- Email: [email protected]
Holographic characterization of azo-dye doped poly (methyl methacrylate) films
ELSEVIER
Thin Solid Films 270
( 1995) 295-299
Holographic characterization of azo-dye doped poly (methyl methacrylate) films Vinh Phuc Pham *, Gurusamy Manivannan Centre d’optique,
Photoniyue
et Laser (COPL). Faculte’des
Sciences et de G&e,
I, Roger A. Lessard
Pavilion Vachon,
Universite' Luval.Citk
universitaire.
Que. GIK 7P4,
Canada
Abstract Many holographic techniques have been developed for non-destructive studies and characterization of materials. In this paper, discussion will be made about the employed holographic technique to characterize the poly( methyl methacrylate) (PMMA) matrices doped with azodyes. In this manner we were able to study the effect of the thickness of the samples, the effect of concentration of the azo-dyes and of PMMA and the effect of aging (storage time) on the holographic efficiency (diffraction efficiency) of these materials. Auto-erasable holographic gratings have been successfully recorded on azo-dye doped PMMA films and the dynamic diffraction efficiency was monitored with light different from that used for the recording. Keywords:
Anisotropy;
Laser irradiation;
Optical properties;
Polymers
1. Introduction The development of holographic imaging techniques is subordinated to the physical characteristics of the recording materials. In this respect, optical interferometry requires not only mechanical stability of optical components to within a fraction of an optical wavelength but also rapid and in-situ processing of the recording layer. Holography must be regarded as a branch of optical interferometry and is closely related to the development of optical recording media. The interferogram recorded by the holographic plate contains fine details, on the scale of the used optical wavelength. Holographic recording materials are traditionally modified silverhalide or dichromate-based systems which meet these high-resolution requirements. In recent years growing interest has been focused on photopolymer-based systems [l-4] which yield near-ideal characteristics. Photographic emulsions and dichromated gelatin must be chemically processed in liquid phase. This treatment is rather cumbersome and the shrinkage accompanying the wet processing phase followed by air drying causes distortion of the observed interferogram. Moreover, repositioning the refer-
* Corresponding author. E-mail: [email protected]. ’ Present address: Department of Chemistry, University of Massachusetts Lowell, One University 0040-6090/95/$09.50
Avenue, Lowell, MA 01854-2881,
USA.
0 1995 Elsevier Science S.A. All rights reserved
SSDIOO40-6090(95)06753-l
ence hologram after the processing is generally a critical operation. Photopolymers were among the first materials used to displace the photographic plate in holography and many variations of these photopolymers have been reported [ 5-101. Among these one important type of photopolymer is that comprising poly( methyl methacrylate) (PMMA) as the base [ 1 l-131. This type of polymer is primarily sensitive to UV light but its sensitivity could be extended to some other regions of the visible spectrum with sensitizers and/or dyes. For the past few decades, photosensitive polymeric systems and mostly azo-dye doped polymers ( ADP) have been in the centre of this feverish activity [ 11. They have shown their impact on optical storage technology for developing high information density and fast access type memories with a high read-out efficiency. Furthermore these polymeric systems allow the manufacturing of reusable films yielding many thousands of write/read/erase (WRE) cycles, involving no chemical or thermal post-recording treatments. In the previous works, real-time dynamic holography with polarized and unpolarized lights has been performed involving PMMA-based recording media, doped with five azo-dyes (AZDl-5) [ 14-171. In this paper focus has been set on the characterization of these azo-dye doped PMMA samples by means of the variation of different parameters (thickness, concentration of PMMA and concentration of azo-dye, etc.).
