However, the recording and reading system was quite complicated for the implementation of 3-D storage devices.
Researchers reached an important milestone in the single-beam 2P recording the following year. They found that the
2P absorption occurs inside the tight focus of a single objective of a large NA instead of two. Since 2P
absorption is a highly nonlinear process,
a high-power pulsed laser is generally
required to facilitate the 2P process. This,
of course, significantly increases the cost for
the device application. Another important
milestone is the discovery that 2P-induced
3-D recording can be introduced using a
high-power continuous wave (CW) laser in
photochromic and photorefractive materials as a replacement for a femtosecond
pulsed laser.
There has been intense research interest
in developing various 2P-induced optical
recording mechanisms—not only permanent ones such as photopolymerizing, photobleaching and
photofabricating voids, but also rewritable ones, including
photorefractivity and photochromism. In 1998, researchers
introduced a rewritable 2P-induced recording in photorefractive crystals.
Later, Day et al. discovered a 2P-induced polymeric photorefractivity, achieving the world’s first erasable 3-D device
with a density of 5 Gbits/cm3. In particular, this group discovered a polarization-sensitive polymeric photorefractivity in
polymer-dispersed liquid crystal materials through a 2P-induced
reorientation mechanism. This discovery builds the conceptual
foundation for multi-dimensional optical storage.
The highly confined 2P fluorescence enhancement allows
rewritable 3-D optical data storage up to 205 Gbits/cm3.
This result is equivalent to 50 times the current DVD capacity. It was the world’s highest 3-D data storage density until
2008, when Walker and colleagues developed a photochromic
polymer-based 3-D disc with a capacity of 1,000 GB/disc, as
reported in a 2008 issue of Nature Photonics. In their system,
the total number of layers is 200, and the
capacity of each layer equals 5 GB.
The idea behind
multi-dimensional
optical data storage
is to multiplex multiple
states of information
in the same 3-D
spatial region of a
recording medium.
Multi-dimensional systems
The storage capacity of 2P-excitation-based
3-D optical memory is still limited by the
resolution of recorded bits, which is in turn
confined by diffraction. The limit of 3-D
storage capacity is approximately 3. 5 Tbits/
cm3, as predicted by the diffraction theory
after aberration correction is considered for
an objective of NA= 1. 4. The ever-increasing
demand for more data capacity is compelling the development of multi-dimensional
optical data storage.
The idea behind multi-dimensional optical data storage is
to multiplex multiple states of information in the same 3-D
spatial region of a recording medium. The information can
be encoded into additional physical dimensions of the writing
beam, such as spectra or polarization, and then individually
addressed. The ground-breaking techniques of polarization and
spectral encoding are the core of third-generation optical data
storage. These approaches, which are not limited by the spatial
resolution of recorded bits, allow capacity to be expanded by
orders of magnitude.
In 2007, Li et al. reported in Optics Letters that the world’s
first erasable four-dimensional (4-D) optical data storage
device adopted the polarization-encoding technique. The
two-state information was multiplexed in the two polarization
states of the writing beams.
Through the 2P-induced
re-orientation of azo dyes
inside a photopolymer,
the researchers recorded
a polarization-sensitive
refractive-index change in
the volume of a recording
medium and retrieved it
back individually. How-
ever, the realization of 4-D
memory in such materials
is limited by the weakness
of 2P-absorption efficiency
of azo dyes. This restriction
has spurred a revolutionary
idea: the 2P-induced photo-
reaction of nanoparticles.
When sizes shrink to the
nanometer scale, intriguing [ Major achievements towards the development of 2P-induced 3-D optical data storage ] Year Achievements Density 1989 Demonstration of 2P-induced 3-D storage with a two-beam approach -- 1991 Demonstration of 2P-induced memory with a single-beam approach -- 1996 2P-induced 3-D voids recording in silica 17 Gbits/cm3 1998 Rewritable 2P recording in a photorefractive crystal 33 Gbits/cm3 1999 Realization 2P recording using a CW laser 3 Gbits/cm3 1999 Rewritable 2P recording in a photorefractive polymer 5 Gbits/cm3 2000 2P recording in photochromic materials by a confocal reflection microscope -- 2002 Polarization-sensitive 2P recording in polymer dispersed liquid crystal materials 205 Gbits/cm3 2002 2P-induced voids recording in polymers 2 Gbits/cm3 2007 2P-induced photochromic recording in a multilayer disc 250 GB/disc 2008 2P-induced photochromic recording in a 200-layer disc 1,000 GB/disc
Group
Parthenopoulos et al.
Strickler et al.
Glezer et al.
Kawata et al.
Gu et al.
Day et al.
Kawata et al.
McPhail et al.
Day et al.
Walker et al.
