Proc. SPIE. 9381, Vertical-Cavity Surface-Emitting Lasers XIX
KEYWORDS: Signal to noise ratio, Diffraction, Glasses, Magnetism, Near field, Transducers, Vertical cavity surface emitting lasers, Multiphoton fluorescence microscopy, Temperature metrology, Near field optics
Heat assisted magnetic recording (HAMR) is a next generation technology proposed for achieving magnetic storage densities beyond 1 Tb/in<sup>2</sup>. However, the commercialization of heat-assisted magnetic recording faces substantial technical challenges that must be resolved before widespread adoption of the technology can commence. Foremost of these challenges is the development of a precise method of delivering light to a very small, sub wavelength bit area with sufficient power to heat a high coercivity magnetic medium above its Curie temperature. Complex fabrication processes, low power transfer efficiency and high heat dissipation are the biggest problems faced in current HAMR light delivery systems. A nano-aperture vertical cavity surface emitting laser (VCSEL) is a potential candidate as a light delivery system in HAMR. We have fabricated 850 nm VCSELs with C-shaped nano-apertures on their output facets to be used as near-field transducers in order to produce a small localized optical spot; we then characterized their performance and compared power requirements with successful HAMR demonstrations with control C-shaped nano-aperture near-field transducers fabricated on glass substrates. Laser light at 850 nm wavelength was focused onto a magnetic medium, through the nano-apertures, and an external magnetic field of magnitude much lower than the coercivity (at room temperature) of the magnetic medium was simultaneously applied. Magnetic force microscopy images of the medium showed that C-apertures are capable of producing a magnetic spot much smaller than the diffraction limit using localized plasmonic effects. The power density required at this wavelength for HAMR process was experimentally measured using a pump-probe optical setup.
We have conducted a thorough experimental analysis of nano-aperture VCSELs for use in heat-assisted magnetic recording (HAMR). To the best of our knowledge, this is the first study to both explore the impact of magnetic media proximity on VCSEL aperture power throughput and to use statistical methods to simultaneously characterize thousands of aperture designs. To achieve areal recording densities beyond 1 Tb/in2, high anisotropy magnetic materials are required to overcome the super-paramagnetic effect. These require high switching fields which are not conventionally available. Heat assisted magnetic recording (HAMR) is a potential technology to reduce the coercivity of the media and thus the required switching field by localized heating to enable writing of bits. The challenges being faced by this technology are to develop a precise method of delivering light to a very small, sub wavelength bit area with sufficient power through a near field aperture, and the fabrication of a laser source which can be integrated with current write heads used in hard disk drives. The focus of our work is to characterize nano-aperture VCSELs and test their potential application to HAMR. We have fabricated 850 nm VCSELs with large arrays of differently shaped nano-apertures in the gold layer on top of each VCSEL. The focusing and transmission characteristics of differently shaped nano-apertures are compared by simulations and experiments. C-shaped and H-shaped nano-apertures have also been fabricated in a gold layer deposited on a SiO<sub>2</sub> substrate to observe the effect of close proximity of magnetic media (FePt) on the performance of nano-apertures, and polarization effects have also been characterized.