Nano-optical transducers have been utilized in existing an emerging applications due to their ability to obtain small optical spots, large transmission efficiency, and narrow and adjustable spectral response. In emerging nano-optical applications, such as heat assisted magnetic recording (HAMR), these features are not sufficient. For example, in HAMR a transducer should also satisfy additional requirements, such as mass-production and integrability with other device components. In this study, the basic principles of Maxwell’s equations and image theory for good metals are utilized to design a perpendicular oriented single-pole nano-optical transducer, which can be integrated into the manufacturing technologies of current hard disk drive heads. The perpendicular oriented single-pole nano-optical transducer is investigated using 3-D finite element methods Gold transducers are investigated for both longitudinal and perpendicular orientations. The optical intensity profiles and spot sizes of longitudinal and perpendicular oriented transducers are compared for various fly heights. It is shown that a perpendicular ridge waveguide provides localized optical spots with intensities comparable to longitudinal transducers.
© 2010 Optical Society of America
Advances in information technology, computer systems, and consumer electronics devices have led to a massive increase in the amount of information that is stored on magnetic data storage systems. The capacity of hard disk drives (HDDs) has been increasing in parallel with this demand. The areal density, which is a measure of the amount of data that can be stored on a HDD, has been increasing at a rate similar to semiconductor devices as defined by Moore’s law . In recent years, the magnetic storage industry has observed a slowdown in the pace of the areal density growth due to the superparamagnetic limit [2, 3]. Currently employed perpendicular recording technology [4, 5] has been predicted to reach an areal density of about 1 Tbit/in.2 by continuous downscaling of the bit dimensions . Thermal stability for small grains can be preserved if the uniaxial anisotropy coefficient of the magnetic material is large. Although recording materials, which have sufficiently large uniaxial anisotropy coefficient for areal densities beyond 1 Tbit/in.2 are known, the coercivity of these materials at room temperature is too large for the available fields from conventional recording heads to switch the magnetic bit. A novel technique is necessary to facilitate recording on a magnetic medium with a large anisotropy.
Heat assisted magnetic recording (HAMR) [7, 8, 9, 10] is a potential technique to extend the physical limits of conventional magnetic recording in hard disk drives beyond 1 Tbit/in.2. In a HAMR system an optical spot is used to heat the recording medium to reduce its coercivity and enable recording by an external field. To achieve a density of 1 Tbit/in.2 an optical spot well below the diffraction limit is required. Recent studies have made progress in addressing this requirement for HAMR. Challener et al.  demonstrated that a 70 nm track width can be recorded on a magnetic medium when a near-field optical transducer is integrated with a magnetic head. Studies in the literature [11, 12] indicate that near-field transducers can further reduce the optical spots in the 30 nm to 40 nm range with transmission efficiencies between 1% and 5%. In addition, recent studies indicate that a recording magnetic medium that is designed based on the basic principles of optical energy coupling and heat transfer can provide superior optical and thermal performance compared to a conventional magnetic medium . Although extremely large optical coupling efficiencies do not seem likely, recent studies [9, 11, 12, 13] indicate that a near-field transducer and recording medium can be tailored to achieve optical spots and thermal profiles for a feasible HAMR system.
For a feasible HAMR system that can be mass-produced for computer systems, a nano-optical transducer should be easily integrated with other device components. This can be achieved if the nano-optical transducer can be produced using lithographic and thin film deposition techniques known from modern integrated circuit processing. Besides manufacturability for a cost-effective system, a planar thin film nano-optical transducer should obtain intense optical spots with small sizes to reach the desired areal density.
In this study a perpendicular oriented ridge waveguide is suggested using the basic principles of Maxwell’s equations and image theory for good metals. Its performance in the vicinity of a magnetic recording medium is investigated for potential utilization in a practical HAMR system. A perpendicular oriented ridge waveguide transducer can be defined using lithographic techniques on the same plane as the magnetic heads. Therefore, it can be more easily integrated into the manufacturing technology of the current hard disk drive heads as compared to its longitudinal counterpart. In this study, the performance of a perpendicular ridge waveguide transducer is compared to that of a longitudinal oriented ridge waveguide transducer for potential utilization in HAMR.