V.P. Pham eral. /Thin Solid Films 270 (1995) 295-299
296
Table 1 Chemical molecular structures of the analysed azo-dyes. 4,8,9.10,1 1 and 12 correspond structure of the used azo-dyes, and A,,, represents the maximum absorption wavelength General chemical structure of the azo-dyes
to the locations of the different chemicals
in the general molecular
(except AZDS)
Azo-dye
4
8
9
10
11
AZDl AZD2 AZD3 AZD4 AZD5
N(CH,C,Hs)(C,H,OH) N(CH,C,HS)(CPH,OH) N(CH,CH,), N( CH,CH,OH),
_ _ _
_ _
COOCH, Cl _
_ _
COOCH, -
Cl
NO,
COOCH, _
12
A,,, (nm)
_
440 518 464 508 504
COOCH, -
N
The symbol - stands for a hydrogen
2. Azo-dye-doped
atom
polymer samples
Azo-dyes are attractive because of their low cost and ease of use. Furthermore both employed azo-dyes and PMMA could be easily dissolved in chloroform and so the preparation of samples is greatly simplified. The azo-dyes were synthesized based on the well-known diazo coupling reaction between the substituted anilines with substituted N,N-dialkyl aminobenzenes or other proper aromatic functional groups [ 181. Their molecular structures are pictured in Table 1 and their absorption spectra are shown in Fig. 1. The method of fabrication of the PMMA-based films can be found in Ref. [ 141. In this study, different sets of doped PMMA films have been prepared and parameters such as the
concentration of PMMA, the concentration of azo-dyes and the thickness of the films have been varied independently for each set. In the first set of samples, from the original chloroform solution of PMMA (20 wt.%), mixtures have been prepared with a constant concentration of azo-dyes ( 10m3 M) giving films with increasing concentration of PMMA (5-15 wt.%). Similarly, for the second set, the azo-dye concentration has been varied in the range of 5.0~10-~ M to 5.0~10-~ M, keeping the PMMA concentration as 10 wt.%. It has been observed that for azo-dye concentrations exceeding 10e2 M, the optical density of the resulting films is too high and yields non-negligible absorption. Finally, a homogeneous solution of constant concentration of PMMA and azo-dyes ( 10 wt.% and 10e3 M respectively) have been prepared and diverse quantities of mixed solutions (2.5-10 ml) were cast on the leveled microscope glass plate. After drying, the thicknesses of the films were measured with the Sloan’s profilometer and found to be 3 f0.5 pm to 15 * 0.5 p.m respectively.
3. Photochemical mechanism
z&l
300
400
500 600 Wavelength (nm)
700
em
Fig. 1. Absorption spectra of the azo-dyes. The writing and the reading wavelengths are indicated by the arrows. The absorption peaks in the region of 200-300 nm are due to PMMA.
The basic mechanism involved in real-time recording and erasing processes leading to photodichroism of azo-dyes is illustrated in Fig. 2. This mechanism is known as the truns + cis photoisomerization [ 19,201, which leads to the change in the refractive index of the polymer matrix, allowing the volume-phase hologram recording in real time. At room temperature, the truns isomer is stable. During the recording process, the molecular conformation of the azo-dyes changes
291
V.P. Pham et al. /Thin Solid Films 270 (1995) 295-299
0 Fig. 2. Complete photochemical
mechanism
involved
10
in real-timerecording
and erasing processes.
20 30 40 50 Exposure power (mW/cm’)
O,oO 60
70
Fig. 3. Maximum diffraction efficiency achieved with increasing writing
when exposed to the writing light. The recording process can be seen as induced by an n + rTTItransition occurring under light illumination when the azo molecules absorb the actinic light. Either by placing in darkness, or when exposed to a light at the wavelength outside the absorption band of the truns isomer, the cis isomer relaxes back to the trans state. 3.1. Real-time dynamic holographic recording
The real-time holographic recording on the ADP films has been performed employing two blue recording beams at 488 nm from an Ar+ laser (SpectraPhysics Model 165 Ion Laser) as shown in Ref. [ 141. The diffraction gratings were recorded using a symmetrical set-up to avoid the rotation of the Bragg planes caused by the thickness variations when measuring with different wavelengths. The interbeam angle was set to 30” (i.e. a frequency of - 1060 lines mm-‘) in all the experiments. Reconstruction was simultaneously carried out using the correct Bragg angle and irradiating with a reading beam at 632.8 nm delivered by a 30 mW He-Ne laser from Melles Griot. The wavelength of the chosen reading beam has been chosen as 632.8 nm, because the dyes do not have significant absorption at that wavelength (Fig. 1). The continuous dynamic reading of the diffracted light with a photometer (EG and G Gamma Scientific Model 550-l) allowed synchronous knowledge of the diffraction efficiency. Taking into account the losses due to reflection at both surfaces of the glass substrate, it is preferable to define the diffraction efficiency 77as [ 211
beam power. The scale on the right side of the figure concerns the dye AZD4.
