Process for the reversible optical storage of data

The invention concerns a process for reversible optical information storage by means of a liquid crystal storage medium, whereby the information is stored in a firm of a liquid crystal side chain polymer in an anisotropic phase by means of locally acting heat source in combination with an electric, magnetic or surfaces active field, through local reorientation.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a process for the reversible optical storage of 
data using polymeric liquid crystals. 
2. Discussion of the Background 
Between the solid crystalline phase and the fluid melt, designated 
hereafter as an isotropic melt, intermediate phases appear in certain 
substances, which from a structural and dynamic viewpoint combine 
properties of both the ordered, crystalline state and the disordered melt 
state within themselves. Thus such phases are fluid, but have, for 
example, optical properties which are characteristic of most of the 
crystalline and partially crystalline substances, i.e., they are 
birefringent. They are called for obvious reasons intermediate phases 
(mesophases) or liquid crystal phases. These intermediate phases may be 
obtained by a temperature variation (thermotropic liquid crystals) or in 
solution by means of concentration variations. Only thermotropic liquid 
crystals shall be considered hereinafter. To characterize the range of 
existence of these intermediate phases, the transition temperatures from 
the crystalline state into the liquid crystal state and from the liquid 
crystal state in the isotropic state clearing temperature, determined, for 
example, by calorimetry or by a polarizing microscope, are generally 
given. In case of the presence of different liquid crystal states, a set 
of corresponding transition temperatures is given. 
The appearance of mesphases is associated with peculiarities in their 
molecular geometry. Spherically symmetric molecules cannot form 
mesophases. In contrast, molecules which may be characterized as 
cylindrical or disc-like can form mesophases. The molecules may be rigid 
and the ratio of their maximum to thrir minimum dimension (for example, 
length of cylinder/diameter of cylinder) must clearly exceed a critical 
value of about 3. 
The structure of such mesophases is characterized in that in the simplest 
case for cylindrical molecules, in the so-called nematic phase, the 
molecular centers are distributed in a disordered manner as in an 
isotropic melt, while the longitudinal axes of the molecule are parallel 
to each other. This differs from the state in the isotropic melt, in which 
the molecular axes are statistically distributed. This results in 
anisotropic mechanical, electrical or optical properties. In the 
cholesteric phase, an additional ordering principle is present, i.e., a 
continously helical variation of the direction of orientation of the 
longitudinal molecular axes appears, leading to particular optical 
properties, such as strong optical activity or the selective reflection of 
light. Finally, in the so-called smectic phases there occurs in addition 
to the aforedescribed orientation order characteristic of the nematic 
state, a uniform arrangement of molecular centers of gravity in space, for 
example, along one spatial axis only, or in other smetic modifications 
along two or even three independent axes. In spite of this, these phases 
are fluid. 
Disc shaped molecules are capable of forming so-called discotic phases, 
wherein either only the disc normals are oriented parallel to each other 
(see the nematic phase) or in which the discs are arranged within columns 
in a regular or irregular manner. These are referred to as columnar 
structures. 
A characteristic value which is highly important for the application of 
liquid crystal structures is the orientation order parameter, which is a 
measure of the quality of the orientation order. 
Its value is between 0, in case of complete disorientation (as in the 
isotropic melt), and 1, when all of the molecular longitudinal axes are 
oriented in a perfectly parallel manner. 
The widespread application of liquid crystal substances in industrial 
products, such as display elements in pocket calculators, wrist watches or 
digital measuring instruments, is based on the particular property that 
the direction of orientation, which may be represented by the so-called 
director, is readily varied by external electrical, magnetic or mechanical 
fields. The resulting changes in optical properties may be used in 
combination with other components, such as polarizers, cell walls, etc., 
in display elements for the visualization of information. The cell walls 
serve to protect the fluid mesophases and determine the macroscopic 
configuration of the liquid crystal film required. 
It has been discovered in recent years that in numerous fields of 
application it may be advantageous to combine the properties of liquid 
crystal phases with those of polymers. The advantageous polymer properties 
are good mechanical properties, making possible the production of thin, 
dimensionally stable films of these substances, together with the 
occurrence of a freezing process (glass transition), whereby the fixation 
of a predetermined orientation structure is made possible. The citation of 
the glass temperature (T.sub.g), determined for example, by calorimetry, 
serves to characterize the range of existence of the solid liquid crystal 
phase. Above this temperature the polymer is in a viscoelastic or viscous 
plastic state. 
Theories of the formation of liquid crystal phases generally and the 
formation of such phases in polymer systems in particular, together with 
experimental results, show that liquid crystal polymers can be produced 
from rigid mesogenic structural units which are characteristic of low 
molecular weight liquid crystals, in combination with flexible spacer 
groups and flexible chain molecules. In this process, different structural 
features are possible. The mesogenic groups are in the case of the class 
of side chain liquid crystals attached to a flexible or semiflexible main 
chain, by means of a spacer or even without one. The mesogenic groups may 
be cylindrical or disc-shaped. The main chain may contain mesogenic groups 
separated by flexible units. Copolymers, characterized in that different 
spacer and/or mesogenic groups appear within a single polymer, are also 
capable of forming liquid crystal phases. 
