Laser Writing of Nanostructures on Magnetic Film Surfaces With Optical Near Field Effects
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Laser Writing of Nanostructures on Magnetic Film Surfaces With Optical Near Field Effects S. M. Huang, M. H. Hong, B. S. Luk`yanchuk, W. D. Song, Y. F. Lu, T.C. Chong Data Storage Institute and Department of Electrical & Computer Engineering, Laser Microprocessing Laboratory, National University of Singapore, 10 Kent Ridge Crescent, Singapore 117608 ABSTRACT Laser directly writing of nanostrucrures on magnetic film surfaces with optical near field effects has been investigated. Spherical 0.99 µm or 0.47 µm silica particles were placed on Cr/CoCrPt multilayers. After laser illumination with an excimer laser for a single shot, pits were obtained at the original position of the particles using different laser fluences or particle size parameters. The mechanism of the formation of nanostructure pattern was discussed and found to be the near-field optical resonance effect induced by particles on the surface. A comparison with accurate theoretical calculations of near-field light intensity distribution showed good agreement with the experiment results. The method of particle enhanced laser irradiation allows the study of field enhancement effects as well as its potential applications for nanolithography. INTRODUCTION The current trend towards submicrometer structures creates the need for new methods and technologies of surface structuring and has attracted intense research in recent years. As the traditional masking approach in optical lithography is limited to the diffraction effects and always relates to the complex system and high cost, a lot of alternative techniques have been developed. One approach in this respect consisted of the illumination of the tip of a scanning tunneling microscope (STM) or the tip of an atomic force microscope (AFM) with a pulsed laser. Structures with lateral dimensions below 30 nm and therefore well below λ/2 could be produced underneath the tip [1-5]. It was proposed that the strong enhancement of electromagnetic field in the vicinity of a tip is responsible for this [5-7]. Thus both setups seemed to be promising for the study of field enhancement effects at sharp tips, a question of great interest in optics and surface structuring applications. A new approach can involve the illumination of micrometer and submicrometer sized spheres, which likely allows the study of field enhancement effects as well as its application for nanolithography process. Some authors have reported the appearance of particle-induced damage in the irradiated surface area during dry laser cleaning of irregularly shaped Al2O3 particles from the glass [8, 9]. Recent experiments have shown that the great light enhancement on the hot spot can produce small pit on the silicon substrate using femtosecond or nanosecond pulsed laser [10,11]. For spherical particles it is known [12] that they may act spherical lenses and therefore increase the laser intensity if their diameter is bigger than the laser wavelength. If their diameter is smaller than the wavelength field enhancement at the particles may happen according to Mie
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