffraction optics.
The drawback of tapered fiber is that most of the power is lost through heat
dissipation in the tapered region of the fiber, because it is no longer a
waveguide. Typically, output powers of up to 50 nW out of 100-nm aperture-
tapered fiber at 514.5 nm can be obtained for 10 mW power input to the fiber.
3
Thus the efficiency is 5 x 10^ . To overcome this problem, the facet of a laser
diode was metal-coated and then a small aperture (250 nm) was created at the