Photosensitive recording material, photosensitive recording medium, and process for producing hologram using this photosensitive recording medium

A photosensitive recording material comprising an alicyclic, solvent-soluble, thermosetting epoxy oligomer capable of cationic polymerization, the oligomer being of a specified structure, an aliphatic monomer having at least one ethylenically unsaturated bond, the monomer being liquid at normal temperature and pressure, having a boiling point of 100.degree. C. or above at normal pressure and being capable of radical polymerization, a photoinitiator selected from the group consisting of i) a first photoinitiator capable of simultaneously generating a radical species that activates radical polymerization and a Br.o slashed.nsted acid or Lewis acid that activates cationic polymerization, upon exposure to actinic radiation, and ii) a second photoinitiator comprised of a radical polymerization photoinitiator capable of generating a radical species that activates radical polymerization upon exposure to actinic radiation and a cationic polymerization photoinitiator capable of generating a Br.o slashed.nsted acid or Lewis acid that activates cationic polymerization upon exposure to actinic radiation, and a spectral sensitizer that sensitizes the first photoinitiator or second photoinitiator; the aliphatic monomer being mixed in an amount of from 20 parts by weight to 80 parts by weight based on 100 parts by weight of the alicyclic epoxy oligomer. This photosensitive recording material is highly effective for producing a volume type phase hologram having superior diffraction efficiency, transparency, weatherability such as thermal resistance, and chemical stability.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a photosensitive recording material and a 
photosensitive recording medium that are used to form a volume type phase 
hologram, and a process for producing a hologram using such a 
photosensitive recording medium. More particularly, it relates to a 
photosensitive recording material and a photosensitive recording medium 
that have a high sensitivity to visible light, in particular, to argon 
laser light and electron rays, also have superior weatherability and 
storage stability and have good hologram characteristic values for, e.g., 
resolution, diffraction efficiency and transparency, and a process for 
producing a hologram using such a photosensitive recording medium. 
2. Description of the Prior Art 
Holograms enable reproduction of three-dimensional stereoscopic images, and 
hence have been hitherto used in covers of books, magazines or the like, 
pop art display, gifts and so forth on account of their attractive 
designability and decorative effect. Holograms can also be said to be 
equivalent to records of information in submicroscopic units, and hence 
they are also used as marks for preventing forgery of marketable 
securities, credit cards and so forth. 
In particular, in volume type phase holograms, spatial interference fringes 
with differences not in optical absorption but in refractive indexes are 
formed in photosensitive recording mediums, whereby phases can be 
modulated without absorption of light beams passing through images. Hence, 
in recent years, they are not only used for display but also expected to 
be applied in hologram optical elements (HOE) as typified by head-up 
display (HUD) on the windshield of cars. 
Now, recording materials for forming the volume type phase holograms are 
required to be highly sensitive to laser light having visible oscillation 
wavelength and besides to show a high resolution. When actually used in 
forming holograms, they are also required to provide holograms having 
characteristics such as diffraction efficiency, wavelength reproducibility 
of reproducing light, band width (half width of a peak of reproducing 
light) and so forth suited for their purposes. In particular, recording 
mediums for HUD holograms should preferably have the properties that the 
diffraction efficiency is 90% or more at spatial frequency of 5,000 to 
6,000 lines/mm, the half width of a peak of reproducing light (the band 
width) is 20 to 30 nm and the peak wavelength of reproducing wavelength is 
within 5 nm of photographing wavelength, and are also required to have a 
good storage stability over a long period of time. 
The general principle concerning the production of holograms is described 
in some publications and technical books, for example, Junpei Tsujinai, 
"Holographic Display", Chapter 2, Sangyo Tosho Co. According to these, one 
beam of a coherent optical system with dual light fluxes, which is 
commonly a laser, is directed to a recording object, and a photosensitive 
recording medium as exemplified by a photographic dry plate is placed at a 
position where the total-reflected light can be received. In addition to 
the beam reflected from the object, another coherent beam is directed into 
the recording medium directly without striking the object. The beam 
reflected from the object is called the object beam, and the beam directed 
to the recording medium, the reference beam. Interference fringes composed 
of the reference beam and the object beam are recorded as image 
information (a hologram). Next, the recording medium having been processed 
is irradiated by light and viewed at a suitable position, where the light 
from an illumination source is diffracted by the hologram so as to 
reproduce the wave front of the reflected light having first reached the 
recording medium from the object at the time of recording. As a result, an 
object image similar to an actual image of the object is 
three-dimensionally seen. Holograms formed by making the object beam and 
reference beam incident on the recording medium from the same direction 
are known as transmission holograms. In contrast thereto, holograms formed 
by making them each other incident from the opposite side of the recording 
medium are commonly known as reflection holograms. The transmission 
holograms can be obtained by known methods as disclosed in, for example, 
U.S. Pat. No. 3,506,327 and No. 3,894,787. The reflection holograms can be 
obtained by known methods as disclosed in, for example, U.S. Patent No. 
3,532,406. 
As a value for comparing holographic characteristics of holograms formed as 
images, refractive index modulation is used. This is a value calculated 
from the measured diffraction efficiency and recording medium thickness, 
the former being the proportion of incident light diffracted by a 
diffraction grating which is prepared while directly irradiating a 
recording medium in the manner that dual light fluxes are at the same 
angles to the recording medium. The refractive index modulation is a 
quantitative measure of the changes in refractive index that occur at 
exposed areas and unexposed areas of a volume hologram, i.e., the portions 
where light rays interfere with one another to become strong or weak in 
intensity, and can be found by the Kogelnik's theoretical formula (Bell. 
Syst. Tech. J., 48, 2909, 1969). In general, the reflection holograms have 
more interference fringes formed per 1 mm than the transmission holograms 
and hence make it difficult to carry out recording, so that it is 
difficult to obtain a high refractive index modulation. 
As recording materials for such volume type phase holograms, photosensitive 
materials of bleached silver salt and dichromated gelatin types have been 
hitherto commonly used. The dichromated gelatin type photosensitive 
materials are materials most widely used in the recording of volume type 
phase holograms, because of their high diffraction efficiency and low 
noise characteristics. Such photosensitive materials, however, have so 
short a storage lifetime that they must be prepared every time the 
holograms are produced. Also, since the development is carried out by the 
wet process, holograms may undergo deformation in the course of swell and 
shrink of the gelatin required when holograms are produced. Hence, such 
materials have the problem that holograms have a poor reproducibility. As 
for the silver salt photosensitive materials, they require complicated 
processing after recording, and they are photosensitive materials that 
cannot be satisfactory in view of stability and workability. These 
aforesaid photosensitive materials also all have the problem that they 
have inferior environmental properties as exemplified by humidity 
resistance and weatherability. 
To overcome such problems, as materials having superior environmental 
properties and other properties to be possessed by hologram recording 
materials, such as a high resolution and a high diffraction efficiency, 
hologram recording materials making use of poly-N-vinylcarbazole are known 
in the art. For example, a hologram recording material comprising a 
cross-linking agent cyclic cis-.alpha.-dicarbonyl compound and a 
sensitizer (Japanese Patent Application Laid-open No. 60-45283), a 
hologram recording material comprising a 
1,4,5,6,7,7-hexachloro-5-norbornene-2,3-dicarboxylic anhydride and a dye 
(Japanese Patent Application Laid-open No. 60-227280), a hologram 
recording material comprising 2,3-bornanedione and Thioflavine (Japanese 
Patent Application Laid-open No. 60-260080), a hologram recording material 
comprising Thioflavine T and iodoform (Japanese Patent Application 
Laid-open No. 62-123489) and so forth are proposed. Since, however, these 
hologram recording materials still also require the wet-process 
development, they require complicated processing steps and have the 
problem of a poor reproducibility. Since also they are photosensitive 
materials mainly composed of poly-N-vinylcarbazole, though being 
chemically stable and having a high resolution and superior environmental 
properties, the poly-N-vinylcarbazole tends to turn white upon 
crystallization, and has the problems that they have a poor 
reproducibility of transparency and solvents are limited. In addition, 
they are desired to be more improved in view of sensitivity 
characteristics. 
As recording materials capable of being photocured at a high sensitivity, a 
photocuring resin composition used in combination of a 3-ketocoumarin dye 
with a diaryl iodonium salt which are constituents of a 
photopolymerization initiator (Japanese Patent Application Laid-open No. 
60-88005) and also a hologram recording material comprised of such a 
photopolymerization initiator and a support polymer poly(methyl 
methacrylate) in combination (Japanese Patent Application Laid-open No. 
