VOLUME PHASE HOLOGRAM AND METHOD FOR PRODUCING THE SAME

Abstract: The hologram comprises: a porous transparent sileceous body having a plurality of mutually interconnected microcavities or pores, the mean radius of which is lower than the wavelength of the hologram recording light and the wavelength of the hologram reading light, a photolysis product of a photolysable material, said product being attached to walls of certain microcavities and being spatially distributed in accordance with the recorded interference pattern, and a solid transparent polymeric material filling said microcavities, said polymeric material exhibiting local variations of refractive index, said variations being spatially modulated in conformity with the recorded interference pattern, said photolysis product being a polymerization modifier for a composition polymerizable into said polymeric filling material.

Description: VOLUME PHASE HOLOGRAM AND METHOD FOR PRODUCING THE SAME The invention is related to holography, more particularly to volume phase holograms. Volume phase holograms consisting of a polymer body with local variations of refractive index are well known. They may be recorded in layers of photopolymers or in bichromated gelatin films. [See : R. J. Collier, C.B. Burckhardt, L.H. Lin "Optical Holography," 1971 , Academic press, New York and London]. They are usually made of organic material with low mechanical properties and low thermostability and are, thus, poorly reliable and durable. Moreover, thick layers of light sensitive materials are not available, which restricts the achievable level of spectral and angular selectivity of the recorded holograms. Also known are volume holograms consisting of a porous highly siliceous body and products of photolysis of organic or inorganic light-sensitive substrate, the spatial modulation of hologram refractive index being obtained by an appropriate body structure modulation or spatial variation of the concentration of the photolysis products of the light-sensitive substrate (See : V. Sukhanov, Heterogeneous recording media, In SPIE, 1989, V. 1238, p. 226-230. The International Unesco Seminar Three-Dimensional Holography Science, Culture, Education. Ed. Tung H. Jeorg, 1989, Kiev, USSR). The physical thickness of such holograms may be as high as 10 3 μm. Moreover, they are diaracterized by a very high thermostability and they are practically shrinkproof. But such holograms have a porous structure that causes a high level of scattering in the blue-green region of spectrum. To avoid this the hologram free volume should be filled with a filling material by means of an immersion treatment. But the closer is the refractive index of the filling material to the body one, the lower is the hologram diffraction efficiency (See: S.A. Kuchinskii et al., "The principal of hologram formation in capillary composites", Laser Physics, 1993, V3, n 6, p. 1114-1123). So, it is not possible to obtain simultaneously a low level of scattering and good diffraction efficiency. The object of present invention is a phase volume hologram exhibiting a remarkable increase of the hologram diffraction efficiency together with a decrease of the losses due to scattering. To achieve this, the body pores are filled with a solid-phase polymer having a spatially modulated refractive index, the period of said refractive index modulation being the same as the period of the spatial distribution of refractive index of a volume phase hologram comprising a porous body the pores of which are filled with an organic filler of uniform refractive index. More particularly, the invention relates to a volume phase hologram having an interference pattern recorded in the form of local variations of refractive index, which comprises: - a porous transparent siliceous body having a plurality of mutually interconnected microcavities or pores the mean radius of which is lower than the wavelength of the hologram recording light and the wavelength of the hologram reading light ; - a photolysis product of a photolysable material, said product being attached to walls of certain microcavities and being spatially distributed in accordance with the recorded interference pattern, and - a solid transparent polymeric material filling said microcavities, characterized in that said polymeric material exhibits local variations of refractive index, said variations being spatially modulated in conformity with the recorded interference pattern, said photolysis product being a polymerization modifier for a composition polymerizable into said polymeric filling material. The invention relates also to a meth\od for producing an hologram according to the invention, which comprises the following steps : a) providing a porous transparent siliceous body having a plurality of mutually interconnected microcavities or pores, the mean radius of which is lower than the wavelength of the hologram