Journal of Photopolymer Science and Technology Volume 4, Number 1(1991) 127-134 DEVELOPMENT PROCESS FOR PVCz HOLOGRAM Yasuo YAMAGISHI, Takeshi ISHITSUKA, and Yasuhiro YONEDA Fujitsu Laboratories Ltd. Morinosato Wakamiya Atsugi, Kanagawa 243-01 Japan We investigated the formation of index modulation for volume holograms consisting of poly-n-vinylcarbazole (PVCz), and achieved a new process that enables large holograms and uniform quality. Conventionally PVCz holograms are developed by two sequential dipping process: first into good solvent, then into poor solvent. When a swollen PVCz film is dipped into poor solvent, PVCz molecules precipitate into small grains. In the upper layer small grains can pack closely, because the solvent in the film can escape outside. However, the solvent in the lower layer is prevented from escaping by the closely packed upper layer, and the remaining solvent forms gaps corresponding to the degree of swelling. As the result of the difference in swelling between highly exposed and low exposed areas, refractive index modulation appears. Based on the investigation above, we found a new process where the holograms are developed by single dipping process into a mixture of volatile good solvent and nonvolatile poor solvent. When the film swollen by the mixture is carried out, the solvent in the film becomes poor solvent rich and small grains are formed. With this new process all areas of a hologram can be developed under same conditions, so large holograms with uniform quality becomes possible. 1. Introduction Volume holograms have been used as high-function optical elements, for such applications bar code readers [1,2] and head-up displays (HUDs) [3], because various optical functions can be integrated into a thin film. They have also created the new field of holographic art. Dichromated gelatin has been widely used for volume holograms because recording films are easily obtained and Received Accepted April 17, May 20, 1991 1991 127
large holograms are possible, even though the material is very susceptible to humidity so needs a tough cover. More stable material is required to make the reliability of equipments up, or to hand down works of art to posterity. The material consisting of poly-n-vinylcarbazole (PVCz) is very resistant to humidity [4], though several problems are present themselves. The most serious problem with PVCz holograms is the difficulties in controlling developing conditions. PVCz holograms are developed by the same process used in dichromated gelatin: first dipping into good solvent, then into poor solvent [4]. In this process, it is difficult to obtain a large hologram, because the upper area dries more than the lower area when the swollen film is carried to poor solvent. In this paper we discuss the formation of refractive index modulation in PVCz hologram, and describe a new development process which enables large hologram with uniform quality. 2. Experimental Material: Poly-N-vinylcarbazole was used as a base polymer. Iodoform and rubren were chosen as the initiator and sensitizer respectively. Polycarbonate was added to improve the diffraction efficiency. A certain amount of these materials are dissolved in the solvent, the mixture of dichlorobenzene and tetrahydrofuran, then filtered. Photosensitive PVCz film was spin-coated on a glass substrate from the solution to a thickness of 6-10 p.m and dried at 60 C. Fabricating method: Latent patterns of transmission holograms or reflection hologram were recorded into photosensitive PVCz films with an Ar ion laser light (~, = 488 or 515 nm) which intensity on the film was about 1 mw/cm2. After exposure iodoform and rubren were extracted by dipping into xylene, then the films were developed following. (a) Conventional process: First the films were swollen with good solvent of xylene toluene mixture. Then the swollen films were raised slowly and dipped into poor solvent of pentane. (b) New process: The films were dipped into a mixture of volatile good solvent and nonvolatile poor solvent. Then swollen films were raised slowly and dried. Measurement: Refractive index and film thickness of PVCz film were measured at the same time before and after development using a prism film coupler (Metricon LTD. PC-2000). In this method laser light (He-Ne laser) is injected into a film through a prism (Fig. 1). The light can be guided in the film when the incident angle is 9m as specified by following equation [5]. 27t1A, d ni cosom - 2 4 12-2 4io = 2 it m m=1, 2, 3, (1) tang t12 = (11e24) / (n12-ne2 ) tang ~12 = (fle2 -not ) / (n12 -ne2 ) ne =n1 sinom 128
