Page 461 - Introduction to Information Optics

446 8. Information Storage with Optics

(Co[P]) exhibit a strong temperature dependence of the coercivity. At a
temperature of about 150°C, the coercivity is decreased by a factor of 3 from
that at room temperature. Thus, binary data can be written with a suitable
field-applied coincident with the laser heating pulse that raises the temperature
of the heated spot to 150°C (which is lower than the Curie point). Once cooled,
the data will not switch because of the surrounding magnetic field, and thus
only the area heated above 150°C is affected.

8.3.13. PHASE-CHANGE MATERIALS

Phase-change recording uses differences of reflected intensity to distin-
guish recorded binary data [30, 32]. The principle underlying optical recording
using phase-change materials is the controlled, reversible switching of a spot
between two states, usually the amorphous and crystalline states. In contrast
to liquid and gaseous states, solids are bodies having constant shape and
volume. However, a distinction is made between crystalline and amorphous
solids. In a crystalline solid, the constituent atoms are in a periodically
repeated arrangement, whereas the atoms are in a random pattern in the
amorphous state. The stored data on the phase-change thin film can be read
by passing a light beam through the thin film. The amorphous state is
transparent while the crystalline state is opaque. On the other hand, the data
can also be read by detecting the reflected light. The reflectivity of the
crystalline state can be four times that of the amorphous state.
A distinctive class of amorphous solids are glasses, which are amorphous
solids obtained by cooling from a melt. Upon slowly cooling below the melting
temperature, T m, the liquid freezes and becomes a crystalline solid. However, if
the liquid is rapidly cooled from the melting temperature down to the glass
transition temperature, T s, crystallization cannot occur. Instead, an amorphous
solid or glass is formed. In principle, to switch from the amorphous state to
the crystalline state, the spot is heated by a laser beam to a temperature well
above T g, but just below T m. The material then has sufficient time to crystallize
when it cools to room temperature through T g. To revert from the crystalline
state to the amorphous state, the spot is heated by a laser beam to a
temperature above T m. In this case, glass will be formed provided that after the
light pulse ends the material is cooled rapidly from T m to T g to prevent
crystallization.

8.4. BIT-PATTERN OPTICAL STORAGE

This and successive sections will overview architectures for optical storage.
Information to be stored and later recalled is a string of bits. Thus, the
information is 1-D, which is related to electronic information processing