**5.2 Relation of holographic characteristics to conditions of sample synthesis**

To obtain a medium with required holographic characteristics, it is necessary to know the effect of synthesis conditions on constructed hologram parameters and be in a position to purposefully change and control them.

The performed experiments have shown the quality of constructed holograms to be strongly affected by physical and mechanical properties of samples, which are determined by their hardness, and the most important holographic characteristic, determined by synthesis conditions for medium samples, to be the dependence of diffraction efficiency*,* of recorded holograms on post-exposure warm-up time. The data given on Fig. 8a, curve 1, show a typical dependence, which is specific for optimal synthesis conditions and can be approximated with two straight lines, as shown on the figure (dashed line).

The first dashed line describes the DE growth from the beginning of post-exposure warm-up (the DE value for latent image hologram) to attainment of the maximum values of DE, its slope being defined by PQ diffusion rate at given warm-up temperature (that is, degradation rate of the grating, formed by PQ unreacted with light). The second dashed line describes the hologram behavior after the maximum values of DE are attained and is, for the optimal synthesis samples, parallel to the abscissa axis. The intersection point of dashed approximation straight lines defines the characteristic sample warm-up time, tch, necessary to attain the maximum efficiency of recorded holograms.

With synthesis conditions, leading to "soft" sample structure, a latent image hologram has high DE values, quickly attains maximum efficiency at warm-up, is unstable and partially degrades at elevated temperature (Fig. 8a, curve 2). With synthesis conditions, leading to "hard" sample structure, PQ molecule diffusion proceeds too slowly, and high values of phase modulation are unattainable under a reasonable temperature-time regime of postexposure treatment (curve 3). The clear-cut dependence of characteristic sample warm-up time on sample synthesis conditions allowed dividing all studied samples into three main groups: optimal synthesis samples, "soft" synthesis samples, and "hard" synthesis samples. Because of necessity to estimate the hardness of samples, a procedure was devised to allow relating physical and mechanical properties of samples to synthesis conditions and holographic process parameters.

Hardness was found by resistance to indentation of diamond pyramid into a material (Vickers test). The said procedure is notable for its relative simplicity, reproducibility and is supported with standard industrial instruments. Samples were tested with the help of PMT-3 hardness tester. Measured for each sample was the diagonal of the diamond pyramid indent in the material, its length depending on the sample hardness. It is the diagonal length, expressed in relative units (tied up with the use of a specific instrument), that is denoted as hardness index, Кh.

Light-Sensitive Media-Composites for

**5.3.1 Modulation transfer function** 

γ = 70 mm-1, 2 – γ = 320 mm-1, 3 – γ = 1100 mm-1.

phase modulation in the region of low spatial frequencies.

theoretically calculated contours for hologram-gratings of given thickness.

**5.3 Main characteristics** 

from exhausted.

**5.3.2 Dynamic range** 

Recording Volume Holograms Based on Porous Glass and Polymer 61

Resolving power of recording material under study is from traditional viewpoint determined by dimensions PQ molecules, whose diameter is less than 2 nm. Hologramgratings on given material were obtained at maximum spatial frequency about 3000 mm-1 with efficiency over 80% (Sukhanov, 1994a), yet it can be said with certainty that the potential of the material in recording of interference structure of high spatial frequency is far

Fig. 9. a – dependence of phase modulation of transmission hologram-gratings φ1 on spatial frequency of recorded interference pattern γ: 1 – latent image hologram (after exposure); 2 – hologram after complete cycle of post-exposure treatment. b – angular selectivity contours of transmission holograms with different spatial frequency, recorded with φ1 < 0.5π: 1 –

Maximum attainable phase modulation amplitude of hologram-gratings, recorded in a medium with diffusion enhancement, depends on grating constant, post-exposure warm-up conditions and is defined by diffusion rate of PQ molecules, which leads to a decrease of

