Planar optical devices for light concentration in heat assisted magnetic recording head

As the area recording density of magnetic recording system increases, the anisotropy of the magnetic media must increase accordingly to maintain data integrity in room temperature. Heat assisted magnetic recording (HAMR) was proposed as an answer to the problem of writing high coercivity media with relatively low field. Optical devices need to be integrated into HAMR head to generate high intensity spots several tens of manometer wide. The focusing optics may include two parts: a non-imaging far-field optic system that focuses light into a spot 100-200 nm in diameter, followed a near-field transducer which will generate several tens of nanometer optical spot. Our work addressed the far-field focusing problem with refractive, diffractive and reflection devices. Specifically, the scattering of guided light at the boundary of waveguide regions with different mode index was modeled with finite difference time difference (FDTD) method and measured experimentally. The effect of taper with different slope was also investigated. The coupling efficiency of linear gratings and curved gratings were calculated with perturbation method and verified with FDTD method. The sensitivity of focusing grating coupler (FGC) to mode index change and misalignment was found to be the limiting factor to achieve high numerical aperture FGC.;In summary, this work includes the design, fabrication and performance characterization of planar optical structures like mode index lens, grating couplers and mirrors. Several novel focusing structures were proposed and preliminary designs were tested and shown good chance of successful.;Mode index lens with thickness modulation was fabricated and tested, Its focusing power is found to be very sensitive to the guiding layer thickness change. Two different solutions were proposed to solve the problem. One design used a stacked solid immersion mirror with a focusing grating coupler to realize achromatic focusing and very small side lobe. The other design used material modulation in the mode index lens to reduce the sensitivity to thickness change. Both designs were fabricated and tested. The material modulated mode index lens was capable of generating very small focusing spot. Trace of reflected light was observed in the stacked solid immersion mirror, Better control of vertical side wall angle is essential to observe the focus spot.