In addition to these side chain liquid crystals, main chain polymers also 
exhibit liquid crystal phases under certain conditions. The conditions for 
this are that either the chains consist entirely of rigid groups, or of 
rigid and flexible groups. Copolymers of different mesogenic groups and/or 
spacer groups can also form liquid crystal phases. Mesogenic groups are of 
a cylindrical or rod-shaped configuration. The nature of the mesophases, 
the range of their existence and that of the glass phase may be adjusted 
approximately by means of the spacer length and flexibility, the 
flexibility of the main chain and its tacticity and length. 
Heretofore only main chain polymers, exclusively with rigid units or 
overwhelmingly with rigid units, have been introduced practically in the 
market. They have extremely high strength and rigidity values. These 
self-strengthening thermoplastic synthetics are known. Their field of 
application consists of mechanical parts requiring extreme mechanical 
properties (Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd ed., 
Vol. 14, pp. 414-421 (1984); J. H. Wendorff, Kunststoffe 73, 524-528 
(1983); M. G. Dobb, J. E. McIntyre, Adv. Polym. Sci. 60/61, 61-98 (1984). 
Polymers with flexible and rigid units have not been used heretofore in 
systems introduced in the market. Their advantage consists of having a 
high orientation parameter value compared to side chain liquid crystals 
(C. Noel, F. Laupretre, C. Friedrich, B. Fagolle, L. Bosio, Polymer 25, 
808-814 (1984); B. Wunderlich, I. Grebowicz, Adv. Polymer Sci. 60/61, 1-60 
(1984). Polymers with mesogenic side chains have also attracted attention 
in recent times (H. Finkelmann in "Polymer Liquid Crystals", Academic 
Press (1982); H. Finkelmann, G. Rehage, Adv. Polym. Science 60/61, 99-172 
(1984); V. P. Shibaev, N. A. Plate, Adv. Polym. Science 60/61, 173-252 
(1984). 
From US 4,293,435 an industrial utilization of the specific behavior of 
liquid crystal polymers, connected with the transition into the glassy 
state, is known. Information is stored by the application of conditions 
which alter the arrangement and orientation of liquid crystal polymers in 
a defined manner (for example, electrical or magnetic fields). This is 
also discussed in GB No. 2,146,787. It is pointed out that the storge of 
the device provided for in U.S. Pat. No. 4,293,435 in the solid state 
below the glass temperature (T.sub.g) signifies that T.sub.g is above the 
usual room temperature (T.sub.a), i.e., that the polymer system is used at 
temperatures which are higher by about 100.degree. C. over T.sub.a, if the 
information is to be stored within a reasonable period of time. Such 
temperatures are unwieldy and lead in the long term to a decomposition of 
the polymer. According to GB No. 2,146,782, these difficulties may be 
avoided by the use of certain polymeric side chain liquid crystals. It is 
then no longer necessary to store the device at a temperature range below 
T.sub.g and stable storage for many years is possible at temperatures 
above T.sub.g and below a temperature (T.sub.f), at which the polymer 
begins to liquify. 
The T.sub.f may be determined by ovserving the passage of light through a 
liquid crystal polymer between two crossed polarizing filters at 
increasing temperatures starting from the glass temperature. A few degrees 
below the smectic-isotropic phase transition the transmission of light 
suddenly increases. This rise is the result of the transition of an 
anisotropic low transparency state to a highly birefringent, transparent 
state of the zone. The temperature range above this temperature T.sub.f is 
designated the "fluid region". The transparency increases with rising 
tempeatures until it attains a maximum at T.sub.m. T.sub.m marks the point 
at which the isotropic (clear) phase first appears. 
Since the appearance of the isotropic phase with crossed polarizers leads 
to the extinction of light, a further increase in temperature results in a 
decrease in the passage of light as the isotropic regions increase, until 
the so-called clearing temperature (T.sub.c) is attained, at which the 
last remnants of the structure responsible for birefringency have 
disappeared. 
GB No. 2,146,787 claims an apparatus with a material layer containing a 
liquid crystal polymer with a mesogenic side chain, together with devices 
for the thermal conversion of at least part of the material from the 
viscous state, in which the temperature of the material is within the 
range of T.sub.g to T.sub.f, to the liquid state. Also claimed are devices 
to affect at least part of the material in the liquid state, whereby a 
selective change in the texture of the molecule in the material is 
effected and information stored, which is retained even after the cooling 
of the liquid region and the return to the viscous state. It is therefore 
an essential condition of GB 2,146,787 to use a polymer material for which 
T.sub.f &gt;T.sub.a &gt;T.sub.g. A device is further described, in which the 
material layer contains a liquid crystal polymer with a smectogenic side 
chain. Particularly preferred are polymeric liquid crystals of the 
polysiloxane type with diphenylcyanogen side chains or benzoric acid ester 
side chains. 
Great interest still exists in optical storage media which in addition to 
high recording densities are also capable of reversible storage. The 
above-described solutions of the problem of optical data storage represent 
relatively narrow technical solutions. Thus, the device according to GB 
No. 2,146,787 is based on the use of liquid crystal side chain polymers 
with the essential condition that the temperature be selected in a manner 
such that the polymeric material is maintained in a viscous state. The 
disclosure extends to polysiloxane liquid crystals, preferably with 
diphenylcyano or benzoic acid ester side chains. The stability of the 
information stored is not unambiguously guaranteed in view of the existing 
molecular mobility and the finite relaxation times and also of possible 
effects on the system, for example, by external interfering fields. There 
remains a need for technical solutions whose limits are not too narrow. 
