Waveguide with phase shifting portions

An apparatus includes a plasmonic transducer with first and second oppositely disposed outer edges. A waveguide is configured to receive light from a light source, the waveguide have first and second portions that deliver first and second portions of the light to the first and second edges of the plasmonic transducer. The first and second portions are different by at least one of a geometry and a construction to cause a relative phase shift between the first and second portions of the light.

SUMMARY

Various embodiments described herein are generally directed to an apparatus having a plasmonic transducer with first and second oppositely disposed outer edges. A waveguide is configured to receive light from a light source, the waveguide have first and second portions that deliver first and second portions of the light to the first and second edges of the plasmonic transducer. The first and second portions are different by at least one of a geometry and a construction to cause a relative phase shift between the first and second portions of the light.

DETAILED DESCRIPTION

The present disclosure relates to waveguide structures that deliver energy (e.g., light) to a near-field transducer (NFT). An NFT and waveguide described herein may be usable in any application where a beam of highly focused and relatively powerful electromagnetic energy is desired. One such application is in heat assisted magnetic recording (HAMR), also referred to as thermally assisted magnetic recording (TAMR). In reference toFIG. 1, a perspective view shows an example HAMR slider100. This example slider100includes a laser diode102located on top of the slider100proximate to a trailing edge surface104of the slider100. The laser diode102delivers light proximate to a HAMR read/write head106, which has one edge on an air bearing surface108of the slider100. The air bearing surface108faces and is held proximate to a moving media surface (not shown) during device operation.

The laser diode102provides electromagnetic energy to heat the media at a point near to the read/write head106. Optical coupling components, such as a waveguide110, are formed integrally within the slider device100to deliver light from the laser102to the media. In particular, a local waveguide110and NFT112may be located proximate the read/write head106to provide local heating of the media during write operations. The laser diode102in this example may be an integral, edge firing device, although it will be appreciated that the waveguide110and NFT112may be used with any light source and light delivery mechanisms. For example, a surface emitting laser (SEL) may be used instead of an edge firing laser.

While the example inFIG. 1shows a laser102integrated with the slider100, the NFT112discussed herein may be applicable to any type of light delivery configuration. For example, in a free-space light delivery configuration, a laser may be mounted externally to the slider, and coupled to the slider by way of optic fibers and/or waveguides. The slider in such an arrangement may include a grating coupler into which light is coupled and delivered to a slider-integrated waveguide110which energizes the NFT112.

A HAMR device utilizes the types of optical devices described above to heat a magnetic recording media (e.g., hard disk) in order to overcome superparamagnetic effects that limit the areal data density of typical magnetic media. When writing to a HAMR medium, the light is concentrated into a small hotspot over the track where writing takes place. The light propagates through a waveguide110where it is coupled to the NFT112, e.g., either directly from the waveguide or by way of a focusing element. Other optical elements, such as couplers, mirrors, prisms, etc., may also be formed integral to the slider. The optical elements used in HAMR recording heads are generally referred to as integrated optics devices.

The field of integrated optics relates to the construction of optics devices on substrates, sometimes in combination with electronic components, to produce functional systems or subsystems. For example, an integrated optics device may transfer light between components via rectangular dielectric slab or channel waveguides that are built up on a substrate using layer deposition techniques. These waveguides may be formed as a layer of materials with appropriate relative refractive indices so that light propagates through the waveguide in a similar fashion as through an optic fiber.

As a result of what is known as the diffraction limit, optical components cannot be used to focus light to a dimension that is less than about half the wavelength of the light. The lasers used in some HAMR designs produce light with wavelengths on the order of 700-1550 nm, yet the desired hot spot is on the order of 50 nm or less. Thus the desired hot spot size is well below half the wavelength of the light. Optical focusers cannot be used to obtain the desired hot spot size, being diffraction limited at this scale. As a result, the NFT112is employed to create a hotspot on the media.

The NFT112is a near-field optics device designed to reach local surface plasmon resonance at a designed wavelength. A waveguide and/or other optical element concentrates light on a transducer region (e.g., focal region) where the NFT112is located. The NFT112is designed to achieve surface plasmon resonance in response to this concentration of light. At resonance, a high electric field surrounds the NFT112due to the collective oscillations of electrons at the metal surface. Part of this field will tunnel into a storage medium and get absorbed, thereby raising the temperature of a spot on the media as it being recorded.

