For 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 a nanoantenna 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 the intensity of the reflected wave from the nanoantenna was also proposed in the same setup as the recording. We found that plasmonic resonance induced in the nanoantenna was enhanced and the intensity of reflected light was also increased when the phase state of nanoparticle was crystalline. Since the sub-diffraction limited size of nanoantenna 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.
© 2013 Optical Society of America
In the field of optical storage, an optical memory system with phase-change optical disk is highly attractive as a reliable and recordable one. In such an optical disk, a reversible crystalline-amorphous transition in the thin film  is used as a rewritable mechanism. Triple-layer Blu-ray phase-change optical disk with 100 GB capacity called BDXL is commercialized . For further large-capacity optical memory, it is effective to increase a recording density by the reduction of a minimum mark length and a track period. The recording density of the BDXL disk is 24 Gb/inch2 and the minimum mark length is 112 nm. Another optical memory system with solid immersion lens (SIL) enables further high density by higher numerical aperture (NA) lens. By using the SIL optical system with NA of 1.6, the minimum mark length in dual-layer phase-change optical disk was reported to be as small as 78 nm . In these optical memory systems, however, the recording density that can be recorded and reproduced is restricted by a diffraction limit determined by a wavelength of λ and an NA of the objective lens (∝ λ/NA).
In recent years, the near-field optical recording technology has broken through the diffraction limit and the recording size of 40 nm was demonstrated in a phase-change thin film using a metallic nanoantenna . However, for a medium with the phase-change thin film covered by a protection layer, the nanoantenna has to be nearly contact (spacing of 3 nm in ) to the medium to achieve as small mark length as nearly an apex size of the nanoantenna, because the near-field light would expand largely with a working distance (WD) between the nanoantenna and the medium. In addition, it is absolutely essential to invent an effective reproduction method to resolve a recording mark smaller than a diffraction limit. A reproduction method of collecting scattered light from such a small mark would be difficult to apply even though near-filed light was used, because the light amount from the recording mark of ~10 to several 10 nm sizes is estimated to be very small in the area of Rayleigh scattering where a scattering coefficient is proportional to the sixth power of a particle size.
We propose near-field optical recording on phase-change nanoparticles and reflective reproduction from a nanoantenna, and study the feasibility. Combination of a nanoantenna and a phase-change nanoparticle so as to enhance plasmonic resonance would be effective solution for high-density optical recording because near-field light tends to concentrate effectively on the nanoparticle. With respect to reproduction, we found that the plasmonic resonance strength in a nanoantenna that was placed longitudinally near a nanoparticle would depend on a phase state (crystalline or amorphous) of the nanoparticle and the intensity of the reflected light from a nanoantenna should change according to the phase state. In addition, since the sub-diffraction limited size of the nanoantenna is not restricted by a recording density and can be larger than a nanoparticle, the signal intensity of the reflected light can be greatly improved as well as without necessity of conversion from propagation light into near-filed light. Therefore the reproduction can be performed by detecting the intensity of reflected light from the nanoantenna. In the point of detecting reflection light, it is the same as the conventional optical pickup and almost the same detection optics can be used. In this paper, we describe the construction of the nanoantenna and the recording medium with phase-change nanoparticles in the proposed optical memory system. Then, we analyze the recording and reproduction performances by using a finite difference time domain (FDTD) method and then discuss the theoretical results.
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 and dual metal layers in the proposed high-density optical memory system. Both for recording and reproduction, the same setup can be used. A triangular metallic nanoantenna is placed longitudinally (floating above the substrate) on a recording medium where working distance of air gap is WD. The nanoantenna has a length of L, an apex angle of θ, apex curvature radii of R (tip) and R1 (bottom), and a thickness of ta. The recording medium consists of a protection layer (thickness h + t6) with a flat surface, an interface layer (thickness t5), and dual metal layers (thickness t4 and t2, respectively) with a space layer (thickness t3) on a substrate (thickness t1). Phase-change nanoparticles embedded in the protection layer (cover thickness t6) are isolated from the upper metal layer by the interface layer that prevents metal migration to the phase-change material. The nanoparticles have vertically long and bell-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 in the 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 plasmonic resonance in the nanoantenna is intensified by the interaction with the nanoparticle. The dual metal layers under the nanoparticle further enhance the plasmonic resonance. The space layer interbedded between metal layers improves symmetrical response in the reproduction by control of the plasmonic resonance to describe it later.
