Near field transducer driven by a transverse electric waveguide for energy assisted magnetic recording

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application Serial No. 62/010,111 filed on Jun. 10, 2014, which is expressly incorporated by reference herein in its entirety.

BACKGROUND

High density storage disks are configured with layers of materials that provide the required data stability for storage. The magnetic properties of the media may be softened when writing to the disk to assist changing the bit state. Energy Assisted Magnetic Recording (EAMR) device or Heat Assisted Magnetic Recording (HAMR) technology provides heat that is focused on a nano-sized bit region when writing onto a magnetic storage disk, which achieves the magnetic softening. A light waveguide directs light from a laser diode to a near field transducer (NFT). The NFT couples the diffraction limited light from waveguide (WG), then further focuses the light field energy beyond diffraction limit down to a highly concentrated (nano-sized) near-field media heating spot enabling EAMR/HAMR writing to the magnetic storage disk. Inefficiencies in the NFT can have a negative impact on the power budget of the laser diode and the EAMR/HAMR system lifetime. Higher NFT efficiency allows for lower laser power demand, relieving EAMR/HAMR system requirement on the total optical power from the laser source, and results in less power for parasitic heating of the EAMR/HAMR head resulting for improved reliability.

The light waveguide may operate primarily in either one of the following modes to drive the NFT. In a transverse electric (TE) mode, the predominant electrical field component is in the transverse direction (i.e., side-to-side, x-axis) above the waveguide core. In a transverse magnetic (TM) mode, and the predominant magnetic field component is in the transverse direction (i.e., side-to-side, x-axis), while the predominant electrical field component above the waveguide core has a significant longitudinal direction (z-axis) component. Driving the NFT using a TM mode waveguide has the advantage of a strong electrical field in the direction along the waveguide for easier coupling to the NFT above the waveguide core. For EAMR/HAMR devices using laser diodes that produce a TE mode wave, a TE mode to TM mode converter is needed to energize a TM mode in a waveguide. An efficient NFT driven by a TE waveguide has the advantage of avoiding the TE mode to TM mode converter, which provides a more compact and higher efficiency light delivery system.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects of the present invention will now be presented in the detailed description by way of example, and not by way of limitation, with reference to the accompanying drawings, wherein:

FIG. 1 shows a diagram of an exemplary hard disk drive.

FIG. 2 shows a diagram of an exemplary embodiment of an NFT formed with two partially sloped plasmonic metal bars disposed above two TE dielectric waveguide cores.

FIG. 3 shows a diagram of an exemplary embodiment of an NFT formed with two fully sloped plasmonic metal bars disposed above two TE dielectric waveguide cores.

FIG. 4 shows a diagram of an exemplary embodiment of a TE waveguide core and NFT that incorporates a heat sink and magnetic pole for the writing head.

FIG. 5 shows a diagram of an exemplary embodiment of a TE waveguide core for driving an NFT formed with a plasmonic metal bar having one sloped edge.

FIG. 6 shows a diagram of an exemplary embodiment of a TE waveguide core for driving an NFT formed with a plasmonic metal bar having two sloped edges.

FIG. 7 shows a diagram of an exemplary embodiment of a TE waveguide core for driving an NFT formed with a plasmonic metal bar disposed above the waveguide core offset from the centerline of the waveguide core.

FIG. 8 shows a diagram of an exemplary embodiment of a TE waveguide core for driving an NFT formed with a plasmonic metal bar disposed above the waveguide core in an askew configuration.

DETAILED DESCRIPTION

The detailed description set forth below in connection with the appended drawings is intended as a description of various exemplary embodiments and is not intended to represent the only embodiments that may be practiced. The detailed description includes specific details for the purpose of providing a thorough understanding of the embodiments. However, it will be apparent to those skilled in the art that the embodiments may be practiced without these specific details. In some instances, well known structures and components are shown in block diagram form in order to avoid obscuring the concepts of the embodiments. Acronyms and other descriptive terminology may be used merely for convenience and clarity and are not intended to limit the scope of the embodiments.

