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We propose a plasmonic data storage medium with a high-transmission metal aperture array embedded in a dielectric material. Bowtie apertures, having an outline of 80 nm and a ridge gap of 30 nm, are arranged in a two dimensional array with a bit pitch of 100 nm and a track pitch of 280 nm. Using the finite differential time domain (FDTD) method, we calculate the exposure power needed to record optical data, the contrast for readability of recorded data, and cross talk between the main track and adjacent tracks. Compared to a conventional blu-ray disc, the exposure power needed to record optical data in the proposed plasmonic data storage medium is less than a quarter of the conventional threshold power, and the density of the data storage is about 1.8 times larger.
©2009 Optical Society of America
1. Introduction
Advances in optical data recording technology have led to an increase in storage density, producing terabyte optical memory systems. In conventional optical systems, the diffraction of light limits the resolution of the optical spot to be used for recording and reading of the optical data. The spot size, in turn, determines the storage density. In practice, we can reduce the optical spot size by using a shorter wavelength light source, such as a blu-ray laser, or a high NA (numerical aperture) optical system. Since the light source for optical memory systems has reached the shortest wavelength of the currently available diode lasers, many studies on the NA of optical systems larger than unity have been carried out as an alternative way to increase the resolution.
In order to achieve a NA larger than one, the optical system must hold all paths of the light propagation in a dielectric medium with a high refractive index. If there is any air gap in the optical path, it should be much smaller than the wavelength of light so that the light can propagate by coupling of the near field which rapidly decays in the gap. For example, a solid immersion lens (SIL) can be used to achieve near field coupling in a high NA optical system [1,2].
After the recent discovery of extraordinary transmission of light through nano-size apertures in a metal film, many researchers have explored different aperture designs [3–8] in an attempt to enhance the aperture transmission. Since the electromagnetic field around the aperture in the metal film is coupled with the localized surface plasmon excited through the aperture hole, we can improve the optical resolution by using nano-size optical aperture or structure, which produces a near field that makes the optical spot size much smaller than the diffraction limit [9,10]. It is well known that high-transmission nano-apertures in a metal film have great potential for high-density optical and magneto-optical data storage.
To realize high optical resolution, the gap distance between the SIL or the aperture and the optical data storage medium should be precisely maintained in the range of a few tens of nanometers, since the coupling of the near field is sensitive to the gap distance [2,4,5]. Also, we expect that an optical data storage device using the near field is extremely sensitive to dust contamination of the surface of the medium. The sensitivity to dust contamination presents a critical challenge in developing a data storage device using the optical near field for practical use [11].
A pioneering concept called super resolution near field structure (Super-RENS), which is capable of overcoming the diffraction limit and the issue caused by near field coupling, has been the subject of many studies [12,13]. Optical resolution was increased by small size aperture generated in a mask layer which blocked a part of the laser spot. However, the method requires a relatively high laser power to generate the aperture in the mask layer not only for recording optical data but also for reading the recorded data. Research is ongoing to solve practical issues related to the mask material and readout durability [14–16].
In the present work, we propose a structure for an optical data storage medium consisting of a high-transmission metallic nano-aperture array in a dielectric layer to avoid the need to control gap distance for the near field coupling problem. Using the finite differential time domain (FDTD) method, we calculate the electric field distribution generated by the nano-aperture array in the proposed data storage medium. The feasibility of the proposed structure is evaluated in terms of its recording power and density for optical data storage.
2. Structure of plasmonic data storage medium
As noted, since the intensity of the localized field underneath a high-transmission metallic nano-aperture rapidly decays, we should precisely control the gap distance between the aperture and the optical data storage medium in the near field region. To avoid the need to dynamically control this distance, we propose a data storage medium in which an array of optical nano-apertures is embedded in a metal film. The basic structure of the plasmonic optical data storage medium proposed in this work is depicted in Fig. 1 . It has a dielectric protection layer, a metal film holding the array of bowtie apertures, a thin layer of phase-change medium for optical data storage, and a dielectric substrate.
Fig. 1 (a) Structure of the proposed plasmonic data storage medium, shown as a top view at top and a cross sectional view at bottom. (b) Total intensity distribution calculated in the cross section.
The cutoff and resonance wavelengths of the bowtie aperture in a metal thin film depend on the outline and the ridge gap [17]. The transmittance of the nano-aperture has Fabry-Pérot-like resonances at certain thickness of the metal film [5]. The dimensions of the bowtie aperture, namely the outline, ridge gap, and the thickness of metal film, are determined in the limited range of design parameters to give the highest transmittance and signal contrast. The outline and the ridge gap are thusly determined to be 80 nm and 30 nm, respectively.
