**5.1 The principle of action**

56 Holograms – Recording Materials and Applications

refractive index of developed high-resolution AgHal materials (film plates PFG-02) in the red region λ ~ 650 nm. Experimental measurements, their results given on Fig. 5b, were performed under different conditions of formation of developed particles and at different developed silver concentrations; the resulting dependence can be represented by the

where ΔnAg is the change of the medium refractive index due to the presence of silver particles; T is the medium thickness; C is the surface silver concentration (silver coverage in g/m2), measured by rhodanine method. Eq. (2) enables estimation of developed silver concentration by measured values of phase incursions in the red region λ ~ 650 nm due to the presence of colloid silver particles, without refinement of their optical parameters. Experimental measurements of a hologram at λ = 1.5 μm are performed in the absence of the amplitude component and allow finding the phase modulation of a hologram at given wavelength, which allows estimating the spectral dependence of amplitude and phase modulation of the studied Ag-PG hologram, φ1(λ) and γ1(λ), by using known wavelength dependence of optical constants of real medium. Calculations of angular selectivity contours of amplitude-phase holograms at 0.63 μm, carried out by experimentally found phase modulation at λ = 1.5 μm, have shown satisfactory agreement with the results of

This situation enables using known value φ1 (0.63-0.65 μm) to estimate the concentration of silver, forming given hologram, with the help of empirical dependence Eq. (2). The performed calculation have shown the volume concentration of silver, forming the Ag-PG hologram, to be C1 = (1÷3) 10-4, which corresponds to surface concentration 1÷5 g/m2, comparable to a similar value in developed holographic film plates PFG-02 and PFG-03. It should be noted that average hologram absorption γ0 (hence average silver concentration in the sample, C0) exceed the estimates found by values γ1 and C1 due to the presence of veil, formed by colloid silver particles, which take no part in hologram construction, yet lead to an increase of sample absorption in the spectral region of the absorption band of colloid

**4.4 Distinctive features of AgHal-porous glass medium-composite in hologram** 

out with samples in the shape of plane-parallel plates or disks 1-3 mm thick.

The proposed version of light-sensitive porous medium-composite on the AgHal base significantly enhances the potential of using traditional AgHal media in holography, which

1. Porous glasses are close in physical and mechanical properties to silicate glass, have close thermal expansion coefficient, and are shrink-proof. Upon placing air-dry samples into liquid medium and using water solutions in post-exposure treatment, geometrical dimension of the framework remain unchanged, while local deformations of solidphase shell inside a pore are substantially less than light wavelength, lack any regular pattern and cause no distortion of hologram structure. The experiments were carried

2. Size of light-sensitive particles and those forming a finished hologram cannot exceed the maximum size of porous framework ducts. When using matrices NPG-17, the size

ΔnAgT/λ = 0.28 C (g/m2) (2)

empirical formula:

experimental measurements.

particles.

**construction** 

is evidenced by its main features.

amounts to 20 nm.

There are at present several modifications of polymeric light-sensitive media on the base of phenanthrenequinone (PQ), which implement the diffusion enhancement principle. The authors devised the technology to obtain a material, whose samples have certain holographic and physical-mechanical parameters, conditioned by modes of sample synthesis and hologram construction. The name Difphen (from words DIFfusion and PHENanthrenequinone) allows singling given material out of variety of other modifications of PQ-based polymeric medium with diffusion enhancement.

Samples of material Difphen (like some other materials of given group) represent a solid solution of organic dye PQ, uniformly distributed in polymethylmethacrylate (PMMA). Light sensitivity of the material results from capacity of PQ to bond to polymer under irradiation, transforming into 9,10-disubstituted derivative of phenanthrene (НPQR) according to schematic diagram (Chercasov et al., 1991):

$$\begin{array}{ccccc} & \text{hv} & \text{RH} & \text{R}^{\bullet} \\ \text{PQ} & \rightarrow & \text{"PQ} & \rightarrow & \text{HPQ}^{\bullet} \rightarrow & \text{HPQR} \end{array} \tag{3}$$

where 3PQ is the triplet-excited PQ molecule, НPQ● is the semiquinone radical, RH и R● are, respectively, the polymer molecule and radical.

