Using Nanotechnology to Increase Data Capacity

Information storage is such an important aspect that it has been pioneering the development of information technology. As one of the most promising subsets, optical data storage has led to a series of revolutionary advances in this area. One of the challenging tasks is to meet the rapid growth in demand for storage capacity. Bit by bit optical data storage such as compact discs (CDs), digital video discs (DVDs) and Blu-ray discs (Blu-rays) emerge as high memory density, high resistance to intense electromagnetic radiation, compact and portable systems. Each technology comes with new expansion of the storage capacity but also with its own limitation.

Current optical data storage media, such as the CDs, DVDs and Blu-rays store data as a series of reflective marks introduced by a focused laser beam on an internal surface of a disc. In all these cases, data is stored within a two-dimensional (2D) layer, where the information occupies less than 0.01% of the volume of a disc¹. Limited by the interaction of recording wavelength and the numerical aperture of the recoding lens, the maximum capacity is approximately 700 Megabytes (MB) for a CD, 4.5 Gigabytes (GB) for a DVD and 25 GB for a Blu-ray disc. It is possible for discs to hold two or even more of these data layers to expand the capacity, but the number of layers is severely limited by the efficiency of delivering addressing laser into a thick volume.

A revolutionary technique of two-photon (2P) excitation by a femtosecond (fs) pulsed laser beam of a pulse duration of 100 fs (1 s = 10¹⁵ fs) has been introduced which lights up the three-dimensional (3D) memory devices or sometimes called 3DCD technology. The beauty of 2P technique is that it allows a better confined focused spot size and a much higher efficiency of penetration depth, which means a higher storage density in each layer as well as a larger number of information layers a disc can hold.

In 1998, the world's first rewritable 3D bit optical data storage device achieved in our group has demonstrated a capacity of 44 Gbits/cm³ by adopting 2P excitation, equivalent to 5 times the current DVD capacity^2,3. Later, our group discovered a new physical mechanism of 2P excitation enhanced fluorescence of liquid crystals and a 3D storage capacity up to 450 Gigabits/cm³ was demonstrated in 2004⁴. This result is equivalent to 50 times the current DVD capacity and was the world’s highest 3D data storage density till 2008.

The data capacity of the 3DCD disc of size of a DVD is predicted by the theoretical limitation approximately Terabytes/disc⁵. To break the data storage limit of the 3DCD technology, our group has developed the ground breaking idea called "polarisation encoding and spectral encoding", which is called multi-dimensional optical data storage. The concept is to record multi-states of information in the same x-y-z spatial region of a recording medium.

Facilitated by the recent advances of nanotechnology, owing to the elegance of a large 2P sensitivity and sharp direction selective excitation properties of nanoparticles⁶, the information can be multiplexed into additional physical dimensions of a recording beam such as spectra or polarisation and addressed individually, as illustrated in Fig. 1. In addition, nanoparticles facilitate multi-dimensional encoding technique with improved sensitivity and much reduced cross talks. It can potentially increase the current storage capacity by orders of magnitude, which is not limited by the spatial resolution of focused spot size.

In 2008, our group has demonstrated the world's first four-dimensional optical data storage devices in quantum rods dispersed polymer materials adopting polarisation encoding technique⁷. Following that principle, our group has achieved a world's record high storage capacity of 1.6 Terabytes/disc in metallic nanorods dispersed medium by applying polarisation and spectra encoding techniques simultaneously⁸.

Figure 1. Scheme of multi-dimensional optical data storage by photoreaction of rod-shaped nanoparticles. (a) Illustration of 2P polarisation-selective excitation and emission dependence of quantum rods on the polarisation state. (b) Polarisation dependence of the fluorescence intensity on excitation polarisation state (red circles) and emission directions (blue squares). Information are polarisation and spectra multiplexed in multilayer inside the medium. One recorded layer indicated by yellow dashed line is addressed using circularly polarised broad band source as illustrated in (c). The multiplexed information can be individually addressed with corresponding polarisation (indicated by the arrow) and wavelength, as illustrated in the (d) and (e).

Our quantum leap achievement has provided the basis for us to embark on an accelerating journey to the new era of multi-dimensional Petabyte optical memory systems (1 Petabyte = 1,000 trillion bytes)), equivalent to 10,000 times the current DVD capacity. This multidimensional optical data storage concept is the core paradigm-shift for optical data storage devices called multi-dimensional CDs (MDCDs), which will emerge in the next 5-10 years. If successful, this new technology will arouse bottom up revolution in every corner of our modern life such as education, portable banking, global e-security and telemedicine as well as lead to enormous economic benefits in Australia.

For example, young people are spending nearly 20 years studying in schools just because of the slow memory process and limited capacity of human brain. By that time the MDCDs are available, a 1Petabytes disc can hold all the information and knowledge one can learn in 20 years’ traditional education system! In other words, a Petabytes disc can liberate young people from boring school life and save them 20 years! If movie is your care, the capacity of a movie needs to be re-defined. 10 years ago, the capacity of a two-hour movie of VCD quality is approximately 5GB. Current, a DVD quality movie is about 15GB and a high-definition movie is about 50GB. Imagine 10 years later, the movie will be 3D displayable, environment simulationable and human brain wave simulationable. We expect the capacity of a movie by that time would be a minimum of 1000GB.

References

1. D. Day, M. Gu, and A. Smallridge, "Review of optical data storage," in Infrared holography for optical communications(Springer Berlin, Heidelberg, 2003), pp. 1.
2. D. Day, M. Gu, and A. Smallridge, "Use of two-photon excitation for erasable-rewritable three-dimensional bit optical data storage in a photorefractive polymer," Opt. Lett. 24, 948 (1999).
3. D. Day, M. Gu, and A. Smallridge, "Rewritable 3D bit optical data storage in a PMMA-based photorefractive polymer," Adv. Mater. 13, 1005 (2001).
4. D. McPhail, and M. Gu, "Use of polarization sensitivity for three-dimensional optical data storage in polymer dispersed liquid crystals under two-photon illumination," Appl. Phys. Lett. 81, 1160 (2002).
5. D. Day, and M. Gu, "Effects of refractive-index mismatch on three-dimensional optical data-storage density in a two-photon bleaching polymer," Appl. Opt. 37, 6299 (1998).
6. X. Li, J. Van Embden, J. W. M. Chon, and M. Gu, "Enhanced two-photon absorption of CdS nanocrystal rods," Appl. Phys. Lett. 94, 103117 (2009).
7. X. Li, J. W. M. Chon, R. A. Evans, and M. Gu, "Quantum-rod dispersed photopolymers for multi-dimensional photonic applications," Opt. Express 17, 2954 (2009).
8. P. Zijlstra, J. W. M. Chon, and M. Gu, "Five-dimensional optical recording mediated by surface plasmons in gold nanorods," Nature 459, 410 (2009).

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