JP2024071169A - Method for manufacturing optical elements - Google Patents

Abstract

To provide a manufacturing method of an optical element, which suppresses coloring in the optical element in which a hologram and the like are recorded.SOLUTION: A manufacturing method of an optical element includes the steps of: recording and exposing a medium having a recording layer including a polymerizable compound and a photoinitiator represented by a formula (1); and post-exposing the medium after recording and exposure with an exposure amount of 100 J/cm2 or more.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method for manufacturing an optical element on which a hologram or the like is recorded.

Holographic recording media, which have been attracting attention in recent years, are recording media that utilize the interference and diffraction phenomena of light. A hologram is a recording technique that three-dimensionally records inside a recording medium the interference pattern created by the interference fringes of two beams of light called reference light and object light (also called information light or signal light). Holographic recording media contain a photosensitive material in the recording layer, and the photosensitive material undergoes a chemical change in response to the interference pattern, causing local changes in its optical properties, thereby recording the interference fringes.

Holographic recording media have been developed for memory applications, but other applications are being considered, such as the use of AR glass light guide plates as optical elements.
In applications, holographically recorded optical elements used in light guide plates (waveguides) are required to have a wide viewing angle, high diffraction efficiency for light in the visible range, and high medium transparency.
Holographic recording media can be divided into several types depending on the type of optical properties they change. Volume holographic media, which record by creating a refractive index difference in a recording layer that has a certain thickness or more, are considered to be advantageous for use in AR glass light guide plates because they can save space and achieve high diffraction efficiency and wavelength selectivity.

An example of a volume holographic recording medium is a write-once type that does not require wet processing or bleaching, and the recording layer is generally composed of a matrix resin and a photoactive compound dissolved in it. For example, it is known to use a photopolymer that combines a photopolymerization initiator and a polymerizable reactive compound capable of radical polymerization or cationic polymerization as a photoactive compound in a matrix resin for the recording layer (Patent Documents 1 to 4).

When recording a hologram, if there is a recording layer made of a photopolymer in the area where the reference light and the object light intersect to form interference fringes, the photopolymerization initiator undergoes a chemical reaction in the area of high light intensity among the interference fringes to become an active substance, which acts on the polymerizable compound to polymerize it. At this time, if there is a difference in refractive index between the matrix resin and the polymerized compound, the interference fringes become a refractive index difference and are fixed in the recording layer. In addition, when the polymerizable compound polymerizes, the polymerizable compound diffuses from the periphery, and a concentration distribution of the polymerizable compound or its polymer occurs inside the recording layer. By this principle, an interference pattern is recorded as a refractive index difference in the hologram recording medium.
Furthermore, by changing the angle at which the reference light and the object light intersect, different data can be recorded in the same position in an overlapping manner.
Hereinafter, the process of changing the optical characteristics of the recording layer by exposing the recording layer of the medium under predetermined conditions as described above will be referred to as "recording exposure".

On the other hand, when reproducing data, only the reproduction light is used, and the irradiated reproduction light is diffracted according to the interference fringes. In this case, even if the wavelength of the reproduction light does not match the wavelength of the recording light, diffraction occurs if the interference fringes and the Bragg condition are established. Therefore, if the corresponding interference fringes are recorded according to the wavelength and incident angle of the reproduction light to be diffracted, diffraction can be caused for reproduction light over a wide wavelength range, and the display color gamut of the AR glasses can be expanded.

After the recording exposure, a post-exposure step is generally performed in which the entire recording layer is irradiated with light.
That is, when a hologram is recorded on a medium by recording exposure, post-exposure is performed by irradiating light to areas other than the hologram recording area (hereinafter also referred to as "non-recording area") (Patent Documents 5 to 7). In this case, post-exposure is intended to fix the concentration distribution of the polymer formed by recording exposure by promoting polymerization of the photopolymer in the non-recording area where no hologram is recorded. Post-exposure is performed for the purpose of preventing the polymerization reaction of the photopolymer caused by light after this process from occurring, thereby preventing the concentration distribution of the polymer formed by recording exposure from being destroyed and preventing the formation of a new concentration distribution.

One of the problems with optical elements recorded in this way is that the optical element itself appears slightly colored. The coloring problem is mainly caused by light absorption originating from the material that constitutes the recording layer. In particular, in the application of AR glass light guide plates, the required optical performance is strict, and even slight coloring can cause a loss of the amount of light guided through the light guide plate and a decrease in image quality, so it is necessary to prevent this.
In particular, when heating was performed as a deterioration test for evaluating the durability of an optical element in long-term use, the coloring was sometimes amplified.

In addition, Patent Document 8 proposes a compound represented by formula (1) described below, which is preferably used in the present invention as a photopolymerization initiator, as a photopolymerization initiator compound used in a composition for a holographic recording medium that maintains excellent recording sensitivity and storage stability while exhibiting little coloring due to irradiation with light after recording. However, there is no suggestion about the relationship between the amount of exposure in the post-exposure after recording exposure and coloring due to heating.

JP 2021-12249 A JP 2007-34334 A Japanese Patent Application Laid-Open No. 8-160842 JP 2003-156992 A Japanese Patent Application Laid-Open No. 10-187013 JP 2014-209217 A JP 2005-134762 A JP 2019-147789 A

The present invention was made in consideration of these problems, and aims to suppress coloration of optical elements on which holograms and the like are recorded, particularly coloration caused by heating.

As a result of extensive research, the inventors have discovered that coloring in optical elements on which holograms or the like are recorded can be suppressed by performing post-exposure under specific conditions after recording exposure.

That is, the gist of the present invention is as follows.
[1] A method for manufacturing an optical element, comprising: subjecting a medium having a recording layer containing a polymerizable compound and a photopolymerization initiator to recording exposure; and further subjecting the medium after recording exposure to an exposure amount of 100 J/cm2 or ^more to [2] post-exposure.
[2] The method for producing an optical element according to [1], wherein the photopolymerization initiator is a compound represented by the following formula (1):

In formula (1), R ¹ represents an alkyl group;
^R2 represents an alkyl group, an aryl group, or an aralkyl group;
^R3 represents a -( _CH2 ) _n- group, n represents an integer of 1 or more and 6 or less;
^R4 represents a hydrogen atom or an arbitrary substituent;
^R5 represents any substituent that does not have a multiple bond conjugated to the carbonyl group to which it is attached.
[3] The method for producing an optical element according to [1] or [2], wherein the recording exposure is a hologram recording exposure.

The manufacturing method of the optical element of the present invention makes it possible to suppress coloration in the optical element on which a hologram or the like is recorded.

FIG. 1 is a schematic diagram showing an example of an apparatus for recording a hologram onto a medium.

The following describes in detail an embodiment of the method for manufacturing optical elements of the present invention. However, the following description is one example (representative example) of an embodiment of the present invention, and the present invention is not limited to these contents as long as it does not exceed the gist of the invention.

[Method of manufacturing optical element of the present invention]
For example, when manufacturing a holographically recorded optical element according to the present invention, first, a reference beam and an object beam (information beam) are crossed on a medium having a recording layer containing a polymerizable compound and a photopolymerization initiator to form interference fringes, and the interference pattern is recorded and exposed on the recording layer. Furthermore, the medium that has been recorded and exposed is post-exposed with an exposure amount of 100 J/ ^cm2 or more.