For the calculation, we assume that the light source is a 405 nm laser beam polarized in the x-direction and a Gaussian beam is focused with a high NA objective lens (NA = 0.9) to make a spot of 450 nm diameter (the e−2 spot size) above the bowtie aperture. We assume the material of the metal film to be aluminum, since it has good optical properties, such as small skin depth and high reflectivity in the relevant wavelength range [18]. Among a number of optical recording materials, we select GeSbTe, which shows the largest refractive index difference during the optical recording process. GeSbTe changes its phase from crystalline to amorphous at approximately 620 °C after being heated by a focused laser beam [19]. The refractive index of GeSbTe depends on the wavelength, the film thickness, and the phase state [20]. For the dielectric material of the protection layer and the substrate, we assume SiO2 due to the high melting temperature of the GeSbTe film. The optical properties of key materials used in the calculation are listed in Table 1 . The calculation is performed with the FDTD method (OptiFDTD, ver. 8.1). The total intensity distribution calculated by this method is plotted in Fig. 1(b), which shows a bright spot formed by the aperture in the layer of data recording material.
Table 1. Optical properties of materials used in the design of the plasmonic data storage medium.
3. Optical data recording and reading
For the calculation, we assume the spacing between bowtie apertures in the aperture array is 100 nm and each aperture can record a single data pit. Threshold intensity Ith means the intensity required to change the phase of the data recording material. Because of the high transmittance of the bowtie aperture, we expect that the incident intensity illuminating the aperture to make a data pit can be smaller than the threshold intensity. We calculate the intensity required for data recording as a function of the data pit size, as shown in Fig. 2 . We estimate the size of the data pit from the profile of the total intensity distribution underneath the bowtie aperture, which corresponds to the profile of threshold intensity distribution. As expected, the intensity needed for recording a data pit less than 70 nm in diameter is much lower than the threshold intensity required for conventional optical data recording.
Fig. 2 Recording intensity and contrast calculated as a function of data pit size. The threshold intensity Ith is the intensity required to record a data pit in the conventional optical storage medium. Iin is the intensity of the incident beam illuminated on the aperture.
Since the ridge gap of the bowtie aperture can generate a bright spot much smaller than that determined by the diffraction limit of light, we can enhance the density of optical data storage. We also expect that the periodic structure of an aperture array in the medium can generate surface plasmon waves on both sides of the metal film, enhancing the transmission of light through the nano-aperture [23]. Therefore, in the present scheme, we achieve a reduction in the laser power needed for recording optical data not only due to the high transmission of a bowtie aperture but also due to the transmission enhancement from the aperture array. In the calculation, we find that the peak intensity underneath the bowtie aperture in the array is enhanced by ~60% compared to that obtained by a single bowtie aperture.
In the proposed medium, we use a laser beam to make a spot to read optical data stored along a track on the data storage medium. To read the information, an optical pickup detects the variation of light intensity reflected at data pits, which have different refractive index due to changing from the crystalline to amorphous state.
Assuming the center of the laser beam is aligned to the entrance of the aperture, we calculate the reflected beam intensity for two different cases shown in Fig. 3 . Ion is the reflected light intensity obtained when a data pit is under the center nano-aperture and Ioff is that obtained when the data pits are under both neighboring apertures. To evaluate data readability, we define the contrast, which indicates the variation of reflected beam intensity [24,25], as the equation:
Fig. 3 Data reading schematics in the proposed plasmonic data storage medium. Ion is the reflected signal obtained when a data pit is under the center nano-aperture and Ioff is that obtained when the data pits are under both neighboring apertures.
The contrast is calculated as a function of data pit size and the results are depicted in Fig. 2 (right-hand axis). The contrast monotonically increases with data pit size until reaching a pit size of 50 nm. Also, we find that small size data pits are created under the adjacent apertures when we record the center data pit of size larger than 50 nm. We thus determine the optimal data pit size to be 50 nm to get the highest contrast without noise from adjacent apertures in the data reading process.
