Page 460 - Introduction to Information Optics

8.3. Optical Storage Media 445

The absorption frequencies of dye molecules (impurities) embedded in a
solid optical matrix are generally shifted due to their interaction with the
surrounding matrix. The absorption lines of the dye molecules give rise to a
broad continuous inhomogeneous optical absorption band. The dye molecules
undergo photochemical reactions when they are irradiated by light of a certain
frequency. After the material sufficiently absorbs optical energy at A 1? the
material can absorb no more light at A l. In other words, the Aj absorption
band is bleached out, so that a spectral hole appears in the broad in-
homogeneous absorption band. The spectral hole can be read using light of the
same wavelength A I. Similar to two-photon absorption materials, two photons
at A 2 and A 3 may be used at the same time for writing, while a photon at A { is
used for reading [23]. At the present time, the main difficulty of these materials,
organic or inorganic, is that they must be kept at a very low temperature
(<100K).

8.3.12. MAGNETO-OPTIC MATERIALS

Magneto-optic (MO) materials store binary information as upward and
downward magnetization [30,31]. The most commonly used MO medium is
a thin film of a manganese bismuth (MnBi) alloy. The recorded data is read
out using a linearly polarized laser beam, which undergoes a small rotation
because of the Faraday or Kerr effect. The polarization of the beam is rotated
to the left or right, depending on whether the magnetization is upward or
downward. In the transmissive Faraday effect, the rotation angle is propor-
tional to the film thickness. In reflective readout, there is a rotation of the
polarization through the Kerr effect. This Kerr rotation is a sensitive function
of the incidence angle. The Kerr rotation seldom exceeds 1 . In principle, the
intensity modulation can be obtained by applying an analyzer.
Ferromagnetism is a property of some substances that are able to retain
magnetization when the external magnetizing field is removed. In ferromag-
netic materials, the magnetic moments of their atoms are aligned in the same
direction. When ferromagnetic materials are heated over a temperature called
the Curie point, atoms start a random motion due to the temperature. The
magnetic moments are in random orientation, and the materials become
paramagnetic. To write binary data onto MnBi film, the temperature of the
MnBi medium at the data spot is raised in excess of the Curie point of the
material (180°C to 360°C). The spot is heated by a focused laser beam. During
cooling from the Curie point, the magnetization of the spot can be determined
by an applied external magnetic field.
At room temperature, the MO medium is resistant to changes in magnetiz-
ation. The reverse magnetic field required to reduce the magnetization of the
recording material is called coercivity. The coercivity of MO film at room
temperature is quite high. Some materials, such as phosphorus-doped cobalt