Measurement results for parameters of holograms, recorded in spatial frequency region 70÷350 mm-1, are given on Fig.9a. Phase modulation of latent image holograms (curve 1) is less than 0.2π and practically independent of spatial frequency at ν > 200 mm-1. For holograms with completed post-exposure treatment cycle (curve 2), phase modulation increases with growing spatial frequency. Thus, it can be concluded that material Difphen with sample thickness on order of 1 mm allows obtaining transmission hologram-gratings with DE over 70 % (φ1 > 0.3π) in spatial frequency region ν > 100 mm-1. When dealing with higher spatial frequency region, the maximum attainable values of hologram phase modulation are strongly affected by factors, determined by the manner of treatment and conditions of hologram recording, such as stability of recording scheme, design features of hologram attachment and so on. Contours of angular selectivity of hologram-gratings with different spatial frequency are given on Fig. 9b and demonstrate agreement with

When using optimal-synthesis samples of material Difphen, the exposure range, wherein the linear mode of recording of latent image holograms can be ensured, is limited by values

Fig. 8. а – dependence of diffraction efficiency η of holograms, recorded on samples of different synthesis, on post-exposure warm-up time (tch is characteristic warm-up time); curve 1 – optimal synthesis, curve 2 – soft synthesis, curve 3 – hard synthesis, spatial frequency of hologram 640 mm-1. b – dependences of relative hardness index Kh\* of samples (curve 1), deviations of synthesis temperature from the given one ΔТ (curve 2), latent image hologram DE (curve 3) on characteristic warm-up time tch of recorded holograms.

Measurements of Кh for samples with different warm-up conditions and shelf lives (up to 10 years) have shown that there is limiting value (Кh)lim, which defines the maximum possible hardness of samples, manufactured by given process. This allowed estimating the degree of sample hardness with the help of relative hardness index Кh\* = (Кh)lim/Кh (curve 1 on Fig. 8b). Sample synthesis conditions were varied by changing some parameters: polymerization initiator concentration, PQ concentration, temperature-time regime of polymerization etc. Fig. 8b (curve 2) shows the dependence of impact of deviation of synthesis temperature from the optimal one (ΔТ = Т – Тopt) on characteristic sample warmup time to be in very good correlation with similar dependence Кh\*(tch).

Dependence η(tch) of latent image holograms (Fig. 8b, curve 3) supplements the notion of existence of a rather narrow range of parameters of sample synthesis, wherein one can obtain holograms with required characteristics: stable parameters in the course of operation, low efficiency of latent image holograms, and high efficiency after warm-up under given temperature-time regime. Samples, obtained under optimal synthesis conditions, are seen from the data of Fig. 8b to exhibit a certain degree of hardness, which allows carrying out preliminary sample quality control without staging a laborconsuming holographic experiment. (Diamond pyramid diagonal is no longer than tenths of a millimeter, which allows making hardness measurements without deterioration of the working sample.)

The fact of existence of limiting values of hardness for studied material samples along with measurements of sample hardness at each stage of hologram construction process have shown that post-exposure sample warm-up can be divided into two basic stages: warm-up-1 till specified hologram parameters are achieved (at temperature 50 °C) and warm-up-2 till limiting values of sample hardness are achieved (additional warm-up at temperature 60- 65 °C, which is higher than at warm-up-1, but below the vitrifying point). Warm-up-2 can take place both before and after sample fixation under conditions that leave hologram parameters unchanged.