In the present invention, the exposure amount means the total exposure energy amount per unit area of light irradiated onto a medium having a recording layer.
Moreover, the light intensity means the intensity (energy) per unit area of light irradiated onto a medium having a recording layer.
In the present invention, the reference light is a light that is a reference when recording an interference pattern on a medium (hereinafter also referred to as "recording exposure"), and is a light that is superimposed with the object light and irradiated onto the recording layer of the medium when exposing the recording layer of the medium. Also, in the present invention, the object light is a light that is irradiated onto the recording layer of the medium in order to holographically record the corresponding interference fringes in the medium by interference with the reference light according to the wavelength and diffraction of the reproduction light required as an optical element used in AR glasses, etc.

<Recording Exposure>
In the present invention, the light intensity used for recording exposure is the sum of the light intensities of the reference light and the object light.
The light intensity during this recording exposure is preferably 2 mW/cm ² or more, more preferably 5 mW/cm ² or more, and further preferably 10 mW/cm ² or more. Within this range, rapid recording exposure is possible, and the manufacturing takt time can be reduced.
On the other hand, the light intensity during this recording exposure is preferably 200 mW/cm ² or less, more preferably 150 mW/cm ² or less, and further preferably 100 mW/cm ² or less. Within this range, the polymerization reaction can be easily controlled by the exposure time, and desired interference fringes tend to be stably recorded.

The exposure dose during recording exposure in the present invention can be arbitrarily selected depending on the type of photopolymerization initiator and the efficiency of the photopolymerization initiator, but is preferably 0.5 J/ ^cm2 or more, more preferably 1 J/ ^cm2 or more. Within this range, the photopolymerization initiator required during recording exposure reacts with light to become an active substance. On the other hand, the exposure dose during recording exposure is preferably 80 J/ ^cm2 or less, more preferably 40
In this range, the tact time of the ^production can be shortened.

The light source that can be used for recording exposure in the present invention can be arbitrarily selected within the absorption wavelength range of the photopolymerization initiator. Particularly suitable light sources include solid-state lasers such as ruby, glass, Nd-YAG, and Nd- _YVO4 ; diode lasers such as GaAs, InGaAs, and GaN; gas lasers such as helium-neon, argon, krypton, excimer, and _CO2 ; and dye lasers having excellent monochromaticity and directivity.

The wavelength of the light source that can be used for recording exposure in the present invention may be in the absorption wavelength range of the photopolymerization initiator, and can be arbitrarily selected from the ultraviolet range to the visible range, but is preferably 300 nm or more, more preferably 350 nm or more. Also, it is preferably 600 nm or less, more preferably 450 nm or less.

<Post-exposure>
In the present invention, the exposure dose in the post-exposure is 100 J/ ^cm2 or more, preferably 200 J/ ^cm2 or more, in order to suppress coloring due to heating. On the other hand, in order to shorten the takt time of production by setting the exposure dose appropriately, the exposure dose in the post-exposure is preferably 5000 J/ ^cm2 or less, more preferably 1000 J/ ^cm2 or less, and even more preferably 500 J/ ^cm2 .

Usually, post-exposure is performed for the purpose of fixing the concentration distribution of the polymer formed by recording exposure by promoting polymerization of the photopolymer in the recorded and non-recorded parts, and preventing the concentration distribution of the polymer formed by recording exposure from being destroyed by light irradiation after this process, so there is no particular consideration given to irradiation with an exposure amount greater than that required for fixing. However, the present inventor has found that when the cause of coloring due to heating is derived from a substance constituting the recording layer, it is possible to suppress coloring after heating by performing post-exposure with an even larger exposure amount in order to render the causative substance harmless.
For this reason, in the present invention, post-exposure is performed with an exposure dose of 100 J/cm ² or more.

In the present invention, the light intensity during post-exposure may be set arbitrarily, but is preferably 2 mW/cm ² or more, more preferably 5 mW/cm ² or more, and further preferably 10 mW/cm ² or more. Within this range, rapid post-exposure is possible, and the tact time of production can be reduced.
On the other hand, the light intensity during this post-exposure is preferably 1000 mW/cm ² or less, more preferably 500 mW/cm ² or less, and even more preferably 200 mW/cm ² or less. Within this range, it is possible to suppress a temperature rise in the medium due to a rapid polymerization reaction caused by post-exposure, and there is a tendency that stable fixation can be achieved without causing disturbance due to a temperature rise in the interference fringes caused by recording.

The post-exposure in the present invention can be carried out by exposing one side of the medium, but it may also be carried out from both sides of the medium, or it may be carried out simultaneously using multiple light sources. This allows the post-exposure of both the recorded and non-recorded areas to be completed in one go in a short period of time, even for holographic recording media with a large area.

In the present invention, it is preferable that at least one of the light sources used for post-exposure is an incoherent light source. By using incoherent light, even when multiple light sources are used, such as in the case of double-sided exposure described above, it is possible to suppress the formation of unnecessary interference fringes in non-recorded areas. Light sources for incoherent light include LEDs, UV lamps, xenon lamps, and mercury lamps, but any light source can be selected as long as it can irradiate light in the absorption wavelength range of the photopolymerization initiator.

The wavelength of the light for post-exposure in the present invention may be within the absorption wavelength range of the photopolymerization initiator, and is preferably 300 nm or more, and more preferably 350 nm or more. Also, it is preferably 600 nm or less, and more preferably 450 nm or less. If it is within this range, the photopolymerization initiator required for post-exposure reacts with the light and becomes an active substance.

[Components of the media]
The medium used in the present invention is useful as a hologram recording medium, and has a recording layer containing at least a polymerizable compound and a photopolymerization initiator. As a composition for forming such a recording layer (hereinafter also referred to as "recording layer forming composition"), for example, a photopolymer is preferably used which is a combination of a polymerizable monomer capable of radical polymerization or cation polymerization as a polymerizable compound, a photopolymerization initiator which promotes the polymerization of the polymerizable monomer, and a polymerization inhibitor which is a substance which inhibits the polymerization of the polymerizable monomer, in an appropriate matrix resin.

[Recording layer]
<Composition for forming holographic recording layer>
The recording layer of the medium according to the present invention is formed from a recording layer-forming composition, which will be described in detail below. Here, the matrix resin is usually present in the recording layer as a crosslinked network structure by polymerizing or crosslinking the recording layer-forming composition after filling the space between the planar substrates, and therefore the recording layer-forming composition contains the matrix resin in the form of a matrix resin composition for forming a matrix resin by polymerization or crosslinking.
In other words, the composition for forming a recording layer according to the present invention can contain a polymerizable monomer, a composition for forming a matrix resin, a photopolymerization initiator, and a polymerization inhibitor, and further a radical scavenger, if necessary. The components of the composition for forming a holographic recording layer are described in detail below.

<<About polymerizable monomers>>
The polymerizable monomer according to the present invention refers to a compound that can be polymerized by a photopolymerization initiator described below. The type of polymerizable monomer is not particularly limited, and can be appropriately selected from known compounds. Examples of the polymerizable monomer include cationic polymerizable monomers, anionic polymerizable monomers, and radical polymerizable monomers. Any of these can be used, or two or more of them can be used in combination.

Examples of cationic polymerizable monomers include epoxy compounds, oxetane compounds, oxolane compounds, cyclic acetal compounds, cyclic lactone compounds, thiirane compounds, thietane compounds, vinyl ether compounds, spiro orthoester compounds, ethylenically unsaturated bond compounds, cyclic ether compounds, cyclic thioether compounds, vinyl compounds, etc. Any one of the cationic polymerizable monomers may be used alone, or two or more of them may be used in any combination and ratio.

