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Experimental study of the acrylamide photopolymer with a pulsed laser
1 February 2001
Optics Communications 188 (2001) 163±166
www.elsevier.com/locate/optcom
Experimental study of the acrylamide photopolymer with a pulsed laser C. Garcõa a,*, I. Pascual a, A. Costela b, I. Garcõa-Moreno b, A. Fimia c, R. Sastre d Departamento Interuniversitario de Optica, Universidade de Alicante, Apdo. 99, E-03080 Alicante, Spain b Instituto de Quõmica-Fõsica ``Rocasolano'', CSIC, Serrano 119, E-28006 Madrid, Spain c Departamento de Ciencia y Tecnologõa de Materiales, Divisi on de Optica, Universidad Miguel Hern andez, Av. del Ferrocarril s/n, E-03202 Elche, Spain d Instituto de Quõmica Org anica e Instituto de Ciencia y Tecnologõa de Polõmeros, CSIC, Juan de la Cierva 3, E-28006 Madrid, Spain a
Received 26 September 2000; accepted 29 November 2000
Abstract We have demonstrated that holograms may be recorded in polyvinyl alcohol/acrylamide photopolymer dry ®lms using pulsed laser exposure with a pulse length of 8 ns. We also studied the eect of the pulse ¯uency together with the number of pulses necessary to obtain maximum diraction eciency. The recording was performed using a holographic copying process. The original was a grating of 1000 lines/mm processed using silver halide sensitized gelatin. Diraction eciencies of 55% were obtained with sensitivities similar to those reached with the same material and cw exposure, without the need for pre-processing or ®nal processing of the gratings. Ó 2001 Published by Elsevier Science B.V. Keywords: Holography; Holographic recording materials; Photopolymers
In previous papers, the optimization and mechanism of hologram recording in polyvinyl alcohol (PVA)/acrylamide (AA) photopolymer material using cw-laser (both Argon [1] and He± Ne [2]) have been analyzed. The formation of gratings in these photopolymerizable materials is obtained by the polymerization of AA in the areas of maximum light intensity. Consequently the index modulation is basically related to the dierence between the index of the polymer formed and that of the unreacted monomer in the non-illuminated zones. The photopolymerization is initiated
*
Corresponding author. Tel./fax: +34-965-903791. E-mail address: [email protected] (C. Garcõa).
by the absorption of light by the initiator, which generates free radicals that react with the monomer and initiate the chain polymerization reaction. These studies assume a stationary state for the production of free radicals that initiate the photopolymerization in the illuminated zones, thereby generating a high modulation of the refractive index enabling diraction eciencies of 85% to be obtained. The formation of dynamic holograms by ultrashort light pulses has been analyzed in dierent holographic recording materials [3±5]. In photorefractive materials [3] the technique used combines a pulsed signal beam with a continuous reference beam. In this way, the diraction eciency can be maximized and greatly increased in
0030-4018/01/$ - see front matter Ó 2001 Published by Elsevier Science B.V. PII: S 0 0 3 0 - 4 0 1 8 ( 0 0 ) 0 1 1 5 5 - X
164
C. Garcõa et al. / Optics Communications 188 (2001) 163±166
comparison with the traditional continuous recording technique. It has been demonstrated that re¯ection holograms may be recorded in Dupont photopolymer ®lms by means of pulsed laser exposure [4], and it was observed that continuous incoherent pre-illumination increased the diraction eciency and the sensitivity. In this paper we present the experimental results obtained when holograms are stored, using pulsed beams, in a PVA/AA photopolymer deposited in the form of a layer. We also study the eect of the pulse ¯uency as well as the number of pulses used during irradiation in order to obtain maximum diraction eciency. The holograms were generated by means of a copying process [6,7]. This method enables transmission gratings to be obtained without having to make the optical paths of the object and reference beams coincide when a pulsed laser is used. The material used was a photopolymer consisting of AA as monomer, triethanolamine (TEA) as radical generator and yellowish eosin (YE) as sensitizer, all on a ®lm of PVA. The photosensitive aqueous solution was prepared when over 40 ml of 10 wt.% PVA (provided by Riedel-de-Haen and Mw 25 000), 2 ml of a solution of YE (provided by Panreac and Mw 691:86) and 6.4 ml of a solution of AA (provided by Sigma and Mw 71:09) and TEA (provided by Sigma and Mw 149:09) were added. The concentration of the composition is summarized in Table 1. The ®nal solution was coated, using an automatic depositor with an initial thickness of 500 lm, on a 20 40 cm2 glass plate, which was left to dry in the dark for 24 h under normal laboratory conditions (T 22°C, RH 60%). The thickness of the layer was measured with a PIG 455 apparatus supplied by Neurtek. The ®nal thickness of the layer was 70 5 lm. The refractive index of the layer was 1.492.