When we illuminate a laser beam on a bowtie aperture for reading or recording an optical datum, the laser beam also covers the two adjacent apertures on either side because of the large beam size and the intensity distribution of a Gaussian beam. We calculate the difference of reflected beam intensity distributions of Ion and Ioff shown in Fig. 3. The resulting quantity, Idiff (= Ion – Ioff), is plotted in Fig. 4(a) . Because of the data pit underneath the bowtie aperture in Fig. 3, we expect that Idiff in the region of adjacent apertures has a reverse sign compared to that in the center region as shown in Fig. 4(a). Therefore, in order to improve the contrast for better readability, we should reduce the detection area, limiting the area of the reflected beam. The contrast calculated for different diameters of the detection area is shown in Fig. 4(b). The contrast gradually decreases with increasing diameter of the detection area, while the magnitude of the total reflected beam intensity increases as a linear function of the diameter. Therefore, we find that we can take smaller diameter of the detection area to get a higher contrast, trading off the magnitude of the total intensity, which corresponds to the signal of an optical pickup.
Fig. 4 (a) The difference between the reflected intensity distributions of Ion and Ioff. Concentric lines indicate the detection areas whose diameters are 100 nm, 200 nm, and 300 nm. (b) Contrast and signal strength (total intensity) of Ion calculated for different detection area diameters.
They commonly use the carrier-to-noise ratio (CNR) to evaluate the performance of optical data storage device, and it is similar to multiply of signal-to-noise ratio (SNR) [26]. When the incident power is sufficiently large, the thermal and the shot noises are negligible [27]. In the typical conditions, a linear relationship between the CNR and the contrast of signal is well established [28] and we estimate the CNR in the present form of the medium to be 30 dB. We expect that the CNR can be improved to meet the specification (45dB) for commercial data storage devices by subtracting floating background signal caused by the reflection at the front surface of the metal film.
4. Density of optical data storage
In our model, we place bowtie apertures in a two-dimensional array with a surface structure as illustrated in Fig. 5(a) . Since each bowtie aperture can hold a bit datum, the bit pitch is defined by the distance between two adjacent apertures. The track pitch indicated in Fig. 5(a) is the distance between two rows of the apertures.
Fig. 5 (a) Track and bit pitches in the plasmonic data storage medium. (b) Crosstalk calculated as a function of the track pitch.
Cross talk is defined as the ratio of the signal arising from recorded marks in adjacent tracks to the signal originating solely from the mark in the main target track [29]. We define the cross talk of our device by the following equation:
where Rm is the reflection when the target track has data pits and the adjacent tracks do not have data pits. Ra is the reflection when both the target track and the adjacent tracks have data pits. In Fig. 5(b), we plot the calculated cross talk as a function of the track pitch. The cutoff criterion in cross talk analysis of conventional optical data storage devices is normally given by −30 dB [26]. Since the cross talk monotonically decreases with the track pitch and becomes −34 dB at the track pitch of 280 nm, we conclude that a track pitch larger than 280 nm meets the cross talk criterion.Based on the cross talk analysis, we choose a track pitch of 280 nm. Based on the track pitch and the bit pitch, we can estimate the data storage density of the proposed data storage medium. If choose a bit pitch of 100 nm, the density along the track is enhanced by 1.5 times compared to the conventional blu-ray disc. Also, we get some gain in track pitch, which is smaller than that of the blu-ray disc. Since the track pitch of the blu-ray disc is 320 nm, the density is enhanced by ~1.2 times by using the smaller track pitch of 280 nm. Overall, the storage density of the proposed plasmonic data storage device can be 1.8 times higher than that of the blu-ray disc.
5. Conclusions
We propose a structure for an optical data storage medium consisting of a high-transmission metallic nano-aperture array in a dielectric layer. This design avoids the difficulty of controlling gap distance in the near field recording system. Using the FDTD method, we determine the optimal dimensions of a bowtie aperture. We also calculate the exposure dose needed for recording of optical data, the contrast for readability of recorded data, and cross talk between the main track and adjacent tracks. Transmission of light through a single bowtie aperture is enhanced by not only the nano-aperture itself but also the arrangement of apertures in an array, due to surface plasmon waves. The recording power required to obtain a 50 nm-size data pit underneath the bowtie aperture is less than a quarter of the threshold dose needed in a conventional optical data storage system. Based on calculation of the cross talk, an optimal track pitch is determined to be 280 nm. We calculate that the density of data storage can be 1.8 times larger than that of a blu-ray disc. Furthermore, we expect that the data storage medium proposed in this work can be fabricated using imprint technique producing a two-dimensional array pattern of nano-apertures in a metal film.
Acknowledgments
This work was supported by the Development Program of Nano Process Equipments of the Korea Ministry of Commerce, Industry and Energy. (Project no. 10030259-2008-02).
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