Examples of anionically polymerizable monomers include hydrocarbon monomers and polar monomers. Examples of hydrocarbon monomers include styrene, α-methylstyrene, butadiene, isoprene, vinylpyridine, vinylanthracene, and derivatives thereof. Examples of polar monomers include methacrylic acid esters, acrylic acid esters, vinyl ketones, isopropenyl ketones, and other polar monomers. Any one of the anionically polymerizable monomers may be used alone, or two or more of them may be used in any combination and ratio.

Examples of radical polymerizable monomers include compounds having a (meth)acryloyl group, (meth)acrylamides, vinyl esters, vinyl compounds, styrenes, spiro ring-containing compounds, etc. Any one of the radical polymerizable monomers may be used alone, or two or more of them may be used in any combination and ratio. Among the above, compounds having a (meth)acryloyl group are more preferred from the viewpoint of steric hindrance during radical polymerization.

Among the above polymerizable monomers, compounds having halogen atoms (iodine, chlorine, bromine, etc.) in the molecule and compounds having heteroatoms (nitrogen, sulfur, oxygen, etc.) are preferred because they have a high refractive index. Among the above, those having a heterocyclic structure are preferred because they can obtain a higher refractive index.

<<About the matrix resin and the composition for forming the matrix resin>>
The matrix resin according to the present invention refers to a cured product having a crosslinked network structure formed by bonds through a polymerization reaction or a crosslinking reaction, and the matrix resin forming composition refers to a matrix resin precursor before bonds are formed by the polymerization reaction and the crosslinking reaction.

Because the matrix resin has a crosslinked network structure, it moderately suppresses the mobility of the polymerizable monomer and photopolymerization initiator, thereby playing a role in stably maintaining the polymerizable monomer and photopolymerization initiator in a spatially uniformly dispersed state in the recording layer. In addition, the matrix resin prevents the recorded information from being erased by suppressing the diffusion of the polymer through entanglement with the polymer generated in the recording layer. Furthermore, because it has a higher elastic modulus than the liquid state, it plays a role in maintaining the physical shape of the recording layer.

The composition for forming the matrix resin is not particularly limited as long as it can maintain sufficient compatibility with the polymerizable monomer or its polymer, and the photopolymerization initiator even after forming bonds by a polymerization reaction or a crosslinking reaction. Preferably, a compound having at least two or more functional groups selected from the group consisting of an isocyanate group, a hydroxyl group, a mercapto group, an epoxy group, an amino group, and a carboxyl group in the molecule may be used alone or in combination.

Examples of the use of these compounds alone or in combination to realize certain chemical bonds that form a crosslinked network structure include the following (1) to (8).
(1) An isocyanurate bond is formed by reacting compounds having isocyanate groups with each other.
(2) A compound having an isocyanate group is combined with a compound having active hydrogen in the molecule, such as a compound containing a hydroxyl group, a mercapto group, an amino group, or a carboxy group, to form a urethane bond, a thiourethane bond, a urea bond, an amide bond, etc.
(3) A compound having a hydroxyl group is combined with a compound having a carboxyl group to form an ester bond.
(4) A compound having an amino group is combined with a compound having a carboxy group to form an amide bond.
(5) An ether bond is formed by reacting compounds having epoxy groups with each other.
(6) An ether bond is formed by combining an epoxy group and a hydroxyl group.
(7) An amine bond is formed by combining a compound having an epoxy group with a compound having an amino group.
(8) Forming multiple types of bonds including those described above in (1) to (7).

Among these, the combination of a compound having an isocyanate group and a compound having two or more hydroxyl groups in the molecule as isocyanate-reactive functional groups is preferred because it allows greater freedom in selecting the matrix resin structure and is odorless.

Compounds having an isocyanate group include, for example, isocyanic acid, butyl isocyanate, octyl isocyanate, butyl diisocyanate, hexyl diisocyanate (HMDI), isophorodiisocyanate (IPDI), 1,8-diisocyanato-4-(isocyanatomethyl)octane, 2,2,4- and/or 2,4,4-trimethylhexamethylene diisocyanate, isomeric bis(4,4'-isocyanatocyclohexyl)methane and isomeric bis(4,4'-isocyanatocyclohexyl)methane with any isomeric content. Mixtures thereof include isocyanatomethyl-1,8-octane diisocyanate, 1,4-cyclohexylene diisocyanate, isomeric cyclohexanedimethylene diisocyanates, 1,4-phenylene diisocyanate, 2,4- and/or 2,6-toluene diisocyanate, 1,5-naphthalene diisocyanate, 2,4'- or 4,4'-diphenylmethane diisocyanate and/or triphenylmethane 4,4',4"-triisocyanate.

It is also possible to use isocyanate derivatives having a urethane, urea, carbodiimide, acrylic urea, isocyanurate, allophanate, biuret, oxadiazinetrione, uretdione and/or iminooxadiazinedione structure. Any one of these may be used alone, or two or more of them may be used in any combination and ratio.

Examples of compounds having two or more hydroxyl groups in the molecule as isocyanate-reactive functional groups include glycols such as ethylene glycol, triethylene glycol, diethylene glycol, polyethylene glycol, propylene glycol, polypropylene glycol, and neopentyl glycol; diols such as butanediol, pentanediol, hexanediol, heptanediol, and tetramethylene glycol; bisphenols or compounds obtained by modifying these polyfunctional alcohols with polyethyleneoxy chains or polypropyleneoxy chains; triols such as glycerin, trimethylolpropane, butanetriol, pentanetriol, hexanetriol, and decanetriol, and compounds obtained by modifying these polyfunctional alcohols with polyethyleneoxy chains or polypropyleneoxy chains; polyfunctional polyoxybutylene; polyfunctional polycaprolactone; polyfunctional polyester; polyfunctional polycarbonate; polyfunctional polypropylene glycol, and the like. Any one of these may be used alone, or two or more may be used in any combination and ratio.

When forming a matrix resin, for example, a combination of a compound having an epoxy group and a compound having an amino group has a relatively high bond formation reaction rate in the composition for forming a matrix resin, and when left at room temperature, the bond formation reaction progresses in a short period of time to form a matrix resin.
On the other hand, for example, a combination of a compound having an isocyanate group and a compound having a hydroxyl group, which is a combination that allows a high degree of freedom in the selection of a matrix resin structure and is preferred as a composition for forming a matrix resin, does not have a high reaction rate for bond formation, and even if left at room temperature for several days, the bond formation may not be completed and a matrix resin may not be formed. When using such a composition for forming a matrix resin with a low bond formation reaction rate, the bond formation reaction of the composition for forming a matrix resin can be promoted by heating, as in a general chemical reaction. Therefore, depending on the composition for forming a matrix resin, it is preferable to fill the recording layer forming composition between both substrates and then heat it appropriately to form a matrix resin.