When the incident wave has power to record, the near-field light changes the phase state (crystalline to amorphous in this case) of the nanoparticle and the information is recorded. A nanoparticle corresponds to a recording mark. By the reverse change of the phase state (amorphous to crystalline), erasing is possible. The structure of a nanoparticle embedded in the dielectric material of the protection layer with low thermal conductivity improves recording characteristics by preventing a lateral thermal propagation in addition to the enhancement of environmental resistance.
For reproduction, the nanoantenna is irradiated with an incident light with less power than for recording, and the reflected light from the nanoantenna is detected. When a nanoparticle is crystalline phase of which the real part of the dielectric constant is negative like a metal, the plasmonic resonance in the nanoantenna is enhanced by the interaction with the nanoparticle, and the intensity of reflected light from the nanoantenna is increased correspondingly. For an amorphous nanoparticle, the reflected light doesn't become strong so much, because the real part of the dielectric constant of the amorphous phase has generally larger value like a dielectric than one of the crystalline phase. By using the intensity difference of the reflected light from the nanoantenna, it is possible to satisfactorily reproduce the information recorded in the nanoparticles of the optical medium. The metal layers in the medium also improve the reproduction signal by enhancing the plasmonic resonance.
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 of the proposed optical memory were analyzed when a Y-polarized plane wave with a wavelength of λ illuminated a nanoantenna and a medium in the – Z direction. In the model shown in Fig. 1, the center of the nanoantenna was taken in the origin of a coordinate system. The computational domain was set as 2.0 μm x 2.0 μm x 2.0 μm (−1.0 μm ≤ x, y, z ≤ 1.0 μm) 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. Only scattered field such as reflected wave from a nanoantenna is present in the area (thickness of 3 cells) between TFSF boundary and PML (see Fig. 6). A plane wave light source with 2-μm width and TFSF boundary were placed at z = 0.928 μm. PML region was at 0.94 μm ≤ |x|, |y|, |z| ≤ 1 μm.
The cell was designed to have the smallest size of 1.0 nm at a central area (cube 500 nm on a side) including a nanoantenna and a recording media (minimum layer thickness of 4 nm in these models) along all axes.
In order to reduce the absorption especially at the tip region of the nanoantenna during recording, which was produced by the plasmonic resonance, a metal of Ag and a wavelength of λ = 780 nm were chosen. In comparison with a typical Au nanoantenna irradiated by the wave of λ = 405 nm, the absorption can be greatly reduced (~1/130 times) and the degradation of the nanoantenna would be prevented. A durability of Ag can be improved by addition of small amount (~1 percent by weight) of metals such as Cu and Pd.
In general, the phase-change material indicates metallic property in a crystalline state at a certain wavelength. We used Ag2Te as an example of phase-change material for an analysis, of which the crystalline phase shows large metallic property at λ = 780 nm in addition to the difference in dielectric constant between phase states. The real parts of the complex relative dielectric constants were measured to be Re[ε*] = −14.9 (crystalline phase) and −5.8 (amorphous phase) in Ag2Te thin film, and the measured values were applied assuming the dielectric constants of the nanoparticle would be the same as those of thin film. Such a metallic property of a crystalline nanoparticle would enhance the plasmonic resonance in a nanoantenna by the interaction. 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 array with 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). By optimizing the various parameters of the model so as to improve both performances of recording and reproduction, the standard values to achieve such density (0.28 Tb/inch2) were determined as follows: L = 152 nm, θ = 70°, R = R1 = 12 nm and ta = 24 nm in the nanoantenna, 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 disk-geometry medium with a diameter of 304 nm. As an initial value, WD was set as the same value (24 nm) as the nanoparticle diameter of d, which is much larger than the conventional value (3 nm) .
3.2 Analytical results of recording characteristics
At first, we describe the recording performances for a medium with 5 x 5 squarely-arranged nanoparticles that are all in a crystalline state. Figure 2 shows the calculated distributions of square of amplitude of electric field (E2), and absorption per unit volume given by nkωε0E2 on XY plane at z = 0 when Y-polarized plane wave with λ = 780 nm and E2 = 1 V2/m2 illuminated, where n is the refractive index, k is the extinction coefficient, ω is the angular frequency, and ε0 is dielectric constant in vacuum. The degree of absorption in the nanoparticle is a measure of the recording sensitivity. It is seen that the near-field light generated from the apex of the nanoantenna was effectively focused on a central nanoparticle. The full width at half maximum (FWHM) of the E2 in the X direction was 24 nm at an exactly one cell upper the protection layer and was reduced to be 18 nm in the SiO2 cover layer to irradiate the central nanoparticle. It is considered that the dielectric polarization at the curved surface of the nanoparticle that was induced by the interaction of the nanoantenna would collect the electric field. The almost uniform absorption in the nanoparticle showed the effective irradiation of the near-field light. The absorption per unit volume of the antenna was much smaller than one of the central nanoparticle. It is noted that the increase of E2 in the interface layer would demonstrate that the plasmonic resonance was intensified by the interaction with the nanoparticle and metal layers. For the recording on a medium with a central crystalline nanoparticle surrounded by all amorphous ones, the plasmonic resonance in the nanoantenna was slightly decreased (0.91 times), but the absorption of the central nanoparticle was almost the same (0.98 times) as for the recording on the medium with all crystalline nanoparticles.
