Page 459 - Introduction to Information Optics

444 8. Information Storage with Optics

is unstable. The excited molecule immediately evolves to the stable state S3
with emission at about 700 nm. In this mechanism the written data can be read
without erasure. However, the real mechanism may involve other effects, since
the written data has been observed to be partially erased when it is read out
[24]. To completely erase the written data, the two-photon-absorption mater-
ial must be heated to about 50°C or irradiated with infrared light. By raising
the temperature, the molecules in the S3 state revert to the original state SI.
Note that ET materials also demonstrate two-photon absorption [25].

8.3.10. BACTERIORHODOPSIN

Bacteriorhodopsin is a biological photochromic material [26]. It is the
light-harvesting protein in the purple membrane of a microorganism called
Halobacterium halobium. This bacterium grows in salt marshes where the salt
concentration is roughly six times that of seawater. Bacteriorhodopsin film can
be made by drying isolated purple membrane patches onto a glass substrate or
embedding them into a polymer. The use of bacteriorhodopsin-based media is
not restricted to the wild-type protein. A set of biochemical and genetic tools
has been developed to greatly modify the properties of the protein.
Bacteriorhodopsin is the key to halobacterial photosynthesis because it acts
as a light-driven proton pump converting light energy into chemical energy.
The initial state of bacteriorhodopsin is called the B state. After absorption of
a photon at 570 nm, the B state changes to the J state, followed by relaxation
to the K and L states. Finally, the L state transforms into the M state by
releasing a proton. The M state is a stable state, which can be driven back to
the B state by absorption of a photon at 412 nm and capturing a proton.
Reversing the M state to the B state can also be performed by a thermal
process. The B and M states are the two distinct stable photochromic states in
bacteriorhodopsin. Instead of absorption of a photon at 570 nm and 412 nrn
for each transition between the two states, the simultaneous absorption of two
photons at 1140 nm and two photons at 820 nm can also stimulate the
transition [27].

8.3.11. PHOTOCHEMICAL HOLE BURNING

In photochromism, the absorption of light at A l changes the absorption
coefficient at A 2, and vice versa. The material has only two absorption bands
at Aj and x 2, of which only one can be in the activated condition. On the other
hand, in photochemical hole burning, a large number of absorption bands,
theoretically as large as from /i^ to A 1000, can exist at the same time and the
same position [28,29].