Near-Field Solid-Immersion Mirror Records Data Optically

Hank Hogan

Investigators at Seagate Technology’s research laboratory in Pittsburgh have demonstrated a near-field planar solid-immersion mirror that could usher in higher optical data storage densities than what magnetic media offer.

The approach offers several advantages over alternative methods, said research manager William A. Challener. For instance, a planar solid-immersion mirror can enable optical recording and reading below the diffraction limit of standard far-field optics while still allowing high throughput. This is not possible with aperture-based near-field techniques. And because it is a mirror, not a lens, it is free of chromatic aberration.

Manufacturing a planar solid-immersion mirror does not require tedious or complex polishing. Instead, its shape is easily and accurately defined by e-beam lithography. Moreover, Challener said, hundreds of thousands of the devices can be made simultaneously on a wafer employing standard fabrication techniques.

In a demonstration of the approach, the researchers fabricated a two-dimensional parabolic mirror in an optical planar waveguide. They built the mirror by thin-film deposition of a Ta₂O₅ high-refractive-index core on a lower-index cladding of SiO₂. The top cladding of the waveguide was air. They deposited this film on an Al₂O₃/TiC substrate. Using an electron beam and subsequent processing, they carved out a planar solid-immersion mirror 100 µm long, with an opening of 50 µm at one end and of 6 µm at the other.

They mounted the mirror on a slider and coupled an 830- or 488-nm laser beam into it with an optical fiber. The waveguide confined the laser radiation, and the parabolic mirror focused it to a spot size of about λ/4. They used the optical recording setup to make marks in phase-change material such as that found in CDs and DVDs.

Challener said that the next step in the work involves building a planar solid-immersion mirror in a recording device and demonstrating recording densities higher than those currently achieved with magnetic methods.