The various exemplary embodiments illustrated in the drawings may not be drawn to scale. Rather, the dimensions of the various features may be expanded or reduced for clarity. In addition, some of the drawings may be simplified for clarity. Thus, the drawings may not depict all of the components of a given apparatus.

Various embodiments will be described herein with reference to drawings that are schematic illustrations of idealized configurations. As such, variations from the shapes of the illustrations as a result of manufacturing techniques and/or tolerances, for example, are to be expected. Thus, the various embodiments presented throughout this disclosure should not be construed as limited to the particular shapes of elements illustrated and described herein but are to include deviations in shapes that result, for example, from manufacturing. By way of example, an element illustrated or described as having rounded or curved features at its edges may instead have straight edges. Thus, the elements illustrated in the drawings are schematic in nature and their shapes are not intended to illustrate the precise shape of an element and are not intended to limit the scope of the described embodiments.

The word “exemplary” is used herein to mean serving as an example, instance, or illustration. Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. Likewise, the term “embodiment” of an apparatus or method does not require that all embodiments include the described components, structure, features, functionality, processes, advantages, benefits, or modes of operation.

As used herein, the term “about” followed by a numeric value means within engineering tolerance of the provided value.

In the following detailed description, various aspects of the present invention will be presented in the context of an interface between a waveguide and a near field transducer used for heat assisted magnetic recording on a magnetic storage disk.

FIG. 1 shows a hard disk drive 111 including a disk drive base 114, at least one rotatable storage disk 113 (e.g., such as a magnetic disk, magneto-optical disk), and a spindle motor 116 attached to the base 114 for rotating the disk 113. The spindle motor 116 typically includes a rotating hub on which one or more disks 113 may be mounted and clamped, a magnet attached to the hub, and a stator. At least one suspension arm 108 supports at least one head gimbal assembly (HGA) 112 that holds a slider with a magnetic head assembly of writer and reader heads. A ramp assembly 100 is affixed to the base 114, and provides a surface for tip of the suspension arm 108 to rest when the HGA 112 is parked (i.e., when the writer and reader heads are idle). During a recording operation of the disk drive 111, the suspension arm 108 rotates at the pivot 117, disengaging from the ramp assembly 100, and moves the position of the HGA 112 to a desired information track on the rotating storage disk 113. During recording, the slider is suspended by the HGA 112 with an air bearing surface of the slider that faces the rotating storage disk 113, allowing the writer head to magnetically alter the state of the storage bit. For heat assisted magnetic recording, a near field transducer (NFT) on the air bearing surface may couple light energy from a waveguide to produce a heating spot on the rotating storage disk 113 for magnetically softening the bit space.

FIG. 2 shows a diagram of an exemplary embodiment of an NFT 200 arranged at an air bearing surface (ABS) 210 of a slider which carries a magnetic head assembly. The ABS 210 is the surface of the slider facing the storage disk 113. As the slider flies over the storage disk 113, a cushion of air is maintained between the slider ABS 210 and the surface of the storage disk 113. As shown, two dielectric waveguide (WG) cores 211, 212 are arranged to each carry light energy to the NFT 200. The light energy may be generated by a TE laser diode source (not shown) that may be split in half by a splitter (not shown). The dielectric waveguide cores 211, 212 may be of equal length to ensure that the combined energy wave at the ABS 210 is in substantial phase alignment for constructive interference and maximum energy emission to the storage disk 113. Alternatively, dielectric waveguide cores 211, 212 may be of unequal length such that the incident energy waves may have a particular phase difference that optimizes constructive interference and maximum energy magnitude at the ABS 210. The two waveguide cores 211, 212 are substantially linear and converge at a junction near the ABS 210 at an interior angle between 0 and 180 degrees, (e.g., approximately 90 degrees as shown in FIG. 2). The dielectric material of the waveguide core may be Ta₂O₅ for example.

