Volume phase type hologram and method of manufacture thereof

Abstract: A volume phase type hologram has a diffraction lattice recorded in a polymer layer where the diffraction lattice is recorded all through the thickness direction of the polymer layer. A method for preparing the volume phase type hologram comprises steps of exposing a photosensitive polymer layer provided on a substrate to an interference pattern; subjecting the polymer layer having been exposed to a first development treatment with a first organic solvent; and peeling the polymer later off the substrate and subjecting the polymer to a second development treatment with a second organic solvent from the side preferentially of the face that had been in contact with the substrate.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a volume phase type hologram and a method of manufacture thereof. Related Background Art Gelatin sensitized by a dichromate has hitherto been widely known and used for recording medium for volume phase type hologram. Another kind of recording mediums are known for forming volume phase type hologram comprising a hydrophobic polymer, exemplified by poly(N-vinylcarbazole), (hereinafter referred to as PVCz), sensitized by addition of an iodine compound, and a volume phase type hologram recorded in the above recording mediums are also known, which are disclosed in detail in prior art publications such as USP 4,172,724, USP 4,258,111 and USP 4,287,277. The volume phase type hologram is employed for recording interference wave of laser beams in a thin film of 20 µm thick or less. In use of visible light laser, the layer of photosensitive material for hologram recording is required to be fixed with positional accuracy of approximately from 0.01 to 0.02 µm during exposure in order to record a Bragg's lattice of approximately 0.1 to 0.2 pm interstice within a film. Further, in the followed-treatment subsequent to the exposure, the photosensitive recording layer is swelled and then shrinked to modulate the refractive index of the portion recorded by laser beam, where the swelling and then the shrinking of the recording layer have to be caused in a direction substantially perpendicular to the recording layer. Because of such restriction both in light exposure and in aftertreatment, conventional volume phase type holograms are tightly fixed on and supported on a rigid substrate plate. The article of Y. Yamagishi et al. in SPIE Vol. 600 "Progress in Holographic Applications" (1985), pp. 14-19, is also related to holographic recording in PVCz. Fig. 2 of this document schematically shows the aforementioned developing process involving swelling and shrinking of a PVCz film while the PVCz film has an exemplified thickness of 2.5 µm. The above-mentioned conventional methods involve a disadvantage that a diffraction lattice cannot be formed satisfactorily in the recording layer in proximity to an interface of the substrate, because the recording layer is supported and fixed by the substrate surface and therefore a solvent cannot penetrate sufficiently to cause necessary swelling and shrinking even though a latent image has been formed in the recording layer near the substrate by exposure. In this respect, it may be referred to the post-published article of Y. Kuwae et al. in "Macromol. Chem. Macromol. Symp." Vol. 37 (1990), pp. 229-238 to be considered as expert knowledge. According to the analysis carried out in that study with the help of electron electroscopy study, it is revealed that recording and developing a PVCz film having a thickness of 6 to 9 µm involving the conventional two-step process of swelling and drying, a diffraction lattice (grating) in the vicinity of a substrate (or the neighborhood or immediate proximity to the interphase of the substrate) cannot be observed. Accordingly, it is stated on p. 232 of this document that the grating in the vicinity of the substrate almost disappeared. To offset the disadvantage, use of a solvent having high swelling power may be considered. However, such a solvent weaken the fixation of the recording layer to the substrate, and makes it inachievable to restrict the direction of swelling and shrinking of the recording layer only to the thickness direction, thus the formation of a desired diffraction pattern becoming difficult. To solve the problem, Patent Laid-open Publication No. 62-502289 (1987) proposes a method in which the photosensivity at the recording layer is reduced in vicinity of the interface to the substrate so as to retard the formation of lattice image near the interface. In this instance, the region decreases which serves practically as hologram in a limited thickness of the recording layer. SUMMARY OF THE INVENTION An object of the present invention is to provide a volume phase type hologram and a method of manufacture thereof, free from the above-mentioned disadvantage of the prior art. According to an aspect of the present invention, there is provided a volume phase type hologram having a diffraction lattice recorded in a form of crosslinked poly(N-vinylcarbazole) in a poly (N-vinylcarbazole) polymer layer, said polymer layer having a thickness in the range from 5 µm to 15 µm and said diffraction lattice being extending throughout the thickness of the polymer layer. According to another aspect of the present invention, there is provided a method for preparing a volume phase type hologram having a diffraction lattice being extending througout the thickness of a poly (N-vinylcarbazole) polymer layer, the method comprising steps of exposing said poly (N-vinylcarbazole) polymer layer provided on a substrate to an interference pattern; subjecting the polymer layer having been exposed to a first development treatment with a first organic solvent; and removing the polymer layer from the substrate and subjecting the polymer to a second development treatment with a second organic solvent on at least the polymer surface which had been in contact with the substrate. