Optical recording medium

Details Optical recording medium Optical recording medium

2001-03-14

2003-06-10

Dinh, Tan (Department: 2653)

Dynamic information storage or retrieval

Storage medium structure

Electrical track structure

C369S126000, C430S001000, C430S290000, C359S003000

Reexamination Certificate

active

06577591

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-098028, filed Mar. 31, 2000, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
The present invention relates to an optical recording medium.
A photorefractive medium is known to the art as one of optical recording media capable of achieving a recording density markedly higher than that of the conventional optical heat phase change type media such as a photomagnetic recording disc or an optical disc. The optorefractive medium, in which data having a large capacity such as a high density image can be recorded, is a medium in which the refractive index of the recording layer is changed by the following mechanism. Specifically, upon irradiation with light, an electric charge is generated within the photorefractive material and the electric charge thus generated is separated in space. The refractive index of the material is changed by the electric field derived from the electric charge distribution. If the electric field generated within the medium is increased, it is possible to obtain a greater change in the refractive index because of the Pockels effect. The photorefractive medium of this type is capable of recording the interference pattern of light directly as a lattice of the refractive index and, thus, is expected to be applied to a holographic memory and to an optical pattern recognition, and holographic associative memories.
In recent years, the photorefractive medium using an organic material is being developed vigorously because the medium can be manufactured easily, as disclosed in, for example, Japanese Patent Publication (Kokoku) No. 6-55901. The photorefractive material using an organic material has a dielectric constant incommensurably smaller than that of an inorganic ferroelectric crystal and is expected to achieve a large performance index and a high response capability. However, in utilizing the particular photorefractive material, it was necessary to mount an electrode for applying a very high electric field from the outside, as disclosed in, for example, Japanese Patent Disclosure (Kokai) No. 6-175167. This is due to the circumstances described below.
Specifically, if the photorefractive medium is irradiated with the interference pattern of light, the carriers the number of which corresponds to the intensity of the light is generated in the photorefractive medium. If an external electric field E
ex
is applied to the photorefractive medium such that the field E
ex
is parallel to the light irradiated plane, the electric field E generated at this time is represented by formula (9) given below:
E=E
0
[(1+
iE
ex
/E
d
)/{1+
iE
ex
/(
E
d
+E
q
)}](
I
1
/I
0
)  (9)
E
0
=iE
d
/(1+
E
d
/E
q
)  (10)
E
d
=(2
&pgr;D
)/(&mgr;&Lgr;)  (11)
E
q
=(
eN
&Lgr;)/(2&pgr;&egr;)  (12)
where I
0
represents a space average of the intensity of the irradiating light, I
1
represents the difference between the maximal value and the minimal value of the intensity of the irradiating light, &Lgr; represents a spatial period at which the light intensity assumes the maximal value, &egr; represents the dielectric constant of the photorefractive medium, N represents the concentration of the space charge, D represents the diffusion coefficient, &mgr; represents the mobility, e represents the elementary charge, and i represents the imaginary unit.
Formulas (9) to (12) given above denote that the phase of the interference pattern of light is deviated from the phase of the electric field E, as described in, for example, “Pochi Yeh, Introduction to Photorefractive Nonlinear Optics, John Wiley & Sons, Inc., 1993, Chapter 3”.
In physics, E
d
represents the electric field produced by the charge diffusion, and E
q
represents the space electric field produced by the ionized impurity and the immovable charge. In general, the Einstein's relation D/&mgr;=kT/e, where k represents the Boltzmann constant and T represents the absolute temperature, is considered to be established between the diffusion coefficient D and the mobility &mgr; and, thus, E
d
is a constant that is not dependent on a substance. Therefore, in order to obtain a large electric field E, it was necessary for E
q
to be sufficiently larger than E
d
and it was also necessary to increase E
ex
.
For making E
q
larger than E
d
, it is necessary to increase the value of &Lgr; or N in formula (12) given above. However, if the value of &Lgr; is increased, the number of sets of the interference patterns recorded in the thickness direction of the film is diminished so as to lower the recording density. On the other hand, where the concentration N of the space charge is increased, a difficulty is generated that the mobility is lowered by the migrating of the charge.
Since the time required for forming the electric field is determined by the drift velocity of the charge, the drop of the mobility implies the drop in the writing rate. It follows that it is necessary to avoid the drop of the mobility as much as possible.
In the substance in which the relationship between the mobility and the diffusion coefficient follows the Einstein's relation, the ratio of the diffusion coefficient to the mobility (D/&mgr;) is very small. For example, in the case of the irradiation of the light intensity pattern of &Lgr;=1 &mgr;m under room temperature 300K, the electric field E
d
due to the diffusion is 0.16 MV/m, which is not sufficiently high. Therefore, where such a substance is used as the photorefractive medium, an electric field not lower than 10 MV/m was applied from the outside, as described in, for example, “W. E. Moerner, and Scott M. Silence, Chem. Rev. 94, pp127-155 (1994)). In a substance having at least 5, preferably at least 10, of the ratio D/&mgr;, it is possible to record the light intensity pattern by generating an internal electric field of this level by extremely diminishing the electric field applied from the outside or, in some cases, without applying the electric field from the outside. Such being the situation, it is of high importance to develop measures for increasing the ratio D/&mgr;.
In the conventional photorefractive medium, however, the ratio D/&mgr; is not sufficiently large, making it impossible to form the space electric field without applying a very high electric field.
BRIEF SUMMARY OF THE INVENTION
An object of the present invention is to provide an optical recording medium capable of recording information with a high recording density by the light irradiation even under the condition that the applied electric field.
According to a first aspect of the present invention, there is provided an optical recording medium having a charge generating ability capable of generating electric charges with different polarities upon irradiation with light and a charge transporting ability capable of transporting at least one of the electric charges to separate specially the electric charges from each other forming an electric field upon irradiation with light, the optical characteristics of the optical recording medium being changed depending on the electric field and the capability of transporting at least one of the electric charges being imparted by a single kind of a charge transporting material,
wherein a light intensity pattern is recorded in the optical recording medium depending on the change in the optical characteristics caused by the electric field, and the electric field is generated by spatially separating the electric charges of the different polarity by light irradiation,
the charge transporting material is formed of a molecule having the charge transporting capability or a polymer containing a monomer unit having the charge transporting capability, and
the charge transporting material has an average intermolecular distance a (nm) and a dipole moment p1 (debye) satisfying at room temperature

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