As shown in the cross-section of FIG. 2, the NFT includes a plasmonic metal bar element 202, which may be disposed above a waveguide core 212 in a longitudinal direction, with centerline of the plasmonic bar element 202 approximately aligned along the centerline the waveguide core 212 surface. Similarly, a plasmonic metal bar 201 may be disposed above a waveguide core 211 as shown in FIG. 2. The optical energy from the dielectric waveguide cores 211, 212 in proximity with of the plasmonic metal bar 201, 202 energizes propagating surface plasmon polaritons (PSPPs) along the plasmonic metal bar 201, 202 surface toward the ABS 210. Thus, each plasmonic metal bar element 201, 202 may function as a PSPP element. As shown in the cross section, a gap (e.g., of about 20 nm) may exist between the plasmonic metal bar 201, 202 and the dielectric waveguide core 211, 212. Alternatively, the gap may be omitted, and the plasmonic metal bar 201, 202 may directly contact the dielectric waveguide core 211, 212, at least for a portion of the plasmonic metal bar 201, 202. The two dielectric waveguide cores 211, 212 and the entire NFT 200 may be encapsulated by a silicon oxide material. The material of the plasmonic metal bars 201, 202 may be a gold alloy, for example. Other examples of plasmonic metals that may be used to form the plasmonic metal bars 201, 202 include silver or copper alloys.

The plasmonic metal bar elements 201, 202 may be configured as shown in FIG. 2, converging at a junction above the junction of the dielectric waveguide cores 211, 212. The junction of plasmonic metal bar elements 201, 202 may occur on a common plane, or may be formed by overlapping one element over the other element. The junction of plasmonic metal bar elements 201, 202 may be formed at the ABS 210. For example, an NFT energy output emitter may be formed at the ABS 210 by the exposed metal bar junction at the ABS 210, from which the maximum energy is propagated across the air cushion and onto the storage disk 113 surface. The physical dimension of the emitter (i.e., the width of the exposed plasmonic metal bar junction) may be approximately equivalent to the size of the focused heating spot on the surface of the disk 113. The target size of the heating spot is dependent on the track size as the slider flies over the track, which may be about 10-70 nm wide for example. The size of the heating spot also depends on the distance between the ABS 210 and the disk 113. The focus of the heating spot may be optimized by minimizing the gap distance.

The plasmonic metal bars 201, 202 may be tapered at a first end that receives the light energy as shown in FIG. 2 so that the electrical field of the light energy in the TE dielectric waveguide mode can propagate to the plasmonic metal in the z-axis direction toward the ABS 210. The width of the plasmonic metal bars 201, 202 is tapered with a sloped edge on one side and a substantially linear edge on the opposite side of the plasmonic metal bar 201, 202. For illustration purpose, the sloped edge is shown in FIG. 2 as the outer edges. However, the plasmonic metal bars 201, 202 may be configured in other variations including the inner edges being sloped, or one of the plasmonic metal bars 201, 202 having a sloped inner edge and the other having a sloped outer edge. With this tapered configuration, the plasmonic metal bars 201, 202 may serve simultaneously as an evanescent coupler and as a polarization converter, transmitting optical power out of the TE dielectric waveguide core 211, 212 into the plasmonic metal bar 201, 202. When operating in a TE mode of a dielectric waveguide, the electrical field above the waveguide core is strongest in the x-axis component and weakest in the z-axis component. The tapering in the plasmonic metal bar 201, 202 is configured for transforming (or rotating) electrical field in the x-axis direction as the light energy propagates into the z-axis direction. As the energy waves reach the fixed width (untapered) portion of the plasmonic metal bar, the excited PSPPs may effectively rotate around the corner at the sloped edge, transferring some of the energy magnitude from the x-direction to the z-direction. As an example, with a TE laser wavelength range of 770-880 nm and the waveguide 550 nm wide and 120 nm thick, the tapering dimensions of the plasmonic metal bar 201, 202 may be approximately 2.5 microns in length for the tapered portion, and the straight portion may be approximately 550 nm wide and 2.0 microns in length.

The width of the plasmonic metal bar element 201, 202 at the widest portion may be approximately equivalent to the width of the dielectric waveguide core 211, 212. Alternatively, the width of the plasmonic metal bar element 201, 202 at the widest portion may be slightly more wide or less wide than the width of the dielectric waveguide core 211, 212. In an embodiment, the width of the plasmonic metal bars 201, 202 may be at least two times the width of the heating spot generated on the storage disk 113. In another embodiment, the width of the plasmonic metal bars 201, 202 may be at least three to six times the width of the heating spot generated on the storage disk 113.

