Metal island films are widely-used as reflection layers in optics and optical data storage. Their optical properties are dominated by the effect of plasmon resonance and, thus, by size and shape of the metal islands. By altering size and shape of these islands, the optical properties of the metal films can be adjusted. Depending on the wavelength range and type of metal, the film reflectance can be either increased or decreased. It is shown that changes in size and shape can be caused by means of a laser. This allows a well-defined adjustment of the optical properties of thin metal films. The optical properties can be adjusted locally. This technique can be used in different fields such as holography, adaptive optics or multi-layer optical data storage.
A vibration non-sensitive lithographic system for writing individual computer-generated micro-holograms into a polymer material for data storage and security application is described. The robust lithograph is the central element of the data storage system consisting of the calculation of a computer-generated micro-hologram (CGH), the writing process and the retrieval of the data stored. The writing process is based on the pattern of a CGH, which projects a data pattern with an optimized signal-to-noise ratio. The hologram, which has the size of one square millimeter, consists of one million dots with a diameter of one micrometer each. In less than one second the laser scanning lithograph transfers the calculated hologram into a polyethelen perephtalate foil (PET) using a high-power single-mode laser diode. A thermal process locally converts the semi-crystalline structure of the foil into an amorphous structure. This leads to a change of the refractive index of more than 0.1 without the need of a post-processing5. Because of the diffractive nature of the introduced optical pattern a relative position accuracy of better than 100 nm has to be accomplished. A high-speed differential optical position detection system guarantees the required position accuracy even in a vibration-afflicted industrial environment. A position deviation of the writing spot caused by vibration is detected by a close correlation between the laser spot in the storage material and a second scanning laser spot on top of a grid mask. The position information is combined with the individual hologram pattern by a fast free-programmable gate-array (FPGA) processor, which again controls the laser diode. The reaction time of 26 ns ensures an interference compensation of up to one MHz. The quality of the reconstructed micro-hologram allows the retrieval of up to one kByte of machine-readable information.
A cylindrical multi-layered optical data storage medium based on polymer films is introduced. The usage of metal island films as tunable absorber system is proposed.
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