"Our main vision for the use of these barcodes is as tags in combinatorial chemistry, so tagging individual molecules with labeled micro-beads," Sam Birtwell, one of the team members at Southampton, told Optics.org. "Also, biological applications, marking of plants, animals, birds, insects and even individual cells can benefit. However, the potential for such tags is also huge in security applications: banknotes, credit cards, product marking, and marking of small electronic components on chips."

Birtwell explains that the code for each tag is stored as the periods of the grating and is read by detecting the angle of the first order beam diffracted from the tag. He adds that one of the main challenges is designing an optical set-up that can collect all of the diffracted beams.

The team used electron beam lithography to deposit 60 nm thick, 50x50 micron chromium diffraction gratings onto glass substrates at a resolution of 100 nm. In this way, it developed a chip library of almost 7,400 unique gratings.

By theoretically superimposing gratings from the chip library, the team then calculated the patterns for higher-order tags. This lead to the possibility of some 68,000 distinguishable tags, manufactured after superimposing up to and including three original gratings.

The number of gratings that can be superimposed is limited by the 100 nm manufacturing resolution. "A tag with a higher number of superimposed gratings generally contains smaller features, which cannot be accurately reproduced," explained Birtwell. "There is also some interplay between gratings on a tag, which may limit the superimposing: this will be the subject of future publication."

In the meantime the group will continue work on improving storage capacities and optimizing the technology for its potential applications. "We are beginning to manufacture the tags on bio-compatible material for biological and chemical applications," added Birtwell. "We are also studying more complicated grating patterns, in order to increase the tag capacity and to make the encoding more optimized."