#### **5.3 Main characteristics**

60 Holograms – Recording Materials and Applications

Fig. 8. а – dependence of diffraction efficiency η of holograms, recorded on samples of different synthesis, on post-exposure warm-up time (tch is characteristic warm-up time); curve 1 – optimal synthesis, curve 2 – soft synthesis, curve 3 – hard synthesis, spatial frequency of hologram 640 mm-1. b – dependences of relative hardness index Kh\* of samples (curve 1), deviations of synthesis temperature from the given one ΔТ (curve 2), latent image hologram DE (curve 3) on characteristic warm-up time tch of recorded

Measurements of Кh for samples with different warm-up conditions and shelf lives (up to 10 years) have shown that there is limiting value (Кh)lim, which defines the maximum possible hardness of samples, manufactured by given process. This allowed estimating the degree of sample hardness with the help of relative hardness index Кh\* = (Кh)lim/Кh (curve 1 on Fig. 8b). Sample synthesis conditions were varied by changing some parameters: polymerization initiator concentration, PQ concentration, temperature-time regime of polymerization etc. Fig. 8b (curve 2) shows the dependence of impact of deviation of synthesis temperature from the optimal one (ΔТ = Т – Тopt) on characteristic sample warm-

Dependence η(tch) of latent image holograms (Fig. 8b, curve 3) supplements the notion of existence of a rather narrow range of parameters of sample synthesis, wherein one can obtain holograms with required characteristics: stable parameters in the course of operation, low efficiency of latent image holograms, and high efficiency after warm-up under given temperature-time regime. Samples, obtained under optimal synthesis conditions, are seen from the data of Fig. 8b to exhibit a certain degree of hardness, which allows carrying out preliminary sample quality control without staging a laborconsuming holographic experiment. (Diamond pyramid diagonal is no longer than tenths of a millimeter, which allows making hardness measurements without deterioration of the

The fact of existence of limiting values of hardness for studied material samples along with measurements of sample hardness at each stage of hologram construction process have shown that post-exposure sample warm-up can be divided into two basic stages: warm-up-1 till specified hologram parameters are achieved (at temperature 50 °C) and warm-up-2 till limiting values of sample hardness are achieved (additional warm-up at temperature 60- 65 °C, which is higher than at warm-up-1, but below the vitrifying point). Warm-up-2 can take place both before and after sample fixation under conditions that leave hologram

up time to be in very good correlation with similar dependence Кh\*(tch).

holograms.

working sample.)

parameters unchanged.

#### **5.3.1 Modulation transfer function**

Resolving power of recording material under study is from traditional viewpoint determined by dimensions PQ molecules, whose diameter is less than 2 nm. Hologramgratings on given material were obtained at maximum spatial frequency about 3000 mm-1 with efficiency over 80% (Sukhanov, 1994a), yet it can be said with certainty that the potential of the material in recording of interference structure of high spatial frequency is far from exhausted.

Fig. 9. a – dependence of phase modulation of transmission hologram-gratings φ1 on spatial frequency of recorded interference pattern γ: 1 – latent image hologram (after exposure); 2 – hologram after complete cycle of post-exposure treatment. b – angular selectivity contours of transmission holograms with different spatial frequency, recorded with φ1 < 0.5π: 1 – γ = 70 mm-1, 2 – γ = 320 mm-1, 3 – γ = 1100 mm-1.

Maximum attainable phase modulation amplitude of hologram-gratings, recorded in a medium with diffusion enhancement, depends on grating constant, post-exposure warm-up conditions and is defined by diffusion rate of PQ molecules, which leads to a decrease of phase modulation in the region of low spatial frequencies.

Measurement results for parameters of holograms, recorded in spatial frequency region 70÷350 mm-1, are given on Fig.9a. Phase modulation of latent image holograms (curve 1) is less than 0.2π and practically independent of spatial frequency at ν > 200 mm-1. For holograms with completed post-exposure treatment cycle (curve 2), phase modulation increases with growing spatial frequency. Thus, it can be concluded that material Difphen with sample thickness on order of 1 mm allows obtaining transmission hologram-gratings with DE over 70 % (φ1 > 0.3π) in spatial frequency region ν > 100 mm-1. When dealing with higher spatial frequency region, the maximum attainable values of hologram phase modulation are strongly affected by factors, determined by the manner of treatment and conditions of hologram recording, such as stability of recording scheme, design features of hologram attachment and so on. Contours of angular selectivity of hologram-gratings with different spatial frequency are given on Fig. 9b and demonstrate agreement with theoretically calculated contours for hologram-gratings of given thickness.