recording light and the wavelength of the hologram reading light, b) coating the walls of said microcavities with a photolysable material a photolysis product of which is a polymerization modifier for at least one predetermined polymerizable composition ; c) recording a hologram interference pattern within said material so as to leave said photolysis product distributed according to said pattern onto walls of certain of said microcavities ; d) removing any unaltered photolysable material ; e) filling the remaining free volume of said microcavities with said at least one polymerizable composition ; and f) polymerizing said composition so as to obtain a volume phase hologram made of a transparent body filled with a solid polymeric material with spatial modulated refractive index the period of modulation of refractive index of said polymer being the same as the period of spatial distribution of said photolysis products. By the expression "polymerization modifier" it is meant any substance which, when a polymerizable composition is polymerized in its presence, gives a polymerized material having a refractive index different from the refractive index of the polymerized material obtained by polymerizing said polymerizable composition in the absence of said polymerization modifier. Other features and advantages of the present invention will appear from the following description and drawings in which : - Fig. 1 represents schematically a volume phase hologram according to the present invention, and - Fig. 2 is a graph of the diffraction efficiency versus the grating power of a transmission hologram, useful to explain the present invention. The hologram shown in Fig. 1 consists of a porous highly siliceous body 1 , comprising a plurality of pores or microcavities having a mean radius lower than the wavelengths of the visible light, i.e. lower than about 0,4μm. The walls of certain microcavities are coated with a coating 2 having a refractive index n c , comprising a product of photolysis of a photolysable material and located in the vicinity of the maxima or minima of a recorded interference pattern. The free volume of the cavities is filled with a filling material 3 having a spatially modulated refractive index, the refractive index of the filling material being equal to r> in the cavities without coating and equal to r> + Δ in all other cavities. It is easy to show that the difference of effective refractive indexes between the exposed (A) and nonexposed (B) regions in the hologram volume is defined by expression : n A - n B = F.f(n c - n f ) + F(1 -f) Δ (1) where : F = relative pore volume ; f = relative volume of coating in pores. The first term in (1 ) describes the amplitude modulation of a porous hologram with a uniform filler, which turns into zero if ri f is equal to n c . But if a space modulation of the refractive index of the filling material takes place this amplitude differs from zero, even in the case r = n c , and is equal to F(1 - f)Δ. The hologram index variation amplitude is K times higher in the case of a spatially modulated filling material than in case of a uniform one (Δ = 0), K being defined by κ = 1 + <!___ ^ (2) f n o - n _r Moreover, it is possible to obtain a grating amplification even with respect to an initially "dry" hologram (i.e. without filling material) for which n c - r reaches its maximum value, by the introduction of the spatially modulated filling material into the free volume of the porous hologram. As it is evident from (1), the amplification coefficient in this case is determinated by the expression : As it is clear from equations (2) and (3), the hologram amplification may take place both in the case of a refractive index modulation of the filling material in phase with the refractive index alteration of unfilled hologram and in the case of the modulation of the same parameter being in antiphase. It is necessary only that this modulation is characterized by the same spatial period of modulation as the recorded interference pattern and that the absolute values of | κ| , \ κ \ determinated from equations (2) and (3) are higher than 1. As it is known, the maximum value of the diffraction efficiency η of transmission holograms is reached at the particular value of the hologram index amplitude modulation that corresponds to the grating power (χ) equal to: = arcsι •n- s / r7 = — π It is obvious that increasing χ K times practically means decreasing the exposure level K times (assuming a linear response of the photolysable material of the hologram) necessary to obtain η = 100%. Thus, it is possible to realize a high efficiency phase hologram with a low level of scattering by means of the space modulation of the refractive index of a filling material, according to the present invention. The realization of the described approach to hologram amplification is based on the difference between polymerization mechanisms