Table 1 Concentration of the composition TEA AA PVA YE
0.199 M 0.446 M 10% 2:5 10
4
M
It is not possible to increase the concentration of AA inde®nitely because the compatibility and the solubility of this monomer in the polymer ®lm are limited. At high concentrations of AA this monomer precipitates over the surface of the ®lm. The maximum concentration of AA without precipitate is of 0.45 M for an initial dissolution of PVA of 10% in weight, this gives rise, when the water has been evaporated, to dry ®lms of 70 lm. The TEA does not has solubility problems in the polymer when the concentration have been increased. As mentioned above, a copying process was used to generate the holograms. We used an original grating of 1000 lines/mm processed in silver halide sensitized gelatin (SHSG), with a beam ratio of 1:2 and total transmittance of 75%. We exposed the samples by recording holograms with a collimated beam from a frequency-doubled Nd±YAG (532 nm) Q-switched laser. The pulse duration was 8 ns and the repetition frequency varied between 3 and 10 Hz. The pulse ¯uency was increased from 0.07 to 3.3 mJ/cm2 . It has been calculated the spatial and temporal coherence factors for the system [7]. These factors are greater of 0.9, what indicates that it is possible to obtain good holographic elements with this holographic system. Firstly, the variation in transmittance of the material for the emission wavelength of the laser, k 532 nm, as a function of the number of pulses and ¯uency per pulse was analyzed. To do so, the laser beam was made to strike the plate perpendicularly. The transmittance was measured with a Cary 3E spectrophotometer to obtain information about the photochemical behavior of the dye. Fig. 1 shows the variation in transmittance of the material as a function of the number of pulses for dierent pulse ¯uencies. This transmittance variation re¯ects that the material is bleached when illuminated with light at a wavelength of 532 nm, which corresponds to the absorption band of YE. Thus incident light produces a consumption of the sensitizer by means of a photoreduction reaction of the dye, giving rise to the formation of free radicals which initiate the polymerization reaction. When the ¯uency per pulse is increased, the variation in transmittance for a given number of pulses increases. This implies a greater consumption of the
C. Garcõa et al. / Optics Communications 188 (2001) 163±166
Fig. 1. Variation in transmittance as a function of the number of pulses for a pulse frequency of 3 Hz.
Fig. 2. Diraction eciency as a function of exposure for different energies per pulse and a frequency of 10 Hz.
dye (bleaching) as well as an increase in the rate of generation of free radicals. Fig. 2 shows the diraction eciency results of the 1 order in the case of dierent energies per pulse, measured with an He±Ne laser with emission at 633 nm, at which wavelength the material does not absorb. It can be seen that diraction eciencies of 55% have been reached for a pulse ¯uency of 0.67 mJ/cm2 . The sensitivity of the material in this case (200 mJ/cm2 ) is similar to that obtained with continuous irradiation [8], if we take the sum of the energy of each pulse. However, the diraction eciency decreases from 80% for continuous irradiation to 60% for irradiation with a
165
Fig. 3. Maximum diraction eciency (s) and sensitivity ( ) as a function of the ¯uency per pulse.