4. Results and discussion 4. I. EfSect of PMMA concentration Auto-erasable holograms have been successfully recorded on these films. The exposure power was kept constant at 36.4 mW cm-* during this experiment. For all the samples, the saturation is reached after an exposure of a few minutes and the maximum attained diffraction efficiency has been determined for films containing 5 wt.%, 10 wt.% and 15 wt.% PMMA. However the saturation level is lower for the samples containing 5 wt.% PMMA than for the other two. The saturation level is the same for the samples containing 10 wt.% and 15 wt.% but the time needed for the sample with the highest concentration of PMMA is longer than for the 10 wt.% film. It has also been observed that the time required for relaxation after light exposure is longer for samples with higher PMMA concentrations (Fig. 4). This is mainly due to the available volume for an azo-dye molecule to move and/or distort freely in the polymer host. As the concentration of PMMA is increased, the room available for a dye molecule to undergo a torsion around its azo
(1) where Z, is the intensity of the first order diffracted beam and Z, is the intensity of the transmitted beam at normal incidence through the unexposed film before recording the holograms. In an earlier study [ 151, the behavior of these systems was examined and their diffraction efficiency was determined for increasing recording beam powers (Fig. 3).
ot,,,,‘,,,,‘,~,,“,‘,‘,,,,‘,,,,i 0
5
10
20
15
Time
25
30
(s)
Fig. 4. Relaxation of the ADP systems after the writing lights are cut off.
298
V.P. Pham et al. /Thin Solid Films 270 (1995) 295-299
From these formulas, the relationship between the angle 0 and the thickness d can then be expressed by + AZD3 --•--AZD4 AZD5
(4)
z
2Lv
____.._...___..............
..
1
_._._.-.* ______.-.-.-. I
0
10
On the basis of this criterion, one can conclude that to a fixed value of the interbeam angle corresponds an optimum thickness of the film. For a value of Q equal to 20 (Bragg regime or volume grating), the expression (4) linking the optimum thickness to the interbeam angle is shown in Fig. 7. It can be seen that to an angle of 30” corresponds an optimum thickness of 8.64 km. Experimentally the maximum diffrac-
50
I
I
I
I
60
Fig. 5. Behaviour of the diffraction efficiency of the ADP samples at different concentrations of azo-dyes.
double-bond axis is reduced. The photochemical process is slower, extending the rise time or relaxation time.
-
tion has been reached for samples which are N 8 pm thick. This interpretation can be applied to the observations made for films with different concentrations of PMMA (Fig. 8).
4.2. Effect of the azo-dye concentration
AZDI
The total power incident on the plate was set to 47.5 mW cmp2 and the time of exposure maintained constant at 120 s. The general trend observed was an increase in the diffraction efficiency for samples with higher concentrations of azo-dyes (Fig. 5). This is mainly due to a higher number of azo-dye molecules participating in the trans + cis photoisomerization.
%
-
AZD2
-
AZD3
-
AZDS
+
AZD4
12
14
6-
&
‘6, 4-
4.3. Role ofjilm thickness The angle separating the reference and object beam was kept constant at 30” and the total exposure was set to 47 mW cm-’ . The diffracted intensity has been determined. For films of growing thicknesses, the diffracted signal increased and reached a maximum for films - 8 km thick. For thicker samples, the intensity of the diffracted signal was found to decrease (Fig. 6). In fact, the volume hologram condition is fulfilled for samples of - 8 pm thick yielding a maximum diffraction efficiency. The distinction between thin hologram and thick hologram regimes is defined as the parameter Q( Q > 10) which satisfies the relation [ 221 2rr&d
Q=n
- 2
4
6
8
10
16
ON
(pm)
Thickness
Fig. 6. Variation of the diffraction efficiency for different thicknesses of the ADP films. The scale on the right side of the figure concerns the dye AZD4.
