Page 457 - Introduction to Information Optics

442 8. Information Storage with Optics

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tion band of the crystal until they become trapped once more by Fe ions in
the low-intensity regions.

8.3.7, PHOTOCHROMIC MATERIALS

Photochromic materials have two distinct stable states A and B called color
centers, since they are associated with absorption bands A x and A 2, respectively
[16]. When illuminated with light of l t , the material undergoes a transition
from stable state A to B. After the transition is completed, the material is no
longer sensitive to k v The absorption band of the material has shifted to A 2.
Thus, optical data can be written using light of ij and read out using light of
1 2. However, when the material is illuminated with light of /1 2, the process
reverses and the material reverts to its stable state A from B. The reversal may
also occur naturally in the dark or it may be caused by heating.
Inorganic photochromic materials are often insulators or semiconductors
with a large energy gap between the valence and conduction bands. The
presence of imperfections in the crystal lattice or impurities cause the appear-
ance of additional localized energy levels, A and B, within the bandgap. When
the material is exposed to light of / 1? electrons are excited from level A into
the conduction band and then captured by electron traps in level B. The
trapped electrons can be returned to level A by irradiating the material with
light of /U, which is the energy required to excite an electron from level B into
the conduction band. Since / 2 > /L 15 in the thermally stable state, traps A will
be occupied in preference to traps B.
Holograms may be recorded in photochromic materials either using A t or
A 2. The recorded hologram can be read using the same wavelength /j or A 2.
The reading process is rather complex, and involves refractive index modula-
tion, which accompanies absorption modulation [17]. In organic photo-
chromic materials, the effect generally involves a change of molecular structure,
such as cis-trans photoisomerization. Isomers are compounds possessing the
same composition and the same molecular weight, but differing in their
physical or chemical properties. Cis-trans isomerism is due to different arrang-
ments of dissimilar atoms or groups attached to two atoms joined by a double
bond. While the change in refractive index of photochromic materials is usually
small, photochromism in cis-trans isomers of stilbene and other organic
materials can produce relatively large refractive index changes [18 j.

8.3,8. ELECTRON-TRAPPING MATERIALS

Electron-trapping or ET materials [19] are similar to photochromies in the
sense that they both have two reversible stable states A and B that can be