of a monomer filling material in exposed and nonexposed regions of the hologram when the photolysis products of a photolysable substance act as a polymerization modifier of said process. As a result the density of the polymer filling material and, as a consequence, its refractive index are spatially modulated in accordance with the recorded light intensity distribution. The porous transparent siliceous body can be, for example, a porous glass produced by leaching a borosilicate glass (See: V.I. Sukhanov. "Porous glass as a storage medium". Optica Applicata, 1994, v. 24, n. 1-2, pp. 13-26.), a porous glass produced by the so-called sol-gel process (See: V.I. Sukhanov et al. "Porous sol-gel glass a halographic recording medium". In : Book of Abstracts. The VIII international Workshop on Glasses and Ceramics from Gels. 1995, Faro, Portugal, September 18-22, p. 331). In both cases the porous body contains mutually interconnected microcavities or pores having a mean radius which is substantially smaller than the wavelengths of acting light (i.e : recording and reading lights), and a high surface area. This fact, on one hand, ensures relatively low level of light scattering and, on the other hand, allows an effective impregnation with said photolysable material and, consequently, a good coating of the walls of said microcavities with said material. As photolysable materials for the recording of holograms, according to the invention, are useful, for example, photolysable materials that give a photolysis produd which substantially induces complex free-radical polymerization. Illustrative examples of suitable photolysable materials are certain inorganic salts of transition metals, such as (NH 4 ) 2 Cr 2 O 7 , Na 2 Cr 2 O 7 , K 2 Cr 2 O 7 and certain organometallic compounds of transition metals such as carbonyl or cydopentadienyl complexes of transition metals, for example Mn 2 (CO)ι 0 , Cr(CO) 6l CcfefCO)* Mo(CO)e or Ti(cycloρentadiene) 2 CI 2 . (See N.F. Borelly and D.L. Morse. hotosensitive impregnated porous glass". Appl. Phys. Lett. 1983, v. 43, n. pp. 992-993 ; "Photosensitive metal-organic systems : mechanistic principles and applications", Ed. Ch. Kutal, N. Serpone. Advances in Chemistry. Ser. 238, 1993, Am. Chem. Soα, Washington, DC 449 p.). The coating step can be carried out in a simple manner by dipping the porous body in a bath of a solution of the photolysable material, then by drying. If required a vacuum can be applied for assisting the impregnation. It should be noted that photolysis of transition metal salts has been already used for hologram recording, in particular, the photolysis of Cr (VI) salts : (See : G. Manivannan et al. "Primary photoprocesses of Cr (VI) in real-time holographic recording : dichromated poly(vinyl alkohol). J. Phys. Chem., 1993, v. 97, n. 28, pp. 7228-7233). The ions of Cr (VI) are reduced into Cr (III) ions in the exposed regions of hologram. When the photolysable transition metal compounds are photolysed within the porous siliceous body during the hologram recording step, their photolysis produds interad with fundional groups (such as -SiOH groups) inherently present on the siliceous surface of the microcavity walls resulting in the formation of a coating of complex molecules chemically bound to the walls of the porous body. Alternatively, a transition metal ion-containing coating chemically bound to the walls of porous glass can be also produced with the use of organometallics compounds of transition metals as disclosed by N.F. Borelly et al., "Photochemical method to produce waveguiding in glass". IEEE. J. Of Quantum Eledronics. 1986, v. QE-22, n. 6, pp. 896-901. Instead of diredly chemically bonding transition metal ions to the walls of the pores of a porous glass, it is possible also to form a coating on the walls of said pores by applying thereon a photoresist layer containing a transition metal compound. In this case, under exposure, a crosslinking of the photoresist takes place and, as a result, on the pore walls a coating is formed which contains ions of a transition metal. Photoresists based on synthetic polymers can be used, in particular, poly(vinyl alcohol) (See : G. Manivanna et al. "Primary photoprocesses of Cr (VI) in real-time holographic recording : dichromated poly(vinyl alcohol). J. Phys. Chem., 1993, v. 97, n. 28, pp. 7228-7233), or on natural polymeric produds, in particular, gelatine or shellac. The hologram recording is made in a conventional manner with a laser light in the visible spedrum. It is only required that the exposure energy be sufficient to generate an amount of photolysis produd suffident for allowing the latter to ad as an effedive polymerization modifier in step (f) and to give rise to a refradive index difference between the exposed and unexposed regions of the body. The removing of any unaltered photolysable material can be carried out by water washing the body