Information Technology Reference
In-Depth Information
several tens of gigabits per second. Estimates also indicate that in the volume of
3cm
3
, a huge amount of information, hundreds of gigabytes, can be stored.
It should be emphasized that the creation of such a system is a major scientific
and technical challenge, with the memory capacity of currently existing prototypes
not exceeding tens of Kbits.
The development of bacteriorhodopsin-based devices and some other studies in
the 1990s brought nearer the industrial use of molecular media in computer
engineering.
3.4 Storage Devices Based on the Effect of “Hole Burning”
in the Absorption Spectra
Along with the volumetric molecular memory, another way to create ultrahigh
storage capacity is of importance. It is based on the principle of frequency-selective
recording.
The mechanism of optical frequency-selective recording exploits the phenome-
non of hole burning in broad inhomogeneously broadened absorption bands of
organic molecules in solid matrices. The essence of this phenomenon, discovered
and investigated by the school of K. K. Rebane, the R. I. Personov group, and some
other authors, is that a narrow band laser creates a narrow dip in the absorption
spectrum of the material which will persist for a rather long time in darkness and at
low (helium) temperatures. For practical implementation of optical frequency-
selective recording of information, it is necessary to have answers to two questions:
how fast is hole burning and what is its life span.
Extensive experimental data indicate that many organic molecules have no
structural optical spectrum. To explain this fact it is usually suggested that molecules
in an amorphous body or in a solid solution are arranged in arbitrary orientations,
which leads to interactions of different strength with the environment. As a result,
the frequencies of vibronic transitions are shifted somewhat in comparison with
those molecules not interacting with the environment. These transitions correspond
to narrow homogeneously broadened bands
ʓ
0
. A single inhomogeneously broad-
ened band
ʓ
H
, which is the superposition of bands of individual molecules, corre-
sponds to the entire ensemble of molecules. 10
2
-10
3
components contribute to
each inhomogeneously broadened band. Their use for recording information allows
to achieve recording density of 10
9
-10
11
bits/cm
2
(Fig.
3.33
).
Each vibronic state will correspond to the band which can be represented as a
sum of two terms:
I
ðÞ
¼
J
ðÞþʦ ðÞ
,
), having the form of a sharp peak, describes the zero-
phonon line directly corresponding to the vibrations of the atoms of the molecule.
where the function
J
(
ˉ