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a rough sketch of an optical system for exposure employed in the Examples of the present invention. Fig. 2A, Fig. 2B, and Fig. 2C respectively outline the method of manufacture of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In the present invention, a volume phase type hologram having a diffraction lattice is recorded all through the thickness direction of the recording layer according to steps of forming a hologram-recording layer from a polymer capable of forming high-strength film, exposing and developing the recording layer conventionally, peeling the recording layer off the substrate, and developing again the face of the recording layer having been in contact with the substrate. Although a latent hologram image is formed all through the thickness direction of the recording layer by exposure, the image is not developed in the vicinity of the interface with the substrate because a solvent cannot penetrate thereto. The diffraction lattice can be formed uniformly throughout the thickness of the recording layer, if the recording layer is once peeled off the substrate and the undeveloped portion (or the recording layer in the vicinity Of the substrate) is subjected to development again. A PVCz polymer preferably employed in the present invention is generally prepared by polymerization of N-vinylcarbazole. The PVCz prepared in a conventional procedure or purchased from the market is applicable in the present invention. However, the conventional PVCz is preferebly purified, for example, by molecular-weight fractionation by solvent to separate off a low molecular fraction and to increase the molecular weight for the use. The comprehensive study by the present inventors reveals that particularly preferable PVCz has a weight-average molecular weight of 1,000,000 or more and the molecular-weight distribution of 3 or less, the use of which as a recording medium of a volume phase type hologram of the present invention will solve the aforementioned various problems. In particular, those having a molecular-weight distribution of 2 or less, those containing PVCz of weight-average molecular weight of 200,000 or less at a content of not more than 1 % by weight, and those containing PVCz having a weight-average molecular weight of 1,000,000 at a proportion of 50% by weight of the whole polymer will accomplish more advantageously the object of the present invention. The weight-average molecular weight described in the present invention are obtained by a gel permeation chromatography in tetrahydrofuran as a polystyrene calibration value at a column temperature of 27°C. The columns used were successive combination of Shodex KF-807 and KF80M, in accordance with polystyrene calibration using standard polystyrene of from 2 x 10 7 to 1.2 x 10 3 of average molecular weight. A procedure of molecular weight fractionation is explained below as one of the embodiment of preparing PVCz of a preferable high molecular weight. Any known conventional molecular-weight fractionation method is applicable to the fractionation such as use of various solvents having different dissolving power toward PVCz, and change of the concentration. As an example, mixing ratio of a good solvent (dioxane) to a poor- or non-solvent (isopropyl alcohol) was varied to change the dissolving power of the mixed solvent toward the PVCz, and a low molecular fraction was removed by dissolution-precipitation fractionation. The method of manufacturing the volume phase type hologram of the present invention is explained below employing the above-mentioned PVCz. The PVCz has no light absorption band at the longer wavelength region than 400 nm, so it is required to be activated to radiation beam of ordinal visible region by adding a halogen compound, preferably an iodine compound and/or a bromine compound as a photosensitizer. The halogen compounds act as a good sensitizer toward PVC Z for visible wavelength region. Specifically the preferable carbon-halogen compounds containing iodine bromine such as carbon tetraiodide, iodoform, tetraiodoethylene, triiodoethane, tetraiodoethane, pentaiodoethane, hexaiodioethane, Carbon tetrabromide, and tetrabromoethylene. The halogen compound is added in an amount of 1 to 0.01 part by weight per 1 part by weight of the aforementioned PVCz. Adding too small amount of the halogen compound is not favorable because it lowers the sensitivity of the recording medium and prolongs the required exposure time, while adding to larger amount of the halogen compound exceeding 1 in the weight ratio is unfavorable because it causes voids in the resulting hologram film after development of the hologram. Additionally, addition of a plasticizer is not preferable in the preparation of the recording medium of the present invention. If a plasticizer