Page 455 - Introduction to Information Optics

440 8. Information Storage with Optics
can be accomplished either through direct interaction of light with the polymer
itself, or through an intermediary such as a photosensitizer.
A commercially available photopolymer contains a monomer and a photo-
sensitizer incorporated with a polymeric binder to form a soft film [11], During
exposure, partial polymerization of the monomer occurs to an extent that is
dependent upon the local intensity of the recording radiation. Since the
distribution of the remaining monomer is not uniform, diffusion of monomer
molecules takes place during and after exposure to regions of low concentra-
tion. The polymer molecules effectively do not move. After polymerization and
diffusion are completed, diffraction efficiency can be considerably increased by
a uniform postexposure using a fluorescent light. The postexposure may also
desensitize the photosensitizer. The refractive index modulation is produced by
the variations in polymer concentration.

8.3.4. PHOTORESISTS

Photoresists are organic photosensitive materials that are commonly used
in photolithography for the fabrication of integrated circuit (1C) chips. There
are two types of photoresists, negative and positive. After exposure to light,
negative photoresists become insoluable in solvent due to polymerization or
other processes. The areas which were not exposed to light are washed away,
leaving a pattern of the exposed image. On the other hand, positive photo-
resists become soluble in the solvent because of depolymerization or other
processes due to the action of light. An image or holographic interference
pattern is recorded as a surface relief pattern of the photoresist layer. The
resulting surface relief patterns enable preparation of reflection holograms
through aluminum coating and mass duplication of holograms through em-
bossing [8,12], The developed photoresist can also be used uncoated as a
phase hologram [12].

8.3.5. THERMOPLASTIC FILM

Thermoplastic film, also known as a photoplastic device, involves the
surface deformation of a transparent layer such that the phase of the light beam
passing through the layer will be modulated. The essential elements in the
thermoplastic process are the creation of an electric field pattern inside the
thermoplastic layer, which is a copy of the incident optical pattern, and the
pulse heating of the thermoplastic layer, which causes the thermoplastic to be
molded according to the electric field pattern [13].
The device is composed of a glass substrate that is coated with a transparent
conducting layer (tin oxide or indium oxide), on the top of which is a layer of