where (12 and ct o are the phase shift of the reflection, ni and no are refractive index of film and substrate respectively, and d is the thickness of film. We observed the cross section of a PVCz hologram by scanning electron microscope (SEM). Here the spatial frequency was 1000 line/mm. The hologram was broken after the cooled by liquid nitrogen. Diffraction efficiency of transmission hologram was measured by He-Ne laser light. The efficiency was defined as the power ratio of 1st order diffraction light for incident light. The efficiency of reflection hologram was measured by white light. Transmitted and reflected light was detected by multichannel spectrophotometer (Otsuka electronics LTD. MCPD-100). 3. Results and discussion Formation of index modulation: Fig. 2 shows the cross section of hologram observed by SEM. Small grains and microgaps between grains are observed in the photograph. The gaps are increasing and decreasing as the cycle of 1.tm which corresponds to the interference fringe. Small gaps or voids reduces the average of refractive index according to the equation (2) [6]. n$2 ={3 n2-2 (n2-1) V} / {3 + (n2-1) V) (2) where na is the average of refractive index, n the index of the material, V the volume ratio of gaps. So microgaps, which are increasing and decreasing corresponds to the interference fringe, are thought to cause the index modulation. To know the reduction of refractive index, we measured the index of PVCz hologram with a prism film coupler. Fig. 3 shows the relationship between mode number and equivalent refractive index (ne), where the transmission hologram with spatial frequency of 2000 line/mm was used. For uniform film, ne decreases gradually and the curve has no inflection point. Though the coated film showed the curve having no inflection point, the curve of the developed PVCz hologram had an inflection point and separated into two areas as shown in Fig. 3. In the area of large 0, (mode number is small), ne appears higher. This is thought to mean that the upper area of the film has higher index than the lower (Fig. 4). Fig. 1. Prism film coupler used for measurements of the refractive index and film thickness. Fig. 2. Cross section PVCz hologram. of a developed 129
J. Photopolym. Sci. Technol., Vol. 4, No.1, 1991 Fig. 3. Equivalent refractive index measured by prism coupler. The solid line is measured and the d otted line is calculated. Fig. 4. Wave guide model for double layered film. When the incident angle is large, the light is guided only in the upper layer. We calculated the wave guide modes using equation (1), assuming double layered structure, and could fit the calculated curve in well with that of actual hologram. From the calculated curve shown as dotted line in Fig. 3, which was most likely fit to the measured curve, we estimated the index 1.648 for the upper layer, 1.569 for the lower layer, and the thickness 2.3 µm for the upper, 4.9 µm for the lower. The estimated index of upper layer is near to the index of PVCz (n=1.672), so it is thought that the grains are packed closely and hologram is not formed in the upper layer. This supposition will be supported by Fig. 5. efficiency. used. Incident angle dependence of the diffraction The same hologram that was shown in Fig. 3 was 130
Fig. 2 where fringes are not observed in the upper area. To conf i rm this supposition we measured the incident angle dependence on diffraction efficiency, because thinner hologram gives broader peak top for the efficiency curve. Fig. 5 shows the diffraction efficiency of the same hologram (same point) as measured in Fig. 3. It also shows the calculated efficiencies for 7.2 µm thick hologram (total thickness of the hologram) and 4.9 µm thick hologram (estimated thickness of the lower layer), using Moharam's way [7]. As shown in the figure, the curve of measured efficiency fitted in well with the calculated curve of 4.9 µm thick hologram. Refractive indices dependence on exposure energy also supports the existence of hologram in the lower layer (Fig. 6). In Fig. 6, PVCz films were developed after exposure with single light beam and holograms were not formed. As shown in the figure, only the indices of lower layers were reduced by the development and depended on the exposure energy. In the lower layer the reduction of index became smaller by light exposure, so a modulation of exposure energy results in a refractive index modulation. Fig. 6. Exposure energy dependence on the refractive indices of upper and lower layers. Interfarent pattern was not recorded. From these results, the refractive index modulation is thought to be formed through following (Fig. 7). When a exposed