In reference now toFIG. 2, a cross-sectional view shows details of a HAMR apparatus200according to an example embodiment. Near-field transducer112is located proximate a media-facing surface202(e.g., ABS), which is held near a magnetic recording media204during device operation. In the orientation ofFIG. 2A, the media-facing surface202is arranged parallel to the x-z plane. A waveguide core206may be disposed proximate the NFT112, which is located at or near the media writing surface214.

The waveguide core206is surrounded by cladding layers208,210that have different indices of refraction than the core206. Light is delivered from the waveguide core206along the negative y-direction where it is coupled to the NFT112. The NFT112delivers surface plasmon enhanced, near-field electromagnetic energy along the y-axis where it exits at the media writing surface214. This may result in a highly localized hot spot (not shown) on the media surface214when the media204placed in close proximity to surface202of the apparatus200. Further illustrated inFIG. 2Ais a recording pole212of the read/write head that is located alongside the NFT112. The recording pole212generates a magnetic field (e.g., perpendicular field) used in changing the magnetic orientation of the hotspot during writing.

In reference now toFIG. 3, a plan view shows an NFT112according to an example embodiment. The NFT112includes a circular disk portion302with a peg portion304that is disposed proximate the media-facing surface (e.g., surface202inFIG. 2). The NFT112is formed from a thin film of plasmonic material (e.g., gold, silver, copper) on a substrate parallel plain of the slider proximate the write pole (e.g., write pole212inFIG. 2). The light delivery waveguide (e.g., waveguide core206inFIG. 2) delivers light306,307to first and second sides302A-302B of the NFT112. The light306,307causes plasmon resonance on the surface of the NFT112, and plasmons308generated by this resonance is emitted from the peg304towards the data storage media where they are absorbed to create a hotspot.

A transducer such as NFT112may utilize a longitudinally polarized focusing field. This can be fulfilled by causing a phase shift between two portions of the light306,307that are incident on the first and second sides302A-302B of the NFT112. For example, it may be desirable to have the light306on one side302A be phase shifted (e.g., π-phase shift) relative to light307on the other side302B.

In the various embodiments described below, coupling arrangements are described that may be used for delivering phase-shifted light from a waveguide to first and second sides of an NFT. These embodiments may use optical focusing components (e.g., lenses, mirrors) to focus light on the NFT, or the light may be delivered directly from portions of a waveguide to the NFT. An arrangement using a mirror is shown inFIG. 4, which is a plan view showing a solid immersion mirror (SIM)402used to focus light onto an NFT112according to an example embodiment. In this example, the SIM402is parabolic, and the NFT is positioned at the focus of the parabola. The SIM402may be formed by creating a parabolic cutout in a substrate, and coating the walls of the cutout with a reflective material. The SIM402may be truncated at the ABS, indicated here by dashed line403.

A waveguide404launches light to the sides of the SIM402, and the rays of light are reflected to the NFT112as represented by lines406-409. The waveguide404may launch light406-409directly to the walls of the SIM402, or via intermediate components (e.g., reflectors, collimators, etc.) may be placed between the waveguide404and the SIM402. Generally, the waveguide404includes first and second portions404A,404B that are different by at least one of a construction or geometry that causes a relative phase shift or mode shift of light that is delivered to sides302A,302B of the NFT112. For example, the first portion404A may have cladding and/or core materials with an index of refraction different than the second portion404B.

Although the waveguide404is shown with an exit portion located outside of the SIM402and NFT112, the waveguide404may extend over or beyond the SIM402and/or the NFT112. For example, the waveguide404may be located above or below the SIM402and/or NFT along the z-axis and include coupling features to direct light to at least side walls of the SIM402. In other arrangements, a core or cladding of the waveguide404may extend into the SIM404. For example, the SIM402may be filled with a waveguide core/cladding material that encompasses the NFT112.

InFIG. 5, a plan view shows first and second portions502,504of a waveguide system that directly delivers light to sides302A,302B of NFT112according to an example embodiment. Generally, the portions502,504may be split from a main waveguide portion (not shown) that is coupled to a slider mounted or external light source (e.g., laser). The first and second portions502,504are different by at least one of a construction or geometry that causes a relative phase shift or mode shift of light that is delivered to sides302A,302B of the NFT112.

