The ability to label chemicals, biological samples, explosives and currency with invisible ID tags would lead to numerous security and safety applications. Now scientists at Corning Inc. in Corning, N.Y., have built bar codes from microscopic slices of glass doped with rare-earth ions that may be used to mark money, drugs and genes.These false color images of rare-earth-doped glass micro bar codes illustrate the tagging devices. The top image shows "one-dimensional" bar codes that feature six color bands. The bottom image displays "two-dimensional" codes, two six- banded fibers fused together. More than a million color combinations are possible with the 1-D bar codes. Introducing 2-D bar codes promises to expand the number of distinguishable combinations to more than a billion. Courtesy of Matthew J. Dejneka. The 20 x 100-µm bar codes feature six bands that fluoresce different colors. To fabricate the codes, Matthew J. Dejneka and his colleagues first create 25 x 25-mm bars of glass, each containing a specific dopant to create a desired color. After drawing the bars into square lengths, they stack the lengths into the color order that they want for a code, fuse them and use conventional optical fiber drawing techniques to create a ribbon fiber. Finally, they score the ribbon every 20 µm with a laser and use ultrasound in water to break the ribbon into the bar codes.In the photonics industry, rare-earth elements such as neodymium have long been used to dope glass to create lasers and other devices. The Corning researchers chose to investigate their use in the micro bar codes because they have narrow emission bands, unlike transitional metal ions, explained Dejneka. "They are efficient, bright and in the right spectral bands," he said.The researchers tested 13 rare-earth ions and found four -- Dy3+, Tm3+, Ce3+ and Tb3+ -- that can be excited with the UV lights used for fluorescence microscopy but that do not interfere with common fluorescent labels. Moreover, they are nontoxic, making them suitable for use in biological applications, unlike quantum-dot based tags, which are made with lead, cadmium, selenium, sulfur and other toxins. The researchers have demonstrated that the bar codes can label genes, which could lead to the development of a system for rapidly sorting and identifying expressed genes.Used discretely in the bar codes, six rare-earth ions yield 2080 nondegenerate combinations in which the foward and backward codes are unique. To increase the number of possible combinations, the group has experimented with double doping the glass to create mixed colors. After ruling out some combinations, this larger palate of nine colors offers 265,000 nondegenerate combinations. Dejneka said that triple doping is possible, bringing the theoretical number of color combinations to more than a million.