A multidye material developed by researchers from the University of Toronto and Princess Margaret Hospital in Toronto promises applications in ultradense optical data storage and document security as well as in the development of tunable lasers, optical limiters and chemical sensors. The approach employs different types of photosensitive dyes in different phases of the periodically structured composite polymer host to minimize crosstalk between the dyes.To produce the material, chemistry professor Eugenia Kumacheva, who holds the Canada research chair in advanced polymer materials at the university, and her colleagues incorporate dyes with different excitation and emission wavelengths in different layers of polymer microbeads. The microspheres feature a solid core doped with one dye and coated with shells of the other dye-doped polymers. The relationship between the composition of the core and shells yields a significantly lower glass transition temperature for the outer polymer shell.Annealing arrays of these microspheres at a temperature below the glass transition temperature of the outer shell produces a composite with one dye in the bulk and the others isolated in regular, spherical domains of other phases. This allows the researchers to record data in multiple storage modes, resulting in spatially superimposed but spectrally distinct patterns. The unwanted dye photobleaching by energy transfer among the dyes is limited to a very narrow interface between the polymer phases, enabling the researchers to sensitize only the desired target.In a demonstration of the technique, the scientists produced a two-dye composite material using anthracene and NBD dyes or NBD and Nile Blue dyes. (A three-dye system could include anthracene, NBD and Nile Blue.) Polymethyl methacrylate and polymethyl methacrylate-co-butyl methacrylate served as the core and shell polymers, respectively, and the team used Carl Zeiss confocal microscopes equipped with 364- and 488-nm argon-ion lasers or with 488-nm argon-ion and 633-nm HeNe lasers as the write/read heads. The setup incorporated an acousto-optic tunable filter to enable them to excite a sample at the two wavelengths simultaneously or sequentially. Information written simultaneously at different wavelengths on different phases in the material displayed no crosstalk.The ability to do so promises to boost the storage capacity of optical media. The material also may find application in security documents, in which visible illumination might reveal information such as a photograph, but excitation at nonvisible wavelengths might expose other personal data such as a signature or a fingerprint.