Furthermore, the bond-forming reaction of the matrix resin-forming composition can be accelerated by using an appropriate catalyst. Examples of such catalysts include onium salts such as bis(4-t-butylphenyl)iodonium perfluoro-1-butanesulfonate and bis(4-t-butylphenyl)iodonium p-toluenesulfonate; zinc chloride, tin chloride, iron chloride, aluminum chloride, BF Catalysts based on Lewis acids such as ₃ ; protonic acids such as hydrochloric acid and phosphoric acid; amines such as trimethylamine, triethylamine, triethylenediamine, dimethylbenzylamine, and diazabicycloundecene; imidazoles such as 2-methylimidazole, 2-ethyl-4-methylimidazole, and 1-cyanoethyl-2-undecylimidazolylium trimellitate; bases such as sodium hydroxide, potassium hydroxide, and potassium carbonate; tin catalysts such as dibutyltin laurate, dioctyltin laurate, and dibutyltin octoate; and bismuth catalysts such as tris(2-ethylhexanoate), tribenzoyloxybismuth, bismuth triacetate, tris(dimethyldicarbamate), and bismuth hydroxide. The catalyst may be any one of the above alone, or two or more may be used in any combination and ratio.

<<About photopolymerization initiators>>
The photopolymerization initiator according to the present invention is a compound that generates cations, anions, and radicals by irradiation with light, and contributes to the polymerization of the above-mentioned polymerizable monomers. The type of photopolymerization initiator is not particularly limited, and can be appropriately selected depending on the type of polymerizable monomer, etc.

The cationic photopolymerization initiator may be any known cationic photopolymerization initiator. Examples include aromatic onium salts. Specific examples include compounds consisting of an anion component such as SbF ₆ ^- , BF ₄ ^- , AsF ₆ ^- , PF ₆ ^- , CF ₃ SO ₃ ^- , and B(C ₆ F ₅ ) ₄ ^- and an aromatic cationic component containing atoms such as iodine, sulfur, nitrogen, and phosphorus. Among these, diaryliodonium salts and triarylsulfonium salts are preferred. The cationic photopolymerization initiators exemplified above may be used alone or in any combination and ratio of two or more.

Any known anionic photopolymerization initiator can be used as the anionic photopolymerization initiator. Examples include amines. Examples of amines include amino group-containing compounds such as dimethylbenzylamine, dimethylaminomethylphenol, 1,8-diazabicyclo[5.4.0]undecene-7, and derivatives thereof; imidazole compounds such as imidazole, 2-methylimidazole, and 2-ethyl-4-methylimidazole, and derivatives thereof; and the like. Any of the anionic photopolymerization initiators listed above may be used alone, or two or more of them may be used in any combination and ratio.

Any known radical photopolymerization initiator can be used as the radical photopolymerization initiator. Examples include phosphine oxide compounds, azo compounds, azide compounds, organic peroxides, organic borates, onium salts, bisimidazole derivatives, titanocene compounds, iodonium salts, organic thiol compounds, halogenated hydrocarbon derivatives, and oxime ester compounds. The above-listed radical photopolymerization initiators may be used alone or in any combination and ratio of two or more.

Other photopolymerization initiators include imidazole derivatives, oxadiazole derivatives, naphthalene, perylene, pyrene, anthracene, coumarin, chrysene, p-bis(2-phenylethenyl)benzene and derivatives thereof, quinacridone derivatives, coumarin derivatives, aluminum complexes such as Al(C ₉ H ₆ NO) ₃ , rubrene, perimidone derivatives, benzopyran derivatives, rhodamine derivatives, benzothioxanthene derivatives, azabenzothioxanthene, phenylpyridine complexes, porphyrin complexes, polyphenylenevinylene-based materials, and the like.

<<Preferable Photopolymerization Initiator>>
One of the substances that can cause discoloration due to heating is derived from decomposition products of a photopolymerization initiator. However, even when irradiated with light with high exposure energy, this has the characteristic that it is unlikely to become a substance that causes discoloration. Therefore, in the present invention, it is particularly preferable to use a compound represented by the following formula (1) (hereinafter, sometimes referred to as "compound (1)") as the photopolymerization initiator.
The compound (1) may be used alone or as a mixture of two or more kinds in any combination and in any ratio.

In formula (1), R ¹ represents an alkyl group;
^R2 represents an alkyl group, an aryl group, or an aralkyl group;
^R3 represents a -( _CH2 ) _n- group, n represents an integer of 1 or more and 6 or less;
^R4 represents a hydrogen atom or any substituent;
^R5 represents any substituent that does not have a multiple bond conjugated to the carbonyl group to which it is attached.

The following describes each of the substituents in formula (1) and the preferred physical properties of compound (1).

(R ¹ )
R ¹ represents an alkyl group. When R ¹ is a group other than an alkyl group such as an aromatic ring group, the sp ² nature of the nitrogen atom to which R ¹ is bonded is enhanced, and the conjugation from the carbazole ring in formula (1) to R ¹ is extended, so that light emission occurs due to the decomposition product after irradiation with light. Therefore, an alkyl group having no such conjugated system is preferable.

Examples of the alkyl group of R ¹ include linear, branched and cyclic alkyl groups such as methyl, ethyl, propyl, isopropyl, cyclopropyl, butyl, isobutyl, sec-butyl, tert-butyl, pentyl, cyclopentyl, hexyl, cyclohexyl, heptyl, octyl, 2-ethylhexyl, 3,5,5-trimethylhexyl, decyl, dodecyl, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cyclopropylmethyl, cyclohexylmethyl, and 4-butylmethylcyclohexyl. Among these, the number of carbon atoms is preferably 1 or more, more preferably 2 or more, and is preferably 18 or less, more preferably 8 or less. In particular, linear alkyl groups having a carbon number in the range of 1 to 4 are preferred from the viewpoint of the crystallinity of compound (1).

The alkyl group may have a substituent, and examples of the substituent that may be included include an alkoxy group, a dialkylamino group, a halogen atom, an aromatic ring group, a heterocyclic group, etc. Among these, an alkoxy group is preferred in terms of ease of adjusting the solubility of compound (1).
Preferred alkoxy groups include a methoxy group, an ethoxy group, an n-propoxy group, an isopropoxy group, a cyclopentyl group, and a cyclohexyl group.
Moreover, an alkyl group substituted with a fluorine atom is preferable from the viewpoint of increasing water repellency and improving the water resistance of compound (1).

( ^R2 )
^R2 represents any one of an alkyl group, an aryl group, and an aralkyl group. ^R2 is a radical moiety generated when the photopolymerization initiator is irradiated with light, and is preferably an alkyl group having a small molecular weight from the viewpoint of ease of movement of the radical in the composition for holographic recording media.
Specific examples of the alkyl group of ^R2 are the same as those of ^R1 , and the substituents that may be possessed are also the same. Among these, the number of carbon atoms is preferably 1 or more, and is preferably 4 or less, more preferably 2 or less, from the viewpoint of radical mobility.

On the other hand, from the viewpoint of improving the water resistance of the compound (1), ^R2 is preferably an aryl group.
Specific examples of the aryl group of ^R2 include a phenyl group, a naphthyl group, an anthryl group, an indenyl group, etc. From the viewpoint of radical mobility, a phenyl group is preferred.
Specific examples of the aralkyl group of ^R2 include a benzyl group, a phenethyl group, a naphthylmethyl group, etc. Radicals such as a benzyl group and a naphthylmethyl group are preferred because they are expected to have specific reactivity.
Examples of the substituent on the aryl ring of the aryl group or aralkyl group of ^R2 include an alkyl group, a halogen atom, an alkoxy group, etc., but from the viewpoint of radical mobility, no substitution is preferred.

