Page 474 - Introduction to Information Optics

8.5. Holographic Optical Storage 459

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hologram. However, when an external electric field of about 5 x 10 V/m is
applied across the crystal in the proper direction, the sensitivity is significantly
increased and enables the recording of a hologram. It is believed that this
applied field aids the rate of photoionized charge separation by providing a
drift field. The drift current attained under conditions of an applied field is
significantly larger than the average diffusion current driven solely by the
spatial variation of the photoionized charge density.
The recorded hologram can be made latent by applying an electric pulse
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that generates an electric field of about 10 V/m that is opposite to the field
used for enhancing the recording sensitivity during recording. In the latent
state, the diffraction efficiency of the reconstructed hologram is very low. The
applied field causes a change of the charge distribution such that the hologram
no longer diffracts the read beam. The change remains after the applied field is
removed. However, the original charge distribution is not lost. The effect of the
original charge is only suppressed and can be returned and even enhanced by
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applying an electric field of about 5 x 10 V/m opposite to the suppressing
field and in the same direction as the field for enhancing the recording.
Therefore, a stack of transparent SBN holographic layers with separated
electric switches can be used as a 3-D optical storage device. Note that
electrodes are at the edges of the SBN layer.

8.5.3.2. Stack of PVA Layers

Some organic photochromic materials, including azo-dye-doped polyvinyl
alcohol (PVA), are sensitive to the polarization of light [50]. A hologram can
be recorded and reconstructed using only light with certain polarization.
Recently, a 3-D storage device formed by a stack of PVA layers and polariz-
ation rotators has been proposed by Chen and Brady [51].
In this 3-D storage, the PVA layer is used to store the hologram. Each PVA
layer is covered by a polarization rotator that consists of a liquid crystal layer
sandwiched between a pair of transparent electrodes. It is well known [2] that
a liquid crystal polarization rotator can rotate the polarization of the incident
light by 90° when no voltage is applied to the electrodes (on state). If voltage
is applied, the liquid crystal polarization rotator will not affect the incident
light (off state).
Consider that the incident light polarization is 0 and the PVA layer
requires 90 polarization for activation. When the liquid crystal polarization
rotators are in the off state, the incident light passes through the polarization
rotators and the PVA layers unaffected. To activate a selected PVA layer, the
liquid crystal polarization rotator just before it is turned on so as to rotate the
light polarization by 90°. After passing the selected PVA layer, the next liquid
crystal polarization rotator is also set on and rotates the polarization by 90