SUMMARY OF THE INVENTION 
Accordingly, one object of the present invention is to provide a device for 
reversible optical information storage which does not depend on the 
polymer material being in a viscous state. 
Another object of the invention is to provide a device for reversible 
optical information storage which is reliable and not subject to the 
influence of spurious fields. 
A further object of the invention is to provide a device for reversible 
optical information storage in which information can be repeatedly stored 
and read without decomposition of the device. 
Still another object of the invention is to provide a device for reversible 
optical information storage which utilizes polymeric liquid crystals. 
Yet another object of the invention is to provide a device for reversible 
optical information storage which is versatile and can be used in a 
variety of applications such as, for example, optical signal processing, 
Fourier transformation and Fourier convolution, the production of image 
and imaging systems, the generation and storage of holograms, and coherent 
optical correlation techniques. 
These objects and other objects of the present invention which will become 
apparent from the following specification have been achieved by the novel 
process of the present invention which utilizes a device comprising (1) a 
substrate, and (2) a liquid crystal side chain polymer film in contact 
with said substrate, comprising the steps of: 
(a) storing information in said device by loaclly reorienting said polymer 
film; and (b) reading stored information from said said device by 
illuminating said polymer film with coherent, monochromatic light.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A proces for the reversible optical storage of information by means of a 
liquid crystal storage medium has now been discovered, wherein information 
is stored in a film comprised of liquid crystal side chain polymers as the 
storage medium by way of local reorientation. The storage medium which 
contains polymeric liquid crystals with mesogenic side chains is part of 
an apparatus. The apparatus is designed for the entry of information by 
means of selective variation of the spatial order and/or the reorientation 
of the polymeric liquid crystals, wherein the film containing the 
polymeric liquid crystals is in the state prepared for the input of the 
information, i.e., during the entry of the information in the 
dimensionally stable range below the glass temperature (T.sub.g) of the 
polymeric liquid crystal or in the viscoelastic state above the glass 
temperature of the polymeric liquid crystals. The selective variation of 
the spatial arrangement and/or reorientation of the polymeric liquid 
crystals is preferably carried out locally by laser irradiation, whereupon 
the local state of order remains frozen after the discontinuation of the 
laser beam in the temperature range below the T.sub.g of the polymeric 
liquid crystals. 
The requirements relative to the structure of liquid crystal polymers for 
the different orientation processes are documented in the literature. 
Thus, for example, a homotropic orientation in the electrical field 
requires a positive dielectric anisotropy for the frequency range 
employed. Homogeneous orientation, on the other hand, may often be 
produced by the limiting surfaces of structured polyimide. This requires 
mesogenic groups of an anisotropic form. See R. Kelker, R,. Hatz, Handbook 
of Liquid Crystals, Verlag Chemie 1981; Pranoto, W. Haase, Mol. Cryst. 
Liq. Crist. 98, 299-308 (1983), R. Zentel, R. Ringsdorf, Makromol. Chem. 
182, 1245-1256 (1982); Liquid Crystals and Ordered Fluids, A. Griffin and 
J. F. Jonnson, Vol. 4, Plenum Press, New York (1984). 
The polymer liquid crystals usable according to the invention consist 
entirely or in part of repeating units which may be represented 
schematically by general formula (I): 
##STR1## 
wherein A--B are the elements of the main chain of the polymer, X is a 
spacer unit and Y a mesogenic side group. The mesogenic side group are 
responsible for the resultant liquid crystal character. Side chains of the 
smectogenic type are of particular interest. 
The elements A--B correspond preferably to units A'.dbd.B' which are 
radically polymerizable (corresponding monomers: A'.dbd.B'--X--Y). 
The A'.dbd.B' units are preferably vinyl groups such as those present in 
radically polymerizable vinyl compounds, for example 
##STR2## 
units wherein R.sub.1 is hydrogen or methyl Q is a function activating the 
double bond, such as the 
##STR3## 
groups, wherein R.sub.2 is hydrogen or an alkyl with 1 to 6 carbon atoms. 
The spacer group X is a flexible chain with 1-chain elements, preferably an 
alkyl group --(CH.sub.2).sub.n --, wherein n is 1 to 14, and in which 
individual chain elements may be optionally substituted, for example, by a 
halogen such as chlorine, or an ether bridge. 
The mesogenic side chain Y optionally contains a function connecting the 
spacer group X with the mesogenic group M itself, such as, for example, an 
##STR4## 
group, in which R.sub.2 has the same meaning as noted above. 
A compilation of suitable mesogenic side groups Y is found in Kelker and 
Hatz, Handbook of Liquid Crystals, Verlag Chenie, ppl 67-113 (1980). 
The mesoigenic group M is bonded preferably by means of the above-mentioned 
connecting function and contains, for example, aromatic radicals, which 
preferably have the following formula: 
##STR5## 
wherein L is a bridge consisting of the radicals: 
##STR6## 
or the radicals 
##STR7## 
m stands for 0 or 1 and R for a radical 
##STR8## 
or if m is 0, R may also signify the radical 
##STR9## 
and L' and m' are the same as L and m and wherein R.sub.3 is hydrogen, 
(O).sub.r --(CH.sub.2).sub.p H--, --COO(CH.sub.2).sub.p' H, --CN or a 
halogen, in particular fluorine, chlorine or bromine and p and p' are an 
integer from 1 to 8, in particular 1 to 6, and r is 0 or 1. 