In addition, the absorption sensitivity (∝ nkωε0E2) of an amorphous nanoparticle for erasing was 1.4 times larger than one for recording in spite of the reduction of the plasmonic resonance and nk. The main reason was estimated that E2 inside the amorphous nanoparticle was larger than E2 inside the crystalline one, because E2 is approximately proportional to 1/|ε*|2 ( = 0.067 for amorphous and 0.033 for crystalline state).
As observed in Fig. 2(b), however, some absorption inside neighbor nanoparticles indicates that the unnecessary plasmonic resonance between the nanoantenna and surrounding nanoparticles caused as we say cross-write by absorption. The cross-write defined by the absorption ratio to an averaged one of the adjacent four nanoparticles was calculated to be 0.35.
In order to verify the validity of the combination of the nanoantenna and the nanoparticle in the proposed system, the recording property for a conventional medium with phase-change thin film was analyzed using the same nanoantenna and the same WD (24nm). The model is shown in Fig. 3, where the recording medium includes Ag2Te phase-change thin film with a thickness of h = 36 nm and a SiO2 protection layer of t6 = 4 nm on a SiO2 substrate.
Figure 4 exhibits the calculated distributions of E2, and nkωε0E2 on XY plane at z = 0 when Y-polarized plane wave with λ = 780 nm and E2 = 1 V2/m2 illuminated. It is seen that the near- field light generated from the apex of the nanoantenna penetrated only near the surface of the phase-change thin film and the absorption per unit volume of the thin film was much smaller than one of the nanoantenna. The FWHM of E2 in the X direction was 47 nm at an exactly one cell upper the protection layer and was much enlarged to be 170 nm in Ag2Te thin-film mainly by an influence of large WD (24 nm). Even if the medium had metal layers, such a recording property was almost the same. When WD decreased to be as small as 2 nm, the FWHM of E2 in the X direction was reduced to be 20 nm in Ag2Te thin-film. This means that near-field recording to the phase-change thin film would demand at least much smaller WD (~1/10 times) to achieve high-density recording in accordance with our proposed recording system.
Figure 5 shows absorption per unit length (sum of absorption per unit volume on XY plane at z = 0) of the central nanoparticle calculated as a function of the length of L, the apex angle of θ of the nanoantenna, the thickness of t3 of the space layer, and WD. The dependence of the cross-write by absorption on WD was also exhibited. The filled circles represent the calculated values. The absorption of the nanoparticle with metal layers was found to be increased by 2.8 times at L = 152 nm in comparison with one without metal layers. It is because of the plasmonic resonance enhanced by the interaction of the metal layers. The absorption of the nanoparticle exhibited a shallow curve with a maximum at L ≈160 nm. The L tolerance to achieve 80% absorption of the maximum value was as broad as 42 nm. With regards to the θ, the absorption of the nanoparticle was almost constant at least for θ ≥ 50°. The absorption of the nanoparticle was gradually decreased with the thickness of the space layer for t3 ≤12 nm. But the absorption for the standard value of t3 = 4 nm was almost the same (0.97 times) as the maximum value at t3 = 0 nm. For an optimum design, a tolerance analysis for the reproduction should also be taken into account as well as one for the recording that was described in this section.
The reduction of WD increased the absorption sensitivity of the nanoparticle by the enhancement of the plasmonic resonance. When WD = 12 nm and 6 nm, for example, the sensitivity was 1.6 and 2.4 times larger than one at WD = 24 nm, respectively. In addition, the cross-write can be also greatly improved by the decrease of WD, since it is determined mainly by the distance ratio from the apex of the nanoantenna to the central nanoparticle and the adjacent one. When WD = 12 nm and 6 nm, for example, the cross-write values were actually reduced to be 0.22 and 0.15, respectively. The cross-write is allowable if the phase states of adjacent nanoparticles are not changed. As a measure of effectiveness, the target cross-write was set to be ≤0.5 here. But, it is necessary to modify it to the optimum value by experimentally elucidating the material characteristics of phase-change nanoparticles.
