for example, reflectivity. Information is later on retrieved by illuminating the spiral
track with a different focused beam and analyzing the reflected light. Strategies
undertaken to increase the storage capacity of these systems have been, among
others, the use of multilayers, or double side systems; however, the most important
one has dealt with the use of light sources with shorter wavelengths (Compact Disc,
CD 780 nm; Digital Versatile discs DVD at 650 nm and blu-ray discs at 405 nm). As
mentioned, the application of linearly polarized light to azobenzene containing
polymers can lead to optical anisotropy in the material, a feature that can be used
to register information. As the magnitude of the photoinduced optical anisotropy in
the material can be controlled by light irradiation conditions, gray-scale recording
was proposed in these systems. In this gray-scale recording strategy, a different
degree of anisotropy can be recorded and read in each point, instead of the traditional
binary scheme in conventional optical discs. Assuming that 2
8 gray values can be
distinguished, a boost of a factor 8 in storage capacity and transfer rate could be
achieved.
Beyond two-dimensional (2D) recording technologies, holographic storage is
able to make use of the whole volume of a thick layer of photosensitive material
enormously increasing storage capacity of a single disc. In addition, recording and
reading of a complete page with thousands of bits can be carried out in one single
illumination event boosting also data transfer rate with respect to the previously
mentioned 2D technologies (Hagen and Bieringer 2001).
Holographic recording makes use of two coherent laser beams that are made to
interfere in the recording layer. One of the beams (signal beam) carries the information as a spatially modulated wavefront. The other beam (reference beam) carries no
information. Both are made to interfere at certain angle in the photosensitive material
that captures the interference pattern as a change of a physical property, for example,
refractive index (Fig. 11a). To read the recorded hologram, the reference beam is
used to illuminate the material appearing a diffracted beam with a spatial modulated
wavefront that carries the information of the previously used signal beam (Fig. 11b).
Holographic recording in thick films (hundreds or thousands of micrometers) allows
to fully benefit from the advantages of this technique. Diffraction is detected at the
Bragg angle and slight detuning results in no diffraction. This feature is used to
record in the same volume different holograms (pages of information) at different
angles, which is known as hologram multiplexing (Fig. 12). The individual holograms can be later independently recovered, in this way increasing the storage
capacity. Despite the obvious advantages of this technique, its introduction in the
market has not crystallized mainly due to the lack of suitable materials fulfilling
requirements of this application. A suitable holographic media should have no
scattering losses, low absorption at the recording wavelength, so recording is
effective through the whole thickness of the film with good sensitivity (large optical
change with low energy illumination), large dynamic range being the media able to
register multiple holograms in the same volume as well as long-term stability of the
recorded holograms (Hvilsted et al. 2009).
The flexibility in the design and synthesis of azobenzene polymeric systems has
made them an interesting and widely studied system for this application. In addition,
16 Light-Sensitive Azobenzene-Containing Liquid Crystalline Polymers
461
track with a different focused beam and analyzing the reflected light. Strategies
undertaken to increase the storage capacity of these systems have been, among
others, the use of multilayers, or double side systems; however, the most important
one has dealt with the use of light sources with shorter wavelengths (Compact Disc,
CD 780 nm; Digital Versatile discs DVD at 650 nm and blu-ray discs at 405 nm). As
mentioned, the application of linearly polarized light to azobenzene containing
polymers can lead to optical anisotropy in the material, a feature that can be used
to register information. As the magnitude of the photoinduced optical anisotropy in
the material can be controlled by light irradiation conditions, gray-scale recording
was proposed in these systems. In this gray-scale recording strategy, a different
degree of anisotropy can be recorded and read in each point, instead of the traditional
binary scheme in conventional optical discs. Assuming that 2
8 gray values can be
distinguished, a boost of a factor 8 in storage capacity and transfer rate could be
achieved.
Beyond two-dimensional (2D) recording technologies, holographic storage is
able to make use of the whole volume of a thick layer of photosensitive material
enormously increasing storage capacity of a single disc. In addition, recording and
reading of a complete page with thousands of bits can be carried out in one single
illumination event boosting also data transfer rate with respect to the previously
mentioned 2D technologies (Hagen and Bieringer 2001).
Holographic recording makes use of two coherent laser beams that are made to
interfere in the recording layer. One of the beams (signal beam) carries the information as a spatially modulated wavefront. The other beam (reference beam) carries no
information. Both are made to interfere at certain angle in the photosensitive material
that captures the interference pattern as a change of a physical property, for example,
refractive index (Fig. 11a). To read the recorded hologram, the reference beam is
used to illuminate the material appearing a diffracted beam with a spatial modulated
wavefront that carries the information of the previously used signal beam (Fig. 11b).
Holographic recording in thick films (hundreds or thousands of micrometers) allows
to fully benefit from the advantages of this technique. Diffraction is detected at the
Bragg angle and slight detuning results in no diffraction. This feature is used to
record in the same volume different holograms (pages of information) at different
angles, which is known as hologram multiplexing (Fig. 12). The individual holograms can be later independently recovered, in this way increasing the storage
capacity. Despite the obvious advantages of this technique, its introduction in the
market has not crystallized mainly due to the lack of suitable materials fulfilling
requirements of this application. A suitable holographic media should have no
scattering losses, low absorption at the recording wavelength, so recording is
effective through the whole thickness of the film with good sensitivity (large optical
change with low energy illumination), large dynamic range being the media able to
register multiple holograms in the same volume as well as long-term stability of the
recorded holograms (Hvilsted et al. 2009).
The flexibility in the design and synthesis of azobenzene polymeric systems has
made them an interesting and widely studied system for this application. In addition,
16 Light-Sensitive Azobenzene-Containing Liquid Crystalline Polymers
461