pulsed laser. If the values of maximum diraction eciency and energetic sensitivity (de®ned as the minimum energy necessary to reach maximum diraction eciency) are plotted against ¯uency per pulse (Fig. 3), it can be seen that there is an optimum ¯uency per pulse (0.67 mJ/cm2 ) at which the maximum diraction eciency with maximum sensitivity is obtained. For energies below 0.67 mJ/ cm2 , lower diraction eciencies and sensitivities are obtained. For energies greater than 0.67 mJ/ cm2 , the diraction eciency does not vary but the sensitivity decreases. Another factor which aects the response of the material is the pulse frequency. In the theoretical models that explain polymerization with continuous irradiation, a stationary state is assumed for the production and consumption of radicals that initiate the polymerization reaction. Therefore, at t 0 the concentration of radicals is zero a minimum time is necessary to reach the stationary state. When the irradiation is pulsed, the generation of free radicals is discontinuous. Consequently, the time during which the sample is not irradiated is important, since the number of radicals decreases during this time and so the rate of polymerization decreases. An increase in the irradiation frequency results in a reduction in the time during which there is no irradiation and therefore comes closer to the stationary state as in the case of continuous irradiation. This can be seen in Fig. 4 which shows the energetic evolution of the diffraction eciency for two dierent repetition frequencies, 3 and 10 Hz and a pulse ¯uency of
166
C. Garcõa et al. / Optics Communications 188 (2001) 163±166
Fig. 4. Diraction eciency as a function of exposure for two dierent frequencies.
Fig. 6. Energetic evolution of diraction eciency for continuous exposure with an incident intensity of 10 mW/cm2 (d) and pulsed exposure with an incident intensity of 8:4 107 mW/cm2 equivalent to a ¯uency per pulse of 0.67 mJ/cm2 (s).
tained using the same material and continuous beams (Fig. 6) without the need for pre-processing or ®nal processing of the so obtained gratings. Preexposure might improve these results. Further studies will be done in order to analyze the role played by the dye as the fundamental component in the activity of these photopolymer materials.
Acknowledgements Fig. 5. Number of pulses (s) and exposure (d) necessary to reach a diraction eciency of 4% as a function of ¯uency per pulse.
3.3 mJ/cm2 . As can be seen, the eciency reached at high frequencies is clearly greater than that obtained at low frequencies, and the sensitivity increases as the frequency is increased. In order to analyze the possibility of storing information, we have determined the pulse ¯uency and number of pulses necessary to obtain gratings with a diraction eciency of 4% (Fig. 5). It can be seen that it is possible to obtain these gratings with a sensitivity of 60 mJ/cm2 and 60 pulses. To conclude, we can say that we have demonstrated the possibility of storing, using pulsed beams, diraction gratings of 1000 lines/mm in a PVA/AA photopolymer with a diraction eciency of 60% and sensitivity similar to that ob-
This work was ®nanced by the Comisi on Interministerial de Ciencia y Tecnologõa (CICYT) of Spain (project MAT 97-0705-C02-02).
References [1] S. Martin, P.E.L.G. Leclere, Y.L.M. Renotte, V. Toal, Y.F. Lion, Opt. Eng. 33 (1994) 3942±3946. [2] S. Blaya, L. Carretero, R. Mallavia, A. Fimia, R.F. Madrigal, Appl. Opt. 38 (1999) 955±962. [3] G. Li, L. Liu, B. Liu, Z. Xu, Opt. Lett. 23 (1998) 1307±1309. [4] K.T. Weitzel, U.P. Wild, V.N. Mikhailov, V.N. Krylov, Opt. Lett. 22 (1997) 1899±1901. [5] A.V. Aristov, Y.E. Burunkova, D.A. Kozlovskii, A.B. Nikolaev, J. Opt. Technol. 66 (1999) 383±386. [6] I. Pascual, A. Belendez, A. Fimia, J. Opt. 22 (1991) 135±140. [7] I. Pascual, A. Belendez, A. Fimia, Appl. Opt. 31 (1992) 3312±3318. [8] C. Garcõa, A. Fimia, I. Pascual, Proc. SPIE 3956 (2000) 367±374.