#### **5.3.2 Dynamic range**

When using optimal-synthesis samples of material Difphen, the exposure range, wherein the linear mode of recording of latent image holograms can be ensured, is limited by values

Light-Sensitive Media-Composites for

**5.3.3 Other characteristics** 

the material in the UV region.

are given on Fig. 11b.

diffraction angle by 0.1÷0.2 mrad.

obtain higher values of Σϕ1 at given sample thickness.

Recording Volume Holograms Based on Porous Glass and Polymer 63

dependence of total phase modulation of superimposed holograms on their total exposure, obtained in experiment on construction of 10 superimposed holograms with individual warm-up. The dependence describes the dynamic range of used RM, being a kind of characteristic curve of light-sensitive material. Maximum values of total phase modulation, attained in the experiment, are as high as Σϕ<sup>1</sup> ≅ 12 rad, which was obtained on a sample 2.3 mm thick. With the use of polymeric media on the base of PMMA with PQ, other authors in work (Steckman et al., 1998) attained values Σϕ<sup>1</sup> ≅ 4.8 rad for a sample 3 mm thick, and in work (Lin et al, 2000) – Σϕ<sup>1</sup> ≅ 14 rad for a sample 8 mm thick. The cited works point out that Σϕ1 grows linearly with growing sample thickness. Comparison of experimental data in the cited works and those by the authors, testifies that using the individual post-exposure warm-up of superimposed holograms can increase dynamic range of the material and

Modulation amplitude of refractive index in holograms. Experimental data on the magnitude of the amplitude of refractive index variation in a hologram and its dependence on wavelength of reconstruction radiation were obtained in study of hologram-gratings, recorded on optimal-synthesis samples of material Difphen with different thickness. Angular selectivity contour of hologram-grating under study was measured using lasers with different wavelengths; at each wavelength, phase modulation amplitude, φ1, was determined and modulation amplitude of the first harmonic of refractive index, n1, of a hologram was calculated by formulas of Kogelnik's theory. Fig. 11a shows calculation results for n1(λ) of studied samples. Dashed lines illustrate the process character and allow making some estimates. Modulation amplitude of refractive index of studied holograms in the visible region is within limits n1 < 10-3; dependence n1(λ) in the studied spectral range (473÷808 nm) displays normal behavior of dispersion, conditioned by absorption band of

The effect of ambient humidity variation on hologram performance. Polymeric RM on the PQ base are commonly considered to demonstrate negligible shrinkage, yet it is known that PMMA samples in atmosphere of increased humidity are capable of absorbing up to 1.5 % of water, which causes the sample thickness and average refractive index to change. Stability of hologram parameters at variations of ambient humidity was estimated by the change in the position of the maximum and in the shape of angular selectivity contour under conditions of different ambient humidity. Use was made of measurement design for angular selectivity contour of volume holograms with divergent radiation beam and data recording on CMOS-matrix. The collected data, processed according to devised procedure,

In the study of holograms with width of angular selectivity contour 1.3 mrad, recorded on a sample ≈ 1 mm thick, the shift of contour maximum position at an abrupt change of ambient humidity from 85 % to 50 % at temperature 21 °С corresponds to a change of radiation

Stability of hologram parameters over a long period of time of storage (6÷9 years) and operation. During storage and observation time, the holograms stayed in a room with temperature (15-40) °С and were used for occasional experiments as optic circuitry elements, for students' laboratory works etc. Used as measured and controlled hologram parameters were DE and angular selectivity contour. The data show that over the time of storage and

Н < 1 J/cm2, as seen from the data of Fig. 10a that shows exposure dependences of hologram parameters after completion of recording (curve 1) and after completion of postexposure treatment cycle (curve 2). At Н > 1 J/cm2, the recording process is of non-linear character, as high-efficiency latent image holograms are formed.

