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.
3D optical data storage is the term
given to any form of optical data storage in which information can be recorded
and/or read with three dimensional resolution
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 capacity2,3. 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
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
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.
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
3. D. Day, M. Gu, and A. Smallridge, "Rewritable 3D bit
optical data storage in a PMMA-based photorefractive polymer," Adv. Mater. 13,
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
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).
Li, J. W. M. Chon, R. A. Evans, and M. Gu, "Quantum-rod dispersed photopolymers
for multi-dimensional photonic applications," Opt. Express 17, 2954
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).
Copyright AZoNano.com, Prof. Min Gu (Swinburne University of
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