after step (c). The filling of the remaining free volume of the microcavities with a polymerizable composition can be done by dipping the body in a dilute solution of said polymerizable composition and the polymerizing step can be performed in a conventional manner, for example by heating in an autoclave of the body soaked with the polymerizable composition. Whatever their form, the transition metal ions, for example Cr (III) ions, modifies the polymerization of a free radical polymerizable composition by influencing the stereoseledivity. As a result, the packing density of the polymer chains formed in the course of the polymerization differs from that of the chains formed by a common free-radical polymerization process (without said ions) and, consequently, a polymer of different refradive index is obtained. Illustrative of polymerizable compositions which can be used in the instant invention are polymerizable compositions based on polyol (allyl carbonate) such as diethylene glycol bis (allyl carbonate) marketed under the trademark CR-39®, and mixtures thereof with copoiymerizable monomers such as vinyl acetate or oligourethanes having terminal dimethacrylate fundionality ; alkyl (meth)acrylates, for example methyl methacrylate (MMA) ; vinyl - containing monomers, for example vinyl acetate (VA), styrenes and mixtures of styrene with copoiymerizable monomers such as MMA, VA, or acrylonitrile. These compositions will comprise usually, in addition to the monomer(s), free-radical initiators such as peroxides or azo compounds. The possibility of pradical realization of the proposed invention is illustrated by the following examples described below. During experiments diffradion efficiency of holograms (η) was measured on different stage of porous glass hologram preparation. The first set of measurements (ηem) was made after exposure and development, with air in the free volume of hologram. The second set of measurements (η rf ) corresponds to a hologram filled with a liquid filling material with refradive index equal to the refradive index of the polymer filling material which will be used in the third stage of hologram preparation. The third set of measurements (η m f) was performed after completion of the polymerization process. The coefficients of hologram amplification defined as : were calculated on the basis of the experimental data obtained. Fig. 2 represents the dependencies of the diffradion efficiency versus the grating power (χ) : circles (•) correspond to a hologram with a polyfdiethylene glycol bis(allyl carbonate)] (PCR-39) filling material and squares (D) to a hologram with a poly(methyl methacrylate) (PMMA) filling material. These data conform to experiments where the maximum values of ^ and K 2 are attained. Table 1 represents the coefficients of amplification for both cases. One can see that coefficients K 2 of amplification of 5 to 70 times were achieved with a spatially modulated index of polymeric filling material, with resped to the same hologram with a liquid filling material of uniform index. It should be noted that by filling the hologram with a spatially modulated polymeric filling material, its amplification with resped to the same hologram filled with air, is equal to - 2.0 for PCR-39 and to 4.5 for PMMA filling material. Data presented in the Table 1 show also that the values of Ki and K do not essentially depend on the type of porous body and on the kind of cation (Na + or NH4 + ) in the photolysable substance. Furthermore, in the case of a PMMA filling material, the values of K, and K 2 increase with a decrease in exposure. Thus the method of preparation of a hologram according to the present invention permits to increase substantially the light-sensitivity of the hologram recording medium. Furthermore, after filling the hologram with a polymeric filling material, the level of scattering decrease significantly. So, the transmittance of the hologram without filling material at λ = 488 nm is equal to τ = 0.75, but after the introdudion of a polymer into the hologram volume, τ increases up to τ = 0.90 Example 1 1. A porous highly siliceous glass (mean pore radius « 60-70 A ; volume fradion of pores « 30 ± 1 %) was used in the form of discs of 40 mm diameter and 1.5 mm thick (Glass of Type 1 in Table 1 ). This glass was obtained by acid leaching of a borosilicate glass of the following composition : SiO 2 - 61.12% ; B 2 O 3 - 28.03% ; Na 2 O - 7.65% ; AI 2 O 3 - 3.17%, in % by weight. 2. The sensitization of said porous glass was done by impregnating it with a 1.25% by weight aqueous solution of (NH ) 2 Cr 2 O 7 in an amount corresponding to a 5-6 - fold excess of the solution volume relative to the porous glass volume and at 20° C until the optical density of the impregnated porous glass reaches 0.25-0.30 at a wavelength of 488 nm. Thereupon the porous glass was dried in air at the room temperature and the pore volume was filled by immersion in isopropyl alcohol to prevent any moisture to spoil the glass. 