Fig. 10. a – exposure dependences of hologram parameters: diffraction efficiency η after completion of recording (curve 1) and phase modulation φ1 after completion of postexposure treatment (curve 2); spatial frequency 640 mm-1. b – dependence of phase modulation amplitude φ1 of superimposed holograms, recorded on the same sample section, on sample warm-up time; for each hologram Нi = 0.8 J/cm2 (holograms are numbered according to order of recording, i = 1, 2, 3,…10). c – dependence of total phase modulation of multiple hologram Σφ1 on total exposure of recorded holograms ΣНi.

The performed experiments on recording of a single hologram on samples 1 mm thick have shown attainability of high enough values of modulation amplitude of the medium, in excess of π radians. Limitations in the case are due to impossibility to obtain latent image holograms with low DE, rather than dynamic range.

Dynamic range of volume RM can be estimated and used in superimposed hologram recording. The potential was demonstrated by superimposed recording of more than 1000 low-efficiency holograms on the same sample section with the view of using it as archive memory (Steckman et al., 1998). Specifics of superimposed recording of high-efficiency holograms is linked with deterioration of holograms, constructed in a medium with nonuniform distribution of light-sensitive particles due to recording of previous holograms. The features of given material allowed overcoming the drawback by way of construction of superimposed holograms with individual post-exposure warm-up: recording 1 – warm-up; recording 2 – warm-up and so on. Fig. 10b shows dependence of phase modulation of superimposed holograms, constructed by angular multiplexing technique, on total time of sample warm-up. Each hologram, regardless of recording number, is recorded in a medium with uniform distribution of PQ in the sample bulk: PQ concentration falls with hologram number growing, which leads, as seen on Fig. 10b, to smaller attainable values of phase modulation of single hologram.

Of much interest in studying the volume RM is quantity Σϕ1, which is a sum of values of phase modulation of all superimposed holograms that constitute given multiple hologram, and describes the dynamic range of used light-sensitive medium. Fig. 10c shows dependence of total phase modulation of superimposed holograms on their total exposure, obtained in experiment on construction of 10 superimposed holograms with individual warm-up. The dependence describes the dynamic range of used RM, being a kind of characteristic curve of light-sensitive material. Maximum values of total phase modulation, attained in the experiment, are as high as Σϕ<sup>1</sup> ≅ 12 rad, which was obtained on a sample 2.3 mm thick. With the use of polymeric media on the base of PMMA with PQ, other authors in work (Steckman et al., 1998) attained values Σϕ<sup>1</sup> ≅ 4.8 rad for a sample 3 mm thick, and in work (Lin et al, 2000) – Σϕ<sup>1</sup> ≅ 14 rad for a sample 8 mm thick. The cited works point out that Σϕ1 grows linearly with growing sample thickness. Comparison of experimental data in the cited works and those by the authors, testifies that using the individual post-exposure warm-up of superimposed holograms can increase dynamic range of the material and obtain higher values of Σϕ1 at given sample thickness.

### **5.3.3 Other characteristics**

62 Holograms – Recording Materials and Applications

Н < 1 J/cm2, as seen from the data of Fig. 10a that shows exposure dependences of hologram parameters after completion of recording (curve 1) and after completion of postexposure treatment cycle (curve 2). At Н > 1 J/cm2, the recording process is of non-linear

Fig. 10. a – exposure dependences of hologram parameters: diffraction efficiency η after completion of recording (curve 1) and phase modulation φ1 after completion of postexposure treatment (curve 2); spatial frequency 640 mm-1. b – dependence of phase modulation amplitude φ1 of superimposed holograms, recorded on the same sample section, on sample warm-up time; for each hologram Нi = 0.8 J/cm2 (holograms are numbered according to order of recording, i = 1, 2, 3,…10). c – dependence of total phase modulation of multiple hologram Σφ1 on total exposure of recorded holograms ΣНi.