4-31590) are proposed. These recording materials are chemically stable and 
have a high resolution and a high sensitivity, but are accompanied by 
formation of holes or pores on account of wet processing. Hence, they have 
the problems that the peak wavelength of reproducing wavelength becomes 
non-uniform, the half width of the peak wavelength expands and also, when 
developed, uneven development tends to occur because of a more or less 
resolution of supporting polymers in swelling solvent, and still also the 
presence of a large number of holes or pores in the hologram results in 
poor thermal resistance and thermopressure resistance. 
As a measure for overcoming such problems, photopolymerization type 
photosensitive materials that enable production of a hologram through a 
sole processing step without any wet processing are disclosed in U.S. Pat. 
No. 3,993,485 and No. 3,658,526. The former discloses two types of 
photosensitive materials. A first example is a photosensitive resin 
composition comprised of combination of i) two polymerizable ethylenically 
unsaturated monomers having different reactivities and refractive indexes 
with ii) a photopolymerization initiator, as exemplified by a cyclohexyl 
methacrylate, N-vinylcarbazole and benzoin methyl ether system, which is 
held between two glass sheets, followed by exposure using a dual light 
flux optical system to record a hologram. A second example is a 
photosensitive resin composition comprised of four components, i.e., a 
polymerizable ethylenically unsaturated monomer and an ethylenically 
unsaturated monomer acting as a cross-linking agent when the former is 
polymerized, both having substantially the same degree of refractive 
index, a non-reactive compound having a different refractive index than 
the two monomers, and a polymerization initiator, as exemplified by a 
butyl methacrylate, ethylene glycol dimethacrylate, 1-phenylnaphthalene 
and benzoin methyl ether system, which can produce a hologram in the same 
manner as the first example. Whichever photosensitive resin compositions 
are used, the polymerization of monomers having higher reactivity proceeds 
at areas where the interference fringes formed by the dual light flux 
optical system have a strong light intensity and at the same time the 
density gradation occurs in monomers to cause the monomers with a high 
reactivity to be diffused in the areas with a strong light intensity and 
cause the monomers with a low reactivity or non-reactive compounds to be 
diffused in the areas with a weak light intensity. Thus, the interference 
fringes are recorded according to differences in refractive indexes to 
form a volume type phase hologram. 
However, such hologram recording photosensitive resin compositions have had 
the following problems. In the composition shown in the first example, the 
monomers with a low reactivity undergo polymerization to a certain degree, 
and no high refractive index modulation can be obtained. In the second 
example, the non-reactive compound 1-phenylnaphthalene is present in the 
system as a compound with a low molecular weight even after the hologram 
has been finished, resulting in no storage stability. Also, in both the 
examples, since they are mixtures having a low molecular weight and have a 
low viscosity, they can be held between substrates with difficulty or can 
form thick films with difficulty, having many problems on workability and 
reproducibility. 
As for the latter U.S. Pat. No. 3,658,526, it discloses a process for 
producing a stable hologram formed of a hologram recording material 
comprising a polymer matrix incorporated with a photopolymerizable 
ethylenic monomer and a photopolymerization initiator, according to which 
a permanent volume type phase hologram can be obtained by one-time 
exposure to actinic radiation. The hologram thus formed is fixed by 
subsequent overall irradiation with actinic radiation. The hologram 
recording material disclosed in that publication aims at many advantages 
in view of workability and reproducibility, but has a low diffraction 
efficiency. In this hologram recording material, the hologram finished has 
a refractive index modulation ranging from 0.001 to 0.003. As a result, 
the reproduced images of the hologram formed can only have a limited 
brightness. The brightness may possibly be improved to a certain extent by 
increasing the thickness of the hologram recorded layer. This measure to 
solve the problem, however, consequently forces manufacturers to use 
hologram recording materials in a large quantity, and also causes a 
difficulty when holograms are used as fixtures in laminated safety glass 
as in HUD on the windshield of cars. It should be also noted that the 
holograms obtained by this process usually cause a decrease in diffraction 
efficiency after storage for a long time. 
Now, improvement techniques including the production of hologram recording 
materials disclosed in this U.S. Pat. No. 3,658,526 are disclosed in U.S. 
Pat. No. 4,942,112 and No. 5,098,803. These publications disclose a 
composition basically consisting of a thermoplastic resin, a polymerizable 
ethylenically unsaturated monomer and a photopolymerization initiator, 
where a compound having an aromatic ring is used in either the 
thermoplastic resin or the polymerizable ethylenically unsaturated monomer 
in order to improve refractive index modulation, so as to provide a 
difference in refractive index. Since, however, similar to what is 
disclosed in U.S. Pat. No. 3,658,526, a resin with a high molecular weight 
is used as a binder matrix, there is a limit on the diffusibility of 
monomers at the time of exposure, so that a large amount of exposure 
becomes necessary and also no high diffraction efficiency can be obtained. 
To eliminate this disadvantage, a non-reactive plasticizer is added. The 
use of such a plasticizer, however, causes a problem on the film strength 
of the hologram formed, and also such a non-reactive plasticizer is 
present in the system as a compound with a low molecular weight even after 
the hologram has been finished, resulting in no storage stability. In 
addition, since the carrier that holds the monomers and so forth is a 
thermoplastic resin, there is a disadvantage that the hologram has a poor 
thermal resistance. 
As a proposal to eliminate such a disadvantage, Japanese Patent Application 
Laid-open No. 5-107999 discloses a recording material in which the 
plasticizer disclosed in the above patent is replaced with a cationic 
polymerizable monomer and a cationic polymerization initiator so that the 
problems caused by the non-reactive plasticizer remaining after the 
formation of holograms can be solved. 
This recording material, however, requires a reasonable irradiation with 
light to fix the hologram after its formation, and also, at the time of 
fixing, the hologram formed may cause a strain because of diffusion of the 
cationic polymerizable monomer with a low molecular weight to make it 
impossible to obtain a high diffraction efficiency. Since also, similar to 
the prior art thereof, the carrier that holds the monomers and so forth is 
a thermoplastic resin, there is a disadvantage that the hologram has a 
poor thermal resistance. Moreover, in a system where no resin binder is 
used as the carrier for holding them, the recording material can be held 
between substrates with difficulty because of a low viscosity or can form 
thick films with difficulty, having many problems on workability and 
reproducibility. 
Under such technical backgrounds, Japanese Patent Application Laid-open No. 
5-94014 discloses, as an improvement of the recording materials disclosed 
in the above U.S. Pat. No. 4,942,112 and No. 5,098,803 and Japanese Patent 
Application Laid-open No. 5-107999, a hologram photosensitive resin 
composition comprised of an epoxy resin, a radical polymerizable 
ethylenically unsaturated monomer and a radical photopolymerization 
initiator. 
So far as seen in Examples disclosed in the above Japanese Patent 
Application Laid-open No. 5-94014, the hologram photosensitive resin 
composition makes use of two kinds of epoxy resins. When, however, 
ultraviolet-curing epoxy resin is used, troublesome operations are 
required such that the radical polymerization and the cationic 
polymerization are carried out under light with different wavelength 
regions, and also, in order to control the diffusibility of monomers, a 
microadjustment control is required such that the viscosity is increased 
by pre-exposure. Thus, this composition still has the problem of 
difficulties in workability and reproducibility. When thermosetting epoxy 
resin and a curing agent are used, it takes a reasonable 
ultraviolet-curing and heating time to cure the epoxy resin for the 
fixing, resulting in a very poor workability. In addition, the improvement 
technique disclosed in this publication has a great problem that no high 
diffraction efficiency can be obtained. 
As discussed above, the photopolymerization type photosensitive materials 
that enable production of a hologram by the sole processing step without 
any wet processing have the problem on polymerizability and diffusibility 
of monomers for obtaining a high refractive index modulation and the 
problem on storage stability caused by the addition of the monomer-holding 
carrier and the non-reactive additive. In addition, they cannot obtain 
photosensitive recording materials and photosensitive recording mediums 
having a good workability for producing holograms and good holographic 
performances such as hologram diffraction efficiency, transparency and 
reproducibility. Thus, it is still sought to provide a photopolymerizable 
composition improved for the hologram recording. In particular, it can be 
said to be natural to do so with regard to HOEs produced using the same. 
SUMMARY OF THE INVENTION 
The present invention was made taking account of the problems as discussed 
above. Accordingly, an object of the present invention is to provide a 
photosensitive recording material and a photosensitive recording medium 
that are used to form a hologram having superior chemical stability, e.g., 
environmental resistance, in particular, thermal resistance, produced by 
dry processing, and having a high resolution, a high diffraction 
efficiency, a high transparency and a superior reproducing wavelength 
reproducibility, and to provide a process for producing a hologram using 
such a photosensitive recording medium. 