3.3 Analytical results of reproduction characteristics
Reproduction can be performed by detecting the intensity of reflected light from the nanoantenna by using almost the same detection system as the conventional optical pickup. An incident wave was focused to the nanoantenna by using a lens, and the reflected wave from the nanoantenna was collected by the same lens to a photo detector. It was assumed that the detected optical power was almost the same when the same NA of a lens was used.
Actual recording medium has various combination of the phase state of nanoparticles. However, 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. Figure 6(a) represents the calculated distribution of total field E2 on YZ plane at x = 0 on a medium with all crystalline nanoparticles when Y-polarized plane wave with λ = 780 nm and E2 = 1 V2/m2 illuminated. It is observed that a standing wave was generated clearly with a period of λ/2 at z > 0 by the interference of the incident wave with a reflected wave mainly from the surface (z = 12 nm) of the nanoantenna. The distribution of scattered field E2 of the reflected wave on the XY detector plane at z = 0.932 μm is exhibited in Fig. 6(b). The sum of scattered field E2 within a detector area with a radius of 0.928 μm was calculated by integrating the E2 distribution. It is equivalent to the light detection by a lens with NA = 0.71 to a photo detector.
In order to consider two cases of reproduction signals described above, calculations were performed on four kinds of media with fundamental arrangements 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, respectively.
Figure 7 shows calculated distributions of scattered field E2 of the reflected wave on the XY detector plane for the four kinds of media. The intensity of the reflected wave depended on the phase state of not only the central nanoparticle but also surrounding ones. It was caused by the reproduction crosstalk that was raised by undesired plasmonic resonance between a nanoantenna and surrounding nanoparticles by the same way as the cross-write on recording.
The reflectance of R was defined by the ratio of the reflected optical power to the incident one. In the calculation model, the incident beam has a square size of 1.856 μm and E2 = 1 V2/m2. It is almost equivalent that the incident wave was focused to the nanoantenna by a lens with NA = 0.19 by reducing the rectangular aperture size. The values of R were calculated to be 0.729, 0.709, 0.673, and 0.651% for the four kinds of media, respectively.
The R values can be improved by increasing NA of the lens for focus and reducing the spot size on the nanoantenna. When the same NA (NA = 0.71) of the lens was used for focus and detection, the FWHM of diffraction-limited focused spot size was given by the equation of 1.03λ/(2NA) and calculated to be 0.566 μm at the nanoantenna. R in this case can be converted easily from the calculated values assuming that incident wave has a diameter of 0.566 μm and E2 = 1 V2/m2. When NA = 0.71 both for focus and detection, the values of R were improved to be 9.98, 9.71, 9.21, and 8.91% for the four kinds of media, respectively. The reflectance difference between all crystalline and center amorphous was ΔR1 = 0.27%, and the difference between center crystalline and all amorphous was ΔR2 = 0.30%. These are actual values enough to detect. This means that the reproduction to resolve the nanoparticle of d = 24 nm was theoretically demonstrated. Hereinafter, we use the reflectance when NA = 0.71 both for focus and detection.
Figure 8 exhibits reflectance differences of ΔR1 and ΔR2 calculated as a function of L, θ, t3 and WD when NA = 0.71 both for focus and detection. At a standard value of L = 152 nm, ΔR1 and ΔR2 of the medium with metal layers were improved by 1.8 times than those of the medium without metal layers. The metal layers enhanced the reproduction performance as well as recording one. ΔR1 and ΔR2 varied depending on these parameters. It is seen that the standard values such as L = 152 nm, θ = 70°, and t3 = 4 nm were also desirable in the reproduction characteristics. Symmetrical response in the reproduction that satisfies the relation of ΔR1 ≈ΔR2 is desirable. Especially forming the space layer between metal layers was effective for controlling the symmetrical response. In addition, it was found that the increase in nanoparticle height of h = 1.5d improved ΔR1 and ΔR2 by 1.3~1.4 times in accordance with h = d. The decrease of WD can improve the reproduction characteristics in the same way as for recording by the enhancement of the plasmonic resonance. When WD = 12 nm, ΔR1 = 0.41% and ΔR2 = 0.44%. ΔR1 = 0.44% and ΔR2 = 0.49% at WD = 6 nm.