Polymers of formula (I), in which M signifies the following mesogenic 
groups, are mentioned especially: 
##STR10## 
Derivatives of (meth)acrylic acid (wherein A'.dbd.B' stands for 
##STR11## 
and derivatives in which the spacer X represents a --(CH.sub.2).sub.n -- 
group with n=1-14, are preferred. 
The polymeric liquid crystal side chain polymers may also be the product of 
a polycondensation. They may belong, for example, to the polyesters or 
polyamides having formula (III): 
##STR12## 
wherein T is a hydrocarban radical, in particular an alkyl group with 1-2 
carbons atoms or an aromatic radical, Z is oxygen or a 
##STR13## 
radical and R.sub.4 a hydrocarbon radical corresponding to the diol of the 
initial compound, with 2 20 carbon atoms, in particular an aromatic, 
specifically a benzyl radical, and --X--Y-- has the significance given for 
general formula (I). Specifically mentioned are polymers wherein the 
mesogenic groups correspond to formula IB and those in which R.sub.3 
stands for a --CN group. Preferred further are spacer groups in which the 
connecting function is an ether group and the spacer group corresponds to 
the formula --[(CH).sub.2 ].sub.6--8 --. 
The preparation of the compound of formula (III) is known. It is related to 
the usual polycondensation processes (Houben-Weyl, 4th ed., Vol. 14/2, 
Georg Thieme Verlag (1961). 
Finally, the units --X--Y may also be introduced by polymer conversion into 
existing polymer chains. Such processes are described, for example, by C. 
M. Paleos et al., in J. Polym. Sci. Polym. Chem. ed. 19, 1427 (1981) and 
H. Finkelmann et al. in Macromol. Chem. Rapid Commun. 1, 31, 733 (1980). 
They may be obtained, for example, by the addition of a suitable compound 
such as, for example, H.sub.2 C.dbd.CH--Y (wherein Y stands for the 
mesogenic group) to a reactive or activatable main chain, for example, to 
poly[oxy(methylsilylene)]. 
In general, the molecular weight of the polymeric liquid crystals are 
within a range of M.sub.W =10.sup.3 to 10.sup.5, generally with 
5,000-200,000, preferably around 100,000 (determined by gel permeation 
chromatography). 
The viscosities of the isotropic phase are generally within a range around 
10.sup.4 poise. 
The glass temperature (T.sub.g) of the liquid crystal polymers used 
according to the invention generally is within -40 to 100.degree. C., in 
particular -10 to 80.degree. C. For glass temperatures see I. Brandrup and 
E. H. Immergut, Polymer Handbook, 2nd ed., II-139, J. Wiler (1975). 
The apparatus 
The process according to the invention is carried out preferably by means 
of the apparatus described below. 
The liquid crystal polymer of the present invention must be adapted with 
regard to its absorption behavior to the wavelength of the recording 
laser. This is effected, for example, either by the admixture of a 
suitable dye or by its polymerization into the polymer chain. In the 
process, the dye itself may possess mesogenic character. Preferably, a 
polymeric liquid crystal may be used, the mesogenic groups of which are 
absorbent in the wavelength range required and which therefore corresponds 
to the extreme case of a mesogenic dye polymerized to 100%. The necessary 
extinction of the storage medium is thus adjusted by means of the dye 
concentration. 
Suitable dyes are known from the literature. Dyes suitable for admixture to 
the liquid crystal phase are those satisfying a series of conditions. (For 
example, B. J. Constant et al., J. Phys.D: Appl. Phys., Vol. 11, pp. 479 
ff (1978), F. Jones et al., Mol. Cryst. Liq. Crystal, Vol. 60, pp. 99 ff, 
1980, EP-A No. 43 904, EP-A No. 55 838, EP-A No. 65 869.) The dyes should 
not be ionized under the effect of an electrical field, should have the 
highest possible molecular extinction coefficient and simultaneously good 
solubility in the liquid crystal matrix used (i.e., the storage medium) 
and must be chemically and photochemically stable. Dyes with such 
properties are found, for example, in the class of anthraquinones (EP-A 
No. 56 492, EP-A No. 44 893, EP-A No. 59 036, EP-A No. 54 217). 
Suitable azo dyes are cited, for example, in DE-A No. 34 06 209. The 
proportion of the dyes in the storage medium is preferably within a range 
of 1 to 50% by weight. 
Polymers with mesogenic groups and dye radicals in the side chains are the 
object of EP-A No. 7 574, EP-A 90 282, EP-A No. 1 40 133. See also H. 
Ringsdorf, H. W. Schmidt, Makromol. Chem. 185, 1327-1334 (1984) and B. 
Reck, H. Ringsdorf, Makromol. Chem. Rapid Commun. 6, 291-299 (1985). 
Similarly to the above-described polymeric liquid crystals of formula (I), 
repeating units 
##STR14## 
may form the main chain elements of the dye containing monomer units. The 
corresponding monomers A'.dbd.B'--X--Y' thus contain the dye radical in 
the Y' group. 