The photosensitive recording material according to the present invention 
comprises as main components: 
an alicyclic, solvent-soluble, thermosetting epoxy oligomer capable of 
cationic polymerization, the oligomer being represented by Formula I: 
##STR1## 
wherein R.sub.1 and R.sub.2 each represent a hydrogen atom, or a 
functional group selected from the group consisting of a methyl group, an 
ethyl group and a trifluoromethyl group; and n is 1 to 20; 
an aliphatic monomer having at least one ethylenically unsaturated bond, 
the monomer being liquid at normal temperature and pressure, having a 
boiling point of 100.degree. C. or above at normal pressure and being 
capable of radical polymerization; 
a photoinitiator selected from the group consisting of i) a first 
photoinitiator capable of simultaneously generating a radical species that 
activates radical polymerization and a Br.o slashed.nsted acid or Lewis 
acid that activates cationic polymerization, upon exposure to actinic 
radiation, and ii) a second photoinitiator comprised of a radical 
polymerization photoinitiator capable of generating a radical species that 
activates radical polymerization upon exposure to actinic radiation and a 
cationic polymerization photoinitiator capable of generating a Br.o 
slashed.nsted acid or Lewis acid that activates cationic polymerization 
upon exposure to actinic radiation; and 
a spectral sensitizer that sensitizes the first photoinitiator or second 
photoinitiator; 
the aliphatic monomer being mixed in an amount of from 20 parts by weight 
to 80 parts by weight based on 100 parts by weight of the alicyclic epoxy 
oligomer. 
The photosensitive recording medium of the present invention comprises: 
a substrate; 
a photosensitive layer formed by coating on the substrate a photosensitive 
solution comprising the above photosensitive recording material, followed 
by drying; and 
an oxygen barrier layer provided on the photosensitive layer. 
The process for producing a hologram using this photosensitive recording 
medium comprises the steps of: 
subjecting the photosensitive layer of the above photosensitive recording 
medium to holographic exposure to form a latent image, substantially 
directly followed by heating at a temperature of from 60.degree. C. to 
120.degree. C. for 1 minute to 30 minutes to produce a volume type phase 
hologram.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention will be described below in detail. 
The photosensitive recording material according to the present invention 
comprises as main components: 
an alicyclic, solvent-soluble, thermosetting epoxy oligomer capable of 
cationic polymerization, the oligomer being structurally specified later; 
an aliphatic monomer having at least one ethylenically unsaturated bond, 
the monomer being liquid at normal temperature and pressure, having a 
boiling point of 100.degree. C. or above at normal pressure and being 
capable of radical polymerization; 
a photoinitiator selected from the group consisting of i) a first 
photoinitiator capable of simultaneously generating a radical species that 
activates radical polymerization and a Br.o slashed.nsted acid or Lewis 
acid that activates cationic polymerization, upon exposure to actinic 
radiation, and ii) a second photoinitiator comprised of a radical 
polymerization photoinitiator capable of generating a radical species that 
activates radical polymerization upon exposure to actinic radiation and a 
cationic polymerization photoinitiator capable of generating a Br.o 
slashed.nsted acid or Lewis acid that activates cationic polymerization 
upon exposure to actinic radiation; and 
a spectral sensitizer that sensitizes the first photoinitiator or second 
photoinitiator; 
the aliphatic monomer being mixed in an amount of from 20 parts by weight 
to 80 parts by weight based on 100 parts by weight of the alicyclic epoxy 
oligomer. 
As the above thermosetting epoxy oligomer, it is possible to use an 
alicyclic epoxy oligomer represented by Formula I: 
##STR2## 
wherein R.sub.1 and R.sub.2 each represent a hydrogen atom, or a 
functional group selected from a methyl group, an ethyl group and a 
trifluoromethyl group; and n is 1 to 20. 
As the aliphatic monomer, it is preferable to use polyethylene glycol 
diacrylate or -methacrylate or polypropylene glycol diacrylate or 
-methacrylate represented by Formula II: 
##STR3## 
wherein R.sub.3 to R.sub.5 each represent a hydrogen atom or a methyl 
group; and m and n are each 0 or more and m+n is 1 to 20. 
The first photoinitiator simultaneously generates a radical species that 
activates radical polymerization and a Br.o slashed.nsted acid or Lewis 
acid that activates cationic polymerization, upon exposure to actinic 
radiation. Compounds which can be used as the first photoinitiator may 
include those selected from iron arene complexes, 
trihalogenomethyl-substituted s-triazines, sulfonium salts, diazonium 
salts, diaryliodonium salts, phosphonium salts, selenonium salts and 
arsonium salts. Specific examples thereof will be shown later. 
The second photoinitiator is specifically a photoinitiator mixture 
comprised of a radical polymerization photoinitiator capable of generating 
a radical species that activates radical polymerization upon exposure to 
actinic radiation and a cationic polymerization photoinitiator capable of 
generating a Br.o slashed.nsted acid or Lewis acid that activates cationic 
polymerization upon exposure to actinic radiation. Specific examples 
thereof will be shown later. 
The spectral sensitizer that sensitizes the first photoinitiator or second 
photoinitiator may include organic compounds selected from cyanine or 
merocyanine derivatives, coumarin derivatives, chalcone derivatives, 
xanthene derivatives, thioxanthene derivatives, azulenium derivatives, 
squarilium derivatives, tetraphenylporphyrin derivatives, 
tetrabenzoporphyrin derivatives and tetrapyrazino derivatives. Specific 
examples thereof will be shown later. 
The photosensitive recording medium of the present invention which is 
obtained using the photosensitive recording material described above 
comprises: 
a substrate; 
a photosensitive layer formed by coating on the substrate a photosensitive 
solution comprising a photosensitive recording material, followed by 
drying; the photosensitive recording material comprising as main 
components an alicyclic, solvent-soluble, thermosetting epoxy oligmer 
capable of cationic polymerization, the oligomer being represented by 
Formula I above, an aliphatic monomer having at least one ethylenically 
unsaturated bond, the monomer being liquid at normal temperature and 
pressure, having a boiling point of 100.degree. C. or above at normal 
pressure and being capable of radical polymerization, a photoinitiator 
selected from the group consisting of i) a first photoinitiator capable of 
simultaneously generating a radical species that activates radical 
polymerization and a Br.o slashed.nsted acid or Lewis acid that activates 
cationic polymerization, upon exposure to actinic radiation, and ii) a 
second photoinitiator comprised of a radical polymerization photoinitiator 
capable of generating a radical species that activates radical 
polymerization upon exposure to actinic radiation and a cationic 
polymerization photoinitiator capable of generating a Br.o slashed.nsted 
acid or Lewis acid that activates cationic polymerization upon exposure to 
actinic radiation, and a spectral sensitizer that sensitizes the first 
photoinitiator or second photoinitiator; the aliphatic monomer being mixed 
in an amount of from 20 parts by weight to 80 parts by weight based on 100 
parts by weight of the alicyclic epoxy oligomer; and 
an oxygen barrier layer provided on the photosensitive layer. 
The process for producing a hologram according to the present invention, 
which produces a volume type phase hologram using this photosensitive 
recording medium, comprises the steps of: 
subjecting a photosensitive layer of a photosensitive recording medium to 
holographic exposure to form a latent image, substantially directly 
followed by heating at a temperature of from 60.degree. C. to 120.degree. 
C. for 1 minute to 30 minutes to produce a volume type phase hologram; the 
photosensitive recording medium comprising; 
a substrate; 
a photosensitive layer formed by coating on the substrate a photosensitive 
solution comprising a photosensitive recording material, followed by 
drying; the photosensitive recording material comprising as main 
components an alicyclic, solvent-soluble, thermosetting epoxy oligomer 
capable of cationic polymerization, the oligomer being represented by 
Formula I above, an aliphatic monomer having at least one ethylenically 
unsaturated bond, the monomer being liquid at normal temperature and 
pressure, having a boiling point of 100.degree. C. or above at normal 
pressure and being capable of radical polymerization, a photoinitiator 
selected from the group consisting of i) a first photoinitiator capable of 
simultaneously generating a radical species that activates radical 
polymerization and a Br.o slashed.nsted acid or Lewis acid that activates 
cationic polymerization, upon exposure to actinic radiation, and ii) a 
second photoinitiator comprised of a radical polymerization photoinitiator 
capable of generating a radical species that activates radical 
polymerization upon exposure to actinic radiation and a cationic 
polymerization photoinitiator capable of generating a Br.o slashed.nsted 
acid or Lewis acid that activates cationic polymerization upon exposure to 
actinic radiation, and a spectral sensitizer that sensitizes the first 
photoinitiator or second photoinitiator; the aliphatic monomer being mixed 
in an amount of from 20 parts by weight to 80 parts by weight based on 100 
parts by weight of the alicyclic epoxy oligomer; and 
an oxygen barrier layer provided on the photosensitive layer. 