The performed experiments on recording of a single hologram on samples 1 mm thick have shown attainability of high enough values of modulation amplitude of the medium, in excess of π radians. Limitations in the case are due to impossibility to obtain latent image

Dynamic range of volume RM can be estimated and used in superimposed hologram recording. The potential was demonstrated by superimposed recording of more than 1000 low-efficiency holograms on the same sample section with the view of using it as archive memory (Steckman et al., 1998). Specifics of superimposed recording of high-efficiency holograms is linked with deterioration of holograms, constructed in a medium with nonuniform distribution of light-sensitive particles due to recording of previous holograms. The features of given material allowed overcoming the drawback by way of construction of superimposed holograms with individual post-exposure warm-up: recording 1 – warm-up; recording 2 – warm-up and so on. Fig. 10b shows dependence of phase modulation of superimposed holograms, constructed by angular multiplexing technique, on total time of sample warm-up. Each hologram, regardless of recording number, is recorded in a medium with uniform distribution of PQ in the sample bulk: PQ concentration falls with hologram number growing, which leads, as seen on Fig. 10b, to smaller attainable values of phase

Of much interest in studying the volume RM is quantity Σϕ1, which is a sum of values of phase modulation of all superimposed holograms that constitute given multiple hologram, and describes the dynamic range of used light-sensitive medium. Fig. 10c shows

character, as high-efficiency latent image holograms are formed.

holograms with low DE, rather than dynamic range.

modulation of single hologram.

Modulation amplitude of refractive index in holograms. Experimental data on the magnitude of the amplitude of refractive index variation in a hologram and its dependence on wavelength of reconstruction radiation were obtained in study of hologram-gratings, recorded on optimal-synthesis samples of material Difphen with different thickness. Angular selectivity contour of hologram-grating under study was measured using lasers with different wavelengths; at each wavelength, phase modulation amplitude, φ1, was determined and modulation amplitude of the first harmonic of refractive index, n1, of a hologram was calculated by formulas of Kogelnik's theory. Fig. 11a shows calculation results for n1(λ) of studied samples. Dashed lines illustrate the process character and allow making some estimates. Modulation amplitude of refractive index of studied holograms in the visible region is within limits n1 < 10-3; dependence n1(λ) in the studied spectral range (473÷808 nm) displays normal behavior of dispersion, conditioned by absorption band of the material in the UV region.

The effect of ambient humidity variation on hologram performance. Polymeric RM on the PQ base are commonly considered to demonstrate negligible shrinkage, yet it is known that PMMA samples in atmosphere of increased humidity are capable of absorbing up to 1.5 % of water, which causes the sample thickness and average refractive index to change. Stability of hologram parameters at variations of ambient humidity was estimated by the change in the position of the maximum and in the shape of angular selectivity contour under conditions of different ambient humidity. Use was made of measurement design for angular selectivity contour of volume holograms with divergent radiation beam and data recording on CMOS-matrix. The collected data, processed according to devised procedure, are given on Fig. 11b.

In the study of holograms with width of angular selectivity contour 1.3 mrad, recorded on a sample ≈ 1 mm thick, the shift of contour maximum position at an abrupt change of ambient humidity from 85 % to 50 % at temperature 21 °С corresponds to a change of radiation diffraction angle by 0.1÷0.2 mrad.

Stability of hologram parameters over a long period of time of storage (6÷9 years) and operation. During storage and observation time, the holograms stayed in a room with temperature (15-40) °С and were used for occasional experiments as optic circuitry elements, for students' laboratory works etc. Used as measured and controlled hologram parameters were DE and angular selectivity contour. The data show that over the time of storage and

Light-Sensitive Media-Composites for

Thickness, mm

photoproduct molecule mobility.

spatial spectrum of radiation.

hologram.