2. Longitudinal oriented transducer
A ridge waveguide has been suggested for data storage as a near-field optical transducer  because of its ability to generate intense fields in the gap between the ridge and the opposite surface. Various ridge waveguide designs have been studied [11, 15, 16, 17, 18] for potential use in data storage devices. The ridge waveguide has provided promising results in terms of optical spot size and intensity in the recording magnetic medium . All of the studies mentioned above utilized the ridge waveguide in a longitudinal orientation with respect to the recording magnetic medium. This orientation with respect to the magnetic medium is illustrated in Fig. 1.
A metallic ridge waveguide primarily utilizes evanescent waves to couple electromagnetic energy to the magnetic recording medium in the near-field. The direction of the polarization of the incident radiation and the ridge waveguide geometry play an important role in this process. If the incident polarization is along the axis of the ridge as shown in Fig. 2, then the incident electromagnetic radiation creates induced currents along this axis. The induced current is the source of charge accumulation on the ridge and the opposite surface with inverse polarity as shown in Fig. 2. The oscillation of the localized charges with inverse polarity is the source of localized near-field electromagnetic radiation. For a longitudinal oriented ridge waveguide, the localized intense near-field radiation is composed of propagating and evanescent components. The optical spot in the recording magnetic medium is obtained by coupling the optical fringe fields onto the magnetic recording medium. If a recording magnetic medium is brought within the vicinity of this localized field around the ridge, an optical field intensity distribution with a size well below the diffraction limit is obtained within the magnetic recording medium.
The optical intensity distribution in the recording magnetic medium is illustrated in Fig. 3 for a longitudinal oriented ridge waveguide transducer. Gold is selected as the material for the ridge waveguide, since silver tarnishes. The optical properties of metals used in this study are retrieved from the literature . The dielectric constant of metals in this study are listed in Table 1 at the wavelengths of 625 nm and 750 nm. The fly height is selected as 8 nm. The optical power absorption profile is obtained using a three-dimensional (3D) finite element method solution of Maxwell’s equations. The dimensions are selected based on a previous study . A wavelength of 625 nm is selected for the longitudinal transducer. The intensity enhancement versus wavelength is illustrated in Fig 4 for the longitudinal transducer. The results show that the longitudinal transducer resonates at 625 nm, which is selected as the operating wavelength for the longitudinal transducer in this study. The thickness of the cobalt magnetic layer and gold heat sink layer are 10 nm and 200 nm, respectively. The intensity in the recording medium is normalized with incident field intensity at the focus. The optical spot size in the recording medium is about 30 nm. The longitudinal waveguide result in Fig. 3 will be used as a benchmark for comparison in the rest of this study. To give a better illustration and comparison of the figures, the scale of the color-map for transducers at 8 nm fly height are unified by selecting the extrema as 0 and 1.9, which is the maximum intensity produced by the perpendicular transducer at 8 nm.
An important component of a heat-assisted magnetic recording (HAMR) system is the magnetic head. For a feasible HAMR system that can be mass-produced for computer systems, a nano-optical transducer should be easily integrated with other device components, in particular the magnetic head. This can be achieved if the nano-optical transducer can be produced using lithographic and thin film deposition techniques currently used to manufacture the magnetic heads. Therefore, a nano-optical transducer should satisfy the following two conditions to be feasible in HAMR:
- the nano-optical transducer can be produced using similar lithographic and thin film deposition techniques currently used to manufacture magnetic heads.
- the nano-optical transducer and magnetic head can be brought into close proximity of each other so that the magnetic head field can be in the vicinity of the optical spot.
