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two-photon absorption cross-section and second-order optical nonlinearity. Figure 1.12 pres-
ents a schematic diagram of a two-photon 3-D optical memory that could be used for reading
and writing of information. The 3-D voxel of information would be located as shown (magni-
fied) within a matrix of stacked 2-D planes of oriented bR membranes located within a poly-
mer matrix. A 3-D disk could be adapted to a rotating disc format and with a total thickness of
1 mm could hold 100 layers of data, assuming that the separation of each data stack is less than
10
m. A 3-D storage format has a tremendous advantage over 2-D storage for equivalent sur-
face area storage devices, since for visible or IR lasers the storage density limit is ~10 8 bits/cm 2
for the 2-D format, but rises to ~10 12 bits/cm 3 for the 3-D case. Because both two-photon exci-
tation and second harmonic generation depend quadratically on intensity, writing and read-
ing with 3-D resolution can be accomplished by this nonlinear process either with a focused
beam or with the overlap of two beams, as shown in Figure 1.12. The writing process would
access a voxel of information to write 0 or 1 by the two-photon process at the depth location
in the disc determined by where the two photons from the separate writing beams were
brought to focus. Everywhere except inside the voxel, the two isolated unfocused beams
would produce no optical state changes and no writing information changes within the disc.
Reading IR
laser beam
Writing
laser beam
BR disk made of
oriented BR in polymer
0 bR
0 M
Filter
Data acquisition,
disk control,
and interface
Detector
FIGURE 1.12
Schematic diagram of a potential two-photon three-dimensional (3-D) optical memory device based upon bR
membranes. The storage medium is a disk made from oriented bR membranes in a polymer matrix. A build-up
of bR membrane stacks creates the 3-D disk, as the right-hand expanded view indicates. The data storage capac-
ity equals the data density of each layer multiplied by the number of layers in the 3-D disk. The writing opera-
tion in this memory is performed by two-photon absorption and reading is accomplished by second harmonic
generation. Reprinted from Chen, Z., Lewis, A., Kumar, J., Tripathy, S.K., Marx, K.A., Akkara, J., Kaplan, D.L.
(1994). Second Harmonic Generation of Bacteriorhodopsin and Its Application for Three-Dimensional Optical
Memory. In: M. Alper, H. Bayley, D. Kaplan, M. Navia, eds. Biomolecular Materials by Design, Proc. Mat. Res.
Soc. , 330:263-268. With permission from the Materials Research Society.
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