is mixed therein, the unextracted remaining plasticizer has to undergo swelling and shrinking similarly to PVCz in the swelling and shrinking process in image development, and may bring about any undesired refraction index distribution in addition to the modulation of refraction index that satisfies the Bragg condition in the recording medium after shrinkage. However, the use is not necessary prohibited within the range where such inconvenience does not arise. The hologram recording medium employed in the present invention is obtained by dissolving or dispersing the above-mentioned PVCz, a halogen compound and other necessary additive in a desired proportion in an appropriate solvent, and then applying it on onto a transparent substrate such as a glass plate and a plastic film, where the applied coating film has a thickness in the range of 5 pm to 15 pm (dry thickness). The recording medium thus formed is exposed to two interfering laser beams of coherent visible light of wavelength upto 560 nm by means of an optical system illustrated in Fig. 1. Subsequently to the exposure, the recording medium is immersed in a solvent which hardly extract PVCz constituting the recording layer as well as crosslinked PVCz formed by photoreaction to remove solely the unreacted halogen compound nearly completely by elution and eliminate color from the recording layer. This step can also serve for swelling as mentioned below. The succeeding development process comprises two steps of swelling and shrinking. The recording layer carrying a latent hologram image formed by the aforementioned light exposure and having made free from the halogen compound is treated with first solvent to cause the recording layer to swell in the direction of the thickness of the recording layer. In the following treatment with a second solvent, the recording layer in a swollen state is made to shrink to fix the hologram in the recording layer. In this state, the recorded interference pattern remains undeveloped in the vicinity of the interface in contact with the substrate as shown schematically in Fig. 2A. The first solvent serving as a liquor for swelling in the hologram developing step is the one which dissolves out within a short time neither the uncrosslinked PVCz nor crosslinked PVCz after the photoreaction between the PVCz and the halogen compound. Such solvent is preferably an organic solvent having solubility of PVCz equal to or lower than the organic solvent employed in the purification of the PVCz mentioned above, and is selected from the organic solvents which are useful for the purification process. The second solvent serving as a liquor for shrinking treatment may be any solvent which does not swell nor dissolve the recording layer and is compatible with the above-mentioned swelling liquor. The solvent or the solvent combination is chosen from non- or poor solvents of PVCz. The treating conditions such as temperature and time length of each of the steps depend on the kind of recording layer or the solvent, etc., and cannot simply be specified. Generally, each step achieves sufficient effect at a temperature of 10 - 70 °C for several seconds to several minutes. In the present invention, the recording layer of the volume phase type hologram as described above is once peeled off the transparent substrate. Any method may be applied which does not impair the recording layer such as a method of peeling by immersion in water of an aforementioned non-solvent. The object of the present invention is accomplished by re-treating the recording layer separated from the transparent substrate with a solvent for swelling and shrinking. In the re-treatment, the recording layer is preferably fixed again on a solvent-resistant substrate such as a glass plate in such a manner that the face having been in contact with the transparent substrate prior to the peeling is brought to the external side as shown schmatically in Fig. 2B so as to prevent the swelling and the shrinking of the recording layer in a plane direction. In the fixing, an adhesive such as epoxy type, urethane type, and α-cyanoacrylate type which is transparent and does not impair the recording layer may be used. The subsequent solvent treatment may be conducted in the same manner as that described above. In particular, the use of the same solvent and the same condition as above are desirable in order to maintain the uniformity of the lattice. In this re-treatment, the treatment of only the surface and the vicinity thereof is enough which has not been sufficiently developed, since the most portion of the recording layer has well already been developed. Such re-treatment will complete the volume phase type hologram having a recorded diffraction lattice all through the thickness direction of the recording layer as shown schematically in Fig. 2C. The above-described manufacturing method may be modified as below. In this modification, the secondary developing treatment is conducted in a condition different from that of the primary swelling and shrinking treatment. For example, the use of a solvent having a swelling power stronger than the primary swelling, or the adoption of higher temperature or longer time with the same solvent allows formation of a new lattice which diffracts light of longer wavelength than the diffraction pattern resulting from the primary development. Otherwise, less swelling than the primary swelling