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196 5 Near Field
effect reaches a maximum with δx =0.8. Figure 5.36b shows the same results
for a = R 0 /2and b = R 0 by changing the mask shape elongated perpendicular
◦
to the track by the mask rotation of 90 . This perpendicularly elongated mask
enables us to cover more of the read spot, resultingin a higher MTF compared
with that of (a). As a result, it is confirmed that the MTF depends strongly
on both the shape and relative position of the mask.

5.4.2 Near-field Recording
Ultrahigh-density optical storage is performed by switching a material phase
or a magnetization point by point using the light energy from an aperture
fiber probe. Heat-assist recordingis particularly useful for magnetic recording
on flux-detectable media because of the high sensitivity of giant magnetore-
sistive (GMR) devices [5.39]. There are mainly three read/write schemes for
an ultrahigh-density optical near-field storage; a scanning probe microscope
(SPM), an optically switched laser (OSL) head, and a solid immersion lens
(SIL).

Scanning Probe Microscopy

There are various types of SPM storage; STM-based, AFM-based, MFM-
based and SNOM-based storages. Figure 5.37 shows an SNOM-based stor-
age [5.40] using an Al-coated fiber probe and a phase change medium (GeS-
bTe). An aperture probe induces an evanescent light, heating the nanometer
area, changing the medium phase from crystal to amorphous such that the
reflectivity of the illuminated area decreases. The smallest mark size obtained
−2
is 60 nm in diameter, which corresponds to 170 Gb in. .

laser light (l=785 nm) 2nd coupler
SNOM tip
1st coupler
core
laser diode
cloud
Al coating film

evanescent light
(writing) detector of
reflected light
(reading)
protect layers
polycarbonate
phase change
phase changed bit recording film
(GeSbTe(30 nm))
Fig. 5.37. SNOM-based storage using Al-coated fiber probe and phase change
medium (GeSbTe). Reprinted from [5.40] with permission by S. Hosaka, Gunma
University, Japan