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Photopolymers in holography
Photopolymers in holography R. L. V A N RENESSE* Photopolymers for application in holography have the advantage over silver halide emulsions that a phase hologram is formed in situ during exposure. The article describes the results of tests concerning the resolving power and photosensitivity of a combination of the monomers acryl amide and methylene-bis-acryl amide. This combination forms a copolymer under the influence of red light, when methylene blue is used as a sensibiliser. In spite of their low sensitivity, photopolymers in some instances appear to be promising substitutes for the conventional silver halide materials.
A drawback of silver halide emulsions in holographic processes is the need to use photographic developing processes, so that it takes a fair amount of time for the hologram to be completely finished. It will take longer if the hologram is also bleached to produce a phase hologram, to achieve greater diffraction efficiency. 1 It is by no means rare for the various stages of processing of the photographic plate from exposure to the end of accelerated drying (alcohol bath, hot air current) to take a total of thirty minutes. Even if a reversing process is applied, in which the developed silver is dissolved, thus directly producing a phase hologram, the complete process may require fifteen minutes. There are two other processes, however, which do not have the above-mentioned drawback: those employing thermoplastics 2'3 and photopolymers 4. Holograms in thermoplastic film can be made in situ, and they offer the advantage that they can be erased, leaving the photosensitive layer to be used for new exposures. Exposure and development take but seconds, and produce a hologram which holds the required information in the form of differences in layer thickness. It is not simple to make up the sensitive layer, however, so that we concentrated our studies first on the use of photopolymers. A hologram made with photopolymers is formed during the exposure, and the information is stored in the form of differences in refractive index of the sensitive layer. The hologram cannot be erased, but the sensitive layer is easy to prepare, so as to eliminate the drawback of time-consuming processing. Preparing the sensitive layer and making the hologram takes approximately one minute. Both methods combine the time saving advantage with that of making the hologram in situ, which is very important for °
24
Technisch Physische Dienst TNO-TH, Stieltjesweg 1 Delft, Postbus 155, The Netherlands
Optics and Laser Technology
February 1972
'real-time' holography. Special formulations and developing cells are unnecessary, and t/ae problems connected with accurate adjustment of the photographic plate, if it is not processed on the spot, are also eliminated. The principle of photopolymerisation is based on the initiation of chemical chain reactions under the influence of light; small molecules form huge molecular chains and space lattices. The conversion of the acrylates of barium, calcium or lead into polyacrylates, and of acrylamide, if necessary combined with methylene-bis-acrylamide, into polyamides, are examples of the aforementioned reactions. For further information regarding the chemical reactions, in which free radicals play a very important part, reference is made to the relevant literature 4 - .8 Table 1 shows the structural formulae of monomers and other chemicals involved in the reactions, together with their molecular weights 9. As far as we could find out, the acrylates were not commercially available. The other chemicals are easy to purchase and are relatively inexpensive. As laboratory preparation of acrylates is a rather costly affair, the studies were provisionally confined to the monomers acryl amide and methylene-bis-acrylamide for holographic purposes. Acrylamides have the drawback of low sensitivity compared with the acrylates ~. The difference may be as great as a factor ten. However, the advantage is that a phase hologram is directly formed, but not entirely so if at all, where acrylates are used, according to Jenney 4. The use of nitro-phenyl acetic acid, which permits fixing the hologram with the aid of ultraviolet light, was decided against because there are several other methods of discontinuing the reaction. The monomer remaining after exposure to ceiling light, can be polymerised less quickly, for instance, so resulting in a lower refractive index, or the reaction may be caused to end through a suitable choice of layer thickness and concentrations.
Acryl .amide polymerises to chains having the following structure • . . -CH2 -CH2 - C O - N H - C H 2 - C H 2 - C O - N H - . . . .
Because the material is sensitive only witlq, n a limited spectral range, bright room lighting in a different spectral range can be used during processing.
The result is a sticky mass, from which threads tens of centimetres long can be drawn. It was found impossible to make holograms with a solution of acrylamide only. Although polyacrylamide with a higher refractive index is produced, there is no diffraction. This is probably attributable to the formation of very long molecular chains in the exposed places, continuing through the unexposed areas. The result is a homogenous mass with insufficient resolving power.