As example of a mesogenic group M simultaneously representing a dye 
radical, the group 
##STR15## 
is cited. Simultaneously a spacer --(CH.sub.2).sub.6 -- is preferred. The 
polymer may be used in principle in the form of a thin layer (film) or a 
laminate, as a coating on a solid or flexible matrix layer. The thickness 
of the film containing the polymeric liquid crystal or consisting of it, 
is preferably between 10.sup.--3 and 10.sup.--6 m. In the embodiment here 
(FIG. 1) the apparatus according to the invention comprises a recording 
cell (1) consisting of two plane-parallel transparent plates (2), 
preferably glass plates suitably spaced apart, generally less than 1 mm, 
preferably about 10 micron. The plate area amounts to a few cm.sup.2 to 
dm.sup.2. The two inner surfaces of the glass plates (2) were coated with 
conducting InO.sub.2 /SnO.sub.2 by vapor deposition and a conductive 
contact with the outside was established. The glass plates (2) prepared in 
this manner were joined together by means of a temperature resistant 
adhesive, for example, a silicone adhesive, so that a cell like, empty 
inner space is formed, with an inlet and an outlet a few mm wide. 
The spacing desired of the two glass plates (2) is secured by means of two 
suitable spacers (3) of the appropriate dimension, preferably made of a 
polyimide plastic. The recording cell futher comprises the electrodes (4). 
Following the drying of the adhesive, the cell is filled on a heated 
deivce with the liquid crystal polymers, preferably of formula (I), in the 
isotropic state. The free space in the cel is filled completely by the 
polymer melt by capillary action. 
The advantage of this process compared with the use of a partially open 
cell is, among others, that the inclusion of air bubbles is reliably 
prevented. Furthermore, standardized cell blanks may be produced at a low 
cost with a geometry variable between certain limits (external dimensions, 
spacings) which then may be filled as needed with the appropriate liquid 
crystal polymers in a second step as described above. Orientation is 
effected in a manner known in itself by the application of an oriented 
field (alignment field), in particular a magnetic and specifically an 
electric field, or by means of surface effects. The necessary orientation 
may be produced further by suitable shearing or drawing. In case of the 
application of an electric field, to the recording cell (1) filled in this 
manner a sinusoidal alternating voltage (V=500 V; freq. =1 kHz) is applied 
at temperatures above T.sub.g and the cell is cooled to room temperature 
while maintaining the applied voltage. The result is an absolutely 
transparent liquid crystal film, which visually does not differ from the 
material in the isotropic state. The glass temperature (T.sub.g) of the 
liquid crystal polymer is higher than the room temperature (T.sub.a). Room 
temperature is assumed to be 20.degree. C. The information may be read by 
illuminating the polymeric film with coherent monochromatic light. To 
store information, different orientations of the liquid crystal polymer 
film are possible in the apparatus of the invention: 
(1) the mesogenic groups are aligned parallel to the surface normal of the 
polymer film in a uniform manner. This may be carried out by the 
application of an alternating electric field to the plates (2) coated with 
(transparent) electrodes, wherein the electric field is parallel in the 
normal of the polymer film layer, by the application of a magnetic field 
or by means of surface treatments. 
(2) The mesogenic groups are oriented parallel or inclined to the film 
plane and parallel to a macroscopically predetermined direction. This may 
be obtained by coating the plates (2) by a suitable material, such as 
polyimide and by structuring this coating along the desired preferential 
orientation, or by a suitable oblique coating for example with silicon 
oxide. The necessary orientation may also be produced by shearing or 
drawing. 
In both cases (1) and (2) orientation takes place in the liquid crystal 
state. 
The orientation is frozen in by cooling to the glass state. The recording 
cell (1) produced as described above represents the storage medium proper 
for the storage of optical information. The variation centers by means of 
a focused laser beam which converts the oriented liquid crystal polymer 
layer locally into the isotropic phase. The locally produced macroscopic 
isotropic region is frozen below the glass temperature. The process is 
preferably carried out as follows. 
According to the invention, the film formed from the liquid crystal polymer 
is heated at the interference maxima of an interferometrically produced 
grid by localized heating from room temperature to the isotropic phase. A 
laser beam, for example light with a wave length of 514.2 nm of an argon 
laser is used. A focused laser beam may also be used, wherein the laser 
beam and the storage medium are moved in a defined manner relative to each 
other. 
The discontinuation of the laser light and subsequent cooling lead to 
stable, disoriented (macroscopically isotropic) regions. The scatter 
centers produced in this manner may be read out as optical information. 
The absorption behavior of the storage medium is preferably chosen so that 
the information may be read in by means of a laser beam of a suitable wave 
length and read out with another laser beam of a different wave length 
without disturbing the information. 
The experimental setup for the evaluation of the storage properties of the 
above-described recording cell is based on a Mach-Zehnder interferometer 
(Encyclopedia of Natural Sciences and Techology, Vol. 2, Moderne Industrie 
Press, 1980). By these means, sinusoidal intensity grids with a line 
spacing of between 100 micron and 1.0 micron, may be produced for the 
superposition of two linearly polarized planar partial waves. By the 
superposition of a planar wave with a sperical wave in combination with a 
convex lens, the intensity distribution similar to a Fresnel zone plate 
may be realized. 