Thus, the process for producing a hologram according to the present 
invention, which can produce a bright hologram on account of a high 
refractive index modulation, can be achieved by carrying out suitable 
holographic exposure to form a latent image, substantially directly 
followed by heating at a temperature of from 60.degree. C. to 120.degree. 
C. for 1 minute to 30 minutes. 
Incidentally, the improvement in diffraction efficiency that is 
attributable to the heat curing of alicyclic thermosetting epoxy oligomers 
cannot be expected when overall exposure to ultraviolet rays or other 
actinic radiations such as electron rays, X-rays, visible light rays or 
infrared rays is carried out after the holographic exposure and before the 
heat treatment. Also, since in the present invention the high refractive 
index modulation is attained by the heat treatment, even the refractive 
index modulation of the level as disclosed in U.S. Pat. No. 3,658,526 can 
be enough at the time of the holographic exposure, and such a high 
refractive index modulation as disclosed in U.S. Pat. No. 4,942,112 and 
No. 5,098,803 is not always necessary. 
In the production processes according to the prior art in the above 
publications, it is explicitly stated that the recording medium is exposed 
to actinic radiation as a processing step and also that the refractive 
index of the hologram is controlled in a heating step subsequent to 
overall exposure. Hence, the photosensitive recording material and 
hologram production process according to the present invention in which 
the improvement in diffraction efficiency that is attributable to the heat 
curing of alicyclic thermosetting epoxy oligomers cannot be expected when 
overall exposure to ultraviolet rays or other actinic radiations is 
carried out after the holographic exposure and before the heat treatment, 
are clearly distinguished from the above prior art. 
In the compositional proportion and production process disclosed in 
Japanese Patent Application Laid-open No. 5-94104, no bright hologram can 
be obtained at all, compared with the case when the photosensitive 
recording material according to the present invention is used. Hence, the 
present invention is also distinguished from the prior art disclosed in 
Japanese Patent Application Laid-open No. 5-94104. 
Now, as shown in FIG. 1A, in a photosensitive layer 3 formed of the 
photosensitive recording material according to the present invention, 
aliphatic monomers 32 having at least one ethylenically unsaturated bond, 
being capable of radical polymerization, and alicyclic, solvent-soluble, 
thermosetting epoxy oligomers 31 capable of cationic polymerization are 
uniformly distributed. In the hologram recording, upon exposure of this 
photosensitive layer 3 to laser interference light (i.e., light of the 
dual light flux optical system), the first photoinitiator or second 
photoinitiator in the photosensitive recording material simultaneously 
generates radical species 34 that activate radical polymerization and Br.o 
slashed.nsted acid or Lewis acid 33 that activates cationic 
polymerization, at portions undergoing a strong light interference action 
among laser irradiated portions (FIG. 1B). The radical species 34 
generated here cause the aliphatic monomers 32 to undergo radical 
polymerization. As the monomers become polymerized, the photosensitive 
recording material causes differences in density in its interior, so that 
aliphatic monomers 32 move from the neighborhood to that portions. That 
is, as shown in FIG. 1B, the density of aliphatic monomers 32 becomes 
higher at the portions undergoing a strong light interference action among 
laser irradiated portions and the density thereof becomes lower at the 
portions undergoing a weak light interference action among laser 
irradiated portions. Thus, differences in refractive index are produced 
between both the portions to effect hologram recording, as so presumed. 
After the exposure to laser interference light, a heat treatment is further 
applied, whereupon the Br.o slashed.nsted acid or Lewis acid 33 
simultaneously generated at the portions undergoing a strong light 
interference action during the laser interference light irradiation acts 
to cause the alicyclic thermosetting epoxy oligomers 31 capable of 
cationic polymerization to undergo the cationic polymerization according 
to light intensity distribution, so that presumably a structure with 
different crosslink density is formed (see FIGS. 1C and 1D) and hence this 
contributes to a larger increase in the differences in refractive index 
between the portions undergoing a strong light interference action and the 
portions undergoing a weak light interference action among the laser 
irradiated portions, thus making it possible to obtain a volume type phase 
hologram having a high diffraction efficiency, different from the prior 
art previously discussed. 
The reason why as previously stated the improvement in diffraction 
efficiency that is attributable to the heat curing of alicyclic 
thermosetting epoxy oligomers cannot be expected when overall exposure to 
ultraviolet rays or other actinic radiations is carried out after the 
exposure to laser interference light and before the heat treatment is 
presumed that the Br.o slashed.nsted acid or Lewis acid is uniformly 
generated as a result of the overall exposure and consequently the 
crosslink density becomes uniform. 
The alicyclic thermosetting epoxy oligomer also serving as an image holding 
matrix turns to have a cross-linked structure attributable to the cationic 
polymerization, so that the weatherability of the resulting volume type 
phase hologram can be improved. 
Since also the first photoinitiator or cationic polymerization 
photoinitiator highly effective for generating the Br.o slashed.nsted acid 
or Lewis acid is used in combination with the spectral sensitizer, the 
Br.o slashed.nsted acid or Lewis acid that acts when the alicyclic epoxy 
oligomer serving as an image holding matrix is made to have a cross-linked 
structure can be generated in a higher efficiency, so that the diffraction 
efficiency of the volume type phase hologram after heating can be further 
improved. 
The photosensitive recording material according to the present invention 
has also a better reproducibility on the peak wavelength of reproducing 
light and the band width thereof than conventional photosensitive 
recording materials, and still also has a superior environmental 
properties. Hence, it can be applied to hologram optical devices. 
The present invention will be further described below in greater detail. 
FIG. 2 cross-sectionally illustrates the construction of the photosensitive 
recording medium according to the present invention. FIG. 3 schematically 
illustrates a dual light flux optical system used in the photographing for 
holograms. 
The alicyclic epoxy oligomer of Formula I above which is one of 
constituents of the photosensitive recording material according to the 
present invention may include, though not restricted, those alicyclic 
epoxy compounds of the following formulae. Each such epoxy compound can be 
produced by subjecting one of various compounds derivable from 
hydrogenating bisphenols to condensation with epichlorohydrin. 
##STR4## 
(In the formulas, polymerization degree n is 1 to 20.) 
The alicyclic thermosetting epoxy oligomer according to the invention 
should preferably have an epoxy equivalent weight of from 400 to 6,000. 
Less than 400 in that equivalent weight needs a larger amount of a 
starting alicyclic epoxy oligomer, in uneconomical manner, so as to 
prepare a photosensitive solution at a given viscosity level and would in 
some cases fail to produce a bright hologram. This is because, due to its 
too low molecular weight, the epoxy oligomer tends to get objectionably 
moved at the time of fixing. More than 6,000 in that equivalent weight 
involves not only difficult synthesis of an alicyclic epoxy oligomer with 
uniform molecular weight and good reproducibility, but also insufficient 
diffusion of the associated monomer, hence poor sensitivity, resulting in 
a hologram of reduced brightness. 
The aliphatic monomer being liquid at normal temperature and pressure, 
having a boiling point of 100.degree. C. or above at normal pressure and 
being capable of radical polymerization has at least one ethylenically 
unsaturated bond in its structural unit, and includes monofunctional vinyl 
monomers and besides polyfunctional vinyl monomers, or mixtures thereof. 
It may specifically include high-boiling point vinyl monomers such as 
acrylic or methacrylic acid, itaconic acid, maleic acid, acryl- or 
methacrylamide, diacetone acrylamide and 2-hydroxyethyl acrylate or 
methacrylate; aliphatic polyhydroxyl compounds as exemplified by di- or 
polyacrylic or methacrylic esters such as ethylene glycol, diethylene 
glycol, triethylene glycol, tetraethylene glycol, propylene glycol, 
dipropylene glycol, tripropylene glycol, tetrapropylene glycol, neopentyl 
glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 
1,10-decanediol, trimethylol propane, pentaerythritol, dipentaerythritol, 
sorbitol and mannitol; and alicyclic monomers such as dicyclopentanyl 
acrylate and dimethyloltricyclodecane diacrylate. Preferably, it may 
include a polyethylene glycol diacrylate or dimethacrylate or 
polypropylene glycol acrylate or methacrylate specified above by Formula 
II. 