3. A transmission hologram was recorded with the use of argon laser light (488 nm) at an angle between the recording beams of 9,2°. The exposure energy was 0.5 J/cm 2 . After recording the porous body was dried in air to remove isopropyl alcohol, then washed with distilled water at 20° C for 15 h, then at 70°C for 30 min. Thereafter, water was removed from the body by drying in air ; then in a oven at 120° C for4h. 4. The diffradion efficiency (η) of the hologram was measured at λ = 633 nm (He-Ne laser) for the dry porous body (ηβ m ) and for the body filled with a non- polymerisable filling material (o-xylene) (r\ m ). The values of the grating power of the hologram (χ) were calculated as follows : χ = arcsin Jη The diffradion efficiency as a fundion of the grating power is shown on Fig. 2 and the values of χ^, χ rf are shown in Table 1. After measurement the body was dried under vacuum (10 2 mm Hg) at room temperature (3h) and at 100°C (3h). 5. The filling of the porous hologram with a polymeric filling material was carried out in the following manner : The dried body with the recorded hologram was impregnated with a 2% by weight solution of a free-radical polymerization initiator (azo-bis-isobutyronitrile) in methyl methacrylate (MMA) at room temperature. Special devices were prepared in advance for carrying out the polymerization. They comprised two silicate glass discs with plane-parallel working planes having a surface of optical quality, which were wrapped at their periphery by a cellophane film (3-4 layers). The working planes of those discs were preliminary treated with dichiorodimethylsilane for decreasing the adhesion of polymer to the silicate surface of the devices. The impregnated body bearing the recorded hologram was located inside the device. Additional initiator-monomer (MMA) solution had been poured into the device so that the body was completely immersed in it. The device was placed in an autodave wherein a pressure of inert gas (Ar ou N 2 ) , 8 atm, was exerted. The polymerization process was achieved by heating the autoclave in a liquid thermostat according to the following temperature schedule : Holding at -0°C - 24 h, Raising temperature to 100° C at 10° C h rate, Holding at 100°C - 2.0 h, and Lowering temperature to room temperature at 10°C h rate. When the pressure in the autoclave was decreased to atmospheric one, the autoclave was opened and the body was pulled out of the device. 6. The diffradion efficiency of the polymer-filled body (ηmf) was measured and the value of • χ fti was calculated (see step A). The amplification coefficients (K 1? K 2 ) were calculated according to expression (4) and are given in Table 1. Example 2 The steps 1 -6 of Example 1 were repeated except for using an exposure energy for the hologram recording equal to 3 J/cm 2 . All experimental data are presented in Table 1. Example 3 The steps 1-6 of Example 1 were repeated except for : - the exposure energy for the hologram recording which was equal to 3 J/cm 2 (step 3). - a 2% by weight initiator (benzoyi peroxide) solution in CR-39 (monomer) was used for impregnating the porous glass and for forming the polymeric filling material in the pore volume (step 5). The process of polymerization was carried out by heating the autoclave in a liquid thermostat according to the following temperature schedule : Holding at 60°C - 24h ; Holding at 80°C - 65 h ; Raising temperature to 100°C at 10°C/h rate ; Holding at 100°C - 1.5 h ; and Lowering temperature to tom at 10°C/h rate. All experimental data are presented on Fig. 2 and in Table 1. [Example 4 The steps 1-6 of example 1 were repeated except that - a porous highly siliceous glass (mean pore radius » 40 - 50 A ; volume fradion of pores « 26 ± 1 %) was used (Glass of Type II in Table 1 ). It was obtained by add leaching of a borosilicate glass of the following composition in % by weight : SiO 2 - 67.5% ; B2θ3-24.6% ; Na 2 θ - 7.9% ; AI 2 O 3 - 0.5% (step 1). - the sensitization of the porous glass was effeded by impregnation with a 10% by weight aqueous solution of Na ∑ C^O ? (step 2). All experimental data are presented in Table 1