In reference now toFIG. 6, a schematic diagram illustrates an example of a split waveguide600according to an example embodiment. The waveguide600may be configured as a channel waveguide that steers a beam from a laser coupler to NFT112. The waveguide600includes an input coupler portion600C that is coupled to a light source, e.g., coupled to a laser via a grating or other optical coupling component. The waveguide600includes first and second portions600A,600B that directly output electromagnetic energy (e.g., light) onto respective first and second sides302A,302B of NFT112. A Y-coupler600D couples the first and second portions600A,600B to the input coupler portions600C.

The waveguide portions600A,600B have different lengths between Y-coupler600D the termination where the waveguide portions600A,600B deliver energy to the NFT112. This difference in path length will induce a relative phase shift between light directed to the different sides302A,302B of the NFT112. In some configurations, it may desirable to induce a phase shift of π, or multiples thereof (nπ). By designing waveguide portions600A,600B with geometries having predefined, different path lengths, any desired phase shift can be obtained.

InFIGS. 7 and 8, waveguides700,800are shown according to alternate embodiments. Waveguides700and800have respective portions700A,700B,800A,800B mode converter from coupling portions700C,800C at Y-couplers700D,800D. Portion700A has a different path length than portion700B, and portion800A has a different path length than portion800B, and so a desired phase shift can be obtained by forming these path length with a predefined path length difference.

In the embodiments shown inFIGS. 6-8, use of a Y-coupler and steering channel waveguide portions can minimize losses that may be induced by other steering and splitting devices, e.g., mirrors, prisms. These channel waveguide embodiments may also be tolerant of processing and wavelength tolerances, as well as operating conditions (e.g., thermal expansion). The illumination angle on the NFT112can also be optimized by positioning and angling the termination of the first and second waveguide portions relative to the sides302A,302B of the NFT112. Additional elements such as planar integrated mirrors or waveguide beam expanders can also be included. The shape of the NFT112can also be altered to optimize efficiency. For example, the NFT302may be asymmetric to account for different illuminations angles of the split waveguide portions.

As mentioned above, mirrors and other devices may be used to direct light to an NFT. In reference now toFIG. 9, a schematic diagram illustrates an arrangement that may be used to deliver light to the previously described NFT112. In this arrangement, light is provided by a laser902mounted on a top surface904of a slider (e.g., slider100inFIG. 1). This arrangement may also be used with free space light delivery, e.g., with a grating on the top surface904, onto which light is projected from an external laser.

The laser902directs light into an input coupler906which directs the light to a beam expander908. Light rays (e.g., represented by paths909) exit the beam expander and are reflected off of first and second mirrors910,912, where they are directed to SIM402, the features of which are discussed above in relation toFIG. 4. The second mirror912may be configured as a collimator, and has two portions912A,912B that are offset by a split distance914. This split914can introduce the desired phase shift between beams directed to left or right sides of the SIM402, which is reflected onto first and second sides of the NFT112.

It should also be noted that being a geometric feature, the split914in mirror912is light wavelength sensitive. As a result, the amount of phase shifting may be dependent on the actual wavelength of light generated by the laser902. Another issue is tight control in masking and fabrication of the mirror912. The size of split914is λ/2neff, where λ denotes light wavelength in free space and neffis the effective mode index. One example is λ=830 nm, neff=1.7287, so split dimension914is 239 nm. Tolerances in this dimension will have a direct effect on the amount of phase shift imparted by the mirror912.

Another issue involved in using a split mirror912is beam and wave-front distortion if the split is far away from the focusing element and/or if the incident beam size is small due to a compact slider format. An incident beam coupled into the waveguide (e.g., at coupler906) is usually in the fundamental mode, which has Gaussian electric field profile (e.g., a single peak) in a direction parallel to the waveguide plane. As the beam propagates through the waveguide and into the focusing element, the beam will be distorted, due to diffraction. The wavefront of the beam will have additional phase variation and the beam shape will also split from single peak into two peaks, which are shifted from the beam center to either side of the beam.