To achieve the required focus of the heating spot width while using a wider plasmonic metal bar, the NFT 200 may be configured with one or more of the following features. The emitter may be configured such that the width of exposed plasmonic metal is approximately equivalent to the desired width of the heating spot. The emitter width may be controlled by lapping the ABS 210 until the width dimension for the junction of the exposed plasmonic metal bars is within the acceptable range. Also, the NFT 210 may be configured with the PSPP elements 201, 202 having unequal lengths to produce a constructive interference at the ABS 210 that gives the desired focus width for the heating spot.

The two-PSPP element configuration as shown in FIG. 2 may provide approximately twice as much electrical field magnitude compared with a configuration of a single PSPP element arranged perpendicular to the ABS 210, driven by a common total input power in the waveguide system. The constructive interference produced by the two PSPP elements 201, 202 allows improved efficiency of energy delivery from the laser diode source, which translates to longer service life of the EAMR/HAMR device.

The NFT 200 embodiment does not have to be limited to two interfering PSPP elements as shown in FIG. 2. In an alternative embodiment, N (a positive integer) PSPP elements interfere at the ABS 210, which may provide approximately N times increase of the electrical field magnitude, driven by a common total input power in the waveguide system. The value of N can increase beyond 2 or 3, until other parasitic interferences within the three dimensional layout the EAMR head becomes a limiting factor. For N≧3, the PSPP elements may be arranged in a three dimensional configuration (i.e., not all NPTs must exist in a common two dimensional plane).

FIG. 3 shows an exemplary embodiment of an NFT 300, which is a variation of the NFT 200 in that the PSPP elements 301, 302 are tapered for the entire length compared to PSPP elements 201, 202 which are only tapered for a portion of the length. The PSPP elements 301, 302 may be disposed above the dielectric waveguide cores 311, 312 in a similar manner as described above for NFT 200.

FIG. 4 shows an exemplary embodiment of an NFT 400, which is a variation to the exemplary embodiment of FIG. 2, by the addition of a plasmonic metal cap 405 in the NFT 400. A magnetic pole 407 for the writer head may be integrated with the plasmonic metal cap 405, and in between, a thin diffusion barrier layer 408 may be disposed to prevent diffusion between the ferrous material of the magnetic pole and the alloy material in the plasmonic metal cap 405. The plasmonic metal cap 405 serves as a heat sink and a light block for the magnetic pole 407. In an alternative embodiment, the pole 407 may be recessed from the ABS 410.

As shown in FIG. 4, the plasmonic metal cap 405 may be configured as a semi-circle having a straight edge substantially aligned with the ABS 410. The size of the metal cap may be, for example, 1000 nm in diameter. The thickness of the plasmonic metal cap 405 is not a significant factor in achieving the precise nano-sized heating spot, and therefore the thickness may be configured according to providing adequate heat transfer for controlling the peak temperature in the NFT 400. As an example, the plasmonic metal cap 405 may be greater than 30 nm in thickness. The metal cap 405 may be configured in shapes other than a semicircle, such as rectangular or polygonal. The plasmonic metal bar elements 401, 402 may be coupled to the plasmonic metal cap 405 above. For illustrative purpose, the plasmonic metal element 405 is depicted as transparent to reveal the metal bars 401, 402 below. As shown in the cross section, a gap (e.g., about 20 nm) may exist between the plasmonic metal bars 401, 402 and the dielectric waveguide cores 411, 412. Alternatively, the gap may be omitted, and the plasmonic metal bar 401, 402 may directly contact the dielectric waveguide core 411, 412, at least for a portion of the plasmonic metal bar 401, 402. The two dielectric waveguide cores 411, 412 and the entire NFT 400 may be encapsulated by a silicon oxide material. The material of the plasmonic metal cap 405 may be a gold alloy, for example. Other examples of plasmonic metals that may be used to form the plasmonic metal cap 405 include silver or copper alloys.