treatment allows the formation of a lattice diffracting a shorter wavelength of light than the primary lattice. Additionally, the hologram prepared thus may naturally be used with the substrate adhered. Further, the holograph film as peeled off the substrate is useful for various uses since the PVCz employed in the present invention is so high molecular that the film itself as peeled off again still has sufficient self-supporting strength. According to the present invention, a volume phase type hologram is provided which has diffraction lattice recording uniformly throughout the direction of the thickness of the recording layer by the steps of forming a recording layer from a polymer capable of forming high-strength film, exposing and developing conventionally, peeling the recording layer off the substrate, and then again subjecting the surface having been in contact with the substrate to developing treatment. The volume phase type hologram of the present invention is improved to exhibit maximum diffraction efficiency of 99.9 %, transmissivity of not less than 92 % at 600 nm, and haze of not more than 2 % for 10 um thickness in comparison with the corresponding values of 98 %, 80 %, and 12 %, respectively of the conventional volume phase type hologram employing PVCz. The present invention is described more specifically referring to examples. EXAMPLE 1 20 grams of PVCz (Rubican® M-170 made by BASF) was put into 600 ml of a mixed solvent of dioxane/isopropyl alcohol (weight ratio: 3/1), heated to 80°C to prepare a homogeneous solution, and then cooled to room temperature. The precipitated PVCz was separated by decantation, and the separated PVCz was washed several times by agitating with 200 ml portions of dioxane/isopropyl alcohol mixture (weight ratio: 3/1) to obtain purified PVCz. A portion of the obtained purified PVCz was dissolved in tetrahydrofuran, and tested for weight-average molecular weight and molecular-weight distribution by means of a gel permeation chormatography at 27°C employing the combination of columns of Shodex KF-807 and KF80M, (made by Showa Denko K.K.), with polystyrene (made by Showa Denko K.K.) as the standard. The weight-average molecular weight was 1,520,000, and the molecular-weight distribution was 1.48. The content of PVCz having a molecular weight of not higher the 200,000 was 0.8 % by weight, and the content of PVCz having a molecular weight of not less than 1,000,000 was 62 % by weight. 2.5 grams of this purified PVCz and 0.25 gram of carbon tetraiodide were put into 40 ml of chlorobenzene and dissolved in the dark, and the liquid was applied onto a cleaned glass plate by means of a spinner (made by Kyowa Semiconductor K.K.) and dried to form a recording layer of 7.2 µm thick. The recording layer was subjected to exposure by means of an optical system as illustrated in Fig. 1 employing Ar laser (488 nm) to record hologram thereon. After the exposure, it is immersed in toluene at 35 °C for 2 minutes, in xylene at 35°C for 2 minutes, and and further in heptane at 20°C for 2 minutes, and dried to obtain volume phase type hologram. Having been kept immersed in water for 30 minutes, the volume phase type hologram film became separated from the substrate. The film having separated was scooped from water by using a polyester film having a surface tension of 40 dyne/cm as measured according to JIS K-6768 in such a manner that the face of the hologram film having been in contact with the glass substrate is brought into contact with the polyester film, and then it was dried. The hologram had a diffraction peak at 520 nm, and exhibited a maximum diffraction efficiency of 98 %, a diffraction peak breadth of 18 nm, and transmissivity of 85 % at 600 nm. The hologram was sticked on a glass plate having been applied with an epoxy-adhesive, and subjected to the developing treatment similar to the above to obtain the volume phase type hologram of the present invention. The reflective diffraction efficiency of the obtained hologram was measured in the same manner as above and was found to be maximum reflective diffraction efficiency of 99.9 %. The transmissivity was 95 %. The haze (total haze value) was 1.3 % by means of a haze meter (made by Nippon Denshoku K.K.) according to JIS K7105. The hologram was kept in an nitrogen atmosphere at 140°C for approximately 5 hours. The hologram characteristics were measured by means of the above-described optical system. Little shift of the peak of the maximum diffraction efficiency was observed, and also little change in the maximum diffraction efficiency and the haze value were observed. EXAMPLE 2 2.5 grams of PVCz having a weight-average molecular weight of 1,520,000 and a molecular-weight distribution of 1.48, and 0.25 gram of carbon tetraiodide were dissolved in chlorobenzene in the dark. The solution was applied on a cleaned glass plate by means of a spinner (made by Kyowa Semiconductor K.K.), and it was dried to form a recording layer of 7.2 µm thick. In this recording layer, a hologram was recorded by exposure to Ar laser (488 nm) by using the optical system illustrated in Fig.1. After the exposure, it is immersed in toluene at 35°C for 2 minutes, in xylene at 35°C for 2 minutes, and further in heptane at 20°C for 2 minutes, and dried to obtain volume phase type hologram. Having been kept immersed in water for 30 minutes, the volume phase type hologram film became