Researchers at the University of Southampton in the UK have taken the familiar bar code to an extreme. They have developed diffraction-based bar codes up to 100 µm in length and only a few microns wide that can be read without contact, in a manner analogous to the way everyday 1-D codes are scanned at the grocery store. The miniature bar codes could be used to generate billions of unique tags for the identification of objects in a variety of applications. Our main vision for the use of these bar codes is as tags in combinatorial chemistry — that is, individual-molecule tagging — as well as biological applications: marking of plants, animals, birds, insects and even individual cells,” said Sam W. Birtwell, a postgraduate researcher at the university. Other potential objects to be tagged include credit cards and electronic components on chips. A scanning electron microscope reveals tiny bar codes constructed by superimposing diffraction grating tags of various spacings. From left, the tags feature a single grating or two, three or four superimposed gratings. As seen in the four-grating tag at far right, nanolithography resolution affects the quality of the bar code and imposes a limit on the technique. Billions of unique tags are possible. Courtesy of the University of Southampton. The technique involves the fabrication of nanoscale gratings on the surface of the tags. When illuminated by a laser, such a structure diffracts the light in a distinctive way, and it is possible to calculate the pitch of the pattern from the angle of the first-order diffracted beam. Higher-order beams also result, but they typically are less intense than the first-order beams, so using an intensity threshold addresses the potential measurement difficulties.From left, the diffraction patterns from three-grating tags show how a progressive decrease in the pitch of one grating changes the diffraction pattern and thereby acts a unique identifier. The encoding capacity of the technique depends on the ability to distinguish the beams generated by different gratings. For bar codes up to 100 µm long that were produced using one grating, the researchers calculated that only a few tens of tags were possible. However, superimposing the gratings increased the number. Using up to five superimposed two-dimensional gratings, for example, theoretically yields 1018 tags, although Birtwell cautioned that this likely cannot be reached.“The maximum figure would be reduced by manufacturing resolution and some other limitations arising from interplay between the superimposed gratings,” he explained.The investigators have demonstrated the concept by manufacturing chromium gratings on a glass substrate using direct-write electron-beam lithography. This bar-code library featured 7400 unique tags, each 50 × 50 µm in size and separated by 200 µm. They read the tags using 633-nm light from a HeNe laser and a CCD detector made by Taos Inc. of Plano, Texas. The proof-of-principle experiments confirmed that they could discern three superimposed patterns, enabling 68,000 tags at a nanofabrication resolution of 100 nm.The group has filed a patent on the technology and is working on a device based on the technique for biochemical applications. Optics Express, Feb. 20, 2006, pp. 13821387.