In reference now toFIG. 10, a schematic diagram illustrates differential waveguide used to phase-shift half of a light beam according to an example embodiment. This example uses a continuous mirror1002(which may be configured as a collimator). A first portion1004of the waveguide between the mirror1002and SIM402has a slight difference in the waveguide core and/or cladding materials relative to a second1006of the waveguide. This induces a phase shift on half of the beam directed to the left part of the SIM402relative to the other half directed to the right side of the SIM402, as indicated by arrows1010-1013.

The use of the differential waveguide portions1004,1006reduces sensitivity to fabrication tolerances and also reduces diffraction distortion. Light propagates through the different portions1004,1006waveguide at differing phase velocity, thereby accumulating a phase shift (e.g., π phase shift). Assuming that the effective mode index is neff1for the left waveguide portion1004and neff2for the right waveguide portion1006, the required length L of the dual waveguide to achieve a particular phase shift depends on the difference between the mode index according to L=λ/(2[neff1−neff2]), which may be much larger than the split914inFIG. 9. Note that the use of dual waveguide portions1004,1006does not increase the sensitivity to light wavelength. In addition, the dual waveguide portions1004,1006may be placed near to the focusing element402, which minimizes the diffraction effect.

In one example, it is assumed that the dual waveguide portions1004,1006include the same core, 125 nm thick Ta2O5, with refractive index n=2.08 and the same bottom cladding layer, Al2O3, n=1.65. The top cladding layer on the left waveguide portion1004is Al2O3, but on the right waveguide portion1006it is SiONx, n=1.68. At λ=830 nm, neff1=1.7287, neff2=1.7745, L=9.057 nm for a π phase shift. The 9057 nm dimension is much larger than the 239 nm dimension of the split914for the same wavelength. As a result, the differential waveguide is less sensitive to fabrication tolerances.

It will be understood that the first and second waveguide portions1004,1006may cause a relative phase shift between first and second portions of light in any light delivery arrangement. For example, the Y-coupler split waveguides shown inFIGS. 6-8may also include regions before or after the Y-coupler that include different core and/or cladding materials. This may be used, for example, to fine-tune the phase shifting that occurs in the mode converter portions following the Y-couplers.

An incident beam coupled into the waveguide (e.g., at coupler906shown inFIG. 9) is usually in the fundamental mode, which has Gaussian electric field profile (e.g., a single peak) in a direction parallel to the waveguide plane. For some NFTs (e.g., funnel-type near-field transducers, lollipop-type near-field transducer shown inFIG. 3), it is desirable to illuminate the NFT with light in a first higher mode, e.g., transverse electric (TE10) mode, for light delivery and NFT excitation efficiency. mode conversion from the fundamental mode (TE00) to the first higher mode is therefore preferred. The TE10mode is an eigen-mode of the planar waveguide. The beam wavefront does not change with propagation, even though the beam size (in-plane) will change.

In reference now toFIG. 11, a schematic diagram illustrates a mode converter waveguide1100that converts fundamental waveguide modes according to an example embodiment. The mode converter waveguide1100includes two three-dimensional channel waveguides: an input waveguide coupler1102and an S-bend waveguide1104that branches from the input waveguide coupler1102. A portion of the waveguide1106extends beyond the junction of the input coupler1102and branching S-bend1106, and is referred to herein as the combined waveguide1106. Dimensions of an example embodiment of the mode converter waveguide are shown in the graph ofFIG. 12.

The mode converter waveguide1100converts the fundamental mode (TE00or TM00) in the input coupler into the first high-order mode (TE10or TM10in the combined waveguide1106. The TE10and TM10modes are eigenmodes of the waveguide1100and will propagate in the waveguide without distortion. These modes have two side-lobes in intensity profile having a π phase difference. As seen inFIG. 12, the channel width W2 of the S-bend waveguide1104is larger than the width W1 of the input coupler1102, and both channel waveguides are single mode. The combined waveguide1106supports the first higher-order mode.

As the two waveguides1102,1104are slowly brought together, light propagating in the input waveguide coupler1102interacts with the S-bend waveguide1104evanescently, and the fundamental mode in the input coupler1102is converted into the first high-order mode of the combined waveguide1106. This conversion is tolerant to variations in polarization and light wavelength, although in some configurations the mode converter angle θ of the two waveguides may need to be carefully controlled. Assuming the mode converter angle is small, the conversion will be fulfilled.