( ^R3 and n)
R ³ represents a -(CH ₂ ) _n - group. When R ³ is a substituent capable of conjugating with the carbazole ring of formula (1), the conjugation of R ³ from the carbazole ring is extended, resulting in a shift in the absorption wavelength to a longer wavelength. Therefore, R ³ is preferably a -(CH ₂ ) _n - group that does not have such a conjugated system.
n represents an integer of 1 or more and 6 or less, and more preferably 2 or more and 4 or less. When n is in the above preferred range, the electronic effect of the substituent ^R4 which is substituted therebefore tends to be more easily reflected.

(R ⁴ )
^R4 represents a hydrogen atom or an arbitrary substituent. The arbitrary substituent is not particularly limited as long as it does not impair the effects of the present invention, and examples of the arbitrary substituent include an alkyl group, an alkoxycarbonyl group, a monoalkylaminocarbonyl group, a dialkylaminocarbonyl group, an aromatic ring group, and a heterocyclic group.
Among these, ^R4 is preferably any one of an alkyl group, an alkoxycarbonyl group, an aromatic ring group, and a heterocyclic group from the viewpoints of availability of raw materials and ease of synthesis, and is particularly preferably an alkoxycarbonyl group from the viewpoint of expression of a remote electronic effect.

Specific examples of the alkyl group of ^R4 are the same as those of ^R1 , and the substituents that may be possessed are also the same. Among these, the number of carbon atoms is preferably 1 or more, and is preferably 8 or less, more preferably 4 or less, from the viewpoint of adjusting the solubility of the compound (1).

Examples of the alkoxycarbonyl group for ^R4 include a methoxycarbonyl group, an ethoxycarbonyl group, and an isopropoxycarbonyl group. The alkoxycarbonyl group preferably has 2 or more carbon atoms and 19 or less, more preferably 7 or less, from the viewpoint of adjusting the solubility of compound (1).
The alkoxycarbonyl group may have a substituent, and examples of the substituent that may be used include an alkoxy group, a dialkylamino group, a halogen atom, etc. Among these, an alkoxy group is preferred in terms of ease of adjusting the solubility of compound (1).

Specific examples of the alkyl group moiety of the monoalkylaminocarbonyl group and dialkylaminocarbonyl group of ^R4 are the same as the alkyl group of ^R1 described above, and the substituents that may be possessed are also the same. Among these, the number of carbon atoms of the alkyl group moiety bonded to the amino group is preferably 1 or more, and preferably 8 or less, more preferably 4 or less. When the number of carbon atoms of the alkyl group moiety bonded to the amino group is within the above-mentioned preferred range, the solubility of the compound (1) tends to be obtained.

Examples of the aromatic ring group of ^R4 include a single ring or a condensed ring such as a phenyl group, a naphthyl group, an anthryl group, etc. Among these, the number of carbon atoms is preferably 6 or more and 20 or less, more preferably 10 or less, from the viewpoint of the solubility of the compound (1).
The aromatic ring group may have a substituent, and examples of the substituent include an alkyl group, an alkoxy group, a dialkylamino group, a halogen atom, etc. Among these, an alkyl group is preferred in terms of ease of adjusting the solubility of compound (1).
Preferred alkyl groups include a methyl group, an ethyl group, an isopropyl group, a cyclohexyl group, an isobutyl group, a 2-ethylhexyl group, and an n-octyl group.

The heterocyclic group of ^R4 is preferably a group containing at least a heterocyclic structure having one or more heteroatoms in the ring. The heteroatoms contained in the heterocyclic group are not particularly limited, and each atom such as S, O, N, and P can be used, but each atom of S, O, and N is preferred from the viewpoints of availability of raw materials and ease of synthesis.

Specific examples of the heterocyclic group include single ring or condensed ring groups such as thienyl, furyl, pyranyl, pyrrolyl, pyridyl, pyrazolyl, thiazolyl, thiadiazolyl, quinolyl, dibenzothiophenyl, and benzothiazolyl. Among these, the number of carbon atoms is preferably 1 or more, more preferably 2 or more, and also preferably 10 or less, more preferably 5 or less, from the viewpoints of availability of raw materials and ease of synthesis.

The heterocyclic group may have a substituent, and examples of the substituent include an alkyl group, an alkoxy group, a dialkylamino group, a halogen atom, etc. Among these, an alkyl group is preferred in terms of ease of adjusting the solubility of compound (1).
Preferred alkyl groups include a methyl group, an ethyl group, an isopropyl group, a cyclohexyl group, an isobutyl group, a 2-ethylhexyl group, and an n-octyl group.

( ^R5 )
^R5 represents any substituent that does not have a multiple bond conjugated to the carbonyl group to which it is bonded. These groups prevent the conjugated system from extending beyond the carbazole ring. Therefore, the conjugated system of the decomposition product generated after light irradiation does not become longer, so the absorption wavelength of the decomposition product does not become longer. This provides an absorption suppression effect after recording.
Examples of ^R5 include an alkyl group, an alkoxy group, a dialkylamino group, an alkylsulfonyl group, and a dialkylaminosulfonyl group. Among these, the alkyl group is preferred in terms of the stability of the compound (1).

Specific examples of the alkyl group for ^R5 are the same as those for ^R1 described above, and the substituents that may be possessed by R5 are also the same. Among these, from the viewpoint of ease of production of compound (1), it is preferable that the alkyl group has a carbon number of preferably 1 or more, more preferably 2 or more, and is preferably 18 or less, more preferably 10 or less, and is a branched alkyl group. Specifically, a cyclopropyl group, a cyclobutyl group, a cyclopentyl group, a cyclohexyl group, a cycloheptyl group, an isopropyl group, a tert-butyl group, an adamantyl group, and a 1-ethylpentyl group are particularly preferable.

Specific examples of the alkyl moiety of the alkoxy group, dialkylamino group, alkylsulfonyl group, and dialkylaminosulfonyl group of ^R5 are the same as those of ^R1 , and the substituents that may be possessed are also the same. Among these, the number of carbon atoms is preferably 3 or more, and preferably 18 or less, more preferably 8 or less, from the viewpoint of the crystallinity of compound (1).

(Preferable properties of compound (1))
The absorption wavelength range of compound (1) is preferably 340 nm or more, more preferably 350 nm or more, and also preferably 700 nm or less, more preferably 650 nm or less. For example, when the light source used for exposure is a blue laser, it is preferable that the absorption is at least at 350 to 430 nm, and when the light source is a green laser, it is preferable that the absorption is at least at 500 to 550 nm. When the absorption wavelength range is different from the above range, it becomes difficult to efficiently use the irradiated light energy for the photopolymerization reaction, and the sensitivity tends to be easily reduced.

The compound (1) preferably has a molar absorption coefficient of 10 L·mol ^-1 ·cm ^-1 or more, more preferably 50 L·mol ^-1 ·cm ^-1 or more, at the recording wavelength of the hologram. Also, it is preferably 20000 L·mol ^{- 1} ·cm ^-1 or less, more preferably 10000 L·mol-1·cm ^-1 or less. By having the molar absorption coefficient in the above range, effective recording sensitivity can be obtained, and the transmittance of the medium is prevented from becoming too low, and sufficient diffraction efficiency for the thickness tends to be obtained.

The solubility of compound (1) in the matrix resin forming composition under conditions of 25° C. and 1 atmospheric pressure is preferably 0.01% by mass or more, and more preferably 0.1% by mass or more.
Among commonly used oxime ester photopolymerization initiators, those having the above-mentioned absorption maximum often have poor solubility in constituent materials of matrix resins, such as isocyanates and polyols, and polymerizable monomers, such as compounds having a (meth)acryloyl group. However, compound (1) has excellent solubility in constituent materials of matrix resins, such as isocyanates and polyols, and polymerizable monomers, such as compounds having a (meth)acryloyl group, and can therefore be suitably used in compositions for holographic recording media.