The first photoinitiator capable of simultaneously generating a radical 
species that activates radical polymerization and a Br.o slashed.nsted 
acid or Lewis acid that activates cationic polymerization, upon exposure 
to actinic radiation, can be exemplified by the compounds disclosed in J. 
Photopolym. Sci. Technol., 2, 283 (1989), and may specifically include 
iron arene complexes, trihalogenomethyl-substituted s-triazines, sulfonium 
salts, diazonium salts, phosphonium salts, selenonium salts, arsonium 
salts and iodonium salts. The diaryliodonium salts may include the 
compounds disclosed in Macromolecules, 10, 1307 (1977), as exemplified by 
chloride, bromide, tetrafluoroborate, hexafluorophosphate, 
hexafluoroarsenate, aromatic sulfonates or the like of diphenyliodonium, 
ditolyliodonium, phenyl(p-anisyl)iodonium, bis(m-nitrophenyl)iodonium, 
bis(p-tert-butylphenyl)iodonium, bis(p-chlorophenyl)iodonium or the like. 
As for the second photoinitiator comprised of a radical polymerization 
photoinitiator capable of generating a radical species that activates 
radical polymerization upon exposure to actinic radiation and a cationic 
polymerization photoinitiator capable of generating a Br.o slashed.nsted 
acid or Lewis acid that activates cationic polymerization upon exposure to 
actinic radiation, it can be exemplified by the following compounds. 
The radical polymerization photoinitiator capable of generating a radical 
species that activates radical polymerization upon exposure to actinic 
radiation may include bisimidazole derivatives such as 
2,2'-bis(o-chlorophenyl)-4,4',5,5'-tetraphenyl-1,1'-bisimidazole and 
(2,4,5-triphenyl)imidazole, N-arylglycine derivatives, and organic azide 
compounds such as 4,4'-diazidochalcone, as well as titanocenes as 
disclosed in Japanese Patent Application Laid-open No. 61-151197 and 
aluminato complexes disclosed in Japanese Patent Application Laid-open No. 
3-209477. Preferred radical polymerization photoinitiators may include 
organic peroxides such as 
3,3',4',4-tetra(tert-butylperoxycarbonyl)benzophenone, and 
N-alkoxypyridinium salts such as 1-methoxy-4-phenylpyridinium 
tetraphenylborate. Without limitation to these examples, other compounds 
may be used so long as the above properties are ensured. 
The cationic polymerization photoinitiator capable of generating a Br.o 
slashed.nsted acid or Lewis acid that activates cationic polymerization 
upon exposure to actinic radiation may include sulfonic esters, 
imidosulfonates, dialkyl-4-hydroxysulfonium salts, 
dialkyl-4-hydroxyphenylsulfonium salts, p-nitrobenzylarylsulfonates, and 
silanol aluminum complexes. Examples thereof include benzoin tosylate, 
pyrogallol trimesylate, o-nitrobenzyl tosylate, 2,5-dinitrobenzyl 
tosylate, N-tosylphthalimide, .alpha.-cyanobenzylidene tosylamine, and 
p-nitrobynzyl-9,10-diethoxyanthracene-2-sulfonate. Without limitation to 
these examples, other compounds may be used so long as the above 
properties are ensured. 
The spectral sensitizer that sensitizes the first photoinitiator or second 
photoinitiator may include organic compounds such as cyanine or 
merocyanine derivatives, coumarin derivatives, chalcone derivatives, 
xanthene derivatives, thioxanthene derivatives, azulenium derivatives, 
squarilium derivatives and porphyrin derivatives. Besides, the dyes and 
sensitizers as disclosed in Shinya Ohkawara et al., "SHIKISO HANDOBUKKU 
(Dye Handbook)", Kodansha Co., 1986; Shinya Ohkawara et al., "KINOUSEI 
SIKISO NO KAGAKU (Chemistry of Functional Dyes)", CMC Co., 1981; and 
Chuzaburo Ikemori et al., "TOKUSHU KINOU ZAIRYO (Special Functional 
Materials)", CMC Co., 1981, may be used. Without limitation to these 
compounds, other dyes and sensitizers may be used so long as they can 
absorb light of visible regions. Specific examples thereof are shown 
below. 
As examples of cyanine or merocyanine derivatives, it is preferable to use, 
but without limitation to, Fluoresceine, Rhodamine, 
2,7-dichlorofluoresceine, 3,3'-dicarboxyethyl-2,2'-thiocyanine bromide, 
anhydro-3,3'-dicarboxyethyl-2,2'-thiocyanine betaine, 
1-carboxymethyl-1'-carboxyethyl-2,2'-quinocyanine bromide, 
anhydro-3,3'-dicarboxyethyl-5,5',9-trimethyl-2,2'-thiacarbocyanine 
betaine, 3,3'-dihydroxyethyl-5,5'-dimethyl-9-ethyl-2,2'-thiacarbocyanine 
bromide, anhydro-3,3'-dicarboxymethyl-2,2'-thiocarbocyanine betaine, 
2-[3-ethyl-4-oxo-5-(1-ethyl-4-quinolinidene)ethylidene-2-thiazolinidene 
methyl]-3-ethylbenzoxazolium bromide, 
3-ethyl-5-[2-(3-ethyl-2-benzothiazolinylidene)ethylidene]rhodanine, 
3-ethyl-5-[2-(3-methyl-2(3H)-thiazolinylidene)ethylidene]-2-thio-2,4-oxazo 
linedione, 3-ethyl-5-(3-ethylbenzothiazolinylidene) rhodanine, 
2-(p-diemthylaminostyryl)-3-ethylbenzothiazolium iodide, 
2-(p-diethylaminostyryl)-1-ethylpyridinium iodide, and 
1,3'-diethyl-2,2'-quinothiacyanine iodide. 
As examples of coumarin derivatives, they may include, but are not limited 
to, 3-(2'-benzimidazole)-7-N,N-diethylaminocoumarin, 
3,3'-carbonylbis(7-diethylaminocoumarin), 3,3'-carbonylbiscoumarin, 
3,3'-carbonylbis(7-methoxycoumarin), 
3,3'-carbonylbis(5,7-dimethoxycoumarin), 
3,3'-carbonylbis(6-methoxycoumarin), 3,3'-carbonylbis(7-acetoxycoumarin), 
3,3'-carbonylbis(5,7-diisopropoxycoumarin), 
3,3'-carbonylbis(5,7-di-n-propoxycoumarin), 
3,3'-carbonylbis(5,7-di-n-butoxycoumarin), 
3,3'-carbonylbis(7-dimethyaminocoumarin), 
7-diethylamino-5',7'-dimethoxy-3,3'-carbonylbiscoumarin), 
3-benzoylcoumarin, 3-benzoyl-5,7-dimethoxycoumarin, 
3-benzoyl-6-methoxycoumarin, 3-benzoyl-7-methoxycoumarin, 
3-benzoyl-8-methoxycoumarin, 3-benzoyl-8-ethoxycoumarin, 
3-benzoyl-6-bromocoumarin, 3-benzoyl-7-dimethylaminocoumarin, 
3-benzoyl-7-diethylaminocoumarin, 3-benzoyl-7-hydroxycoumarin, 
3-acetyl-7-diethylaminocoumarin, 3-acetyl-7-methoxycoumarin, 
3-acetyl-5,7-dimethoxycoumarin, 7-dimethylamino-3-(4-iodobenzoyl)coumarin, 
7-diethylamino-3-(4-iodobenzoyl)coumarin, and 
7-diethylamino-3-(4-diethylaminobenzoyl)coumarin. 
As examples of chalcone derivatives, they may include, but are not limited 
to, the following compounds. 
##STR5## 
As examples of porphyrin derivatives, it is preferable to use, but without 
limitation to, 9,10-dihydroporphyrin, 5,9,15,10-tetramethylporphyrin, 
4,5,14,15-tetrahydro-4,9,14,19-tetramethyl-2,7,12,17-tetrazaporphyrin, 
meso-tetraphenylporphyrin, 4,5,9,10,14,15,19,20-octamethylporphyrin, 
5,9-diacetyl-4,10,14,15,19,20-hexamethylporphyrin, 
5,9-diacetyl-14-ethyl-4,10,15,19,20-pentamethylporphyrin, 
4,9,14,19-tetramethyl-5,10,15,20-tetrapropylporphyrin, 
2-amino-4,5,9,10,14,15,19,20-octaethylporphyrin, 
2-nitro-4,5,9,10,14,15,19,20-octaethylporphyrin, 
meso-diphenyltetrabenzoporphyrin, 4,5-dibromo-9,10-,14,15-, 
19,20-tribenzo-2,7,12,17-tetrazaporphyrin, 
4,5,9,10,14,15,19,20-octaphenylporphyrin, 
tetrakis(3,4-dimethoxyphenyl)porphyrin, 
4,5,9,10,14,15,19,20-octa(p-methoxyphenyl)porphyrin, and copper, cobalt, 
nickel, zinc, platinum, magnesium and like metal complexes thereof. 