Sample type

Recording Volume Holograms Based on Porous Glass and Polymer 65

the constructed holograms. Comparative characteristics of bulk and film samples (with the same optical density at the recording wavelength), derived by using a unified procedure of holographic testing, are given in Table 1. Quantitative estimation of enhancement of hologram-gratings, Q, used the value of modulation amplitude of refractive index, n1, obtained after warm-up and after fixation relative to the value of n1 of latent image

Bulk 1.45 ~ 20 0.94 5.1 5.0 Film 0.18 ~ 20 0.43 4.0 4.5 Table 1. Comparison of parameters of holograms, constructed on bulk and film samples of

Investigation of stability characteristics of parameters of holograms, recorded on film-type samples with thickness on order of 100 μm, in the course of long-term storage and operation has shown that some studied holograms, recorded at exposures over 0.3 J/cm2, exhibited a DE increase beyond the limits of predicted experimental errors (see Table 1). The effect of DE growth during storage of holograms, recorded on film samples, may be due to a host of

Performed experiments have shown the fruitfulness of the devised approach to obtaining polymeric holographic film-type materials and the necessity to carry on the research and to improve parameters of film samples and procedure for their use with the view of creating an assortment of materials for hologram recording with wide spectrum of parameters.

Material Difphen belongs to the group of polymeric recording materials, implementing the diffusion enhancement principle, which is at present fruitfully used by various authors in a number of science and engineering projects. Material samples have certain holographic, and physical and mechanical parameters, determined by the devised modes of sample synthesis

1. Resolving power of material exceeds 3000 mm-1 and is limited by PQ molecule size and

2. The modulation transfer function is untypical for traditional light-sensitive materials. In the low spatial frequency region (less than 50 mm-1), hologram construction on given material is impossible; phase modulation amplitude of recorded holograms grows with increasing spatial frequency of a hologram. The use of such media can be very helpful in information recording that requires canceling the low-frequency component of

3. Feasibility of construction of latent image holograms and their subsequent enhancement under conditions of unchanged interference structure, formed at the recording stage,

4. Wide dynamic range and feasibility of construction of high-efficiency superimposed

which ensures linearity of information recording in a wide dynamic range.

holograms owing to the use of individual post-exposure warm-up.

DE (λ = 633 nm) Enhancement, Q (λ = 633 nm)

> After fixation

After warm-up

Transmission (λ = 488 nm), %

polymeric material with PQ (measurements at λ = 633 nm).

causes, including, apparently, plastic nature of RM samples used.

**5.5 Main special features of samples of volume polymer medium** 

and of hologram construction. The following main special features can be noted.

operation of studied holograms there is no noticeable and systematic degradation of their parameters in a wide range of used exposures and spatial frequencies.

Fig. 11. a - wavelength dependence of modulation amplitude of the first harmonic of refractive index n1 for holograms constructed on different samples of material Difphen: 1-3 – bulk-synthesis samples 2.3 mm, 2.0 mm, 1.3 mm thick, 4 – film sample 0.1 mm thick. b – position of the maximum of contour of studied hologram-grating under stable conditions (1) and at abrupt change in ambient humidity from 85 % to 50 % (2). Dashed lines illustrate the character of processes.

### **5.4 Film-type samples of material Difphen**

Of great interest is to obtain samples of given material, which have thickness in the range 50÷500 μm, since the available product line of RM for holography has such samples represented by isolated laboratory-made specimens (e.g., Mahilny et al, 2006). The devised technological mode is unsuitable to obtain samples about 100 μm thick by means of bulk polymerization.

Film samples were obtained by the technique of pouring from solution, which is in some cases used to accomplish similar tasks under laboratory conditions. The basic components were PQ, PPMA and organic solvent. This resulted in homogeneous, uniformly colored films of size greater than 10 × 10 cm and thickness from 80 to 350 μm, whose elasticity allowed samples of various shape to be cut out of them. Here, volume PQ concentration in film samples was several times that in bulk ones. Performed tests have shown that samples, manufactured by given process, retain light-sensitive properties after storing at room temperature for a year.