Optics Communications 188 (2001) 163±166
www.elsevier.com/locate/optcom
Experimental study of the acrylamide photopolymer with a pulsed laser C. Garcõa a,*, I. Pascual a, A. Costela b, I. Garcõa-Moreno b, A. Fimia c, R. Sastre d Departamento Interuniversitario de Optica, Universidade de Alicante, Apdo. 99, E-03080 Alicante, Spain b Instituto de Quõmica-Fõsica ``Rocasolano'', CSIC, Serrano 119, E-28006 Madrid, Spain c Departamento de Ciencia y Tecnologõa de Materiales, Divisi on de Optica, Universidad Miguel Hern andez, Av. del Ferrocarril s/n, E-03202 Elche, Spain d Instituto de Quõmica Org anica e Instituto de Ciencia y Tecnologõa de Polõmeros, CSIC, Juan de la Cierva 3, E-28006 Madrid, Spain a
Received 26 September 2000; accepted 29 November 2000
Abstract We have demonstrated that holograms may be recorded in polyvinyl alcohol/acrylamide photopolymer dry ®lms using pulsed laser exposure with a pulse length of 8 ns. We also studied the eect of the pulse ¯uency together with the number of pulses necessary to obtain maximum diraction eciency. The recording was performed using a holographic copying process. The original was a grating of 1000 lines/mm processed using silver halide sensitized gelatin. Diraction eciencies of 55% were obtained with sensitivities similar to those reached with the same material and cw exposure, without the need for pre-processing or ®nal processing of the gratings. Ó 2001 Published by Elsevier Science B.V. Keywords: Holography; Holographic recording materials; Photopolymers
In previous papers, the optimization and mechanism of hologram recording in polyvinyl alcohol (PVA)/acrylamide (AA) photopolymer material using cw-laser (both Argon [1] and He± Ne [2]) have been analyzed. The formation of gratings in these photopolymerizable materials is obtained by the polymerization of AA in the areas of maximum light intensity. Consequently the index modulation is basically related to the dierence between the index of the polymer formed and that of the unreacted monomer in the non-illuminated zones. The photopolymerization is initiated
*
Corresponding author. Tel./fax: +34-965-903791. E-mail address: [email protected] (C. Garcõa).
by the absorption of light by the initiator, which generates free radicals that react with the monomer and initiate the chain polymerization reaction. These studies assume a stationary state for the production of free radicals that initiate the photopolymerization in the illuminated zones, thereby generating a high modulation of the refractive index enabling diraction eciencies of 85% to be obtained. The formation of dynamic holograms by ultrashort light pulses has been analyzed in dierent holographic recording materials [3±5]. In photorefractive materials [3] the technique used combines a pulsed signal beam with a continuous reference beam. In this way, the diraction eciency can be maximized and greatly increased in
0030-4018/01/$ - see front matter Ó 2001 Published by Elsevier Science B.V. PII: S 0 0 3 0 - 4 0 1 8 ( 0 0 ) 0 1 1 5 5 - X
164
C. Garcõa et al. / Optics Communications 188 (2001) 163±166
comparison with the traditional continuous recording technique. It has been demonstrated that re¯ection holograms may be recorded in Dupont photopolymer ®lms by means of pulsed laser exposure [4], and it was observed that continuous incoherent pre-illumination increased the diraction eciency and the sensitivity. In this paper we present the experimental results obtained when holograms are stored, using pulsed beams, in a PVA/AA photopolymer deposited in the form of a layer. We also study the eect of the pulse ¯uency as well as the number of pulses used during irradiation in order to obtain maximum diraction eciency. The holograms were generated by means of a copying process [6,7]. This method enables transmission gratings to be obtained without having to make the optical paths of the object and reference beams coincide when a pulsed laser is used. The material used was a photopolymer consisting of AA as monomer, triethanolamine (TEA) as radical generator and yellowish eosin (YE) as sensitizer, all on a ®lm of PVA. The photosensitive aqueous solution was prepared when over 40 ml of 10 wt.% PVA (provided by Riedel-de-Haen and Mw 25 000), 2 ml of a solution of YE (provided by Panreac and Mw 691:86) and 6.4 ml of a solution of AA (provided by Sigma and Mw 71:09) and TEA (provided by Sigma and Mw 149:09) were added. The concentration of the composition is summarized in Table 1. The ®nal solution was coated, using an automatic depositor with an initial thickness of 500 lm, on a 20 40 cm2 glass plate, which was left to dry in the dark for 24 h under normal laboratory conditions (T 22°C, RH 60%). The thickness of the layer was measured with a PIG 455 apparatus supplied by Neurtek. The ®nal thickness of the layer was 70 5 lm. The refractive index of the layer was 1.492.