(Photopolymerization initiator used in combination with compound (1))
In the present invention, the compound (1) may be used alone as the photopolymerization initiator, but may also be used in combination with other photopolymerization initiators, if necessary.
Examples of other photopolymerization initiators that can be used in combination include acetophenones, benzophenones, hydroxybenzenes, thioxanthones, anthraquinones, ketals, hexaarylbiimidazoles, titanocenes, acylphosphine oxides, halogenated hydrocarbon derivatives, organic borate salts, organic peroxides, onium salts, sulfone compounds, carbamic acid derivatives, sulfonamides, triarylmethanols, etc. These may be used alone or in any combination and ratio of two or more.

When other photopolymerization initiators are used, the amount of the other photopolymerization initiators (the total amount when two or more types are combined) may be any amount within the range that does not impair the function of compound (1), but the ratio to compound (1) is usually 0.001% by mass or more, preferably 0.01% by mass or more, and usually 100% by mass or less, preferably 80% by mass or less, and more preferably 50% by mass or less.

(Specific examples of compound (1))
Specific examples of compound (1) are shown below, but the present invention is not limited thereto.

<<About polymerization inhibitors>>
The polymerization inhibitor according to the present invention refers to a substance that inhibits the polymerization reaction of the polymerizable monomer, and can be used as necessary. For example, in a recording layer before holographic recording, the polymerization reaction of the polymerizable monomer is initiated by radicals generated by a polymerization initiator or a polymerizable monomer due to slight light or heat in a storage environment, and the polymerization inhibitor has an effect of inhibiting the progress of an unexpected polymerization reaction, thereby improving the storage stability of the medium before holographic recording.

The type of polymerization inhibitor is not particularly limited as long as it can inhibit the polymerization reaction of the polymerizable monomer. Specifically, for example, phosphinates such as sodium phosphate and sodium hypophosphite; mercaptans such as mercaptoacetic acid, mercaptopropionic acid, 2-propanethiol, 2-mercaptoethanol, and thiophenol; aldehydes such as acetaldehyde and propionaldehyde; ketones such as acetone and methyl ethyl ketone; halogenated hydrocarbons such as trichloroethylene and perchloroethylene; terpenes such as terpinolene, α-terpinene, β-terpinene, and γ-terpinene; 1,4-cyclohexafluoropropane, 1,4-cyclohexafluoropropane, and 1,4-cyclohexafluoropropane; non-conjugated dienes such as diene, 1,4-cycloheptadiene, 1,4-cyclooctadiene, 1,4-heptadiene, 1,4-hexadiene, 2-methyl-1,4-pentadiene, 3,6-nonanediene-1-ol, and 9,12-octadecadienol; linolenic acids such as linoleic acid, γ-linolenic acid, methyl linoleate, ethyl linoleate, isopropyl linoleate, and linoleic anhydride; linoleic acids such as linoleic acid, methyl linoleate, ethyl linoleate, isopropyl linoleate, and linoleic anhydride; eicosanols eicosapentaenoic acids such as sapentaenoic acid and ethyl eicosapentaenoate; docosahexaenoic acids such as docosahexaenoic acid and ethyl docosahexaenoate; phenol derivatives such as 2,6-di-t-butyl-p-cresol, p-methoxyphenol, diphenyl-p-benzoquinone, benzoquinone, hydroquinone, pyrogallol, resorcinol, phenanthraquinone, 2,5-toluquinone, benzylaminophenol, p-dihydroxybenzene, 2,4,6-trimethylphenol, and butylhydroxytoluene. Conductors: nitrobenzene derivatives such as o-dinitrobenzene, p-dinitrobenzene, and m-dinitrobenzene; N-phenyl-1-naphthylamine, N-phenyl-2-naphthylamine, cupferron, phenothiazine, tannic acid, p-nitrosamine, chloranil, aniline, hindered aniline, iron(III) chloride, copper(II) chloride, triethylamine, hindered amine, nitroxy radicals including 2,2,6,6-tetramethylpiperidine 1-oxyl, triphenylmethyl radical, and oxygen. These components may be any one of conventionally known materials used alone, or two or more of them may be used in any combination and ratio.

<<About radical scavengers>>
In holographic recording, a radical scavenger may be added to accurately fix the interference light intensity pattern as a polymer distribution in the holographic recording medium. The radical scavenger is preferably one having both a functional group that captures radicals and a reactive group that is covalently fixed to the matrix resin. An example of the functional group that captures radicals is a stable nitroxyl radical group.

Examples of reactive groups that are covalently fixed to the matrix resin include hydroxyl groups, amino groups, isocyanate groups, and thiol groups. Examples of such radical scavengers include 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl free radical (TEMPOL), 3-hydroxy-9-azabicyclo[3.3.1]nonane N-oxyl, 3-hydroxy-8-azabicyclo[3.2.1]octane N-oxyl, and 5-HO-AZADO: 5-hydroxy-2-azatricyclo[3.3.1.1 ^3,7 ]decane N-oxyl.
The above-mentioned various radical scavengers may be used either alone or as a mixture of two or more kinds in any combination and in any ratio.

<<Other additives>>
Other components contained in the composition for forming a recording layer include a solvent, a plasticizer, a dispersant, a leveling agent, an antifoaming agent, an adhesion promoter, a compatibilizer, a sensitizer, etc. These components may be any one of conventionally known materials used alone, or two or more of them may be used in any combination and ratio.

<<Content of each material>>
The content of each component in the composition for forming a recording layer in this embodiment may be any content as long as it is not contrary to the spirit of the present invention, but it is preferable that the ratio of each component is in the following range based on the total mass of the composition.

The total content of the matrix resin is usually 0.1% by mass or more, preferably 10% by mass or more, and more preferably 35% by mass or more. It is also usually 99.9% by mass or less, preferably 99% by mass or less, and more preferably 98% by mass or less. By making the content of the matrix resin equal to or greater than the above lower limit, it becomes easier to form the recording layer.

The content of the catalyst used to promote the formation of the matrix resin is preferably determined taking into consideration the bond formation rate of the matrix forming components, and is usually 5% by mass or less, preferably 4% by mass or less, and more preferably 1% by mass or less. It is usually preferable to use 0.005% by mass or more.

The content of the polymerizable monomer is usually 0.1% by mass or more, preferably 1% by mass or more, and more preferably 2% by mass or more. It is also usually 80% by mass or less, preferably 50% by mass or less, and more preferably 30% by mass or less. If the content of the polymerizable monomer is equal to or more than the above lower limit, sufficient diffraction efficiency is obtained, and if it is equal to or less than the above upper limit, the compatibility of the recording layer is maintained.

The content of the photopolymerization initiator is usually 0.1% by mass or more, preferably 0.3% by mass or more, and usually 20% by mass or less, preferably 18% by mass or less, and more preferably 16% by mass or less. If the content of the photopolymerization initiator is equal to or greater than the above lower limit, sufficient recording sensitivity can be obtained. If the content of the photopolymerization initiator is equal to or less than the above upper limit, the amount of substances that cause coloring can be reduced, which is preferable.

