S21

Near-Field Optical Recording on Phase-Change
Nanoparticles and Reflective Reproduction
from Nanoantenna using Plasmonic Resonance
Teruhiro Shiono
R & D Division, Panasonic Corporation
Kyoto 619-0237, Japanshiono.teruhiro@jp.panasonic.com
Key words: Phase-change nanoparticles, Near-field optical recording, Nanoantenna, Plasmonic resonance.

ABSTRACTFor high-density optical memory system, near-field recording on a medium with phase-change nanoparticles and dual
metal layers was proposed. A finite difference time domain (FDTD) analysis demonstrated that a combination of ananoantenna with such a medium so as to enhance plasmonic resonance would enable effective recording with larger(10 times) working distance (WD) than for a conventional medium. A reproduction method of detecting theintensity of the reflected wave from the nanoantenna was also proposed in the same setup as the recording. We foundthat plasmonic resonance induced in the nanoantenna was enhanced and the intensity of reflected light was also
increased when the phase state of the nanoparticle was crystalline. Since the sub-diffraction limited size ofnanoantenna is larger than a nanoparticle, the detected signal intensity can be greatly improved. Calculated results
showed that our proposed system and methods for recording and reproduction would have a potential to become
effective solutions for terabyte-class optical memory system.
1. Introduction
In the field of optical storage, an optical memory system with phase-change optical disk is highly attractive as areliable and recordable one. The recording density of the BDXL disk is 24 Gb/inch2 and the minimum mark length is112 nm. By using the SIL optical system with numerical aperture (NA) of 1.6, the minimum mark length in dual-layer
phase-change optical disk was reported to be as small as 78 nm [1]. In these optical memory systems, however, therecording density is restricted by a diffraction limit determined by a wavelength of  and an NA ( /NA).
In recent years, the near-field optical recording technology has broken through the diffraction limit and therecording size of 40 nm was demonstrated in a phase-change thin film using a metallic nanoantenna [2]. However, fora medium with the phase-change thin film covered by a protection layer, the nanoantenna has to be nearly contact tothe medium (WD= 3 nm in [2]) to achieve as small mark length as nearly an apex size of the nanoantenna.
We propose high-density optical memory system with larger WD using plasmonic resonance [3]. In this paper, theconstructions of the optical memory system are described. Then, the performances of recording and reproduction areanalyzed by using a FDTD method, and the theoretical results are discussed.
2. Construction and operation of high-density optical memory system
Figure 1 shows the construction of a nanoantenna and the recording medium with phase-change nanoparticles anddual metal layers in the proposed optical memory system. Both for recording and reproduction, the same setup can beused. A triangular metallic nanoantenna is placed longitudinally on a recording medium with working distance of WD.
The nanoantenna has a length of L, and an apex angle of . The recording medium consists of a protection layer, aninterface layer, and dual metal layers with a space layer on a substrate. The nanoparticles have vertically long andbell-shaped structure with a curvature radius of R and a height of h, where their period is p and the diameter is d.
Illumination of an incident wave polarized in the Y direction to the nanoantenna induces surface plasmon resonance inthe metallic nanoantenna and generates a near-field light at the apex of the nanoantenna. When a nanoparticle
especially in a crystalline state is closely aligned with the nanoantenna in the polarization (Y) direction, the plasmonicresonance in the nanoantenna is intensified by the interaction with the nanoparticle. The dual metal layers under thenanoparticle further enhance the plasmonic resonance.
When the incident wave has power to record, the near-field light changes the phase state (crystalline phasetransition in this case) of the nanoparticle and the information is recorded. For reproduction, the nanoantenna isirradiated with an incident light with less power than for recording, and the reflected light from the nanoantenna isdetected. When a nanoparticle is crystalline phase of which the real part of the dielectric constant is negative like a

