4,842 research outputs found
Laser microsculpting for the generation of robust diffractive security markings on the surface of metals
AbstractWe report the development of a laser-based process for the direct writing (‘microsculpting’) of unique security markings (reflective phase holograms) on the surface of metals. In contrast to the common approaches used for unique marking of the metal products and components, e.g., polymer holographic stickers which are attached to metals as an adhesive tape, our process enables the generation of the security markings directly onto the metal surface and thus overcomes the problems with tampering and biocompatibility which are typical drawbacks of holographic stickers. The process uses 35ns laser pulses of wavelength 355nm to generate optically-smooth deformations on the metal surface using a localised laser melting process. Security markings (holographic structures) on 304-grade stainless steel surface are fabricated, and their resulted optical performance is tested using a He–Ne laser beam of 632.8nm wavelength
Automorphisms of Partially Commutative Groups II: Combinatorial Subgroups
We define several "standard" subgroups of the automorphism group Aut(G) of a
partially commutative (right-angled Artin) group and use these standard
subgroups to describe decompositions of Aut(G). If C is the commutation graph
of G, we show how Aut(G) decomposes in terms of the connected components of C:
obtaining a particularly clear decomposition theorem in the special case where
C has no isolated vertices.
If C has no vertices of a type we call dominated then we give a semi-direct
decompostion of Aut(G) into a subgroup of locally conjugating automorphisms by
the subgroup stabilising a certain lattice of "admissible subsets" of the
vertices of C. We then characterise those graphs for which Aut(G) is a product
(not necessarily semi-direct) of two such subgroups.Comment: 7 figures, 63 pages. Notation and definitions clarified and typos
corrected. 2 new figures added. Appendix containing details of presentation
and proof of a theorem adde
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