Amit Vasant Itagi

Imaging of optical field confinement in ridge waveguides fabricated on very-small aperture laser

Optical field confinement in a ridge waveguide nanostructure (“C” aperture) designed for ultrahigh-density recording was observed using an apertureless near-field scanning optical microscope. The aperture was fabricated on a commercial edge-emitting semiconductor laser as the light source. High-contrast near-field images at both 1× and 2× lock-in detection frequencies were obtained. The emission patterns are in agreement with theoretical simulation of such structures. A 90 nm×70 nm full width half maximum spot size was measured and is comparable to the ridge width of the aperture.

HAMR Recording Limitations and Extendibility

Heat-assisted magnetic recording (HAMR) limitations and extendibility are studied in light of the recent 1.0 Tb/in2 technology demonstration. The paper examines HAMR specific technology challenges, including switching field distributions at elevated temperature, saturation noise, and near-field transducer (NFT) thermal spot-size limits. While current HAMR recording density ( ~ 1 Tb/in2) is limited by switching field distribution and thermal spot size, ultimate HAMR density (up to 5 Tb/in2) is determined by achievable recording-layer magnetic anisotropy and grain size.

Ridge waveguide as a near-field optical source

We investigate the feasibility of using a ridge waveguide at optical frequencies as a near-field optical transducer, using the finite difference time domain method. The complete electromagnetic field picture of the ridge waveguide, in the absence and presence of the irradiated medium, is presented. A power efficiency of 7% and an optical spot with full width half maximum of 50 nm×80 nm is obtained in the medium. We show that impedance considerations play a major role in the transducer-medium optical coupling.

An Integrated Read/Write Head for Hybrid Recording

A proposed integrated read/write head for hybrid recording is described. This design features a thin film waveguide to deliver light to an aperture. This aperture is described in terms of the propagating modes of a ridge waveguide with a reduced cutoff frequency. Also included in this integrated head is a geometry that maintains the necessary coincidence of the optical and magnetic fields. The essential features of this design are very compatible with existing fabrication methods.

Optical Transducers for Near Field Recording

Optical transducers that concentrate optical energy in the near field to dimensions much smaller than the standard diffraction limit are often classified as apertures or antennas. For near field recording the transducer must obviously operate with a recording medium in its immediate vicinity which can strongly interact with the transducer. Transducers composed of gold and with a minimum dimension of 20 nm are spaced 7.5 nm from the medium in this study. Even simple apertures when optimized are able to couple ?1.5% of the incident power into optical spots with a full width at half maximum diameter of less than 50 nm or a tenth of a wavelength. More sophisticated apertures and antennas can improve this coupling efficiency by a factor of three or more, although not by orders of magnitude. Such transducers may have applications in optical or heat assisted magnetic data storage.

MOBILITY OF 180 DOMAIN WALLS IN CONGRUENT LITAO3 MEASURED USING REAL-TIME ELECTRO-OPTIC IMAGING MICROSCOPY

We report the electric-field dependence of 180° domain-wall mobility in congruent LiTaO3 measured at room temperature using in situ electro-optic imaging microscopy. The measured sideways domain-wall velocity of serrated domain fronts formed upon merger of domains was an order of magnitude larger than that for independently growing domains. The wall velocities also show a strong dependence on the nature of the applied electric field, being an order of magnitude larger for steady-state voltages as compared to pulsed voltage measurements. This is shown to be due to wall stabilization between applied voltage pulses resulting in an inertial delay in moving a domain wall which has been at rest for many seconds.

Near-Field Optics for Heat-Assisted Magnetic Recording (Experiment, Theory, and Modeling)

One application of near-field transducers (NFT) is in heat-assisted magnetic recording (HAMR). HAMR is similar to conventional magneto-optical (MO) recording in that the data are stored in magnetic bits on a disk by heating the area of the bit with a laser beam in the presence of an external field to set the magnetic orientation of the bit as it cools. The optical head in conventional MO recording is mounted on an actuator and optical feedback signals are used to maintain a constant spacing between the head and the recording medium, which is generally on the order of tens or hundreds of nanometers. Also, for conventional MO recording the applied magnetic field is very small (approximately 0.02 T), typically generated by a large fixed external magnet, and the laser energy rather than the magnetic field is modulated with the input data stream. On the other hand, for HAMR the integrated optical–magnetic head is mounted on a slider, which flies over the surface of the recording medium at 10 nm or less. The applied field for HAMR is highly localized, very large in magnitude (up to 1 T or more), and generated by a miniature recording pole positioned within tens of nanometers of the optical spot. For HAMR the magnetic field from the pole is modulated with the input data stream, while the laser energy on the medium can remain constant.

Refraction Theory for Planar Waveguides: Modeling of a Mode Index Integrated Solid Immersion Lens

The framework for modeling mode index lenses in planar waveguides is developed. It is used for modeling an integrated solid immersion lens for application in data storage.

The road to HAMR

Heat assisted magnetic recording (HAMR) was initially proposed in the 1990s to achieve storage densities not limited by superparamagnetism. The key to HAMR has been to find an efficient near field transducer that can operate with a nearby magnetic recording pole. An integrated HAMR head has now been demonstrated which can record at a track width of 50 nm and an areal density of ∼240 Gb/in2 on high coercivity FePt media.

A Study of Near-field Aperture Geometry Effects on Very Small Aperture Lasers (VSAL)

We have investigated nano-apertures with different geometries on VSALs using far-field measurements, near-field measurements, and finite difference time domain (FDTD) simulation methods. We were able to quantitatively verify the aperture geometry dependent power throughput in all three methods. From both far-field measurements and FDTD simulation results, we conclude that for the apertures of the same area, a rectangular aperture with the long side perpendicular to the active layer has the largest throughput, while a circular aperture has the second largest, and the rectangular aperture with the long side parallel to the active layer has the least throughput among the three. We have attempted to correlate the relationship between far-field power and near-field power. Employing an apertureless near-field scanning optical microscopy (NSOM), we found that for the two rectangular apertures being studied, the near-field power throughput results was consistent to that of far-field measurement. Using VSALs as a near-field aperture testbed was also proposed and demonstrated.