These spectral sensitizers may be selected so as to be adapted to the 
wavelengths of radiation sources serving as light sources, according to 
the purposes for which holograms are used. Depending on their uses, two or 
more kinds of them may be used in combination. 
The photosensitive recording material may be further optionally 
incorporated with known additives such as a heat polymerization inhibitor, 
a chain transfer agent and an antioxidant. 
The photosensitive recording material according to the present invention 
comprises, as described above, as main components the alicyclic, 
solvent-soluble, thermosetting epoxy oligomer capable of cationic 
polymerization, the oligomer being specified above by Formula I; the 
aliphatic monomer having at least one ethylenically unsaturated bond, the 
monomer being liquid at normal temperature and pressure, having a boiling 
point of 100.degree. C. or above at normal pressure and being capable of 
radical polymerization; the photoinitiator selected from the group 
consisting of i) the first photoinitiator capable of simultaneously 
generating a radical species that activates radical polymerization and a 
Br.o slashed.nsted acid or Lewis acid that activates cationic 
polymerization, upon exposure to actinic radiation, and ii) the second 
photoinitiator comprised of a radical polymerization photoinitiator 
capable of generating a radical species that activates radical 
polymerization upon exposure to actinic radiation and a cationic 
polymerization photoinitiator capable of generating a Br.o slashed.nsted 
acid or Lewis acid that activates cationic polymerization upon exposure to 
actinic radiation; and the spectral sensitizer that sensitizes the first 
photoinitiator or second photoinitiator; where the aliphatic monomer is 
mixed in an amount of from 20 to 80 parts by weight based on 100 parts by 
weight of the alicyclic thermosetting epoxy oligomer, which may 
particularly preferably be in an amount of from 40 to 70 parts by weight. 
If it is in an amount less than 20 parts by weight, the quantity of 
aliphatic monomers which undergo polymerization upon holographic exposure 
using a laser may become short and hence no high refractive index 
modulation can be obtained even after the heat treatment, making it 
impossible to obtain bright holograms. If it is in an amount more than 80 
parts by weight, the quantity of aliphatic monomers may become excess, so 
that no polymerization may take place upon the initial holographic 
exposure and aliphatic monomers may remain in the system in a large 
quantity. As the result, in the heat treatment in the production steps, 
the residual aliphatic monomers may cause polymerization while diffusing, 
to disturb the interference fringes of holograms once formed, and hence no 
high refractive index modulation can be obtained to make it impossible to 
obtain bright holograms. 
The first photoinitiator or second photoinitiator may be used in an amount 
of from 0.1 to 20 parts by weight, and preferably from 1 to 10 parts by 
weight, based on 100 parts by weight of the alicyclic epoxy oligomer. The 
spectral sensitizer may be used in an amount of from 0.1 to 10 parts by 
weight, and preferably from 0.5 to 2 parts by weight, based on 100 parts 
by weight of the alicyclic epoxy oligomer. The amount of these components 
is governed by the thickness of the photosensitive layer formed and the 
optical density of that layer, and hence the amount may preferably be in 
such a range that the optical density becomes not higher than 2. 
(Alternatively in terms of transmittance, the amount may preferably be in 
such a range that the transmittance of irradiation light at the 
photographing for holograms becomes not less than 1%) If the amount is 
outside this range, it becomes difficult to obtain a high diffraction 
efficiency, also resulting in a lowering of sensitivity characteristics. 
Thus, these components are appropriately selected and mixed in the desired 
proportions to obtain a photosensitive solution, which is then coated in 
film form on a substrate 2 such as a glass plate, a polycarbonate plate, a 
poly(methyl methacrylate) plate or a polyester film by a known coating 
means such as a spin coater, a roll coater or a bar coater to form a 
photosensitive layer 3. The product thus obtained is the photosensitive 
recording medium 1, used in the photographing of holograms according to 
the present invention (see FIG. 2). On the photosensitive layer 3, a 
protective layer 4 is further provided as an oxygen barrier layer. In the 
protective layer 4, the same material as the substrate 2 or an optically 
transparent material, e.g., a plastic of polyolefin, poly(vinyl chloride), 
poly(vinylidene chloride), poly(vinyl alcohol) or polyethylene 
terephthalate, or glass, is used, and the layer is superposed so as to 
hold the photosensitive layer between it and the substrate, or formed by 
lamination using an extruder or the like or by coating a solution of such 
a material. When the photosensitive solution is coated, the solution may 
be optionally diluted with a suitable solvent. In such a case, after 
coated on the substrate, the coating must be dried. The photosensitive 
solution may also preferably be prepared in the manner that the 
transmittance of photographing irradiation light may be not less than 1%. 
The solvent that can be used in the present invention is exemplified by 
dichloromethane, chloroform, acetone, 2-butanone, cyclohexanone, ethyl 
acetate, 2-methoxyethanol, 2-ethoxyethanol, 2-butoxyethanol, 2-ethoxyethyl 
acetate, 2-butoxyethyl acetate, 2-methoxyethyl ether, 2-ethoxyethyl ether, 
2-(2-ethoxyethoxy)ethanol, 2-(2-butoxyethoxy)ethanol, 
2-(2-ethoxyethoxy)ethyl acetate, 2-(2-butoxyethoxy)ethyl acetate, 
1,4-dioxane and tetrahydrofuran. 
FIG. 3 schematically illustrates a dual light flux optical system used in 
the photographing for a hologram, where a laser beam 6 is shed from a 
laser 5, and is directed to the photosensitive recording medium 1 by 
mirrors 7 and a beam splitter 8 through spatial filters 9 and lenses 10. 
In the present invention, after the photographing for a hologram is 
carried out by exposure, the fixing is carried out by a dry process. The 
present invention can also be applied to the production of transmission 
holograms, detailed description and illustration of which are herein 
omitted, and a transmission hologram having a superior holographic 
performance can be obtained. 
When hologram images are recorded in this photosensitive recording medium 
1, laser irradiation is applied in accordance with the desired images. 
More specifically, in the photosensitive layer (photosensitive recording 
material) 3 of the photosensitive recording medium 1, the aliphatic 
monomers capable of radical polymerization and the alicyclic thermosetting 
epoxy oligomers capable of cationic polymerization are uniformly 
distributed. In the hologram recording, upon exposure of this 
photosensitive layer 3 to laser interference light (i.e., light of the 
dual light flux optical system), the first photoinitiator or second 
photoinitiator in the photosensitive recording material of the 
photosensitive layer 3 simultaneously generates radical species that 
activate radical polymerization and Br.o slashed.nsted acid or Lewis acid 
that activates cationic polymerization, at portions undergoing a strong 
light interference action among laser irradiated portions. The radical 
species generated here cause the aliphatic monomers to undergo radical 
polymerization. As the monomers become polymerized, the photosensitive 
recording material causes differences in density in the photosensitive 
layer 3, so that aliphatic monomers move from the neighborhood to those 
portions. That is, the density of aliphatic monomers becomes higher at the 
portions undergoing a strong light interference action among laser 
irradiated portions and the density thereof becomes lower at the portions 
undergoing a weak light interference action among laser irradiated 
portions. Thus, differences in refractive index are produced between both 
the portions to effect hologram recording, as so presumed. 
After the exposure to laser interference light, a heat treatment is further 
applied, whereupon the Br.o slashed.nsted acid or Lewis acid 
simultaneously generated during the laser interference light irradiation 
acts to cause the alicyclic thermosetting epoxy oligomer capable of 
cationic polymerization to undergo the cationic polymerization according 
to light intensity distribution, so that presumably a structure with 
different crosslink density is formed and hence this contributes to a 
larger increase in the differences in refractive index between the 
portions undergoing a strong light interference action and the portions 
undergoing a weak light interference action among the laser irradiated 
portions, thus making it possible to obtain a volume type phase hologram 
having a high diffraction efficiency. 
The alicyclic thermosetting epoxy oligomer also serving as an image holding 
matrix turns to have a cross-linked structure attributable to the cationic 
polymerization, so that the weatherability and chemical stability of the 
resulting volume type phase hologram can be improved. 