R1
metal, the plasmonic resonance
YYIncident Wave
Nanoantennain the nanoantenna is enhancedXNanoantennaZL
Lby the interaction with theZ
X
nanoparticle, and the intensity of
Reflected Wave
Phase-changetaPhase-change
reflectedlightfrom
theNanoparticles
RNanoparticles
WDWDR
t6
nanoantenna
isincreased Protection Layer
hcorrespondingly.
For
anInterface Layert5
pt4
damorphous nanoparticle, thet3
Space Layert2
reflected light doesn't become
Metal Layers
RecordingMediumstrong so much, because the realt1
part of the dielectric constant of
Substrate
the amorphous phase has(a)(b)
generally larger value like aFig.
1.
Constructionofananoantenna
andarecording
mediumwithphase-change
dielectric than one of thenanoparticles
anddualmetallayers
intheproposed
opticalmemory
system:crystalline phase. By using thecross-sectional views on (a) XY plane at z=0, and (b) YZ plane at x=0.
intensity difference of thereflected
lightfromthenanoantenna, it is possible to reproduce the information recorded in the nanoparticles of the optical medium.

3. FDTD analysis of recording and reproduction performances
3.1 Analytical parameters
By an electromagnetic theory of a FDTD method in combination with commercial software and self-produced
programs, recording and reproduction performances were analyzed when a Y-polarized plane wave illuminated ananoantenna in the – Z direction. The computational domain was set as 3.0 m x 3.0 m x 2.0 m with a non-uniform
cell size. The domain has total field / scattered field (TFSF) formulation for the plane wave source, and perfectly
matched layer (PML) as an absorbing boundary condition. The cell was designed to have the smallest size of 1.0 nm
at a central area along all axes.
Ag2Te was used as an analytical example of phase-change material, of which the crystalline state shows large
metallic property at 780 nm in addition to the difference in dielectric constant between phase states. The real partsof the complex relative dielectric constants were measured to be Re[*] = -14.9 (crystalline phase) and -5.8(amorphous phase) in Ag2Te thin film. A dielectric of SiO2 was applied as a material of a protection layer, an interface
layer, a space layer and a substrate. Ag thin films were used as metal layers.
The period of p of the nanoparticles is an important parameter to determine the recording density. The square arraywith p=48 nm and d=24 nm gives a recording density of 0.28 Tb/inch2 (capacity of 0.40 TB as a 12-cm diameter disk).
The standard values were determined by optimization as follows: L=152 nm,=70, R=R1=12 nm and ta=24 nm in thenanoantenna, and h=36 nm (1.5d), t1=100 nm, t2=20 nm, t3=4 nm, t4=8 nm, t5=4 nm and t6=4 nm in the medium.
3.2 Analytical results of recording characteristics
At first, the recording performances are described for a medium with 5 x 5 squarely-arranged nanoparticles that are all
in a crystalline state. Figure 2 (a) and (b) show the calculated distributions of E2 (square of amplitude of electric field),and nk0E2 (absorption per unit volume) by on XY plane at z=0, respectively when Y-polarized plane wave with
=780 nm and E2=1 V2/m2 illuminated, where n is the refractive index, k is the extinction coefficient, is the angularfrequency, and 0 is dielectric constant in vacuum. The degree of absorption in the nanoparticle is a measure of theAgAg
Near-field
Near-fieldLightLightY

200
Ag2Te
Ag2Te
Nanoparticles
Nanoparticles
SiO2SiO
Ag 2SiOAg
2SiO2E2Ag2Te

Nanoparticles
SiO22AgSiO2YXnkE
3Thin Film
SiO22[nW/m ]48 nm

Z00(b)

nkE 2[nW/m3]

SiO2YX

48 nm

Z0(a)

Ag

Ag2Te2[V /m ]

48 nm
48 nm

X
Z

420

420
Ag(c)

Fig. 2. Distributions of (a) E2, (b) nk0E2 (absorption per unit volume) on XY plane at z=0 for a medium with phasechange nanoparticles, and (c) nk0E2 on XY plane at z=0 for a medium with phase-change thin film when WD=24 nm.