Resolving power A study of the resolving power of a photosensitive monomer layer was conducted with the arrangement shown in Fig. 1. For this experiment a 500 mW krypton laser was used, set at the 647.1 nm line.
Addition of methylene-bis-acrylamide to the solution not only speeds up the polymerisation reaction, but also permits the making of phase holograms. A molecular space lattice is formed, and in this the methylene-bis-acrylamide builds cross-links between the polyacrylamide chains to form a copolymer. The transparent mass thus formed is much more rigid and has a grainy structure.
Formulations and processing
M
o
As the polymerisation speed generally increases along with the concentration of the substances involved, the solutions were made in the highest possible concentrations:
Photo - sensitive moteriol
Solution A - monomer
iO °
Acrylamide (saturated) Methyle ne-bis-acrylamide Distilled water to make Refractive index
7M-50g 10g 100 cm 3 1.44
Solution B - photo-catalyst
4-toluene-sulphinic acid (saturated) Methylene blue (for medical use) Distilled water to make
approx. 0.1 M - 1.5 g 0.75 g 100 cm 3
Fig. 1 Schematic set-up of the optical system used to determine the resolution of photosensitive materials. The spatial frequency of the inter- • ference pattern varies between 270 lines mm "~ and 1780 lines mm -l.
Table 1 Structural formulae of chemicals in reactions
Chemical Acrylamide
Combination of solutions A and B produces a photosensitive liquid which has limited keeping properties. The separate solutions have a long shelf life ~°. The photosensitive layer is made by causing a few drops of the mixture.to run out between two glass slides, to obtain a liquid layer 10 - 20pm thick. An optimum mixing ration for A/B was found to be approximately 2, at which the photosensitive mixture has a refractive index o f n = 1.415. When the solution polymerises under the effect of light, the refractive index increases, and may after some time reach about 1.5. If the crosslinker methylene-bis-acrylamide is left out of solution A, the photosensitive mixture has a refractive index of 1.408, while after polymerisation under the same conditions a refractive index of about 1.44 was observed. The spectral sensitivity of the mixture A + B is determined by the spectral absorption of the sensibiliser. If'methylene blue is used, the mixture is only sensitive to red light, but with a suitable other dye the mixture can be rendered sensitive to other spectral ranges6.
Molecular weight
Structural formula H2 C=CH--CO--N H2
Methylene-bisacrylamide
(H2C=CH-CO-NH)2CH2
Methylene blue
(OH3)2 N - [ ~
71.08 154.17
sN ~ ] = N + ' ( C H 3 ) 2 319.85
Toluene-4-sulphinic H3C- ~ acid
-SO2 H
156.20
4-nitro-phenyl acetic acid
NOa- @-CH2CO2
Barium acrylate
(H2 C=CH-CO212 Ba
279.42
Lead acrylate
(H2 C=CH-CO2 )2 Pb
349.27
Calcium acrylate
(H2 C=CH-COz )2 Ca
182.14
Optics and Laser Technology
H
F e b r u a r y 1972
181.14
25
Cl-
t
I000
The angle between the object and reference beams varied between 10 ° and 70 ° over the hologram surface, owing to which the spatial frequency of the interference pattern increased from approximately 270 lines mm "1 to 1780 lines mm-L
500 '1', E u
The polymerisation process transfers the intensity modulation of the interference pattern into a phase modulation of the photo-sensitive layer.
E
50
iw
Since for small phase variations ( A~0 < 0.6), by approximation, l :
~= ( ~ ) 2
I0
8 x LI..I
5
l
in which ~" is the diffraction efficiency caused by phase differences A~o, X/~" as a function o f the spatial frequency of the interference pattern is a direct measure of the modulation transfer of the monomer-polymer system. In order to comply with the above-mentioned requirement of linearity, a low intensity modulation of the interference pattern was used. The irradiance of the object beam at the edge of the hologram being smaller on account of the large angle of incidence (o:) to the normal, the values found for the diffraction efficiency were adjusted by a factor cos-~ c~. The results of the measurements are set out in Fig. 2; the modulation transfer was assumed to be 1 for low spatial frequencies. Although diffraction effects on photopolymer gratings up to 3000 lines mm -1 were observed, the resolving power appeared to be virtually limited to about 2000 lines mm -~. This limit is apparently caused by the size of the copolymer clusters. Sensitivity
It is known that in photography the sensitivity of a material is very often not entirely independent of the irradiance. This deviation from Bunsen-Roscoe's reciprocity law means that from an energy consideration very high and very low irradiance levels result in an unfavourable exposure (E=It).
