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198 5 Near Field
Servo detector
Laser diode

-
+
MO signal

Photo diode
First-focusing lens
Hemi-spherical Suspension
SIL slider
) Recording medium
RE-TM amorphous recording film
Electromagnet
Fig. 5.38. Heat-assisted magnetic recording (HAMR), avoiding superparamagnetic
limit of HDD. A SIL has a higher NA [5.39]. Courtesy of H. Sukeda, Hitachi, Japan

SIMs have been suggested for delivering a substantial amount of optical power.
However, many practical technologies for HAMR have remained unsolved to
date.

5.4.3 Super-RENS Optical Disk

The near-field recordingschemes described earlier have difficulties in fabricat-
inga nanometer-size probe with reproducibility and distance control between
a probe and a recordingmedium. Why is not the conventional superresolution
scheme usinga mask layer employed? Tominaga et al. [5.33] proposed beyond
the diffraction limit optical readout in the near field usingsuperresolution
structure in 1998. Transmission efficiency of very small apertures is thought
to decay as d −4 , where d is the aperture size, but it can be markedly enhanced
with the aid of surface plasmons and localized surface plasmons [5.49,5.50].
There are three types of beyond the diffraction limit optical readout using
super-RENS. The first is by a transparent aperture formed in an Sb mask
layer [5.33]. The mask layer is uniformly crystallized but the high tempera-
ture region of the super-RENS readout power opens a small aperture sim-
ilar to the superresolution technique [5.38]. This type optical disk consists
of PC-substrate (0.6 mm)/SiN (170 nm)/Sb (15 nm)/SiN (30 nm)/GeSbTe
(15 nm)/SiN (20 nm). The success of 90 nm mark length readout and direct
observation of the near-field aperture [5.51] formed on the super-RENS, the
phase change mechanism for two layers (mask and recording layers) [5.52]
and thermal lens model of the Sb thin film [5.53] have been reported, but the
carrier to noise ratio (CNR) has been poor (approximately 15 dB for 100 nm
marks).