Cross-write by Absorption

Absorption of Nanoparticle [nW/m]

0.5
1.5recording sensitivity. It is seen that the near-field light
generated from the apex of the nanoantenna was effectively
0.4
focused on a central nanoparticle. It is considered that thedielectric polarization at the curved surface of the1.0
0.3
L=152 nmnanoparticle that was induced by the interaction of the
=70 degnanoantenna would collect the electric field. The almost
t3=4 nm0.2
uniform absorption in the nanoparticle in Fig. 2 (b)0.5demonstrated the effective irradiation of the near-field light.
0.1
Figure 2 (b) also shows cross-write by absorption thatcauses some absorption inside neighbor nanoparticles.
0.00.00102030
Cross-write is defined by the absorption ratio to an averaged
WD [nm]one of the adjacent four nanoparticles. It was calculated tobe 0.35.
Fig. 3. Absorption of the central nanoparticle andRecording property for a conventional medium withcross-write by absorption calculated as a function ofAg2Te phase-change thin film (thickness 36 nm) and a SiO2WD.
protection layer (thickness 4 nm) was analyzed as shown inFig. 2 (c). At WD=24 nm, the absorption was much smaller and the full width at half maximum (FWHM) of E2 wasmuch larger (170 nm) in Ag2Te thin-film. When WD decreased to be as small as 2 nm, the FWHM of E2 was reduced
to be 20 nm in Ag2Te thin film. This means that effective recording with larger (10 times) WD would be enabled in
the proposed system than for the medium with phase-change thin film.
Figure 3 shows absorption per unit length (sum of absorption per unit volume on XY plane at z=0) of the central
nanoparticle and cross-write by absorption calculated as a function of WD. The reduction of WD increased theabsorption sensitivity of the nanoparticle by the enhancement of the plasmonic resonance. In addition, the cross-write
can be also greatly improved by the decrease of WD (0.15 at WD=6 nm, for example).

3.3 Analytical results of reproduction characteristics
Reproduction can be performed by detecting the intensity of reflected light from the nanoantenna depending on thephase state of a target nanoparticle. An incident wave was focused using a lens with NA to the nanoantenna, and thereflected wave from the nanoantenna was collected by the same lens with the same NA to a photo detector.
In order to evaluate the potential of our reproduction method, let us consider simple two cases of reproduction
signals when the phase state of only the central nanoparticle was changed in a medium with all crystalline ones and all
amorphous ones, respectively. Corresponding to the above two cases, calculations were performed on four kinds ofmedia of which nanoparticles were all crystalline, center amorphous (central amorphous nanoparticle surrounded by
all crystalline ones), center crystalline (central crystalline nanoparticle surrounded by all amorphous ones) and all
amorphous. The reflectance difference between all crystalline and center amorphous was designated as R1, and one
between center crystalline and all amorphous was R2.
Figure 4 shows R1 and R2 calculated as a function of NA of the lens for focus and detection. R1=0.27% andR2=0.30% at NA=0.71. It is noted that R1 and R2 increased drastically with NA. When NA rose from 0.71 to 0.84,
for example, R1 and R2 increased to be 0.57% and 0.62% (2.1 times), respectively. These are actual values enough todetect. This means that the reproduction to resolve the nanoparticle with a diameter of d=24 nm was theoreticallydemonstrated.
Figure 5 exhibits R1 and R2 calculated as a function of WD when NA=0.71. The decrease of WD can improve
the reproduction characteristics in the same way as for recording by the enhancement of the plasmonic resonance.
0.6
0.5
0.4

0.6
Difference of Reflectance [%]Difference of Reflectance [%]0.7
p=48 nmd=24 nm
WD=24 nm
R1
R2

0.3
0.2
0.1
0.0

NA=0.710.5
0.40.3

R1
R20.2

L=152 nm=70 deg
t3=4 nm

0.1
0.0

0.4

0.5

0.6

0.7

0.80.9NA of Lens for Focus and Detection

Fig. 4. R1 and R2 calculated as a function of NAof the lens for focus and detection.