o
]
0.01
8
I
I
I
0.05 0-1
I
O.S 1.0
I
I
S
I0
I
I
50 I00
Irradiance [ mW cm-2 ] Fig. 3
Reciprocity failure of the monomer-polymer system is demonstrated by the relation between exposure and irradiance. Beyond an irradiance of 0.2 mW cm "2 the relationship becomes exponential. The Schwarzschild-exponent is then p,=3.
As this reciprocity failure is very pronounced in this photopolymer the krypton laser was employed to determine the sensitivity at irradiances between approx 0.04 - 20 mW cm -2. For this purpose, holograms were made of a diffuse object, the criterion for the exposure time being the point at which a specific arbitrarily chosen efficiency was reached. The results of this test are shown in Fig. 3, in which the exposure is plotted as a function of the irradiance. The figure indicates that beyond an irradiance of 0.2 mW cm -2 there is an exponential relationship: E=C1
m ,
in which m is the intensity exponent, and C a constant depending on the sensitivity. The value of the intensity exponent appears to be m = 0.67. The sensitivity of the material is represented by the horizontal section of the curve, where I t = constant. The sensitivity of the described mixture appears to be approx 5 mJ cm "2. As a comparison: the sensitivity of Scientia 8E75 is about 0.0075 mJ cm -2.
I'0 i =-
O.S tUO. Ill
>
_o IZ
0"0
j
I00
0
I 0' 0 0
2 0'0 0
3C~XD
Spatial frequency [lines mm"~]
Exposure times within the range of constant exposure may be as long as several minutes however, so that the material will not be practically suitable until the exponential range is reached, where exposure times of a few seconds are feasible. This requires irradiances of over 5 mW cm "2. For comparison of the test material with the conventional photographic emulsions, the Schwarzschild exponent is an important factor. The S-exponent (p) is defined as the power to which the exposure time must be raised in order to calculate the exposure time for various irradiances to achieve a constant result: I tP = constant
From Equations 2 and 3 follows, after a few simple steps: Fig. 2
26
Modulation transfer of the monomer-polymer system. The resolving power is limited by the size of the macromolecular copolymer clusters.
Optics and Laser Technology
February 1972
p. = i/(1 - m) in which m 0 for very high irradiances. Substituting the experimental value for
2
the intensity component (m=0.67) in Equation 4, an S-exponent p = 3 is found. In view of the long exposure times involved, no attempts were made to find out how the curve of Fig. 3 would proceed at very low irradiances. The S-exponents for the various types of silver halide emulsions vary from 1 to a maximum of about 3 in extreme cases; this warrants the conclusion that the behaviour of the examined material is relatively unfavourable compared with the conventional materials.
K9.O00 5000 E
u --}
E
I000 500
Q. x t~
IOO O'1
It is especially this inefficiency caused by the reciprocity failure which makes brief exposure times very difficult to realize. However, this need not always be an objection to using the material. It has been found possible to make holograms of diffuse objects with exposure times of several tens of seconds, using a 20 mW He-Ne laser. This means that the use of photopolymers need not be limited to facilities which have lasers with extremely high output power available. As mentioned in the description of the formulations, the sensitivity o f the material likewise depends on the concentrations of the substances taking part in the reaction. Fig. 4 shows for instance the exponential relationship between the exposure and the concentration of methylene blue. The measurements were conducted at an irradiance of approx 35 mW cm -2. Up to a concentration of 7.5 mg cm "3 the sensitivity increases exponentially, but addition of more methylene blue has no further effect. The concentration of the methylene blue in the formulation used was defined by reference to these results.
=
0"5
t
I
I
I
I-O
5
IO
50
I
IOO
Concentration [ mg cm -3 ] Fig.
4 When the concentration of the methylene blue is increased, the sensitivity increases exponentially. Maximum sensitivity is obtained at a concentration of approx 7.5 mg cm "3. Measurements were conducted at an irradiance of 35 mW cm "2 .
