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Fig. 3.33 Recording and
reading information in a
volumetric molecular
memory on the basis of
bacteriorhodopsin
The function
) describes the phonon wing which accompanies the zero-phonon
line. It arises as a result of collective molecular vibrations in a solid. Suppose that
the broad band in the spectrum of a chemical compound is the result of superposing
zero-phonon lines corresponding to different local conditions for individual mole-
cules in the matrix. This is possible when the molecule has a spectrum consisting of
a zero-phonon line and a weak phonon wing.
It is in this case that a hole can be burnt in the absorption band by monochro-
matic laser radiation. This phenomenon occurs due to high intensity of the zero-
phonon line if, upon light absorption, there exists nonzero probability that some-
thing will happen to the molecule and it will not return to its exact original state.
Regardless of the specific nature of phototransformation, it is essential that the
energy transition either disappears altogether or has a shift in the spectrum of the
photoproduct exceeding the uniform width of the initial transition. An additional
requirement for observing the effect of hole burning is for phototransformation to
be irreversible on the time scale required for the registration of the spectrum.
Experimental evidence shows that for a number of investigated molecules the
lifetimes of holes are in the order of at least hours (in the dark, at helium temper-
ature). As for the duration of hole burning, in currently known experimental studies,
it is in the range of seconds to tens of seconds. Naturally, this excludes the
possibility of recording sufficiently large volumes of information (albeit at rela-
tively low laser intensities used in experimental studies). However, it was suggested
that the recording time of information can be reduced to 30 ns/bit.
In the 1980s of the last century at the IBM Research Laboratory in San Jose
(California, USA), experimental studies as well as materials science and
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