Claims: CLAIMS 1. A volume phase hologram having an interference pattern recorded in the form of local variations of refradive index, which comprises : - a porous transparent siliceous body having a plurality of mutually interconneded microcavities or pores the mean radius of which is lower than the wavelength of the hologram recording light and the wavelength of the hologram reading light, - a photolysis produd of a photolysable material, said produd being attached to walls of certain microcavities and being spatially distributed in accordance with the recorded interference pattern, and - a solid transparent polymeric material filling said microcavities, charaderized in that said polymeric material exhibits local variations of refradive index, said variations being spatially modulated in conformity with the recorded interference pattern, said photolysis produd being a polymerization modifier for a composition polymerizable into said polymeric filling material. 2. A hologram according to claim 1 , wherein said polymeric material is a polymer or copolymer of a polyol (allyl carbonate) monomer. 3. A hologram according to claim 2, wherein said polyol (allyl carbonate) monomer is diethylene glycol bis-(allyl carbonate). 4. A hologram according to claim 1 , wherein said polymeric material is a polymer of at least one alkylacrylate. 5. A hologram according to claim 1 , wherein said polymeric material is a polymer or copolymer of at least one vinyl monomer. 6. A hologram according to claim 2, wherein said polymeric material is a copolymer of a polyol (allyl carbonate) monomer with at least another copoiymerizable material seleded from the group consisting essentially of vinyl acetate and oligo-urethanes having terminal dimethacrylate fundionality. 7. A hologram according to claim 1 , wherein said polymeric material is a copolymer of styrene with at least one other copoiymerizable monomer seleded from the group consisting essentially of methyl methacrylate, vinyl acetate and acrylonitrile. 8. A hologram according to any of claims 1 to 7, wherein said porous body is made of porous highly siliceous glass produced by acid etching of a phase separated glass. 9. A hologram according to any of claims 1 to 7, wherein said porous body is made of a porous glass produced by the sol-gel process. 10. A hologram according to any of claims 1 to 9, wherein said polymerisation modifier comprises ions of transition metals. 11. A hologram according to any of claims 1 to 10, wherein said polymerisation modifier is a produd of a photolysable organometallic compound. 12. A hologram according to claim 11 , wherein said organometallic compound is seleded from the group consisting of (Mn 2 (CO)ι 0 , CO 2 (CO 8 ), Cr(CO) 6 , Mo(CO) 6 , and Ti(cyclopentadiene) 2 C.2)). 13. A hologram according to any of claims 1 to 10, wherein said polymerization modifier is a photolysis produd of a Cr salt. 14. A hologram according to claim 13, wherein said Cr™ salt is seleded from the group consisting essentially of (NH 4 ) 2 Cr 2 O 7 , Na 2 Cr 2 O 7 , and f^Cr O 7 . 15. A hologram according to any of daims 1 -14, wherein said polymerization modifier is diredly attached to the walls of microcavities of said body. 16. A hologram according to any of claims 1-14, wherein said polymerization modifier is contained in a layer coating the walls of microcavities of said body. 17. A hologram according to claim 16, wherein said layer is comprised of a photoresist material. 18. A hologram according to daim 17 wherein said photoresist material is made of gelatin and said polymerization modifier is Cr 1 " ions. 19. A hologram according to claim 17, wherein said photoresist material is made of poly(vinyl alcohol). 20. A hologram according to daim 17, wherein said photoresist material is made of Shellac. 21. An optical device containing a light source and at least one optical element charaderized in that said at least one optical element is an hologram according to any of claims from 1 to 21. 22. A method for producing the hologram according to anyone of the preceding claims, comprising the following steps: a) providing a porous transparent siliceous body having a plurality of mutually interconneded microcavities or pores, the mean radius of which is lower than the wavelength of the hologram recording light and the wavelength of the hologram reading light ; b) coating the walls of said microcavities with a photolysable material a photolysis produd of which is a polymerization modifier for at least one predetermined polymerizable composition ; c) recording a hologram interference pattern within said material so as to leave said photolysis produd distributed according to said pattern onto walls of certain of said microcavities ; d) removing any unaltered photolysable material ; e) filling the remaining free volume of said microcavities with said at least one polymerizable composition ; and f) polymerizing said composition so as to obtain a volume phase hologram made of a transparent body filled with a solid polymeric material with spatial modulated refractive index the period of modulation of reftactive index of said polymer being the same as the period of spatial distribution of said photolysis products. Table 1

Inventors: KURSAKOVA ALYA M, SHELEKHOV NIKITA, SUKHANOV VITALY I, KURSAKOVA, ALYA M., SHELEKHOV, NIKITA, SUKHANOV, VITALY I.
Applicants: CORNING INC, KURSAKOVA ALYA M, SHELEKHOV NIKITA, SUKHANOV VITALY I, CORNING INCORPORATED, KURSAKOVA, ALYA M., SHELEKHOV, NIKITA, SUKHANOV, VITALY I.
Application year: 1998
Main classification: G03H1
IPC codes: G01H1, G03F7, G03H1