Nonlinear optical effect (Optically induced Frederiks transition) 
The electric field of the linearly polarized laser writing light source 
induces a local modulation of the refractive index on the basis of the 
positive polarizability anisotropy of the liquid crystal molecules, which 
may be read out by a second laser beam (linearly polarized wave of a 
helium-neon laser) as a phase object. This is obtained by means of a 
directional local reorientation of the liquid crystal molecule in the 
optical field. The reorientation (optically induced Frederiks transition) 
takes place above the glass temperature of the liquid crystal polymer in 
an anisotropic phase. During subsequent cooling the refractive index is 
frozen in below T.sub.g as the phase object. The film may remain visually 
entirely clear. 
The temperature may be adjusted externally by heating, wherein the 
frequency of the writing laser is of no decisive importance. Preferably, 
the temperature is set by the absorption of the writing laser and cooling 
is effected by turning off the laser. The writing laser should satisfy two 
conditions simultaneously: 
(a) the optical fireld strength at the interference maxima must be above 
the "Frederiks threshold voltage" for the liquid crystal polymer chosen, 
and 
(b) the intensity must be chosen so thatthe temperature is increased at the 
interference maxima by absorption in a manner such that the polymer is 
heated over the glass temperature but not above the nematic-isotropic 
phase transition (T.sub.NI). 
By the suitable correlation of the laser wave length and the liquid crystal 
polymers different degrees of reorientation are possible, corresponding to 
a continuous phase modulation in addition to purely binary information 
(maximum possible reorientation). 
An aspect of the present process of extreme industrial interest is the 
ability to produce a phase object with high diffraction efficiencies, 
which is highly important for otical analog technology (conventional 
holography, synthetic holography). 
The experimenhtal setup for the evaluation of the storage propoerties of 
the afore-described recording cell is based on a Mach-Zehnder 
interferometer (Encyclopedia of Natural Science and Technology" Vol. 2, 
Moderne Industrie Press 1980). By the superposition of two linearly 
polarized planar partial waves sinusoidal intensity grids may be produced 
with line spacings between 100 micron and 1.0 micron. An intensity 
distribution similar to a Fresnel zone plate is obtained by superposing a 
spherical wave on a planar wave in combination with a convex lens. 
Extinction of stored information 
In principle, the information stored may be erased by an increase in 
temperature (above T.sub.NI) and cooling in an electric or magnetic field. 
The extinction of stored information may be effected locally by an 
increase in temperature and subsequent cooling in an electric or magnetic 
field with the restoration of the original state of orientation in local 
areas. Alternatively, all of the information stored may be erased and the 
original state restored, by increasing the temperature of the storage 
medium and cooling it in an electric or magnetic field. 
The process is preferably carried out as follows. 
Similarly to the preparation of the first writing process the information 
stored in the liquid crystal polymer is erased by heating the recording 
cell (1) above T.sub.g and subsequently cooling it with the application of 
an alternating voltage (V=500 V, freq.=1 kHz). Following multiple 
repetitions of the writing and erasing processes, it was determined that 
none of the steps carried out caused irreversible changes in the recording 
cell. 
Reversible analog data storage 
As set forth above, the application of the nonlinear optical effect makes 
it possible to store data optically in an analog manner, read them 
optically, erase them as needed and store data repeatedly. The data may be 
stored by holographic methods in the storage medium according to the 
invention. As a rule, the information to be stored concerns reproducible 
material structures, for example objects or two-dimensional objects, such 
as printed pages or graphical images. The structure to be stored is 
illuminated by means of a coherent, monochromatic source of light. The 
interference pattern, which is determined by the direction, amplitude and 
phase location of the light from the structure to be stored relative to a 
reference light wave originating in the same source of light, is recorded 
holographically in the preferably macroscopically oriented film of a 
liquid crystal polymer and stored. The thickness of the liquid crystal 
polymer film here again is preferably between 1 and 20 micron. The plane 
parallel transparent plates may be made of transparent plastics such as 
PMMA or preferably of inorganic glasses. 
Preferably, the storage medium contains dyes. The dye molecules may be 
components of the liquid crystal polymer or they may be mixed into the 
storage medium and distributed therein. The glass temperature (T.sub.g) of 
the liquid crystal polymer is higher than the room temperature (T.sub.a). 
The information may be read out by illuminating the polymer film with a 
monochromatic coherent light. Different orientations of the liquid crystal 
polymer film are possible for the storage of the information in the 
apparatus according to the invention (FIG. 1): (1) The mesogenic groups 
are aligned parallel to the surface normal of the polymer film layer in a 
uniform manner. This may be effected by the application of an alternating 
electric field to the plates (2) coated with (transparent) electrodes, 
with the electric field parallel to the normal of the polymer film layer, 
by the application of magnetic field or by a surface treatment. (2) the 
mesogenic groups are oriented parallel or inclined to the plane of the 
film and parallel to a macroscopically predetermined direction. This may 
be carried out either by the coating of the plates (2) with a suitable 
material, such as polyimide, and by the structuring of the coating along 
the preferred orientation desired, or by an oblique coating of the 
substrate with silicon oxide. The necessary orientation may also be 
obtained by suitable shearing or drawing. The orientation is frozen into 
the glass state by cooling. 
The storage is effected in the above-described manner with a laser being 
used as the monochromatic source of light, the wave length of which is 
within the absorption range of the storage medium. Reading is by means of 
a laser the wave length of which is absorbed to a much lesser extent by 
the storage medium. The storing and reading may take place at room 
temperature with the solid film. The information is erased by heating the 
specimen in the anisotropic or isotropic range above the glass 
temperature. 