To carry out holographic tests of film samples, custom designed cartridges were used: hologram recording, post-exposure treatment and measurement of parameters proceeded in the stable-state mode of the film sample, fixed between glass plates to exclude local deformations. Measurement results for DE of hologram-gratings, recorded at λ = 488 nm at spatial frequency 360 mm-1 with different exposures, have shown that values of DE on order of 50 % were achieved on test samples of film 180 μm thick (measurements at 633 nm). In a number of cases, a decrease of hologram DE under fixation was observed, which never occurred in working with bulk samples.

Obtained film samples permitted to run hologram recording in the "latent image" mode (with low values of DE), enhance holograms in the course of post-exposure warm-up and fix

operation of studied holograms there is no noticeable and systematic degradation of their

Fig. 11. a - wavelength dependence of modulation amplitude of the first harmonic of

character of processes.

polymerization.

temperature for a year.

occurred in working with bulk samples.

**5.4 Film-type samples of material Difphen** 

refractive index n1 for holograms constructed on different samples of material Difphen: 1-3 – bulk-synthesis samples 2.3 mm, 2.0 mm, 1.3 mm thick, 4 – film sample 0.1 mm thick. b – position of the maximum of contour of studied hologram-grating under stable conditions (1) and at abrupt change in ambient humidity from 85 % to 50 % (2). Dashed lines illustrate the

Of great interest is to obtain samples of given material, which have thickness in the range 50÷500 μm, since the available product line of RM for holography has such samples represented by isolated laboratory-made specimens (e.g., Mahilny et al, 2006). The devised technological mode is unsuitable to obtain samples about 100 μm thick by means of bulk

Film samples were obtained by the technique of pouring from solution, which is in some cases used to accomplish similar tasks under laboratory conditions. The basic components were PQ, PPMA and organic solvent. This resulted in homogeneous, uniformly colored films of size greater than 10 × 10 cm and thickness from 80 to 350 μm, whose elasticity allowed samples of various shape to be cut out of them. Here, volume PQ concentration in film samples was several times that in bulk ones. Performed tests have shown that samples, manufactured by given process, retain light-sensitive properties after storing at room

To carry out holographic tests of film samples, custom designed cartridges were used: hologram recording, post-exposure treatment and measurement of parameters proceeded in the stable-state mode of the film sample, fixed between glass plates to exclude local deformations. Measurement results for DE of hologram-gratings, recorded at λ = 488 nm at spatial frequency 360 mm-1 with different exposures, have shown that values of DE on order of 50 % were achieved on test samples of film 180 μm thick (measurements at 633 nm). In a number of cases, a decrease of hologram DE under fixation was observed, which never

Obtained film samples permitted to run hologram recording in the "latent image" mode (with low values of DE), enhance holograms in the course of post-exposure warm-up and fix

parameters in a wide range of used exposures and spatial frequencies.

the constructed holograms. Comparative characteristics of bulk and film samples (with the same optical density at the recording wavelength), derived by using a unified procedure of holographic testing, are given in Table 1. Quantitative estimation of enhancement of hologram-gratings, Q, used the value of modulation amplitude of refractive index, n1, obtained after warm-up and after fixation relative to the value of n1 of latent image hologram.


Table 1. Comparison of parameters of holograms, constructed on bulk and film samples of polymeric material with PQ (measurements at λ = 633 nm).

Investigation of stability characteristics of parameters of holograms, recorded on film-type samples with thickness on order of 100 μm, in the course of long-term storage and operation has shown that some studied holograms, recorded at exposures over 0.3 J/cm2, exhibited a DE increase beyond the limits of predicted experimental errors (see Table 1). The effect of DE growth during storage of holograms, recorded on film samples, may be due to a host of causes, including, apparently, plastic nature of RM samples used.

Performed experiments have shown the fruitfulness of the devised approach to obtaining polymeric holographic film-type materials and the necessity to carry on the research and to improve parameters of film samples and procedure for their use with the view of creating an assortment of materials for hologram recording with wide spectrum of parameters.