separated from the substrate. The film was scooped out from the water by using a polyester film having a surface tension of 40 dyne/cm as measured according to JIS K-6768 in such a manner that the face of the hologram film having been in contact with the glass substrate is brought into contact with the polyester film, and dried. The hologram film was then sticked to a glass plate on which an epoxy-adhesive had been applied, and was adhered thereto and dried. It is then immersed in toluene at 30°C for 1 minute, in xylene at 30°C for 1 minute, and further in heptane at 20°C for 1 minutes, and dried to obtain volume phase type hologram of the present invention. The reflection diffraction efficiency of the above-described hologram was measured to be 83 % at 520 nm, and 42 % at 490 nm. EXAMPLE 3 A volume phase type hologram was prepared from a recording medium similar to the one of Example 2 by exposing it to light, immersing it in toluene at 30 C for 2 minutes, in xylene at 30°C for 2 minutes, and further in hexane at 18°C for 2 minutes. The hologram exhibited the maximum diffraction efficiency at 495 nm, diffraction peak breadth of 23 nm, and transmissivity of 83 %. The hologram was treated in the same manner as in Example 2 for film separation and re-fixing. It was then immersed in dioxane at 35°C for 1 minute and in ethyl ether at 15°C for 1 minute to obtain the volume phase type hologram of the present invention. The hologram had diffraction peaks of 78 % at 495 nm, and 52 % at 590 nm as the result of reflective diffraction efficiency measurement. A volume phase type hologram has a diffraction lattice recorded in a polymer layer where the diffraction lattice is recorded all through the thickness direction of the polymer layer. A method for preparing the volume phase type hologram comprises steps of exposing a photosensitive polymer layer provided on a substrate to an interference pattern; subjecting the polymer layer having been exposed to a first development treatment with a first organic solvent; and peeling the polymer later off the substrate and subjecting the polymer to a second development treatment with a second organic solvent from the side preferentially of the face that had been in contact with the substrate.

Claims: A volume phase type hologram having a diffraction lattice recorded in a form of cross-linked poly (N-vinylcarbazole) in a poly (N-vinylcarbazole) polymer layer, said polymer layer having a thickness in the range from 5 µm to 15 µm and said diffraction lattice being extending throughout the thickness of the polymer layer. The volume phase type hologram of claim 1, wherein the polymer has a weight-average molecular weight of not less than 1,000,000 and a molecular-weight distribution of not more than 3. The volume phase type hologram of claim 1, wherein the polymer layer is supported by a substrate. The volume phase type hologram of claim 1, wherein the polymer layer is supported by a substrate with interposition of an adhesive. The volume phase type hologram of claim 1, wherein the polymer layer is a self-supporting film. The volume phase type hologram of claim 1, wherein the hologram has diffraction peaks at two different wavelengths. A method for preparing a volume phase type hologram having a diffraction lattice being extending throughout the thickness of a poly(N-vinylcarbazole) polymer layer, the method comprising steps of exposing said poly(N-vinyl carbazole) polymer layer provided on a substrate to an interference pattern; subjecting the polymer layer having been exposed to a first development treatment with a first organic solvent; and removing the polymer layer from the substrate and subjecting the polymer to a second development treatment with a second organic solvent on at least the polymer surface which had been in contact-with the substrate. The method of claim 7, wherein the polymer layer removed is attached to the substrate with a face opposite to the side that had been in contact with the substrate, and then the polymer layer is subjected to said second development treatment. The method of claim 7, wherein the substrate is a glass plate. The method of claim 7, wherein the polymer layer is swollen and shrunk in both of the first and the second development treatments. The method of claim 7, wherein the first and the second development treatments are conducted under the same development condition. The method of claim 7, wherein the first and the second treatments are conducted respectively under different development conditions. The method of claim 7, wherein the polymer layer has a thickness in the range from 5 µm to 15 µm. The method of claim 7, wherein the polymer has a weight-average molecular weight of not less than 1,000,000 and a molecular-weight distribution of not more than 3. The method of claim 7, wherein the polymer is sensitive in the visible wavelength spectrum. The method of claim 7, wherein the polymer layer is removed by immersion in water. The method of claim 7, wherein the polymer layer has a uniform thickness throughout. The method of claim 7, wherein the polymer layer has diffraction peaks at two different wavelengths.

Inventors: KUSHIBIKI NOBUO, KUWAYAMA TETSURO, TANIGUCHI NAOSATO, YOSHINAGA YOKO, KUSHIBIKI, NOBUO, KUWAYAMA, TETSURO, TANIGUCHI, NAOSATO, YOSHINAGA, YOKO
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1989
Main classification: G03H1
IPC codes: G03H1, G03F7