3.4 Further high-density examination
Magnetic recording assisted by near-field light and magnetic reproduction with high-density≥ ~1Tb/inch2 was reported [5, 6]. In our proposed optical memory, optical recording and optical reproduction without magnetic components would simplify the construction of the system. In order to evaluate the potential for high-density terabyte-class optical memory, we examined two models of media by using the FDTD method. One medium has a 5 x 5 square array of nanoparticles with p = 24 nm and d = 12 nm that provides the recording density of 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 as a 12-cm diameter disk). Corresponding to d = 12 nm, WD was set as the same value (12 nm) as d, and the nanoantenna had a conical apex with R = 6 nm. In these media, t3, t5 and t6 were reduced to be 2 nm in addition to h = 18 nm (1.5d) both for 1.12 Tb/inch2 and 2.91 Tb/inch2 media. Other parameters were the same as for 0.28 Tb/inch2 medium that was described in 3.1. In accordance with the thin-layer thickness 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 analysis.
Figure 9 shows calculated distributions of absorption per unit volume of nkωε0E2 on XZ plane at the middle of nanoparticles (all crystalline phase) in the Y direction when Y-polarized plane wave with λ = 780 nm and E2 = 1 V2/m2 illuminated.
Figure 10 plotted absorption per unit length of the central nanoparticle and cross-write by absorption calculated as a function of recording density and recording capacity (as a 12-cm diameter disk), and reflectance differences of ΔR1 and ΔR2 calculated as a function of recording density and recording capacity, in addition to values for 0.28 Tb/inch2 medium that were described in 3.2 and 3.3.
Since the central nanoparticle represented the largest absorption among them as shown in Fig. 9, the near field recording was confirmed for both media. Cross-write was as good as 0.43 in 1.12 Tb/inch2 medium, while it degenerated to 0.58 in 2.91 Tb/inch2 one, which was brought about by the tight arrangement. When WD = 6 nm, for example, the cross-write values were improved to be 0.28 and 0.41, respectively. On condition of WD = 6 nm, the calculated cross-write (0.41) for high-density of 2.91 Tb/inch2 satisfied our target value (≤0.5).
Thermal analysis is necessary to estimate the limit of the recording density. We have made a start to solve heat conduction equations based on the calculated absorption distributions. The heat generated in phase-change nanoparticle tended to propagate to metal layers in the –Y direction. It means that the thermal cross-write would be close to the cross-write by absorption. In the near future, the limit of the recording density will be discussed based on the thermal calculated results.
In reproduction for 1.12 Tb/inch2 medium at WD = 12 nm, the reflectance of R was calculated to be 9.89, 9.77, 9.20 and 9.05% for the four kinds of media, respectively. The reflectance differences of ΔR1 and ΔR2 were 0.12% and 0.15%, respectively. When WD = 6 nm, ΔR1 and ΔR2 were 0.18% and 0.22%, respectively. In reproduction for 2.91 Tb/inch2 medium, R was calculated to be 10.03, 9.98, 9.30 and 9.22%, respectively. And ΔR1 and ΔR2 were 0.05% and 0.08%, respectively. When WD = 6 nm, ΔR1 and ΔR2 were 0.08% and 0.13%, respectively. These results showed that ΔR1 and ΔR2 decreased with the increase of recording density and can be increased by the reduction of WD. By the increase of NA for focus and detection and further optimization of parameters in high-density models, ΔR1 and ΔR2 would be improved.
Reflectance difference (equivalent to ΔR1) for 2 T mark (minimum mark length is 112 nm) in a practical system of BD-RE XL is calculated to be 0.3%. In order to estimate the limit of ΔR1 and ΔR2 in reproduction for a practical system, the various experimental evaluations are necessary including noise reduction method suitable for our system. This is a future research to be solved.
High density optical memory system with a metallic nanoantenna and phase-change nanoparticle medium was proposed. Performances of recording and reproduction were examined in the same setup of which the parameters were optimized by using FDTD method. It was demonstrated 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 a crystalline 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 and reproduction 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 . As the next step, we will approach the experimental examination of a basic operation in the proposed optical memory.
We thank Masahiro Birugawa and Noboru Yamada for valuable advices about optical memory concept. We also thank Norihito Fijinoki for useful discussions about thermal recording characteristics of nanoparticles, Akio Tsuchino for help in the measurement of optical constants of phase-change material, and Kenichi Nishiuchi for his support.
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