The content of the polymerization inhibitor is usually 0.001% by mass or more, preferably 0.005% by mass or more. Also, it is usually 30% by mass or less, preferably 10% by mass or less. When the content of the polymerization inhibitor is within the above range, it is possible to inhibit the progress of unexpected polymerization reactions initiated by radicals generated by slight light or heat.

The content of the radical scavenger, in terms of molar amount per unit weight of the matrix-forming component, is preferably 0.5 μmol/g or more, more preferably 1 μmol/g or more, and is preferably 100 μmol/g or less, more preferably 50 μmol/g or less.
When the content of the radical scavenger is equal to or higher than the lower limit, the radical scavenging efficiency is excellent, and the polymer with a low polymerization degree does not diffuse, and the components that do not contribute to the signal tend to be reduced. On the other hand, when the content of the radical scavenger is equal to or lower than the upper limit, the polymerization efficiency of the polymer is excellent.

The total content of other components is usually 30% by mass or less, preferably 15% by mass or less, and more preferably 5% by mass or less.

<Film thickness of recording layer>
The thickness of the recording layer in the present invention is preferably 0.1 mm or more and 3.0 mm or less, more preferably 0.3 mm or more and 2 mm or less.Within this range, when multiplex recording is performed in the holographic recording medium, the selectivity of each hologram tends to be high, and the degree of multiplex recording tends to be high.In addition, the light transmittance of the recording layer at the recording light wavelength can be kept high, and uniform recording can be performed on the entire recording layer in the thickness direction, and multiplex recording with a high S/N ratio tends to be realized.

[Other layers]
The medium of the present invention includes a recording layer and, if necessary, a support or other layers. Usually, the medium includes a support, and the recording layer and other layers are laminated on the support to form the medium. However, if the recording layer or other layers have the strength and durability required for the medium, the medium does not need to include a support. Examples of other layers include a protective layer, a reflective layer, an anti-reflective layer (anti-reflective film), and the like.

<Support>
There are no particular limitations on the details of the support, and any support can be used as long as it has the strength and durability required for the medium.
The shape of the support is not limited, but is usually formed into a flat plate or film.
There is no limitation on the material constituting the support, and it may be either transparent or opaque.

Examples of transparent materials for the support include organic materials such as acrylic, polyethylene terephthalate, polyethylene naphthoate, polycarbonate, polyethylene, polypropylene, amorphous polyolefin, polystyrene, and cellulose acetate; and inorganic materials such as glass, silicon, and quartz. Among these, polycarbonate, acrylic, polyester, amorphous polyolefin, and glass are preferred, and polycarbonate, acrylic, amorphous polyolefin, and glass are particularly preferred.

On the other hand, examples of opaque support materials include metals such as aluminum; the above-mentioned transparent supports coated with metals such as gold, silver, and aluminum, or with dielectrics such as magnesium fluoride and zirconium oxide.

There is no particular limit to the thickness of the support, but it is usually preferable to set it in the range of 0.05 mm or more and 1 mm or less. If the thickness of the support is equal to or more than the above lower limit, the mechanical strength of the medium can be obtained and warping of the substrate can be prevented. Also, if the thickness of the support is equal to or less than the above upper limit, the amount of light transmission can be maintained and costs can be prevented from increasing.

The surface of the support may also be subjected to a surface treatment. This surface treatment is usually performed to improve the adhesion between the support and the recording layer. Examples of surface treatments include subjecting the support to a corona discharge treatment, or forming an undercoat layer on the support in advance. Examples of compositions for the undercoat layer include halogenated phenols, partially hydrolyzed vinyl chloride-vinyl acetate copolymers, polyurethane resins, etc.

Furthermore, the surface treatment may be performed for purposes other than improving the adhesiveness. Examples of such purposes include a reflective coating treatment for forming a reflective coating layer made of a metal such as gold, silver, or aluminum; and a dielectric coating treatment for forming a dielectric layer made of magnesium fluoride, zirconium oxide, or the like. These layers may be formed as a single layer or as two or more layers.
These surface treatments may also be performed for the purpose of controlling the gas or moisture permeability of the substrate. For example, the reliability of the medium can be further improved by providing the support sandwiching the recording layer with a function of suppressing the gas or moisture permeability.

The support may be provided only on either the upper side or the lower side of the recording layer of the medium of the present invention, or on both sides. However, when supports are provided on both the upper and lower sides of the recording layer, at least one of the supports is configured to be transparent so as to transmit active energy rays (excitation light, reference light, reproduction light, etc.).
In the case of a medium having a support on one or both sides of the recording layer, a transmission or reflection hologram can be recorded, and in the case of a medium having a support with reflective properties on one side of the recording layer, a reflection hologram can be recorded.

Furthermore, the support may be provided with a pattern for data addresses. In this case, there are no limitations on the patterning method. For example, the support itself may be provided with unevenness, a pattern may be formed in the reflective layer described below, or a combination of these methods may be used.

<Protective Layer>
The protective layer is a layer for preventing the effects of oxygen and moisture on the recording layer, such as a decrease in sensitivity and deterioration in storage stability. There is no limitation on the specific structure of the protective layer, and any known material can be used. For example, a layer made of a water-soluble polymer, organic/inorganic material, etc. can be formed as the protective layer.
The position where the protective layer is formed is not particularly limited, and may be formed, for example, on the surface of the recording layer, between the recording layer and the support, on the outer surface side of the support, or between the support and another layer.

<Reflective Layer>
The reflective layer is formed when the medium is constructed as a reflective holographic medium. In the case of a reflective holographic recording medium, the reflective layer may be formed between the support and the recording layer, or may be formed on the outer surface of the support, but it is usually preferable that the reflective layer is formed between the support and the recording layer.
As the reflective layer, any known material can be used, for example, a thin metal film or the like can be used.

<Anti-reflection coating>
When the medium of the present invention is configured as a transmission type or reflection type holographic recording medium, an anti-reflection film may be provided on the side where the object light and the readout light enter and exit, or between the recording layer and the support. The anti-reflection film improves the light utilization efficiency and suppresses the occurrence of ghost images.
As the antireflection film, any known antireflection film can be used.

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples as long as they do not deviate from the gist of the invention.

[Ingredients used]
The composition raw materials used in the examples and comparative examples are as follows.
(Compound having an isocyanate group)
Duranate ^™ TSS-100: Hexamethylene diisocyanate-based polyisocyanate (NCO 17.6%, manufactured by Asahi Kasei Corporation)
Takenate 600: Aliphatic diisocyanate (manufactured by Mitsui Chemicals)
(Compounds Having Isocyanate-Reactive Functional Groups)
Plaxel PCL-305: Polycaprolactone triol (molecular weight 550, manufactured by Daicel Corporation)
Plaxel PCL-204HGT: Polycaprolactone diol (molecular weight 400, manufactured by Daicel Corporation)
(Polymerizable Monomer)
HLM303A: 2-[[2,2-bis[(2-dibenzothiophen-4-ylphenyl)sulfanylmethyl]-3-(1,3-benzothiazo-2-ylsulfanylmethyl)propoxy]carbonylamino]ethyl acrylate HLM303B: 2-[[2,2-bis[(2-dibenzothiophen-4-ylphenyl)sulfanylmethyl]-3-(1,3-benzothiazo-2-ylsulfanylmethyl)propoxy]carbonylamino]ethyl acrylate (HLM303A and HLM303B are the same compound. HLM303A contains impurities that become colored when heated, but if media using the respective polymerizable monomers are compared, this does not affect the effect of the post-exposure amount of the present invention.)
(Photopolymerization initiator)
HLI02: 1-(9-ethyl-6-cyclohexanoyl-9H-carbazol-3-yl)-1-(O-acetyloxime) methyl glutarate (curing catalyst)
Bismuth tris(2-ethylhexanoate) in octylic acid solution (active ingredient amount: 56% by mass)
(Radical Scavenger)
4-Hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl free radical (TEMPOL, manufactured by Tokyo Chemical Industry Co., Ltd.)