Reversible digital data storage 
A further embodiment of the invention concerns digital data storage by 
optical means, with here again optical reading and erasing and reentry of 
information being provided. In this process, in the optically clear, 
preoriented liquid crystal polymer film of the storage medium a digital 
phase structure is produced by means of a monochromatic laser beam. The 
laser beam and the storage medium are moved relative to each other in a 
defined manner and the internsity of the laser beam is modulated. The 
stored information is read by the defined relative motion of the storage 
medium and a laser beam of constant intensity and a suitable wave length, 
which leaves the stored information unaffected. 
The technical preparation of the storage medium (alignment of the polymers) 
is similar to the reversible analog data storage. Information storage is 
effected in the above-described manner with a laser being used as the 
monochromatic source of light, the wave length of which is within the 
absorption range of the storage medium. Reading is by means of a laser, 
the wave length of which is absorged to a much lesser extent by the 
storage medium used. Storing and reading may take place at room 
temperature with the solid film. The information is erased by heating the 
specimen in the anisotropic or isotropic range above the glass 
temperature. 
Reversible synthetic holography 
In this process a phase structure is produced in a preoriented liquid 
crystal plastic film by digital means, in the above-described (for 
reversible digital data storage) manner, by the defined relative motion of 
the writing laser beam and storage medium. Reading is effected not as in 
the case of digital storage by a defined relative motion of the reading 
beam and the storabe medium but by the complete illumination of the 
synthetic hologram with a reference wave. The necessary information for 
the determination of the intensity modulation required must be determined 
by calculation. The process described makes it passible to procuce phase 
structures with defined optical properties, such as lenses and the like. 
As this is accomplished by computation in a digital form, complex 
processing processes (glass grinding, polishing) may be substantially 
simplified. The lighter weight of these optical components (eyeglasses, 
lenses) is also highly important. 
Advantageous effects 
The apparatus according to the invention for reversible optical data 
storage is highly suitable for use in the field of reversible digital 
information storage (EDRAW). 
A further extremely interesting potential application is found in the field 
of analog information processing (holography). This field is concerned 
primarily with industrial process control. By means of the optical analog 
technology, important phases of the product control such as recognition, 
sorting and testing, may be carried out on the basis of coherent optical 
correlation very rapidly and efficiently. 
Preparation of the liquid crystal polymers 
The polymers that may be used according to the invention, in particular 
those of formula (I), may be prepared in a known manner. See De-A No. 27 
22589, DE-A No. 28 31 909, DE-A No. 30 20 645, DE-A No. 30 27 757, DE-A 
No. 32 11 400, and EU-A No. 90 282. 
Copending application Ser. No. 262,031 filed Oct. 26, 1988 discloses the 
preparation of liquid crystal polymers and is incorporated herein by 
reference. 
EXAMPLE 1 
Direct connection of the spacer mesogenic groups 
As an example, the preparation of compounds of Type IA is described (V.P. 
Shibaev et al., Eur. Polym. J. 18, 651 (1982). In this process, a compound 
of formula (III) 
EQU Br--(CH.sub.2).sub.n' --COOH) (III) 
wherein n'=n-1is reacted with an organic acid chloride, such as SOCl.sub.2, 
preferably in DMF, to produce compound (IV). 
EQU Br--(CH.sub.2).sub.n' --COCl (IV) 
Compound (IV) is reacted in a Friedel-Crafts reaction, for example in 
nitrobenzene with biphenyl to produce a compound of formula (V) 
EQU Br--(CH.sub.2).sub.n' --CO--C.sub.6 H.sub.4 --C.sub.6 H.sub.5(V) 
which may be reduced to compound (VI) for example with lithium-aluminum 
hydride. 
EQU Br--(CH.sub.2).sub.n --C.sub.6 H.sub.4 --C.sub.6 H.sub.5 (VI) 
This compound is then reacted with Cl.sub.2 CHOC.sub.4 H.sub.9 and titanium 
tetrachloride to form compound (VII). 
##STR16## 
Compound (VII) is converted preferably with hydroxylamine salt in the 
presence of a base such as pyridine to an oxime and the oxime is converted 
into nitrile (VIII) by elimination of water, for example with the aid of 
an anhydride, such as acetic anhydride. 
##STR17## 
By the reaction of a salt of (meth)acrylic acid, for example in DMF with 
compound (VIII), a type IAa compound is obtained. 
##STR18## 
wherein R.sub.1 stands for hydrogen or methyl. 
In a directly analogous manner compound IAb may be obtained for example 
from compound (V) and a salt of (meth)acrylic acid. 
##STR19## 
EXAMPLE 2 
Spacer-mesogenic groups connection by means of an ether group 
As an example. the preparation of a compound of Type IA is described. (V.P. 
Shibaev, loc. cit., N.A. Plate, V.P. Shibaev, J. Polym. Sci. Polym. Symp. 
(IU 1978) 67, 1 (1980). 
Compound (IV) is reduced for example with lithium-aluminum hydride in ether 
to compound 
EQU Br--(CH.sub.2).sub.n --OH (IX) 
which with compound (XI) 
EQU KO--C.sub.6 H.sub.4 --C.sub.6 H.sub.4 --CN (XI) 
is converted for example in methanol to compound (XII). 
EQU HO--(CH.sub.2).sub.n --O--C.sub.6 H.sub.4 --C.sub.6 H.sub.4 --CN(XII) 
The hydroxy compound (XII) is reacted with the chloride of (meth)acrylic 
acid in the presence of an acid acceptor, such as for example a tertiary 
amine to form compound IAc. 
