Information bits are polarization- and spectra-multiplexed in
multiple layers inside the medium. (Center) One recorded layer
indicated by the red dashed line is addressed using a randomly polarized broad band source. (Right) The multiplexed
information can be individually addressed with corresponding polarization (indicated by the arrow) and wavelength, as
illustrated in the right column.
optical properties of nanoparticles emerge, such as a large 2P
sensitivity, wavelength-tunable absorption and emission over
their sizes, and a sharp polarization sensitivity, depending on
their shapes. With the significantly enhanced 2P sensitivity,
2P-induced 3-D recording can be efficiently implemented
using quantum dots as the energy transfer donors.
To this end, rod-shape semiconductor nanoparticles
have been incorporated into photopolymers to introduce a
polarization-sensitive energy transfer, therefore enabling the
world’s first nanoparticle-enabled polarization-sensitive 4-D
optical data storage device. On the other hand, when the sizes
of metallic nanoparticles shrink, their absorption and scattering cross-sections can be significantly enhanced at the surface
plasmon resonance wavelength. These appealing properties
make nanoparticles suitable to fulfill the application in both
spectral and polarization encoding techniques with higher 2P
sensitivity and less cross-talking.
When photoexcited at the surface plasmon resonance
wavelength, the nanorods can raise the temperature above the
melting point and re-shape into spheres. As a consequence of
the shape change, surface plasmon bands are blue-shifted. This
phenomenon can be used as a permanent recording mechanism, and it contributes a key technique for spectral encoding
that uses nanorods of different sizes. In a 2009 Nature article,
Zijlstra and colleagues demonstrated that gold nanorods,
combined with the sharp 2P-induded polarization sensitivity,
enable information to be recorded in five dimensions.
According to the global data storage market projection by the
International Data Corporation, the amount of information that
can be captured has been increasing explosively—six-fold within
every four years. In the near future, the information generated
each year will much overwhelm available storage capacity.
The urgent demand for more capacity compels the development of ultra-high-density storage devices. The major
achievement of 5-D optical data storage marks the beginning
a journey to the new era of multi-dimensional petabyte optical
memory systems ( 1 exabyte = 1,000 petabytes; 1 petabyte =
1,000 terabytes)—which are equivalent to 10,000 times the
current DVD capacity.
These multi-dimensional optical data storage devices are
called multi-dimensional CD (MD CDs). They should emerge
within the next 5-10 years. If successful, this new technology will underpin every sector of our modern life, with roles
in remote education, portable banking, global e-security and
telemedicine. It will also lead to enormous economic benefits.
Multi-dimensional optical drives
In the future, we hope to see the development of a multi-dimensional optical drive that is not only compatible with the
current DVD and Blu-ray discs, but also able to record and
read polarization and spectral information. The signal noise
ratio and bit error rates of multiplexed data decoding should
be tested. The writing and reading speeds can be optimized
to make large-scale petabyte capacity feasible, provided that
parallel binary optics can be adopted.
Engineering of multi-dimensional
point spread function
In a 3-D optical system, the point spread function is considered only in the x-y-z spatial dimensions. However, for a 5-D
optical system, the point spread function must be engineered
within 4 years
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