Q=ZO
80
(2)
0
where A,, d and no are respectively the employed recording wavelength, the thickness and the refraction index of the film (no = 1.49 for PMMA [ 231) and A is defined by Bragg’s relation 2_4 no sin: = A0 8 being the interbeam angle.
(3)
Of
5
10
15
20
25
30
35
40
Interbeam angle (“) Fig. 7. Theoretical curve linking the recording interheam angle to the thickness of the holographic film. The arrows show the corresponding optimum thickness for an interbeam of 30” used in the holographic arrangement of this study.
V.P. Pham ef al. /Thin Solid Films 270 (1995) 295-299
U + -
possible interpretations been proposed.
AWZ AZD3 AW4
299
of the occurring
phenomena
have
Acknowledgements
0~‘)““““‘~““““‘)” 6 [I&/$;(% 4 Fig, 8. Variation
of the diffraction
::.,
efficiency
14
16
The azo-dyes are a courtesy of Dr. R. P6 and Dr. G. Bornengo of Enichem (Italy). We would like to thank Mr. L. Turgeon for his highly appreciated technical help. This research is financially supported by the National Sciences and Engineering Research Council of Canada (Under Grants NSERC-A0360 and STRO 118289) and the Gouvernement du QuCbec (Under grant 93-ER-0344).
as a function of the PMMA
concentration.
References [l] G. Manivannan and R.A. Lessard, Trends Polym. Sci., 2( 8) ( 1994)
"0
40
10 ;:Ine
50
(s;
Fig. 9. Effect of storage time of the doped films on the relaxation occurring after the recording lights are cut off.
process
ESfect of storage time Freshly prepared azo-dye doped PMMA films have been compared with old samples preserved for one year. The old films are showing a longer relaxation interval than recentlyprepared films (Fig. 9). The same trend has been followed by the other azo-dyes. The residual solvent may play a role.
5. Conclusion Characterization has proceeded on PMMA-based recording media doped with new azo-dye molecules. Real-time dynamic auto-erasable holograms have been successfully recorded on these films. Different parameters such as the concq&ation of PMMA, the concentration of the azo-dyes and the thickness of the films have been varied and their resulting effects have been analysed by measuring the diffraction efficiency in real time. Based on the results obtained,
282. [2] W.K. Smothers, B.M. Monroe, A.M. Weber and D.E. Key, Proc. SPIE, 1212 (1990) 20. [3] V.A. Barachevsky, V.T. Gvozdovsky and Yu.V. Shulev, Proc. SPIE, 1213 (1990) 62. [4] K.L. Mittal (ed.), Polymer in Informafion Sforage Technology, Plenum Press, New York, 1990. [5] D.H. Close, A.D. Jacobson, J.D. Margerum, R.G. Brauh and F.J. McClung, Appl. Phys. L&r., 14 (1969) 159. [6] J.A. Jenney, J. Opt Sot. Am., 60 (1970) 1155. [7] R.L. Van Renesse, Opt. Laser Technol., 4 (1972) 24. [8] N. Sadlej, Opt. Laser Technol., 5 (1973) 230. [9] S. Sugarawa, K. Murase and T. Kitayama, Appl. Opt,, 14 (1975) 378. [ 101 N. Sadlej and B. Smolinska, Opt Laser Technol., 7 ( 1974) 175. [ 111 W.J. Tomlinson, I.P. Kaminow, E.A. Chandross, R.L. Fork and W.T. Silfvast, Appl. Phys. Lert, 16 (1970) 486. [ 121 J.M. Moran and I.P. Kaminov, Appl. Opt., 12 (1963) 1964. [13] M.J. Bowden, E.A. Chandross and I.P. Kaminov, Appl. Opt, 13 (1974) 112. [ 141 V.P. Pham, G. Manivannan, R.A. Lessard, G. Bomengo and R. P6, Appl. Phys. A, 60 (1995) 239. [15] V.P. Pham, G. Manivannan and R.A. Lessard, Proc. SPIE, 2-105 (1995) 133. [ 161 V.P. Pham, G. Manivannan, R.A. Lessard