Table 1 Concentration of the composition TEA AA PVA YE
0.199 M 0.446 M 10% 2:5 10
4
M
It is not possible to increase the concentration of AA inde®nitely because the compatibility and the solubility of this monomer in the polymer ®lm are limited. At high concentrations of AA this monomer precipitates over the surface of the ®lm. The maximum concentration of AA without precipitate is of 0.45 M for an initial dissolution of PVA of 10% in weight, this gives rise, when the water has been evaporated, to dry ®lms of 70 lm. The TEA does not has solubility problems in the polymer when the concentration have been increased. As mentioned above, a copying process was used to generate the holograms. We used an original grating of 1000 lines/mm processed in silver halide sensitized gelatin (SHSG), with a beam ratio of 1:2 and total transmittance of 75%. We exposed the samples by recording holograms with a collimated beam from a frequency-doubled Nd±YAG (532 nm) Q-switched laser. The pulse duration was 8 ns and the repetition frequency varied between 3 and 10 Hz. The pulse ¯uency was increased from 0.07 to 3.3 mJ/cm2 . It has been calculated the spatial and temporal coherence factors for the system [7]. These factors are greater of 0.9, what indicates that it is possible to obtain good holographic elements with this holographic system. Firstly, the variation in transmittance of the material for the emission wavelength of the laser, k 532 nm, as a function of the number of pulses and ¯uency per pulse was analyzed. To do so, the laser beam was made to strike the plate perpendicularly. The transmittance was measured with a Cary 3E spectrophotometer to obtain information about the photochemical behavior of the dye. Fig. 1 shows the variation in transmittance of the material as a function of the number of pulses for dierent pulse ¯uencies. This transmittance variation re¯ects that the material is bleached when illuminated with light at a wavelength of 532 nm, which corresponds to the absorption band of YE. Thus incident light produces a consumption of the sensitizer by means of a photoreduction reaction of the dye, giving rise to the formation of free radicals which initiate the polymerization reaction. When the ¯uency per pulse is increased, the variation in transmittance for a given number of pulses increases. This implies a greater consumption of the
C. Garcõa et al. / Optics Communications 188 (2001) 163±166
Fig. 1. Variation in transmittance as a function of the number of pulses for a pulse frequency of 3 Hz.
Fig. 2. Diraction eciency as a function of exposure for different energies per pulse and a frequency of 10 Hz.
dye (bleaching) as well as an increase in the rate of generation of free radicals. Fig. 2 shows the diraction eciency results of the 1 order in the case of dierent energies per pulse, measured with an He±Ne laser with emission at 633 nm, at which wavelength the material does not absorb. It can be seen that diraction eciencies of 55% have been reached for a pulse ¯uency of 0.67 mJ/cm2 . The sensitivity of the material in this case (200 mJ/cm2 ) is similar to that obtained with continuous irradiation [8], if we take the sum of the energy of each pulse. However, the diraction eciency decreases from 80% for continuous irradiation to 60% for irradiation with a
165
Fig. 3. Maximum diraction eciency (s) and sensitivity ( ) as a function of the ¯uency per pulse.
