Abstract:
An apparatus includes dielectric waveguide cores operating in transverse electric (TE) mode configured to receive incident light energy from an energy source and direct the incident light energy to a target. A near field transducer (NFT) is configured to focus the light energy received from the waveguide cores and to transmit the focused light energy onto a storage disk surface to generate a heating spot. The NFT includes propagating surface plasmon polariton (PSPP) elements that are energized by the light energy from the waveguide cores. Each PSPP element has a plasmonic metal bar disposed above a surface of a single waveguide core in a longitudinal alignment with the waveguide core, a first end that receives the light energy from the waveguide core, and a second end exposed to the air bearing surface. The width of each metal bar tapers toward the first end.

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 Ta 2 O 5  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.”