and R. P6, Opr. Mater., 4 (1995) 467. [ 171 V.P. Pham, G. Manivannan. R.A. Lessard and R. P6, Proc. Sot. Opt Quantum Electron, 1995, in press. [ 181 G. Bomengo, R. P6, G. Agnes, E. Occhielo and F. Garbassi, Italian Patent No. 003473/A 91, 199 1. [ 191 D.L. Ross and J. Blanc, in G.H. Brown (ed.), Phorochromism, Chap. 5, Wiley, New York, 1971. [20] H. Rau, in H. Diirr and H. Bouas-Laurent (eds.), Photochromism, Elsevier, Amsterdam, 1990, p. 165. [21] G. Manivannan, R. Changkakoti and R.A. Lessard, Opt. Eng., 32 (1993) 671. 1221 P. Hariharan, in P.L. Knight and S.D. Smith (eds.), Optical Holography. Principles, Techniques and Applicalions, Cambridge Studies in Modem Optics, Cambridge, 1987, p, 58. [231 I.Bohn, in J. Brandrupand E.H. Immergut (eds.), PolymerHandbook, 2nd edn., Wiley, New York, 1975, Chap. III, p, 241.
Thin Solid Films 270
( 1995) 295-299
Holographic characterization of azo-dye doped poly (methyl methacrylate) films Vinh Phuc Pham *, Gurusamy Manivannan Centre d’optique,
Photoniyue
et Laser (COPL). Faculte’des
Sciences et de G&e,
I, Roger A. Lessard
Pavilion Vachon,
Universite' Luval.Citk
universitaire.
Que. GIK 7P4,
Canada
Abstract Many holographic techniques have been developed for non-destructive studies and characterization of materials. In this paper, discussion will be made about the employed holographic technique to characterize the poly( methyl methacrylate) (PMMA) matrices doped with azodyes. In this manner we were able to study the effect of the thickness of the samples, the effect of concentration of the azo-dyes and of PMMA and the effect of aging (storage time) on the holographic efficiency (diffraction efficiency) of these materials. Auto-erasable holographic gratings have been successfully recorded on azo-dye doped PMMA films and the dynamic diffraction efficiency was monitored with light different from that used for the recording. Keywords:
Anisotropy;
Laser irradiation;
Optical properties;
Polymers
1. Introduction The development of holographic imaging techniques is subordinated to the physical characteristics of the recording materials. In this respect, optical interferometry requires not only mechanical stability of optical components to within a fraction of an optical wavelength but also rapid and in-situ processing of the recording layer. Holography must be regarded as a branch of optical interferometry and is closely related to the development of optical recording media. The interferogram recorded by the holographic plate contains fine details, on the scale of the used optical wavelength. Holographic recording materials are traditionally modified silverhalide or dichromate-based systems which meet these high-resolution requirements. In recent years growing interest has been focused on photopolymer-based systems [l-4] which yield near-ideal characteristics. Photographic emulsions and dichromated gelatin must be chemically processed in liquid phase. This treatment is rather cumbersome and the shrinkage accompanying the wet processing phase followed by air drying causes distortion of the observed interferogram. Moreover, repositioning the refer-
* Corresponding author. E-mail: [email protected]. ’ Present address: Department of Chemistry, University of Massachusetts Lowell, One University 0040-6090/95/$09.50
Avenue, Lowell, MA 01854-2881,
USA.
