Faceted gratings for an optical security feature.

A method of optimizing and manufacturing a diffractive blazed grating array (DBA) is proposed to create a visual security feature when illuminated by a divergent light-emitting diode source. A pure phase grating array serving as the optical security component consists of blazed grating cells with the same size, 75 µm. After a divergent spherical wave is decomposed into harmonic waves, each grating cell of the DBA locally deflects the harmonic-waves into predefined directions and forms a feature pattern on the target plane. Particularly, a two-step optimization method is further developed for optimizing the period and orientation of each grating cell. The DBA sample is fabricated by using our home-built parallel direct-write photoplotter with a resolution of 0.75 µm. Both numerical simulations and optical experiments are demonstrated to validate the proposed model. Since the optical security component developed is a surface relief structure of a single polymer material, it can be replicated for mass production by using standard roll to roll nanoimprint technology. The design algorithm proposed in this work will enable the extension of the optical security elements to a broader realm and facilitate extensive developments in other research fields of the optics community, such as light-shaping, specific illumination for lithography, and microscope systems.

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