Page 456 - Introduction to Information Optics

8.3. Optical Storage Media 441

photoconducting material, followed by a layer of thermoplastic. For the
photoconductor, poly-n-vinyl carbazole (PVK) sensitized with trinitro-
fluorenone (TNF) can be used with an ester resin thermoplastic [14].
Before exposure, the thermoplastic layer is charged by a corona device while
the transparent conducting layer on the glass substrate is grounded. The
positive ions from the ionized air are deposited on the surface of the thermo-
plastic layer. The voltage is capacitively divided between the thermoplastic and
photoconducting layers. After the charging process, the thermoplastic layer can
be exposed to the signal light. The illuminated region displaces the charge from
the photoplastic interface to the transparent conducting layer through the
photoconducting layer, which in turn reduces the surface potential of the outer
surface of the thermoplastic. The thermoplastic layer is then recharged to the
original surface potential. The electric field across the thermoplastic increases
in the illuminated area as a result of the second charging, although the surface
potential is now uniform. The thermoplastic is developed by raising its
temperature to the softening point and then rapidly reducing it to room
temperature. Surface deformation caused by the electrostatic force then pro-
duces a phase recording of the intensity of the input light. This recording can
be erased by raising the temperature of the thermoplastic above the melting
point, thus causing the surface tension of the thermoplastic to flatten the
surface deformation and erase the recording.

8.3.6. PHOTOREFRACTIVE MATERIALS

Holographic storage in lithium niobate (LiNbO 3), which is a photorefrac-
tive crystal, was first demonstrated by Chen et al, in 1968 [15]. Photorefractive
materials are crystals that contain electron traps. Exposure of photorefractive
materials to light excites electrons from traps to the conduction band, where
electrons can freely move. The free electrons diffuse thermally or drift under
applied or internal electric fields and become retrapped, preferably in regions
of low-intensity light. The space charge buildup continues until the field
completely cancels the effect of diffusion and drift; i.e., the current is zero
throughout. The resulting charge distribution modifies the refractive index of
the material by the linear electro-optic effect.
The electron traps are generated by intrinsic defects and residual impurities.
The sensitivity of the crystal can be dramatically increased by adding iron to
the crystal lattice. For example, Fe 2O 3 is added when a crystal of lithium
niobate (LiNbO 3) is grown by means of the Czochralski process. Iron enters
2 +
3 +
the crystal lattice as Fe and Fe ions. These two species of iron ions are
2+
initially evenly distributed throughout the crystal. When Fe is exposed to
the input light, an electron is excited to the conduction band of the crystal
2+ 3 +
lattice, and Fe becomes Fe . Migration of electrons occurs in the conduc-