##STR20## 
The following synthesis process permits great variability relative to 
structure IA and IE-IJ (M. Portugall, H. Ringsdorf, R. Zentel, Makromol. 
Chem. 183, 2311 (1982); Ringsdorf, A. Schneller, Brit. Polym. J. 13, 43 
(1981). 
In the process, compound (XIII) 
##STR21## 
is reacted with compound (XIV) in the presence of the base 
EQU HO--(CH.sub.2).sub.n --Cl (XIV) 
to prepare the acid (XV). 
##STR22## 
This acid is reacted with (meth)acrylic acid in the presence of an acid 
catalyst, for example with ptoluenesulfonic acid in chloroform to form the 
(meth)acrylic ester (XVI) 
##STR23## 
Compound (XVI) may be converted similarly to (III) into the acid chloride, 
which may be reacted with phenol or with a phenol substituted in the para 
position to form compound IE 
##STR24## 
or compounds IE, IF, IG, IH, or IJ. 
EXAMPLE 3 
Spacer-mesogenic group connection by an ester group 
As an example, the preparation of a compound of the IA type is described. 
(Plate et al. loc. cit.; Shibaev et al. loc. cit.), but the process is 
generally applicable to the IA-IJ mesogenic radicals. 
Compound (IV) may be reacted with a para-phenol corresponding to one of the 
mesogenic groups, to obtain for example compound (XVII). 
##STR25## 
preferably in the presence of an acid receptor and in an inert solvent, 
such as THF. 
Further conversion is carried out with a salt of (meth)acrylic acid. The 
following synthesis is mentioned as an example of compounds being a chiral 
center of formula (XVIII) 
##STR26## 
is reacted with the bromide of formula (XIX) in the presence of an 
alkaline alcoholate 
##STR27## 
to produce the phenol (XX). 
##STR28## 
By reaction of (XX) with compound (XIV) in ethanol with the addition of a 
base, the alcohol (XXI) is obtained 
##STR29## 
which is esterified with (meth)acrylic acid 
In a manner similar to the (XIV-XV) reaction, the reaction of compound 
(XIV) with the phenol of formula (XXII) in the presence of a base, such as 
for example potassium carbonate or potassium hydroxide, in acetone 
##STR30## 
produces the alcohol of formula (XXIII) 
##STR31## 
which may be converted by reaction with (meth)methacrylic acid chloride 
into a compound of type IB. 
Acid amides of formula (I) may be obtained for example by reacting 
(meth)acrylic acid chloride with the amine of formula (XXIV) 
EQU R.sub.2 --HN--(CH.sub.2).sub.n --COOH (XXIV) 
to obtain (meth)acrylic acid amide (XXV) 
##STR32## 
which may be converted into type ID compounds for example by reaction with 
##STR33## 
in the presence of an inert base with a phenol containing the mesogenic 
group, such as for example 
##STR34## 
Polymerization of the monomers 
In the polymerization of monomers, for example of Type I, state of the art 
polymerization methods may be used. See Houben-Weyl, 4th ed., Vol. 14/1, 
Georg Thieme Press (1961); H. Rauch-Puntigam, Th. Volker "Acryl and 
Methacryl Verbindungen", Springer Berlin (1967); Kirk-Othmer, 3rd ed. Vol. 
18, J. Wiley (1982). Schildknecht, Skeist, Polymerization Processes, Vol. 
29 of "High Polymers", p. 133 Wiley-Interscience (1977). 
Radical polymerization may be used to polymerize the compounds of formula 
(I). As examples, polymerization in solution, in a suspension/emulsion or 
bead polymerization, may be mentioined. 
Conventional radical initiators are used, such a azo- or peroxy-compounds 
(Rauch-Puntigam, Volker, loc. cit. or Brandrup, Immergut, Polymer 
Handbook, loc. cit.) in the amounts specified, for example 0.1 to 1% by 
weight with respect to the monomers. Examples include azoisobutyronitrile 
dibenzoylperoxide and dilauroyl-peroxide. 
Optionally, the polymerization may be controlled by the use of regulators, 
such as for example conventional sulfur regulators, generally in 
proportions of 0.5 to about 2% by weight with respect to the monomers 
(DE-A No. 10 83 548). 
The usual temperatures and processing methods may be employed. 
The process is illustrated below with the example of the (meth)acrylic acid 
derivatives of formula (I), i.e. 
##STR35## 
Polymerization process in solution 
Approximately 0.35 mole of the monomer of formula (I) are dissolved in 850 
ml toluene. 1.8 mmole azoisobutyronitrile is added and the solution is 
heated for about 8 hr under an inert gas at about 333.degree. K. 
The polymer formed is obtained by precipitation with a precipitating agent, 
such as for example 1200 ml of methanol, separated and purified by 
solution in a suitable solvent, such as for example dichloromethane and 
reprecipitation with methanol. 
The material obtained in this manner, which usually is of the consistency 
of a powder, may be dried in a water jet vacuum at about 303.degree. K. to 
constant weight. 
The glass temperature and the clarification temperature are determined by 
thermal analysis. 
Obviously, numerous modifications and variations of the present invention 
are possible in light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims, the invention may 
be practiced otherwise than as specifically described herein.