A longitudinal oriented ridge waveguide transducer cannot be easily produced using the current manufacturing techiques of magnetic heads. A magnetic head pole and a ridge waveguide are illustrated in Fig. 5. As shown in Fig. 5, the magnetic pole and longitudinal transducer are vertical with respect to each other. Therefore, the ridge waveguide cannot be processed using lithographic techniques, since it is vertical to the lithography plane. In addition, a longitudinal transducer cannot be brought in close proximity of the magnetic pole without changing the shape and structure of the longitudinal transducer. A perpendicular oriented ridge waveguide transducer, which will be discussed in the next section, can be defined using lithographic techniques on the same plane as the magnetic pole. Therefore, it can be more easily integrated into the manufacturing technology of the current hard disk drive heads as compared to its longitudinal counterpart.
3. Perpendicular oriented transducer
In this section, the basic principles of Maxwell’s equations and image theory for good metals are utilized to design a perpendicular oriented single-pole nano-optical transducer. The equivalence principle for Maxwell’s equations states that the equivalent sources can be placed in certain regions of space . The goal is to obtain the same solution of the original problem within the desired part of the space. In the other parts of the space in which the solution is not of interest, the original problem and the equivalent problem yield different solutions. Therefore, there are multiple ways of constructing an equivalent problem.
One of the well-known ways to construct an equivalent problem is image theory . An equivalent problem can be constructed for an electric monopole that is placed in the vicinity of a conducting half-space according to image theory. The original problem is illustrated in Fig. 6. An electric monopole is placed at a distance L from a conducting half-space, which occupies the region defined as z < 0. The equivalent problem is illustrated in Fig. 7. The conducting half-space is replaced with a vacuum in the equivalent problem. Another electric monopole with a charge of equal magnitude and opposite sign is placed in the lower half-space at a distance L from the interface. The original and equivalent problems illustrated in Figs. 6 and 7 yield an identical electric field distribution at the upper half-space defined as z > 0.
A nano-optical transducer can be constructed using the fundamentals of equivalence principle and image theory summarized in the previous paragraph. As described in Sect. 2, incident electromagnetic radiation creates an induced current on the ridge waveguide transducer, which causes a charge accumulation on the ridge and the opposite surface. An alternative way to define an opposite surface for the ridge is to utilize a separate semi-infinite conducting layer as shown Fig. 8. The transducer and semi-infinite conducting layer are in a perpendicular orientation with respect to each other in Fig. 8. For a perpendicular oriented ridge waveguide transducer, the heat sink layer under the magnetic medium forms the opposite surface of the waveguide. The operation principle of a perpendicular transducer in the vicinity of a semi-infinite conducting layer can be best interpreted by image theory using Figs. 9 and 10. As shown in Fig. 9 the charge is accumulated on the ridge for a perpendicular ridge waveguide. The configuration in Fig. 10 yields the same solution to the problem in Fig. 9 in the upper-half space due to the equivalence principle and image theory. For a perpendicular oriented ridge waveguide, the intense fields are formed in the magnetic medium since it is in between the ridge and its image in the heat sink underlayer. In other words, the magnetic medium is located in the gap region of the perpendicular oriented ridge waveguide transducer. The gap distance is determined by a combination of the thickness of the magnetic layer and the fly height.
To produce an induced charge accumulation shown in Fig. 9 and Fig. 10 for the perpendicular oriented transducer, it needs to be illuminated with an incident electromagnetic radiation with a strong component in the perpendicular direction. In other words, the polarization of the incident electromagnetic radiation should be parallel to the z-axis of the sample structure depicted in Fig. 8. This can be achieved by using one of the following three methods: (a) a radially polarized donut mode [21, 22, 23, 24, 25], (b) a focused waveguide mode obtained using a planar solid immersion mirror with a dual offset grating [9, 26], or (c) an apertureless excitation . In this study, the first type of excitation, i.e. a radially polarized donut mode, is utilized to excite the structures to achieve an induced charge accumulation shown in Fig. 9 and Fig. 10.