A main advantage is that an excellent phase hologram can be made in situ in a very short time and at very low cost. Exposure times of a few to several tens of seconds are feasible at irradiances varying from 0.1 to 10 mW cm -2. The resolving power of the material is also quite sufficient for most holographic applications. Acknowledgements
Keeping
properties
Holograms in layers of photopolymers do not appear to keep well. After some time the adhesion of the plastic layer to the glass deteriorates, and air penetrates between the glass and the polymer, always at the edges first. Lacquering the edges is not sufficient to keep air out. Adhesion is very much improved, however, by thorough cleaning of the glass surfaces. It is also necessary to convert the remaining monomer into polymer, as the chemicals will otherwise crystallise between the glass surfaces after some time and adversely affect the signal-to-noise ratio. With these provisions, a hologram could be kept in good condition for about a week. Conclusions
In spite of the relatively poor keeping properties (a few days) and the considerable reciprocity failure (Schwarzschild exponent, p = 3), combined with a low sensitivity (5 mJ cm -2) of the photomonomers acryl amide and methylene-bisacryl amide, the material promises to be an excellent substitute for the conventional silver halide materials in certain applications.
The work described in this paper has been carried out in the Optical Department of our Institute, under supervision of H.J. Raterink. The investigation of photopolymers is part of a research programme, directed at industrial application of laser techniques for vibration and deformation analysis.
References 1 2 3 4 5 6 7 8 9 10
Kogelnik,H. (1967) Proc of the Symposium on Modern Optics (J. Fox, ed, Polytechnic Press, Brooklyn) Urbach, John. C. and Meier, Reinhard. W. (1966) Appl Opt 5(4) 666 Lin, L.H. and Beauchamp, H.L. (1970) Appl Opt 9(9) 2088 Jenney, J.A. (1970) J O S A 60(9) 1155 Burnett, G.M. and Melville, H.W. (1947) Proc Roy Soc London A189 456 Oster, G. and Mark, H. (1953) J O S A 43(4) 283 Bunn,C.W. and Carner, E.V. (1947) Proc Roy Soc London A189 39 Stong, C.L. (December 1969) Scientific American 128 Handbook of Chemistry and Physics (R.C. Weast, ed, (68/69) The Chemical Rubber Co, Cleveland) KeepSolution A out of direct sunlight because it may polimerise with violence.
Optics and Laser Technology
February 1972
27
A drawback of silver halide emulsions in holographic processes is the need to use photographic developing processes, so that it takes a fair amount of time for the hologram to be completely finished. It will take longer if the hologram is also bleached to produce a phase hologram, to achieve greater diffraction efficiency. 1 It is by no means rare for the various stages of processing of the photographic plate from exposure to the end of accelerated drying (alcohol bath, hot air current) to take a total of thirty minutes. Even if a reversing process is applied, in which the developed silver is dissolved, thus directly producing a phase hologram, the complete process may require fifteen minutes. There are two other processes, however, which do not have the above-mentioned drawback: those employing thermoplastics 2'3 and photopolymers 4. Holograms in thermoplastic film can be made in situ, and they offer the advantage that they can be erased, leaving the photosensitive layer to be used for new exposures. Exposure and development take but seconds, and produce a hologram which holds the required information in the form of differences in layer thickness. It is not simple to make up the sensitive layer, however, so that we concentrated our studies first on the use of photopolymers. A hologram made with photopolymers is formed during the exposure, and the information is stored in the form of differences in refractive index of the sensitive layer. The hologram cannot be erased, but the sensitive layer is easy to prepare, so as to eliminate the drawback of time-consuming processing. Preparing the sensitive layer and making the hologram takes approximately one minute. Both methods combine the time saving advantage with that of making the hologram in situ, which is very important for °
24
Technisch Physische Dienst TNO-TH, Stieltjesweg 1 Delft, Postbus 155, The Netherlands
Optics and Laser Technology
February 1972
'real-time' holography. Special formulations and developing cells are unnecessary, and t/ae problems connected with accurate adjustment of the photographic plate, if it is not processed on the spot, are also eliminated. The principle of photopolymerisation is based on the initiation of chemical chain reactions under the influence of light; small molecules form huge molecular chains and space lattices. The conversion of the acrylates of barium, calcium or lead into polyacrylates, and of acrylamide, if necessary combined with methylene-bis-acrylamide, into polyamides, are examples of the aforementioned reactions. For further information regarding the chemical reactions, in which free radicals play a very important part, reference is made to the relevant literature 4 - .8 Table 1 shows the structural formulae of monomers and other chemicals involved in the reactions, together with their molecular weights 9. As far as we could find out, the acrylates were not commercially available. The other chemicals are easy to purchase and are relatively inexpensive. As laboratory preparation of acrylates is a rather costly affair, the studies were provisionally confined to the monomers acryl amide and methylene-bis-acrylamide for holographic purposes. Acrylamides have the drawback of low sensitivity compared with the acrylates ~. The difference may be as great as a factor ten. However, the advantage is that a phase hologram is directly formed, but not entirely so if at all, where acrylates are used, according to Jenney 4. The use of nitro-phenyl acetic acid, which permits fixing the hologram with the aid of ultraviolet light, was decided against because there are several other methods of discontinuing the reaction. The monomer remaining after exposure to ceiling light, can be polymerised less quickly, for instance, so resulting in a lower refractive index, or the reaction may be caused to end through a suitable choice of layer thickness and concentrations.