pulsed laser. If the values of maximum diraction eciency and energetic sensitivity (de®ned as the minimum energy necessary to reach maximum diraction eciency) are plotted against ¯uency per pulse (Fig. 3), it can be seen that there is an optimum ¯uency per pulse (0.67 mJ/cm2 ) at which the maximum diraction eciency with maximum sensitivity is obtained. For energies below 0.67 mJ/ cm2 , lower diraction eciencies and sensitivities are obtained. For energies greater than 0.67 mJ/ cm2 , the diraction eciency does not vary but the sensitivity decreases. Another factor which aects the response of the material is the pulse frequency. In the theoretical models that explain polymerization with continuous irradiation, a stationary state is assumed for the production and consumption of radicals that initiate the polymerization reaction. Therefore, at t 0 the concentration of radicals is zero a minimum time is necessary to reach the stationary state. When the irradiation is pulsed, the generation of free radicals is discontinuous. Consequently, the time during which the sample is not irradiated is important, since the number of radicals decreases during this time and so the rate of polymerization decreases. An increase in the irradiation frequency results in a reduction in the time during which there is no irradiation and therefore comes closer to the stationary state as in the case of continuous irradiation. This can be seen in Fig. 4 which shows the energetic evolution of the diffraction eciency for two dierent repetition frequencies, 3 and 10 Hz and a pulse ¯uency of
166
C. Garcõa et al. / Optics Communications 188 (2001) 163±166
Fig. 4. Diraction eciency as a function of exposure for two dierent frequencies.
Fig. 6. Energetic evolution of diraction eciency for continuous exposure with an incident intensity of 10 mW/cm2 (d) and pulsed exposure with an incident intensity of 8:4 107 mW/cm2 equivalent to a ¯uency per pulse of 0.67 mJ/cm2 (s).
tained using the same material and continuous beams (Fig. 6) without the need for pre-processing or ®nal processing of the so obtained gratings. Preexposure might improve these results. Further studies will be done in order to analyze the role played by the dye as the fundamental component in the activity of these photopolymer materials.
Acknowledgements Fig. 5. Number of pulses (s) and exposure (d) necessary to reach a diraction eciency of 4% as a function of ¯uency per pulse.
3.3 mJ/cm2 . As can be seen, the eciency reached at high frequencies is clearly greater than that obtained at low frequencies, and the sensitivity increases as the frequency is increased. In order to analyze the possibility of storing information, we have determined the pulse ¯uency and number of pulses necessary to obtain gratings with a diraction eciency of 4% (Fig. 5). It can be seen that it is possible to obtain these gratings with a sensitivity of 60 mJ/cm2 and 60 pulses. To conclude, we can say that we have demonstrated the possibility of storing, using pulsed beams, diraction gratings of 1000 lines/mm in a PVA/AA photopolymer with a diraction eciency of 60% and sensitivity similar to that ob-
This work was ®nanced by the Comisi on Interministerial de Ciencia y Tecnologõa (CICYT) of Spain (project MAT 97-0705-C02-02).
References [1] S. Martin, P.E.L.G. Leclere, Y.L.M. Renotte, V. Toal, Y.F. Lion, Opt. Eng. 33 (1994) 3942±3946. [2] S. Blaya, L. Carretero, R. Mallavia, A. Fimia, R.F. Madrigal, Appl. Opt. 38 (1999) 955±962. [3] G. Li, L. Liu, B. Liu, Z. Xu, Opt. Lett. 23 (1998) 1307±1309. [4] K.T. Weitzel, U.P. Wild, V.N. Mikhailov, V.N. Krylov, Opt. Lett. 22 (1997) 1899±1901. [5] A.V. Aristov, Y.E. Burunkova, D.A. Kozlovskii, A.B. Nikolaev, J. Opt. Technol. 66 (1999) 383±386. [6] I. Pascual, A. Belendez, A. Fimia, J. Opt. 22 (1991) 135±140. [7] I. Pascual, A. Belendez, A. Fimia, Appl. Opt. 31 (1992) 3312±3318. [8] C. Garcõa, A. Fimia, I. Pascual, Proc. SPIE 3956 (2000) 367±374.