The optical intensity distribution in the recording magnetic medium for a perpendicular oriented ridge waveguide is illustrated in Fig. 11. The perpendicular ridge waveguide thickness is selected as 20 nm, since it is a crucial parameter that determines the optical spot size for a perpendicular transducer. The length and width of the waveguide are selected as 420 nm and 20 nm, respectively. A wavelength of 750 nm is selected for the perpendicular transducer. The intensity enhancement versus wavelength is illustrated in Fig 4 for the perpendicular transducer. The results show that the perpendicular transducer resonates at 750 nm, which is selected as the operating wavelength for the perpendicular transducer in this study. Other system related parameters, such as the fly height, and the thickness of magnetic and heat sink layers, are selected as identical to the simulation in Fig. 3. The intensity in the recording medium is normalized with the incident field intensity at the focus. The spot size in Fig. 11 is about 30 nm, similar to the spot size in Fig. 3. The intensity for the perpendicular transducer is slightly larger when both transducers are made of gold.
Optical intensity distributions in the recording magnetic medium are illustrated when the fly height is 15 nm in Figs. 12 and 13 for longitudinal and perpendicular transducers, respectively. To give a better illustration and comparison of the figures, the scale of the color-map for transducers at 15 nm fly height are unified by selecting the extrema as 0 and 0.35, which is the maximum intensity produced by the longitudinal transducer at 15 nm. Spots sizes in Figs. 12 and 13 are 45 nm and 47 nm for longitudinal and perpendicular transducers, respectively. A comparison of Figs. 3, 11, 12, and 13 suggests that the optical spots are bigger for larger fly heights for both orientations. Larger fly heights also cause the optical field intensities in the magnetic medium to drop sharply in Figs. 12 and 13 for both orientations when compared to Figs. 3 and 11. This sharp drop can be explained by the intense electric fields around the ridge having an evanescent component for both the perpendicular and longitudinal orientations. As the recording medium is located farther from the ridge by increasing the fly height, the evanescent field rapidly decays. Therefore, optical field coupling into the recording magnetic medium reduces for both the longitudinal and perpendicular orientations. The perpendicular optical transducer is more affected by the fly height. This is analogous to the spacing loss in conventional magnetic recording. In conventional magnetic recording, perpendicular magnetic heads are more susceptible to spacing loss as compared to longitudinal magnetic heads. Analogous to that, the perpendicular oriented ridge waveguide transducer is more sensitive to the spacing loss compared to the longitudinal ridge waveguide transducer.
In Fig. 14, the peak optical intensity distributions for the longitudinal and perpendicular oriented ridge waveguide transducers are plotted as a function of fly height. The results of the longitudinal orientation indicate that the optical intensity in the magnetic medium increases even for very small fly heights. For the perpendicular oriented ridge waveguide transducer, however, there is a sharp drop of optical intensity in the magnetic medium for fly heights smaller than 4 nm. The perpendicular ridge waveguide provides an optical intensity in the recording medium with intensity similar to that of the longitudinal oriented ridge waveguide for fly heights larger than 4 nm.
In summary, a perpendicular oriented single-pole nano-optical transducer, which can be more easily integrated into the manufacturing technologies of current hard disk drive heads, was suggested based on the basic principles of Maxwell’s equations and image theory. The performance of the perpendicular transducer was demonstrated by comparing the near-field radiation from perpendicular and longitudinal ridge waveguide transducers in the vicinity of a recording magnetic medium for various fly heights. A perpendicular oriented ridge waveguide transducer provided similar performance to a longitudinal transducer in terms of spot size and intensity. A perpendicular ridge waveguide can be defined on the same plane as the magnetic heads using lithographic techniques; therefore, it can be more easily integrated with the manufacturing technologies of the current hard disk drive heads.
This work is supported by TUBITAK under project number 108T482 and by Marie Curie International Reintegration Grant (IRG) under project number MIRG-CT-2007-203690. Kursat Sendur acknowledges partial support from the Turkish Academy of Sciences.
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