Acryl .amide polymerises to chains having the following structure • . . -CH2 -CH2 - C O - N H - C H 2 - C H 2 - C O - N H - . . . .
Because the material is sensitive only witlq, n a limited spectral range, bright room lighting in a different spectral range can be used during processing.
The result is a sticky mass, from which threads tens of centimetres long can be drawn. It was found impossible to make holograms with a solution of acrylamide only. Although polyacrylamide with a higher refractive index is produced, there is no diffraction. This is probably attributable to the formation of very long molecular chains in the exposed places, continuing through the unexposed areas. The result is a homogenous mass with insufficient resolving power.
Resolving power A study of the resolving power of a photosensitive monomer layer was conducted with the arrangement shown in Fig. 1. For this experiment a 500 mW krypton laser was used, set at the 647.1 nm line.
Addition of methylene-bis-acrylamide to the solution not only speeds up the polymerisation reaction, but also permits the making of phase holograms. A molecular space lattice is formed, and in this the methylene-bis-acrylamide builds cross-links between the polyacrylamide chains to form a copolymer. The transparent mass thus formed is much more rigid and has a grainy structure.
Formulations and processing
M
o
As the polymerisation speed generally increases along with the concentration of the substances involved, the solutions were made in the highest possible concentrations:
Photo - sensitive moteriol
Solution A - monomer
iO °
Acrylamide (saturated) Methyle ne-bis-acrylamide Distilled water to make Refractive index
7M-50g 10g 100 cm 3 1.44
Solution B - photo-catalyst
4-toluene-sulphinic acid (saturated) Methylene blue (for medical use) Distilled water to make
approx. 0.1 M - 1.5 g 0.75 g 100 cm 3
Fig. 1 Schematic set-up of the optical system used to determine the resolution of photosensitive materials. The spatial frequency of the inter- • ference pattern varies between 270 lines mm "~ and 1780 lines mm -l.
Table 1 Structural formulae of chemicals in reactions
Chemical Acrylamide
Combination of solutions A and B produces a photosensitive liquid which has limited keeping properties. The separate solutions have a long shelf life ~°. The photosensitive layer is made by causing a few drops of the mixture.to run out between two glass slides, to obtain a liquid layer 10 - 20pm thick. An optimum mixing ration for A/B was found to be approximately 2, at which the photosensitive mixture has a refractive index o f n = 1.415. When the solution polymerises under the effect of light, the refractive index increases, and may after some time reach about 1.5. If the crosslinker methylene-bis-acrylamide is left out of solution A, the photosensitive mixture has a refractive index of 1.408, while after polymerisation under the same conditions a refractive index of about 1.44 was observed. The spectral sensitivity of the mixture A + B is determined by the spectral absorption of the sensibiliser. If'methylene blue is used, the mixture is only sensitive to red light, but with a suitable other dye the mixture can be rendered sensitive to other spectral ranges6.