As an example, assuming that the three-dimensional input waveguide1102is made of a Ta2O5core of refractive index n=2.08 and Al2O3cladding, n=1.65. It is 0.7 μm wide (along the x-direction, see W1inFIG. 12) and 0.15 μm thick along the z-direction (out of the plane of the page). The S-bend waveguide1104has a cosine shape. The lower boundary of the S-bend waveguide can be expressed as: x(y)=0.5*Offset[1+cos(πy/L)]. The definition of offset and L is seen inFIG. 12. The variable W2denotes the S-bend core width. For this example, L=100 μm, offset=1 μm, W2=1 μm.

An analysis was performed assuming light (wavelength λ=0.83 μm) is coupled into the input waveguide from an edge-emitting laser diode in TE00mode. Both waveguides only permit the fundamental mode. InFIG. 13, a graph shows the profiles of electric field at the joint of two waveguides, y=L. The peak field is seen in regions1300,1302ofFIG. 13. The graph inFIG. 14shows phase of electric field at y=L. Region1400inFIG. 14is about 0 degrees, and region1402is about −180 degrees. The graph inFIG. 15shows the electric field after 20 μm propagation through the combining waveguide (y=L+20 μm). Maximum field inFIG. 15is seen in regions1500and1502.

It can be seen inFIGS. 13 and 15that the amplitude profile has two maxima andFIG. 14shows the phase shifts between two spots, which is the signature of TE10mode. After 20 μm propagation in the combining waveguide, there is little change in the mode profile. Note that it is not necessary to have the full S-bend to achieve mode conversion. The two waveguides do not interact until z=60 μm. This means that about half of the S-bend wavelength may be used for acceptable results. The graphs inFIGS. 16 and 17show mode conversion is still fulfilled with variances in waveguide width and offset. Maximum field inFIG. 16is at regions1600and1602, and maximum field inFIG. 17is at regions1700and1702. The results inFIG. 16are for W1=0.5 μm, W2=0.8 μm, offset=1.2 μm. The results inFIG. 17are for W1=0.5 μm, W2=1.0 μm, offset=1.2 μm.

It will be understood that the mode converter waveguide shown inFIGS. 11 and 12can be used with any optical energy coupling arrangement known in the art. For example, the mode converter waveguide1100may be used with a split mirror912as shown inFIG. 11, or with an un-split mirror1002as shown inFIG. 10. The mode converter waveguide1100may also be used with Y-coupled waveguides as shown inFIGS. 6-8. Additional embodiments of a mode converter waveguide are shown inFIGS. 19 and 20.

InFIG. 19, a schematic diagram illustrates a slider assembly with a laser902and NFT112that may both be similar to analogous components described elsewhere herein. The laser902is mounted on slider body1902wherein it is coupled to a waveguide input coupler1904. A mode converter waveguide1906is coupled to the input coupler1904and delivery waveguide1908, the delivery waveguide1908delivering light to the NFT112directly (e.g., without focusing mirrors or other intermediary optical devices). In this example, the mode converter waveguide1906, e.g., branching S-bend/cosine curve waveguide, is configured as a mode converter, e.g., TE00to TE10.

InFIG. 20, a schematic diagram illustrates a slider assembly with a laser902and NFT112that may both be similar to analogous components described elsewhere herein. The laser902is mounted on slider body2002wherein it is coupled to a waveguide input coupler2004. A mode converter waveguide2006is coupled to the input coupler2004at a delivery region2008, where light is launched to a SIM2010which focuses light onto the NFT112. The delivery region2008may include a beam expander (not shown) which expands light by half-angle θmax. In this example, the mode converter waveguide2006e.g., S-bend/cosine curve waveguide, is configured as a mode converter, e.g., TE00to TE10.

In reference now toFIG. 18, a flowchart illustrates a method according to an example embodiment. The method involves receiving1800light at a waveguide from a light source. First and second portions of the light are caused to propagate1802along first and second portions of the waveguide, wherein the first and second portions of the waveguide are different by at least one of a geometry and a construction to cause a relative phase shift between the first and second portions of the light. The first and second portions of the light are directed1804to first and second oppositely disposed outer edges of a near field transducer.