2005 Conference on Lasers and Electro-Optics Europe

Multidimensional optical data storage

Min GuCentrefor Micro-Photonics, Faculty ofEngineering and Industrial Sciences

Swinburne University of Technology, Hawthorn V'ic 3122 Australia

The concept of optical data storage is based on the use of a laser beam that is focused onto a recording materialto produce a spot where physical or chemical properties of the material are changed. The first generation of the opticaldata storage devices is the two-dimensional (2-D) devices such as compact disks (CDs) and single layer digital videodisc (DVDs) with a 2-D storage density of up to 0.8 Gbits/cm2 for a visible laser beam. The idea of using the thirdspatial dimension has led to the generation of multi-layered DVDs with capacity of up to 100 Gigabytes per disk. Theutilisation of the two-photon excitation technique, in which two incident photons of a recording beam of an infraredwavelength are simultaneously absorbed by a recording medium to induce a local physical and chemical alteration, canbreak this limitation. In previous efforts we saw successful developments of various types of two-photon-inducedrecording techniques, i.e., the localised photorefractivity in photorefractive polymer [1], and the localised polarisationsensitivity in a polymer-dispersed liquid crystal (PDLC) material for rewritable (or erasable) 3-D bit optical storage. Asa result, one could project a 3-D data density of up to I Tbits/cm3, which is equivalent to 300 times the information in acurrent DVD.

However, due to the problems of aberration and scattering of a recording beam inside the medium, it is likelythat the 3-D optical storage technique will not be able to reach its proposed data density capacity. Furthermore, even ifit met its capacity, it will be the ultimate upper capacity limit for such techniques and further improvement in datadensity will be stalled due to the same problem that we are facing today: spatial limit imposed by the recording mediumand diffraction limited bit size. One possible way of overcoming the "universal" spatial limit in improving the datadensity is by incorporating other physical dimensions (i.e. spectral domain), hence increasing the dimensionality of thestorage methods.

The QDs have received so much attention because of their interesting properties such as the emission wavelengthtunability with size, narrow emission bandwidths and discrete atom-like energy level structures. When two differentsizes of QDs are mixed, the differences in energy level structures can be manifested by variation in relative intensitiesof the two fluorescence wavelengths with respect to the excitation wavelengths. This feature is shown in Figure 1,where the fluorescence spectrum of green QDs (- 3 nm in diameter) mixed together with red QDs (- 6 nm diameter) isshown, excited by various near-infrared wavelengths (two-photon absorption). The implication of this phenomenon isthat the improved absorption could induce more interactions with excitation light such as bleaching or quenching, sothat in optical storage, marks can be recorded on one type of QDs at a certain wavelength without affecting the otherQDs - the property that could be used as a foundation for spectral encoding [2]. In addition, the shape of the QDs leadsto a polarisation sensitivity, which is another physical dimension data encoding.

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Emissiowaveengt (rmn)Fig. 1 Two-photon fluorescence spectra of mixed nanocrystals in polymer at difference excitationwavelengths. Note the relative change in intensities of two different sizes of nanocrystals.

ReferencesI. D. Day, M. Gu and A. Smallridge, Opt. Lett., 24 (1999) 948.2. J. Chon, P. Zijlstra, M. Gu, J. Embden, P. Mulvaney, Appl. Phys. Lett., 85 (2004), 5514.

0-7803-8974-3/05/$20.00 @2005 IEEE 713