Light sources in the step of interference pattern exposure, usable as the 
light source suited for the photosensitive recording material of the 
present invention, include, but are not limited to, a helium cadmium 
laser, an argon laser, a krypton laser and a helium neon laser. 
Thus, the photosensitive recording material according to the present 
invention can achieve in dry processing a superior sensitivity and 
resolution in visible light regions, because it comprises as main 
components the alicyclic, solvent-soluble, thermosetting epoxy oligomer 
capable of cationic polymerization as specified by Formula I, the 
aliphatic monomer having at least one ethylenically unsaturated bond, the 
monomer being liquid at normal temperature and pressure, having a boiling 
point of 100.degree. C. or above at normal pressure and being capable of 
radical polymerization; the photoinitiator selected from the group 
consisting of i) the first photoinitiator capable of simultaneously 
generating a radical species that activates radical polymerization and a 
Br.o slashed.nsted acid or Lewis acid that activates cationic 
polymerization, upon exposure to actinic radiation, and ii) the second 
photoinitiator comprised of a radical polymerization photoinitiator 
capable of generating a radical species that activates radical 
polymerization upon exposure to actinic radiation and a cationic 
polymerization photoinitiator capable of generating a Br.o slashed.nsted 
acid or Lewis acid that activates cationic polymerization upon exposure to 
actinic radiation; and the spectral sensitizer that sensitizes the first 
photoinitiator or second photoinitiator; where the aliphatic monomer is 
mixed in an amount of from 20 to 80 parts by weight based on 100 parts by 
weight of the alicyclic epoxy oligomer. 
Advantageously, the present invention provides a volume type phase hologram 
having superior diffraction efficiency, transparency, weatherability such 
as thermal resistance, and chemical stability. 
Since also the first photoinitiator or cationic polymerization 
photoinitiator highly effective for generating the Br.o slashed.nsted acid 
or Lewis acid is used in combination with the spectral sensitizer, the 
Br.o slashed.nsted acid or Lewis acid that acts when the thermosetting 
epoxy oligomer serving as an image holding matrix is made to have a 
cross-linked structure can be generated in a higher efficiency, so that 
the diffraction efficiency of the volume type phase hologram after heating 
can be further improved. 
Hence, the present invention has the advantage that it can be applied to 
photosensitive recording materials suited for producing hologram optical 
devices required to have a very high performance. 
The present invention will be still further described below in greater 
detail by giving Examples. 
EXAMPLE 1 
To 100 parts by weight of 2-butanone were added 100 parts by weight of an 
alicyclic thermosetting epoxy oligomer ST-5080 (trade name; available from 
Toto Kasei K.K.; degree of polymerization: n=2.9; epoxy equivalent weight: 
550-650), 50 parts by weight of triethylene glycol diacrylate, 5 parts by 
weight of diphenyliodonium hexafluorophosphate and 1 part by weight of 
3,3'-carbonylbis(7-diethylaminocoumarin), and the whole was dissolved to 
prepare a photosensitive solution. This photosensitive solution was coated 
with a 3-mil applicator onto a glass plate to thereby form a 
photosensitive layer. The surface of the photosensitive layer was then 
covered with a film of poly(vinyl alcohol) (PVA) to produce a 
photosensitive recording medium. 
The photosensitive recording medium thus obtained was exposed to light by 
means of the dual light flux optical system for hologram photographing as 
shown in FIG. 3, using an argon laser (514.5 nm) as a light source, 
followed by heating at 100.degree. C. for 30 minutes to produce a 
hologram. 
The diffraction efficiency of the resulting hologram was measured using a 
spectrophotometer manufactured by Nihon Bunko Kogyo K.K. This 
spectrophotometer is so designed that a photomultimeter with a slit of 3 
mm wide can be placed on an area of 20 cm radius in circumference around a 
sample. The diffraction efficiency was measured by allowing monochromatic 
light of a beam width of 0.3 mm to incident at an angle of 45 degrees and 
then by detecting the light diffracted from the sample. The ratio of the 
greatest value other than those of specular reflected light to the value 
measured when the incident light was directly received without the sample 
placed was regarded as diffraction efficiency. The diffraction efficiency 
before heating was also measured in the same manner. Results of the 
evaluation of diffraction efficiency are shown in Table 1. 
EXAMPLES 2 to 6 
The procedure of Example 1 was followed except that the triethylene glycol 
diacrylate (TEGDA) was replaced with diethylene glycol diacrylate (DEGDA), 
neopentyl glycol diacrylate (NPGDA), ethylcarbitol acrylate (EKA), 
1,6-hexanediol diacrylate (HDDA) and triethylene glycol dimethacrylate 
(TEGDMA), respectively, thereby producing holograms of the invention. The 
diffraction efficiencies were measured similarly. Results of the evalution 
are shown in Table 1. 
TABLE 1 
______________________________________ 
Amount of 
Aliphatic exposure Diffraction efficiency (%) 
monomer (mJ/cm.sup.2) 
Before heating 
After heating 
______________________________________ 
Examples: 
1 TEGDA 20 11.2 70.3 
2 DEGDA 20 
18.3 
81.3 
3 NPGDA 20 
7.2 
70.3 
4 EKA 20 
7.9 
69.6 
5 HDDA 20 
10.6 
67.2 
6 TEGDMA 20 20.6 
76.4 
______________________________________ 
TEGDA: Triethylene glycol diacrylate 
DEGDA: Diethylene glycol diacrylate 
NPGDA: Neopentyl glycol diacrylate 
EKA: Ethylcarbitol acrylate 
HDDA: 1,6Hexanediol diacrylate 
TEGDAM: Triethylene glycol dimethacrylate 
EXAMPLES 7 TO 12 
The procedure of Example 1 was followed except that the alicyclic epoxy 
oligomer ST-5080 (trade name; available from Toto Kasei K.K.; degree of 
polymerization: n=2.9; epoxy equivalent weight: 550-650) was replaced with 
an alicyclic thermosetting epoxy oligomer ST-5100 (trade name; available 
from Toto Kasei K.K.; degree of polymerization: n=5.6; epoxy equivalent 
weight: 900-1100), thereby producing holograms of the invention. The 
diffraction efficiencies were measured similarly. Results of the 
evaluation are shown in Table 2. 
TABLE 2 
______________________________________ 
Amount of 
Aliphatic exposure Diffraction efficiency (%) 
monomer (mJ/cm.sup.2) 
Before heating 
After heating 
______________________________________ 
Examples: 
7 TEGDA 30 12.3 79.3 
8 DEGDA 30 18.9. 
85.3 
9 NPGDA 30 10.6 
75.3 
10 EKA 30 
10.8 
74.9 
11 HDDA 30 
12.6 
75.0 
12 TEGDMA 30 14.7 
74.9 
______________________________________ 
TEGDA: Triethylene glycol diacrylate 
DEGDA: Diethylene glycol diacrylate 
NPGDA: Neopentyl glycol diacrylate 
EKA: Ethylcarbitol acrylate 
HDDA: 1,6Hexanediol diacrylate 
TEGDAM: Triethylene glycol dimethacrylate 
EXAMPLES 13 to 17 
The procedure of Example 1 was followed except that the diphenyliodonium 
hexafluorophosphate as a photoinitiator was replaced with diphenyliodonium 
tetrafluoroborate, diphenyliodonium hexafluoroarsenate, diphenyliodonium 
hexafluoroantimonate, an iron arene complex (hexafluorophosphate salt) and 
1,3,5,-trichloromethyl triazine, respectively, thereby producing holograms 
of the invention. The diffraction efficiencies were measured similary. 
Results of the evaluation are shown in Table 3. 
TABLE 3 
______________________________________ 
Amount of 
Photo- exposure 
Diffraction efficiency (%) 
initiator (mJ/cm.sup.2) 
Before heating 
After heating 
______________________________________ 
Examples: 
13 DPITFB 25 9.8 70.6 
14 DPIHFA 20 10.6 
75.9 
15 DPIHFS 20 10.8 
78.9 
16 IAHFP 35 10.9 
72.6 
17 TCT 40 
8.6 
69.8 
______________________________________ 
DPITFB: Diphenyliodonium tetrafluoroborate 
DPIHFA: Diphenyliodonium hexafluoroarsenate 
DPIHFS: Diphenyliodonium hexafluoroantimonate 
IAHFP: Iron arene complex (hexafluorophosphate salt) 
TCT: 1,3,5Trichloromethyl triazine 
EXAMPLES 18 to 22 
The procedure of Example 1 was followed except that the 
3,3'-carbonylbis(7-diethylaminocoumarin) as a spectral sensitizer was 
replaced with 2-benzoyl-3-(p-dimethylaminophenyl)-2-propenenitrile, Rose 
Bengale, 4,4-bis(dimethylamino)benzalacetone, 3,3'-oxacarbocyanine iodide 
and 2,4,6-triphenylthiapyrylium perchlorate, respectively, thereby 
producing holograms of the invention. The diffraction efficiencies were 
measured similarly. Results of the evaluation are shown in Table 4. At the 
time of exposure, a light of 488 nm was used in place of an argon laser of 
514.5 nm. 