Molecular weight
Structural formula H2 C=CH--CO--N H2
Methylene-bisacrylamide
(H2C=CH-CO-NH)2CH2
Methylene blue
(OH3)2 N - [ ~
71.08 154.17
sN ~ ] = N + ' ( C H 3 ) 2 319.85
Toluene-4-sulphinic H3C- ~ acid
-SO2 H
156.20
4-nitro-phenyl acetic acid
NOa- @-CH2CO2
Barium acrylate
(H2 C=CH-CO212 Ba
279.42
Lead acrylate
(H2 C=CH-CO2 )2 Pb
349.27
Calcium acrylate
(H2 C=CH-COz )2 Ca
182.14
Optics and Laser Technology
H
F e b r u a r y 1972
181.14
25
Cl-
t
I000
The angle between the object and reference beams varied between 10 ° and 70 ° over the hologram surface, owing to which the spatial frequency of the interference pattern increased from approximately 270 lines mm "1 to 1780 lines mm-L
500 '1', E u
The polymerisation process transfers the intensity modulation of the interference pattern into a phase modulation of the photo-sensitive layer.
E
50
iw
Since for small phase variations ( A~0 < 0.6), by approximation, l :
~= ( ~ ) 2
I0
8 x LI..I
5
l
in which ~" is the diffraction efficiency caused by phase differences A~o, X/~" as a function o f the spatial frequency of the interference pattern is a direct measure of the modulation transfer of the monomer-polymer system. In order to comply with the above-mentioned requirement of linearity, a low intensity modulation of the interference pattern was used. The irradiance of the object beam at the edge of the hologram being smaller on account of the large angle of incidence (o:) to the normal, the values found for the diffraction efficiency were adjusted by a factor cos-~ c~. The results of the measurements are set out in Fig. 2; the modulation transfer was assumed to be 1 for low spatial frequencies. Although diffraction effects on photopolymer gratings up to 3000 lines mm -1 were observed, the resolving power appeared to be virtually limited to about 2000 lines mm -~. This limit is apparently caused by the size of the copolymer clusters. Sensitivity
It is known that in photography the sensitivity of a material is very often not entirely independent of the irradiance. This deviation from Bunsen-Roscoe's reciprocity law means that from an energy consideration very high and very low irradiance levels result in an unfavourable exposure (E=It).
o
]
0.01
8
I
I
I
0.05 0-1
I
O.S 1.0
I
I
S
I0
I
I
50 I00
Irradiance [ mW cm-2 ] Fig. 3
Reciprocity failure of the monomer-polymer system is demonstrated by the relation between exposure and irradiance. Beyond an irradiance of 0.2 mW cm "2 the relationship becomes exponential. The Schwarzschild-exponent is then p,=3.
As this reciprocity failure is very pronounced in this photopolymer the krypton laser was employed to determine the sensitivity at irradiances between approx 0.04 - 20 mW cm -2. For this purpose, holograms were made of a diffuse object, the criterion for the exposure time being the point at which a specific arbitrarily chosen efficiency was reached. The results of this test are shown in Fig. 3, in which the exposure is plotted as a function of the irradiance. The figure indicates that beyond an irradiance of 0.2 mW cm -2 there is an exponential relationship: E=C1
m ,
in which m is the intensity exponent, and C a constant depending on the sensitivity. The value of the intensity exponent appears to be m = 0.67. The sensitivity of the material is represented by the horizontal section of the curve, where I t = constant. The sensitivity of the described mixture appears to be approx 5 mJ cm "2. As a comparison: the sensitivity of Scientia 8E75 is about 0.0075 mJ cm -2.
I'0 i =-
O.S tUO. Ill
>
_o IZ
0"0
j
I00
0
I 0' 0 0
2 0'0 0
3C~XD
Spatial frequency [lines mm"~]
Exposure times within the range of constant exposure may be as long as several minutes however, so that the material will not be practically suitable until the exponential range is reached, where exposure times of a few seconds are feasible. This requires irradiances of over 5 mW cm "2. For comparison of the test material with the conventional photographic emulsions, the Schwarzschild exponent is an important factor. The S-exponent (p) is defined as the power to which the exposure time must be raised in order to calculate the exposure time for various irradiances to achieve a constant result: I tP = constant
From Equations 2 and 3 follows, after a few simple steps: Fig. 2
26
Modulation transfer of the monomer-polymer system. The resolving power is limited by the size of the macromolecular copolymer clusters.