FIG. 5 shows a diagram of exemplary embodiment of an NFT 500 having a single plasmonic metal bar 501 disposed above a dielectric waveguide core 511 in a longitudinal direction that is substantially perpendicular to the ABS 113. The NFT 500 of this embodiment is a variation of the NFT 200 shown in FIG. 2, except that a single PSPP element 501 is arranged in the NFT. The plasmonic metal bar 501 is sloped to form the tapered width at the one end that receives the light energy from the waveguide core, similar to the plasmonic metal bar 201.

FIG. 6 shows a diagram of a variation of the exemplary embodiment of the NFT shown in FIG. 5. For NFT 600 as shown in FIG. 6, both sides of a portion of the plasmonic metal bar 601 may be sloped to form the tapered end. The tapering dimensions control the focus of the electrical field energy at a side edge of the plasmonic metal bar at the ABS 610, such that the width of the electrical field is approximately equal to the required heating spot width (e.g., 10-70 nm) on the recording medium. The shape and configuration of plasmonic metal bar 601 may also be implemented as a two-PSPP element NFT as shown in FIG. 2, or an NFT having N PSPP elements as described above.

FIG. 7 shows a diagram of an exemplary embodiment of an NFT 700 having a single PSPP element that is aligned with a TE waveguide core, and substantially perpendicular to the ABS 710. A lengthwise portion of the PSPP element 701 extends beyond the perimeter of the facing surface (i.e., the top surface edge) of the dielectric waveguide 711. The centerline of the PSPP element 701 may be offset from the centerline 720 of the TE waveguide core 711 as shown in FIG. 7. Since the z-axis component of the electrical field in a TE waveguide core is strongest at the side edges of the TE waveguide core (i.e., where the z-axis projects into the waveguide core aligned with centerline 720), exposing a portion of the plasmonic metal bar 701 (i.e., that portion extending beyond the edge of the TE waveguide core) to the side edge 721 of the waveguide core 711, enhances capture and coupling of the electrical field. The shape and configuration of plasmonic metal bar 701 may also be implemented as a two-PSPP element NFT as shown in FIG. 2, or an NFT having N PSPP elements as described above.

FIG. 8 shows a diagram of exemplary embodiment of an NFT 800 that combines the tapering effect as described for the embodiments shown in FIG. 2-6, and the offset effect as described for the embodiment shown in FIG. 7. As shown in FIG. 8, the plasmonic metal bar 801 is longitudinally askew with respect to the centerline 820 of the waveguide core 811 below, such that the side edge 822 is sloped in the longitudinal direction with respect to the centerline 820 of the waveguide core 811. Also, some of the plasmonic metal bar element 801 extends beyond the side edge 821 of the waveguide core 811, exposing the plasmonic metal bar 801 to the area of the TE waveguide core 811 where strongest z-axis component of the electrical field resides. The shape and configuration of plasmonic metal bar 801 may also be implemented as a two-PSPP element NFT as shown in FIG. 2, or an NFT having N PSPP elements as described above.

The embodiments described above employ a plasmonic metal bar having a robust width that can better withstand the harsh service duty conditions while still capable of delivering a precise focus of the heating spot required at the storage disk surface, compared to much smaller dimensioned plasmonic elements used in typical EAMR/HAMR devices. The plasmonic metal bar is configured as a PSPP element to interface with a dielectric waveguide core operating in TE mode.

The various aspects of this disclosure are provided to enable one of ordinary skill in the art to practice the present invention. Various modifications to exemplary embodiments presented throughout this disclosure will be readily apparent to those skilled in the art, and the concepts disclosed herein may be extended to other devices. Thus, the claims are not intended to be limited to the various aspects of this disclosure, but are to be accorded the full scope consistent with the language of the claims. All structural and functional equivalents to the various components of the exemplary embodiments described throughout this disclosure that are known or later come to be known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the claims. Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the claims. No claim element is to be construed under the provisions of 35 U.S.C. §112(f) unless the element is expressly recited using the phrase “means for” or, in the case of a method claim, the element is recited using the phrase “step for.”