Research Highlight

Subject Category: Optics, photonics and optoelectronics

NPG Asia Materials research highlight; doi:10.1038/asiamat.2010.49
Published online 29 March 2010

Optical memory: What a difference

Photonic crystals can increase the brightness and contrast of emissive optical memory.

Compact discs store information in the degree of reflectivity of each area of the disc: high reflectivity represents a ‘1’, and low reflectivity a ‘0’. An alternative form of optical information storage is to use the strength of fluorescent emission, rather than reflectivity. This type of memory requires a high degree of contrast between the high- and low-emission states. Now, Yanlin Song, Rongming Wang and colleagues at the Institute of Chemistry in Beijing and the Beijing University of Aeronautics and Astronautics in China have increased the contrast ratio of fluorescent optical information storage by a factor of seven.1

The Beijing team accomplished this by placing the fluorescent dye on top of a photonic crystal — a textured surface with a repeating pattern (Fig. 1). This photonic crystal enhanced the fluorescence from the dye by increasing its effective surface area, which reduces the probability that the emissions of different dye molecules could interfere with each other — a phenomenon known as concentration quenching. It also increased the fluorescence emission by a second mechanism: when the dye's emission was tuned to a frequency in resonance with that of the photonic crystal’s structure, it caused the dye to emit more brightly.

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Fig. 1: A photonic crystal (right) can increase the brightness and contrast of emissive optical information storage, which could look like a conventional compact disc.

Together, the two effects increased the dye's overall emission by a factor of 40. More importantly, since the high- and low-intensity states of the dye also had slightly different frequencies, the high-intensity state was enhanced more strongly than the low-intensity state. This served to further improve the contrast by a factor of seven.

The photonic crystal was fabricated by self-assembling latex spheres and then forming a polymer layer over the top. This process has the advantages of being simple, cheap and capable of treating a large area, all of which are key requirements for any commercially viable memory technology.

The development of such two-state fluorescent systems with high brightness and contrast should hasten the development of emissive optical storage devices. Further contrast enhancements may be possible through optimization of the photonic crystal. The results may also have relevance to other applications. “We are planning to introduce photonic crystals into other fluorescence-based detection systems, such as DNA detection or trace explosives detection,” says Song.

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References

  1. Li, H., Wang, J., Lin, H., Xu, L., Xu, W., Wang, R., Song, Y. & Zhu, D. Amplification of fluorescent contrast by photonic crystals in optical storage. Adv. Mater. Published online: 15 Dec 2009; doi:10.1002/adma.200903105. | Article |

This research highlight has been approved by the author of the original article and all empirical data contained within has been provided by said author.

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