0 1995 Elsevier Science S.A. All rights reserved
SSDIOO40-6090(95)06753-l
ence hologram after the processing is generally a critical operation. Photopolymers were among the first materials used to displace the photographic plate in holography and many variations of these photopolymers have been reported [ 5-101. Among these one important type of photopolymer is that comprising poly( methyl methacrylate) (PMMA) as the base [ 1 l-131. This type of polymer is primarily sensitive to UV light but its sensitivity could be extended to some other regions of the visible spectrum with sensitizers and/or dyes. For the past few decades, photosensitive polymeric systems and mostly azo-dye doped polymers ( ADP) have been in the centre of this feverish activity [ 11. They have shown their impact on optical storage technology for developing high information density and fast access type memories with a high read-out efficiency. Furthermore these polymeric systems allow the manufacturing of reusable films yielding many thousands of write/read/erase (WRE) cycles, involving no chemical or thermal post-recording treatments. In the previous works, real-time dynamic holography with polarized and unpolarized lights has been performed involving PMMA-based recording media, doped with five azo-dyes (AZDl-5) [ 14-171. In this paper focus has been set on the characterization of these azo-dye doped PMMA samples by means of the variation of different parameters (thickness, concentration of PMMA and concentration of azo-dye, etc.).
V.P. Pham eral. /Thin Solid Films 270 (1995) 295-299
296
Table 1 Chemical molecular structures of the analysed azo-dyes. 4,8,9.10,1 1 and 12 correspond structure of the used azo-dyes, and A,,, represents the maximum absorption wavelength General chemical structure of the azo-dyes
to the locations of the different chemicals
in the general molecular
(except AZDS)
Azo-dye
4
8
9
10
11
AZDl AZD2 AZD3 AZD4 AZD5
N(CH,C,Hs)(C,H,OH) N(CH,C,HS)(CPH,OH) N(CH,CH,), N( CH,CH,OH),
_ _ _
_ _
COOCH, Cl _
_ _
COOCH, -
Cl
NO,
COOCH, _
12
A,,, (nm)
_
440 518 464 508 504
COOCH, -
N
The symbol - stands for a hydrogen
2. Azo-dye-doped
atom
polymer samples
Azo-dyes are attractive because of their low cost and ease of use. Furthermore both employed azo-dyes and PMMA could be easily dissolved in chloroform and so the preparation of samples is greatly simplified. The azo-dyes were synthesized based on the well-known diazo coupling reaction between the substituted anilines with substituted N,N-dialkyl aminobenzenes or other proper aromatic functional groups [ 181. Their molecular structures are pictured in Table 1 and their absorption spectra are shown in Fig. 1. The method of fabrication of the PMMA-based films can be found in Ref. [ 141. In this study, different sets of doped PMMA films have been prepared and parameters such as the
concentration of PMMA, the concentration of azo-dyes and the thickness of the films have been varied independently for each set. In the first set of samples, from the original chloroform solution of PMMA (20 wt.%), mixtures have been prepared with a constant concentration of azo-dyes ( 10m3 M) giving films with increasing concentration of PMMA (5-15 wt.%). Similarly, for the second set, the azo-dye concentration has been varied in the range of 5.0~10-~ M to 5.0~10-~ M, keeping the PMMA concentration as 10 wt.%. It has been observed that for azo-dye concentrations exceeding 10e2 M, the optical density of the resulting films is too high and yields non-negligible absorption. Finally, a homogeneous solution of constant concentration of PMMA and azo-dyes ( 10 wt.% and 10e3 M respectively) have been prepared and diverse quantities of mixed solutions (2.5-10 ml) were cast on the leveled microscope glass plate. After drying, the thicknesses of the films were measured with the Sloan’s profilometer and found to be 3 f0.5 pm to 15 * 0.5 p.m respectively.
3. Photochemical mechanism
z&l
300
400
500 600 Wavelength (nm)
700
em
Fig. 1. Absorption spectra of the azo-dyes. The writing and the reading wavelengths are indicated by the arrows. The absorption peaks in the region of 200-300 nm are due to PMMA.