[Preparation of TEMPOL Masterbatch]
0.03 g of TEMPOL was dissolved in 2.97 g of Duranate ^™ TSS-100, and then 0.0003 g of a solution of tris(2-ethylhexanoate)bismuth in octylic acid was dissolved therein, and the mixture was stirred at 45° C. under reduced pressure and reacted for 2 hours.

[Preparation of composition for recording layer and production of holographic recording medium]
<Holographic recording medium 1>
Liquid A was prepared by dissolving 0.5952 g of Takenate 600, 1.2315 g of polymerizable monomer HLM303A, 0.0387 g of photopolymerization initiator HLI02, and 1.4531 g of TEMPOL master batch in 1.9341 g of Duranate ^™ TSS-100.
Separately, 3.6289 g of Plaxel PCL-204HGT and 0.4032 g of Plaxel PCL-305 were mixed (Plaxel PCL-204HGT:Plaxel PCL-305=90:10 (mass ratio)), and 0.00024 g of an octylic acid solution of tris(2-ethylhexanoate)bismuth was dissolved therein to prepare liquid B.

Liquid A was degassed under reduced pressure for 2 hours, and liquid B was degassed under reduced pressure at 45° C. for 2 hours, after which 3.2829 g of liquid A and 2.5202 g of liquid B were mixed together with stirring.
Next, the above mixture was poured onto a glass slide with 0.5 mm-thick spacer sheets placed on two opposing edges, and then the glass slide was placed on top of it, the periphery was fixed with clips, and the mixture was heated at 80° C. for 24 hours to produce a holographic recording medium sample. This evaluation sample had a recording layer with a thickness of 0.5 mm formed between the glass slides as a cover.

<Holographic recording medium 2>
A holographic recording medium sample was prepared in the same manner as in Holographic Recording Medium 1, except that HLM303B was used as the polymerizable monomer.

[Manufacturing of Optical Elements: Hologram Recording Exposure on Medium]
<Hologram Recording Device>
FIG. 1 is a diagram showing an outline of the configuration of an apparatus used for recording a hologram on a medium.
In Fig. 1, S is a sample of a holographic recording medium, and M1 and M2 are both mirrors. CS is an exposure time control shutter, and BS is a beam shutter. PBS is a polarizing beam splitter, LED is a light source for post-exposure (a THORLAB LED with a central wavelength of 405 nm), LD is a recording light laser light source that emits light with a wavelength of 405 nm (a single mode laser manufactured by TOPTICA Photonics that can obtain light with a wavelength of around 405 nm), and PD1 and PD2 are photodetectors.

<Hologram recording exposure>
The light with a wavelength of 405 nm generated from the LD was split by the PBS, and the two beams were regarded as object light (M1 side) and reference light (M2 side), and the two beams were irradiated so that the angle between them was 59.3° on the recording surface. At this time, the bisector of the angle between the two beams (hereinafter referred to as the optical axis) was made perpendicular to the recording surface of the recording layer of the holographic recording medium, and the vibration plane of the electric field vector of the two beams obtained by the splitting was made perpendicular to the plane containing the two intersecting beams. The above case was set to 0°, the beam shutter BS was opened, and the direction of the holographic recording medium was changed to an angle of 24° while keeping the incidence direction of the two beams fixed, and the exposure time control shutter was opened for 350 milliseconds as a pre-treatment exposure, and the object light and reference light were irradiated, and the deactivation of oxygen acting as a radical quencher in a dark state was waited for 10 seconds. After that, multiple exposure recording was performed while changing the angle of the recording surface to the optical axis by 0.2°.

<Post-exposure>
After the multiple recording exposure, the rotation angle was returned to 0°, and a signal growth time of 120 seconds was waited in a dark state. The light intensity of the post-exposure light source LED was set to 20 mW/ ^cm2 , and light irradiation was performed by changing the post-exposure time so that the post-exposure amount (the total exposure energy amount per unit area of light irradiated to a medium having a recording layer) became the value shown in Table 1, thereby producing an optical element.

[Post-exposure of the Example]
In Examples 1 to 3, holographic recording medium 1 or holographic recording medium 2 was used, and light irradiation was performed with post-exposure doses adjusted to 105, 216, and 432 J/cm ² .

[Post-exposure of Comparative Example]
In Comparative Examples 1 and 2, holographic recording medium 1 or holographic recording medium 2 was used, and light irradiation was performed with a post-exposure amount adjusted to 36 J/cm ² .

[Heat Deterioration Test]
As a post-exposure heat deterioration test, the holographic recording medium was heated at 75° C. for 500 hours.

[Evaluation of Coloration of Optical Elements]
The holographic recording medium was subjected to full-surface exposure immediately after the post-exposure and after the heat deterioration test, and ΔE00 was measured in a 10-degree visual field using a spectrophotometer SE7700 manufactured by Nippon Denshoku Industries Co., Ltd., based on JIS: Z8722: 2009. As a reference, one sheet of Corning EAGLE XG glass was used.

[Evaluation results]
The changes in ΔE00 before and after the heat deterioration test and the changes in ΔE00 due to heating (ΔE00 after heating at 75° C. for 500 hours−ΔE00 immediately after post-exposure) in Examples 1 to 3 and Comparative Examples 1 and 2 are shown below.

From Example 1 and Comparative Example 1, it can be seen that by setting the post-exposure dose to 108 J/cm ² or more, the increase in ΔE00 of the obtained optical element due to heating at 75° C. for 500 hours is significantly reduced, that is, coloring can be suppressed. Moreover, from Examples 2 and 3 and Comparative Example 2, it can be seen that by setting the post-exposure dose to 216 J/cm ² or more, coloring can be further suppressed.

The method for manufacturing optical elements of the present invention is particularly useful as a means for reducing coloration in optical elements with recorded holograms, such as those used in AR glass light guide plates (waveguides).

S Hologram recording medium M1, M2 Mirror LD Semiconductor laser light source for recording light PD1, PD2 Photodetector LED Post-exposure LED light source BS Beam shutter PBS Polarizing beam splitter CS Exposure time control shutter

Claims (3)

A method for producing an optical element, comprising: exposing a medium having a recording layer containing a polymerizable compound and a photopolymerization initiator to recording light; and post-exposing the medium after the recording light exposure with an exposure amount of 100 J/cm ² or more. The method for producing an optical element according to claim 1 , wherein the photopolymerization initiator is a compound represented by the following formula (1):

In formula (1), R ¹ represents an alkyl group;
^R2 represents an alkyl group, an aryl group, or an aralkyl group;
^R3 represents a -( _CH2 ) _n- group, n represents an integer of 1 or more and 6 or less;
^R4 represents a hydrogen atom or an arbitrary substituent;
^R5 represents any substituent that does not have a multiple bond conjugated to the carbonyl group to which it is attached. The method for producing an optical element according to claim 1 , wherein the recording exposure is a hologram recording exposure.

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