film is dipped in good solvent, low exposed area swells more than highly exposed area, because photo exposure reduces the solubility of PVCz for solvents. Next swollen film is carried to poor solvent, and solvent mixing starts from the upper layer. When the poor solvent reaches a critical density, PVCz molecules precipitate into small grains. In the upper layer small grains can pack closely, because the solvent in the medium can escape outside. On the other hand, the solvent in the lower layer will be prevented from escaping by the closely packed upper layer, and the remaining solvent will form gaps, As the amount of remaining solvent is thought to correspond to the degree of swelling, refractive index modulation will be formed. 131
I Photopolym. Sci. Technol., Vol. 4, No.1, 1991 Fig. 7. Formation of micro gaps during the development process. New developing process: Based on the results above, a new process has been developed. In the new process, an exposed film is swollen by the mixture of a volatile good solvent and a nonvolatile poor solvent. The mixture works as a good solvent. During the swollen film is raised, the mixture in the film becomes poor solvent and PVCz precipitates in small grains, because the good solvent vaporizes more than the poor solvent. In this way, swelling and precipitation are controlled to same condition in all area, so a large hologram with uniform quality can be realized. Table 1 shows the combinations of good and poor solvents which were able to develop PVCz hologram by the new process. Dichloromethane, dichloroethane and chlorobenzene were used as good solvents with different boiling point. Hydrocarbons of pentane, hexane, heptane, octane and nonane were used as poor solvents with different boiling point. As shown table 1, holograms were developed by the new process, when a volatile good solvent and a nonvolatile poor solvent were used. Table 1. Combinations of the solvents for new process. 132
Fig. 8 shows the diffraction efficiency of transmission type holograms with the spatial frequency of 2200 lines/mm. More than 70% of efficiency, which was almost same as that of the hologram developed by conventional process, was obtained. Fig. 9 shows the diffraction efficiency and transmittance of reflection hologram. The transmittance of violet light tended to decrease due to scattering caused by small grains. However this decrease is not thought to make serious problem in use of HUDs or Lipman hologram, because sensitivity of violet light is low for human eyes. Fig. 8. Diffraction efficiency of the developed by our new process. transmission holograms Fig. 9. Diffraction efficiency of the reflection holograms developed by our new process. 133
4. Conclusion We investigated the origin of the refractive index modulation of a PVCz hologram. Micro gaps between small grains reduce the refractive index. As the light exposure represses the reduction, refractive index modulation appears. We also found that a developed film has double layered structure and hologram is formed in the lower layer. Based on these results, we developed a new process, where hologram is developed by single dipping into a mixture of volatile good solvent and nonvolatile poor solvent. This new process enables large hologram with uniform quality, so PVCz hologram will be applied to various optical elements and holographic arts. Acnowledgment: The authors would like to thank H. Ikeda, F. Yamagishi and H. Okuyama for their suggestion and encouragement during the course of this work. Thanks also to Y. Kuramitsu, M. Tani and N. Ikeda for their discussion and support. w 6. Reffrence 1. K. Yamazaki et. al. "New holographic technology for a compact POS scanner," App!. Opt. Vol. 29, No. 11, pp. 1666-1670, April 1990. 2. H. Ikeda et. al. "Shallow-type truncated symbol-reading point-of-sale hologram scanner," Appl. Opt. Vol.24, pp. 1366-1370, 1985. 3. R. B. Wood and M. A. Thomas "A holographic head-up display for automotive applications," SPIE Vol. 958, June 1988. 4. Y. Yamagishi et. al. "Holographic recording material containing poly-n-vinylcarbazole," Proceedings of the SPIE, Vol. 600, pp.14-19, 1985. 5. R. Th. Kersten "Numerical solution of the mode-equation of planar dielectric waveguides to determine their refractive index and thickness by means of a prismfilm coupler," Opt. Commun. Vol. 9, No. 4, pp. 427-431, December 1973. K. Kinoshita "Progress in optics IV," pp. 115, 1965, North Holland Publishing Company, Amsterdam. 7. M. G. Moharam and T. K. Gaylord "Rigorous coupled-wave analysis of planar-grating diffraction," J. Opt. Soc. Am. Vol. 71, No. 7, pp. 811-818, July 1981. 134