The basic mechanism involved in real-time recording and erasing processes leading to photodichroism of azo-dyes is illustrated in Fig. 2. This mechanism is known as the truns + cis photoisomerization [ 19,201, which leads to the change in the refractive index of the polymer matrix, allowing the volume-phase hologram recording in real time. At room temperature, the truns isomer is stable. During the recording process, the molecular conformation of the azo-dyes changes
291
V.P. Pham et al. /Thin Solid Films 270 (1995) 295-299
0 Fig. 2. Complete photochemical
mechanism
involved
10
in real-timerecording
and erasing processes.
20 30 40 50 Exposure power (mW/cm’)
O,oO 60
70
Fig. 3. Maximum diffraction efficiency achieved with increasing writing
when exposed to the writing light. The recording process can be seen as induced by an n + rTTItransition occurring under light illumination when the azo molecules absorb the actinic light. Either by placing in darkness, or when exposed to a light at the wavelength outside the absorption band of the truns isomer, the cis isomer relaxes back to the trans state. 3.1. Real-time dynamic holographic recording
The real-time holographic recording on the ADP films has been performed employing two blue recording beams at 488 nm from an Ar+ laser (SpectraPhysics Model 165 Ion Laser) as shown in Ref. [ 141. The diffraction gratings were recorded using a symmetrical set-up to avoid the rotation of the Bragg planes caused by the thickness variations when measuring with different wavelengths. The interbeam angle was set to 30” (i.e. a frequency of - 1060 lines mm-‘) in all the experiments. Reconstruction was simultaneously carried out using the correct Bragg angle and irradiating with a reading beam at 632.8 nm delivered by a 30 mW He-Ne laser from Melles Griot. The wavelength of the chosen reading beam has been chosen as 632.8 nm, because the dyes do not have significant absorption at that wavelength (Fig. 1). The continuous dynamic reading of the diffracted light with a photometer (EG and G Gamma Scientific Model 550-l) allowed synchronous knowledge of the diffraction efficiency. Taking into account the losses due to reflection at both surfaces of the glass substrate, it is preferable to define the diffraction efficiency 77as [ 211
beam power. The scale on the right side of the figure concerns the dye AZD4.
4. Results and discussion 4. I. EfSect of PMMA concentration Auto-erasable holograms have been successfully recorded on these films. The exposure power was kept constant at 36.4 mW cm-* during this experiment. For all the samples, the saturation is reached after an exposure of a few minutes and the maximum attained diffraction efficiency has been determined for films containing 5 wt.%, 10 wt.% and 15 wt.% PMMA. However the saturation level is lower for the samples containing 5 wt.% PMMA than for the other two. The saturation level is the same for the samples containing 10 wt.% and 15 wt.% but the time needed for the sample with the highest concentration of PMMA is longer than for the 10 wt.% film. It has also been observed that the time required for relaxation after light exposure is longer for samples with higher PMMA concentrations (Fig. 4). This is mainly due to the available volume for an azo-dye molecule to move and/or distort freely in the polymer host. As the concentration of PMMA is increased, the room available for a dye molecule to undergo a torsion around its azo
(1) where Z, is the intensity of the first order diffracted beam and Z, is the intensity of the transmitted beam at normal incidence through the unexposed film before recording the holograms. In an earlier study [ 151, the behavior of these systems was examined and their diffraction efficiency was determined for increasing recording beam powers (Fig. 3).
ot,,,,‘,,,,‘,~,,“,‘,‘,,,,‘,,,,i 0
5
10
20
15
Time
25
30
(s)
Fig. 4. Relaxation of the ADP systems after the writing lights are cut off.
298
V.P. Pham et al. /Thin Solid Films 270 (1995) 295-299
From these formulas, the relationship between the angle 0 and the thickness d can then be expressed by + AZD3 --•--AZD4 AZD5
(4)
z
2Lv
____.._...___..............
..
1
_._._.-.* ______.-.-.-. I
0
10
On the basis of this criterion, one can conclude that to a fixed value of the interbeam angle corresponds an optimum thickness of the film. For a value of Q equal to 20 (Bragg regime or volume grating), the expression (4) linking the optimum thickness to the interbeam angle is shown in Fig. 7. It can be seen that to an angle of 30” corresponds an optimum thickness of 8.64 km. Experimentally the maximum diffrac-
50
I
I
I
I
60
Fig. 5. Behaviour of the diffraction efficiency of the ADP samples at different concentrations of azo-dyes.
double-bond axis is reduced. The photochemical process is slower, extending the rise time or relaxation time.