0102030

WD [nm]Fig. 5. R1 and R2 calculated as a function of WD
when NA=0.71.

When WD was reduced to be 6 nm, for example, values of R1 and R2 increased to be 0.44% and 0.49% (1.6 times).
3.4 Further high-density examination

Cross-write by Absorption

In order to evaluate the potential for high-density terabyte-class optical memory, two models of media were examined.
One medium has a 5 x 5 square array of nanoparticles with p=24 nm and d=12 nm that provides the recording densityof 1.12 Tb/inch2 (capacity of 1.6 TB as a 12-cm diameter disk). The other one has a 7 x 7 hexagonal close-packed
array with p=16 nm and d=12 nm that gives the recording density of 2.91 Tb/inch2 (capacity of 4.1 TB). In thesemedia, t3, t5 and t6 were reduced to be 2 nm in addition toRecording Capacity [TB]
h=18 nm (1.5d) both for 1.12 Tb/inch2 and 2.91 Tb/inch2
0.4 0.6 0.8 1246 8
0.8
media. The nanoantenna had a conical apex with R=6 nm.
Other parameters were the same as for 0.28 Tb/inch2 medium
WD=12 nmthat was described in 3.1. In accordance with the thin-layer
0.6
WD= 6nmthickness t3, t5 and t6 = 2 nm in these models, the smallest
cell size of 0.5 nm was used in the Y direction for the FDTD
0.4analysis.
Figure 6 plotted the cross-write calculated as a functionof recording density and recording capacity. At WD=12 nm,
0.2
cross-write by absorption was as good as 0.43 in 1.12
Tb/inch2 medium, while it degenerated to 0.58 in 2.91
0.0
Tb/inch2 one. When WD=6 nm, for example, the cross-write
0.2
0.4 0.6 0.8 12462
values were improved to be 0.28 and 0.41, respectively.
Recording Density [ Tb/inch ]Thermal analysis is necessary to estimate the limit of therecording density. By an analysis to solve heat conduction Fig. 6. Cross-write calculated as a function ofequations based on the calculated absorption distributions, it recording density and recording capacity (12 cm).
was found that thermal cross-write was close to the crosswrite by absorption. It is because the heat generated in phase-change nanoparticle tended to propagate to metal layers
in the –Y direction.
In reproduction at WD=12 nm for 1.12 Tb/inch2 medium, R1 and R2 were 0.12% and 0.15% for NA=0.71, andwere improved to 0.26% and 0.30% for NA=0.84, respectively. In reproduction at WD=12 nm for 2.91 Tb/inch2medium, R1 and R2 were calculated to be 0.05% and 0.08% for NA=0.71 and were 0.11% and 0.16% for NA=0.84,
respectively. By the decrease of WD and further optimization of parameters in high-density models, the values of R1and R2 would be improved.
4. Conclusion
Performances of recording and reproduction were examined in the same setup by using FDTD method. It wasdemonstrated that a combination of a nanoantenna with such a medium so as to enhance plasmonic resonance enabled
effective near-field recording with 10 times larger WD than for a conventional medium with phase-change thin film.
A reproduction method of detecting the intensity of the reflected light from the nanoantenna was also proposed. For acrystalline nanoparticle, plasmonic resonance induced in the nanoantenna was enhanced and the intensity of reflected
light was also increased. Calculated results showed that our proposed system and methods for recording andreproduction would have a potential for terabyte-class optical memory system. The research on phase-change
nanoparticles has started and their fabrication process and experimental crystallization properties were reported [4, 5].
As the next step, the experimental examination will be approached in the proposed optical memory.
Acknowledgment
I thank Keiichi Matsuzaki for help in an initial-stage reproduction analysis, and Shigeru Furumiya for useful
discussion especially about reproduction crosstalk. I also thank Norihito Fijinoki for valuable advices about thermal
recording characteristics of nanoparticles.
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