Holograms
Hologram is for displaying natural three-dimensional (3D) images without
eyeglasses.
When the technologies are considered, two types of holograms are known;
• Raman-Nath type
• Bragg type
Generally, the Raman-Nath hologram is recorded in a thin film, which can cause
multiple diffraction of an incident beam, and diffraction efficiency remains low (<40%).
The Bragg hologram is recorded in thick film showing a single diffraction, which
enables achievement of 100% diffraction efficiency.
For photoactive LCP, the situation is different.
• Mesogens are known to amplify the external stimuli by way of cooperative
molecular motion.
• The value of birefringence increases with length of the mesogenic core.
• In case of delocalization of π-electrons through phenyl rings, enhanced optical
anisotropy is observed.
Raman-Nath-type hologram can be easily fabricated without much difficulty. For
Bragg type it is not easy to fabricate a high-performance Bragg hologram using the
azobenzene-containing LC polymers because of heavy light scattering. Further, the
absorbance is usually very high which prevents incident writing beams penetrating
deep into the thick films. If the concentration of the azobenzene moiety is lowered, it
lowers the change in refractive index. There is always a trade-off between the content of
the azobenzene and change in refractive index. This is solved by adding one
non-mesogenic component.
Holography on photoactive elastomer: Elastomer has the property of large and
reversible deformation, and chromophore has photoactivity attributes. When combined the holographic properties are amplified many times because:
• Deformation can be utilized to enhance diffraction efficiency. Spatial modulation
of refractive index (RI) can be enhanced by deformation.
Reversible deformation adds tunability to holographic grating and controls
changes in the period of grating and RI modulation depth.
Multiple Holograms in Single Holographic Medium
In Bragg hologram the angular multiplicity plays great role for high-density data storage
application. Multiple holograms can be recorded in a single spot. After recording the
first hologram, the film is rotated to record the next hologram. By repeating this
402
A. B. Samui
Hologram is for displaying natural three-dimensional (3D) images without
eyeglasses.
When the technologies are considered, two types of holograms are known;
• Raman-Nath type
• Bragg type
Generally, the Raman-Nath hologram is recorded in a thin film, which can cause
multiple diffraction of an incident beam, and diffraction efficiency remains low (<40%).
The Bragg hologram is recorded in thick film showing a single diffraction, which
enables achievement of 100% diffraction efficiency.
For photoactive LCP, the situation is different.
• Mesogens are known to amplify the external stimuli by way of cooperative
molecular motion.
• The value of birefringence increases with length of the mesogenic core.
• In case of delocalization of π-electrons through phenyl rings, enhanced optical
anisotropy is observed.
Raman-Nath-type hologram can be easily fabricated without much difficulty. For
Bragg type it is not easy to fabricate a high-performance Bragg hologram using the
azobenzene-containing LC polymers because of heavy light scattering. Further, the
absorbance is usually very high which prevents incident writing beams penetrating
deep into the thick films. If the concentration of the azobenzene moiety is lowered, it
lowers the change in refractive index. There is always a trade-off between the content of
the azobenzene and change in refractive index. This is solved by adding one
non-mesogenic component.
Holography on photoactive elastomer: Elastomer has the property of large and
reversible deformation, and chromophore has photoactivity attributes. When combined the holographic properties are amplified many times because:
• Deformation can be utilized to enhance diffraction efficiency. Spatial modulation
of refractive index (RI) can be enhanced by deformation.
Reversible deformation adds tunability to holographic grating and controls
changes in the period of grating and RI modulation depth.
Multiple Holograms in Single Holographic Medium
In Bragg hologram the angular multiplicity plays great role for high-density data storage
application. Multiple holograms can be recorded in a single spot. After recording the
first hologram, the film is rotated to record the next hologram. By repeating this
402
A. B. Samui