-
tion has been reached for samples which are N 8 pm thick. This interpretation can be applied to the observations made for films with different concentrations of PMMA (Fig. 8).
4.2. Effect of the azo-dye concentration
AZDI
The total power incident on the plate was set to 47.5 mW cmp2 and the time of exposure maintained constant at 120 s. The general trend observed was an increase in the diffraction efficiency for samples with higher concentrations of azo-dyes (Fig. 5). This is mainly due to a higher number of azo-dye molecules participating in the trans + cis photoisomerization.
%
-
AZD2
-
AZD3
-
AZDS
+
AZD4
12
14
6-
&
‘6, 4-
4.3. Role ofjilm thickness The angle separating the reference and object beam was kept constant at 30” and the total exposure was set to 47 mW cm-’ . The diffracted intensity has been determined. For films of growing thicknesses, the diffracted signal increased and reached a maximum for films - 8 km thick. For thicker samples, the intensity of the diffracted signal was found to decrease (Fig. 6). In fact, the volume hologram condition is fulfilled for samples of - 8 pm thick yielding a maximum diffraction efficiency. The distinction between thin hologram and thick hologram regimes is defined as the parameter Q( Q > 10) which satisfies the relation [ 221 2rr&d
Q=n
- 2
4
6
8
10
16
ON
(pm)
Thickness
Fig. 6. Variation of the diffraction efficiency for different thicknesses of the ADP films. The scale on the right side of the figure concerns the dye AZD4.
Q=ZO
80
(2)
0
where A,, d and no are respectively the employed recording wavelength, the thickness and the refraction index of the film (no = 1.49 for PMMA [ 231) and A is defined by Bragg’s relation 2_4 no sin: = A0 8 being the interbeam angle.
(3)
Of
5
10
15
20
25
30
35
40
Interbeam angle (“) Fig. 7. Theoretical curve linking the recording interheam angle to the thickness of the holographic film. The arrows show the corresponding optimum thickness for an interbeam of 30” used in the holographic arrangement of this study.
V.P. Pham ef al. /Thin Solid Films 270 (1995) 295-299
U + -
possible interpretations been proposed.
AWZ AZD3 AW4
299
of the occurring
phenomena
have
Acknowledgements
0~‘)““““‘~““““‘)” 6 [I&/$;(% 4 Fig, 8. Variation
of the diffraction
::.,
efficiency
14
16
The azo-dyes are a courtesy of Dr. R. P6 and Dr. G. Bornengo of Enichem (Italy). We would like to thank Mr. L. Turgeon for his highly appreciated technical help. This research is financially supported by the National Sciences and Engineering Research Council of Canada (Under Grants NSERC-A0360 and STRO 118289) and the Gouvernement du QuCbec (Under grant 93-ER-0344).
as a function of the PMMA
concentration.
References [l] G. Manivannan and R.A. Lessard, Trends Polym. Sci., 2( 8) ( 1994)
"0
40
10 ;:Ine
50
(s;
Fig. 9. Effect of storage time of the doped films on the relaxation occurring after the recording lights are cut off.
process
ESfect of storage time Freshly prepared azo-dye doped PMMA films have been compared with old samples preserved for one year. The old films are showing a longer relaxation interval than recentlyprepared films (Fig. 9). The same trend has been followed by the other azo-dyes. The residual solvent may play a role.
5. Conclusion Characterization has proceeded on PMMA-based recording media doped with new azo-dye molecules. Real-time dynamic auto-erasable holograms have been successfully recorded on these films. Different parameters such as the concq&ation of PMMA, the concentration of the azo-dyes and the thickness of the films have been varied and their resulting effects have been analysed by measuring the diffraction efficiency in real time. Based on the results obtained,
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