Optics and Laser Technology
February 1972
p. = i/(1 - m) in which m
2
the intensity component (m=0.67) in Equation 4, an S-exponent p = 3 is found. In view of the long exposure times involved, no attempts were made to find out how the curve of Fig. 3 would proceed at very low irradiances. The S-exponents for the various types of silver halide emulsions vary from 1 to a maximum of about 3 in extreme cases; this warrants the conclusion that the behaviour of the examined material is relatively unfavourable compared with the conventional materials.
K9.O00 5000 E
u --}
E
I000 500
Q. x t~
IOO O'1
It is especially this inefficiency caused by the reciprocity failure which makes brief exposure times very difficult to realize. However, this need not always be an objection to using the material. It has been found possible to make holograms of diffuse objects with exposure times of several tens of seconds, using a 20 mW He-Ne laser. This means that the use of photopolymers need not be limited to facilities which have lasers with extremely high output power available. As mentioned in the description of the formulations, the sensitivity o f the material likewise depends on the concentrations of the substances taking part in the reaction. Fig. 4 shows for instance the exponential relationship between the exposure and the concentration of methylene blue. The measurements were conducted at an irradiance of approx 35 mW cm -2. Up to a concentration of 7.5 mg cm "3 the sensitivity increases exponentially, but addition of more methylene blue has no further effect. The concentration of the methylene blue in the formulation used was defined by reference to these results.
=
0"5
t
I
I
I
I-O
5
IO
50
I
IOO
Concentration [ mg cm -3 ] Fig.
4 When the concentration of the methylene blue is increased, the sensitivity increases exponentially. Maximum sensitivity is obtained at a concentration of approx 7.5 mg cm "3. Measurements were conducted at an irradiance of 35 mW cm "2 .
A main advantage is that an excellent phase hologram can be made in situ in a very short time and at very low cost. Exposure times of a few to several tens of seconds are feasible at irradiances varying from 0.1 to 10 mW cm -2. The resolving power of the material is also quite sufficient for most holographic applications. Acknowledgements
Keeping
properties
Holograms in layers of photopolymers do not appear to keep well. After some time the adhesion of the plastic layer to the glass deteriorates, and air penetrates between the glass and the polymer, always at the edges first. Lacquering the edges is not sufficient to keep air out. Adhesion is very much improved, however, by thorough cleaning of the glass surfaces. It is also necessary to convert the remaining monomer into polymer, as the chemicals will otherwise crystallise between the glass surfaces after some time and adversely affect the signal-to-noise ratio. With these provisions, a hologram could be kept in good condition for about a week. Conclusions
In spite of the relatively poor keeping properties (a few days) and the considerable reciprocity failure (Schwarzschild exponent, p = 3), combined with a low sensitivity (5 mJ cm -2) of the photomonomers acryl amide and methylene-bisacryl amide, the material promises to be an excellent substitute for the conventional silver halide materials in certain applications.
The work described in this paper has been carried out in the Optical Department of our Institute, under supervision of H.J. Raterink. The investigation of photopolymers is part of a research programme, directed at industrial application of laser techniques for vibration and deformation analysis.
References 1 2 3 4 5 6 7 8 9 10
Kogelnik,H. (1967) Proc of the Symposium on Modern Optics (J. Fox, ed, Polytechnic Press, Brooklyn) Urbach, John. C. and Meier, Reinhard. W. (1966) Appl Opt 5(4) 666 Lin, L.H. and Beauchamp, H.L. (1970) Appl Opt 9(9) 2088 Jenney, J.A. (1970) J O S A 60(9) 1155 Burnett, G.M. and Melville, H.W. (1947) Proc Roy Soc London A189 456 Oster, G. and Mark, H. (1953) J O S A 43(4) 283 Bunn,C.W. and Carner, E.V. (1947) Proc Roy Soc London A189 39 Stong, C.L. (December 1969) Scientific American 128 Handbook of Chemistry and Physics (R.C. Weast, ed, (68/69) The Chemical Rubber Co, Cleveland) KeepSolution A out of direct sunlight because it may polimerise with violence.
Optics and Laser Technology
February 1972
27