TABLE 4 
______________________________________ 
Amount of 
Spectral exposure Diffraction efficiency (%) 
Sensitizer (mJ/cm.sup.2) 
Before heating 
After heating 
______________________________________ 
Examples: 
18 BDMAPPN 30 10.2 68.0 
19 RB 30 
72.4 
20 BDMABA 30 75.1 
21 DEOCCI 30 70.3 
22 TPTPPC 30 65.9 
______________________________________ 
BDMAPPA: 2benzoyl-3-(p-dimethylaminophenyl)-2-propenenitrile 
RB: Rose Bengale 
BDMABA: 4,4bis(dimethylamino)benzalacetone 
DEOCCI: 3,3oxacarbocyanine iodide 
TPTPPC: 2,4,6triphenylthiapyrylium perchlorate 
EXAMPLE 23 
To 100 parts by weight of 2-butanone were added 100 parts by weight of an 
alicyclic thermosetting epoxy oligomer ST-5080 (trade name; available from 
Toto Kasei K.K.; degree of polymerization: 2.9; epoxy equivalent weight: 
550-650), 50 parts by weight of triethylene glycol diacrylate, 5 parts by 
weight of 2,2',5,5'-tetra(tert-butylperoxycarbonyl)benzophenone, 3 parts 
by weight of p-nitrobenzyl-9,10-dianthracene-2-sulfonate and 1 part by 
weight of 3,3'-carbonylbis(7-diethylaminocoumarin), and the whole was 
dissolved to prepare a photosensitive solution. This solution was coated 
with a 3-mil applicator onto a glass plate to thereby form a 
photosensitive layer. The surface of the photosensitive layer was then 
covered with a PVA film to produce a photosensitive recording medium. 
The photosensitive recording medium thus obtained was exposed to light by 
means of the dual light flux optical system for hologram photographing as 
shown in FIG. 3, using an argon laser (514.5 nm) as a light source, 
followed by heating at 100.degree. C. for 30 minutes, thereby producing a 
hologram of the invention. 
The diffraction efficiency of the resulting hologram was measured 
similarly. Results of the evaluation are shown in Table 5. 
EXAMPLES 24 to 28 
The procedure of Example 23 was followed except that the 
3,3'-carbonylbis(7-diethylaminocoumarin) as a spectral sensitizer was 
replaced with 2-benzoyl-3-(p-dimethylaminophenyl)-2-propenenitrile, Rose 
Bengale, 4,4'-bis(dimethylamino)benzalacetone, 3,3'-oxacarbocyanine iodide 
and 2,4,6-triphenylthiapyrylium perchlorate, respectively, thereby 
producing holograms of the invention. The diffraction efficiencies were 
likewuse measured together with those obtained before heating. Results of 
the evaluation are shown in Table 5. At the time of exposure, a light of 
488 nm was used in place of an argon laser of 514.5 nm. 
TABLE 5 
______________________________________ 
Amount of 
Spectral exposure Diffraction efficiency (%) 
Sensitizer (mJ/cm.sup.2) 
Before heating 
After heating 
______________________________________ 
Examples: 
23 KCD 30 9.9 72.2 
24 BDMAPPN 30 8.6 
68.0 
25 RB 30 
9.2 
69.5 
26 BDMABA 30 8.9 
70.8 
27 DEOCCI 30 7.7 
65.3 
28 TPTPPC 30 7.2 
64.1 
______________________________________ 
KCD: 3,3Carbonylbis(7-diethylaminocoumarin) 
BDMAPPN: 2Benzoyl-3-(p-dimethylaminophenyl)-2-propenenitrile 
RB: Rose Bengale 
BDMABA: 4,4bis(Dimethylamino)benzalacetone 
DEOCCI: 3,3oxacarbocyanine iodide 
TPTPPC: 2,4,6Triphenylthiapyrylium perchlorate 
EXAMPLES 29 to 34 
The procedure of Example 23 to 28 was followed except that the 
p-nitrobenzyl-9,10-dianthracene-2-sulfonate as a photoinitiator was 
replaced with DNB-105 (trade name; sulfonic acid ester; available from 
Midori Kagaku K.K.), thereby producing holograms of the invention. The 
diffraction efficiencies for before and after heating were measured 
similarly. Results of the evaluation are shown in Table 6. 
TABLE 6 
______________________________________ 
Amount of 
Spectral exposure Diffraction efficiency (%) 
Sensitizer (mJ/cm.sup.2) 
Before heating 
After heating 
______________________________________ 
Examples: 
29 KCD 30 9.5 71.3 
30 BDMAPPN 30 8.2 67.4 
31 RB 30 8.5 69.6 
32 BDMABA 30 8.1 70.2 
33 DEOCCI 30 7.3 64.9 
34 TPTPPC 30 6.9 60.7 
______________________________________ 
KCD: 3,3Carbonylbis(7-diethylaminocoumarin) 
BDMAPPN: 2benzoyl-3-(p-dimethylaminophenyl)-2-propenenitrile 
RB: Rose Bengale 
BDMABA: 4,4bis(dimethylamino)benzalacetone 
DEOCCI: 3,3oxacarbocyanine iodide 
TPTPPC: 2,4,6triphenylthiapyrylium perchlorate 
All of the holograms provided by the present invention and shown in 
Examples 1 to 34 have proved to be highly satisfactory in respect of 
diffraction efficiency. They caused no decline in such efficiency even 
after standing at 25.degree. C. and at 60% RH for 180 days and also at 
150.degree. C. for 10 hours. 
COMATIVE EXAMPLE 1 
In 100 parts by weight of hydroxypropyl acrylate and 25 parts by weight of 
2-butanone, 35 parts by weight of a bisphenol-A type epoxy oligomer 
EPIKOTE 1001 (EP-1001; trade name; available from Yuka Shell Epoxy K.K.; 
degree of polymerization: n=2.4; epoxy equivalent weight: 450-500) and 14 
parts by weight of an epoxy curing agent FUJICURE FXR-1030 (trade name; 
available from Fuji Kasei Co., Ltd.) were dissolved to prepare a solution, 
to which 5 parts by weight of 
3,3',4,4'-tetra(tert-butylperoxycarbonyl)benzophenone and 0.2 part by 
weight of 3,3'-carbonylbis(7-diethylaminocoumarin) were further added to 
obtain a photosensitive solution. This photosensitive solution was 
interposed between two sheets of transparent glass plates to form a 
photosensitive layer with a layer thickness of 19.5 .mu.m. The 
photosensitive layer thus obtained was irradiated with ultraviolet rays to 
be pre-polymerized until the layer becomes non-fluid, followed by 
holographic exposure in the same manner as in Example 1, and also 
subjected to overall irradiation with ultraviolet rays, followed by 
heating at 80.degree. C. for 30 hours. As a result, a hologram with a 
diffraction efficiency of 35.6% was obtained. 
On account of such hologram characteristic values, it was ascertained that 
the compositional proportions of materials and production process as 
disclosed in Japanese Patent Application Laid-open No. 5-94014 could not 
provide such bright holograms as in Examples of the present invention. 
COMATIVE EXAMPLE 2 
In 100 parts by weight of hydroxypropyl acrylate, 50 parts by weight of a 
cationic polymerization type ultraviolet-curable epoxy resin OPTOMER 
KR-600 (trade name; available from Asahi Denka Kogyo K.K.) was dissolved 
to obtain a solution, to which 5 parts by weight of 
3,3',4,4'-tetra(tert-butylperoxycarbonyl)benzophenone and 0.2 part by 
weight of 3,3'-carbonylbis(7-diethylaminocoumarin) were further added to 
obtain a photosensitive solution. 
The subsequent steps of Comparative Example 1 were followed to form a 
photosensitive layer, which was then subjected to the pre-exposure and the 
holographic exposure, followed by the overall irradiation with ultraviolet 
rays. As a result, a hologram with a diffraction efficiency of 42.3% was 
obtained. 
On account of such hologram characteristic values (diffraction efficiency: 
42.3%), it was ascertained that the compositional proportions of materials 
and production process as disclosed in Japanese Patent Application 
Laid-open No. 5-94014 could not provide such bright holograms as in 
Examples of the present invention.