Samples of recording medium 1÷5 mm thick are obtained by means of bulk polymerization of PQ solutions in methylmethacrylate (MMA) between molding glass plates. Fig. 7a

Light-Sensitive Media-Composites for

purposefully change and control them.

figure (dashed line).

holograms.

holographic process parameters.

denoted as hardness index, Кh.

Recording Volume Holograms Based on Porous Glass and Polymer 59

The work used samples obtained by means of casting polymerization in the shape of disks with diameter 20-40 mm and thickness 1-4 mm. Fixation of samples under study was affected by radiation of mercury lamp at 436 nm or that of "blue" LED with maximum of radiation band at 470 nm. Schematic diagram of information recording in such medium is

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

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

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

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

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

shown on Fig. 7b by the example of constructing a hologram-grating.

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

(curve 1) shows PQ absorption spectrum, measured in a sample with concentration 8,5·10-3M, with clearly defined long-wave maximum of absorption in the visible region (λ = 405÷410 nm). Under the impact of radiation, PQ changes its chemical structure, while the formed photoproduct (semiquinone radical) bonds to a PMMA molecule and loses its mobility. Absorption spectrum of photoproduct differs from that of PQ: the long-wave maximum vanishes (Fig.7a, curve 2). The dissimilarity of absorption spectra of PQ and its photoproduct preconditions the difference in their refraction indices and determines the efficiency of recorded latent image hologram (without effect of post-exposure treatment) at given wavelength.

Fig. 7. a – absorption spectra of PQ (curve 1) and photoproduct (curve 2) in polymeric matrix. b – diagram to explain hologram construction process in polymeric medium with diffusion enhancement: distribution of molecules of PQ ( ) and photoproduct ( ) across the sample bulk in the initial state (1), after recording of hologram-grating (2), after warmup (3), after fixation (4).

The process of post-exposure warm-up leads to redistribution of concentration of unexposed PQ molecules uniformly in the sample bulk, which ensures enhancement of recorded interference pattern, formed by photoproduct, that is, hologram "development". The sample after fixation becomes non-light-sensitive, the unreacted PQ, distributed uniformly across the sample volume, transforms into non-light-sensitive photoproduct with the same distribution, therefore, the fixation process leaves n1 unchanged. The presence of photoproduct molecules in the medium makes the maximum values of n1 of constructed hologram dependent on recording radiation intensity, spatial frequency of interference pattern being recorded and initial PQ concentration.

Thus, hologram construction on given material includes the following basic stages:


(curve 1) shows PQ absorption spectrum, measured in a sample with concentration 8,5·10-3M, with clearly defined long-wave maximum of absorption in the visible region (λ = 405÷410 nm). Under the impact of radiation, PQ changes its chemical structure, while the formed photoproduct (semiquinone radical) bonds to a PMMA molecule and loses its mobility. Absorption spectrum of photoproduct differs from that of PQ: the long-wave maximum vanishes (Fig.7a, curve 2). The dissimilarity of absorption spectra of PQ and its photoproduct preconditions the difference in their refraction indices and determines the efficiency of recorded latent image hologram (without effect of post-exposure treatment) at

Fig. 7. a – absorption spectra of PQ (curve 1) and photoproduct (curve 2) in polymeric matrix. b – diagram to explain hologram construction process in polymeric medium with diffusion enhancement: distribution of molecules of PQ ( ) and photoproduct ( ) across the sample bulk in the initial state (1), after recording of hologram-grating (2), after warm-

Thus, hologram construction on given material includes the following basic stages:

• hologram fixation by incoherent radiation with wavelength in the PQ absorption region

• hologram recording by radiation with wavelength 400 < λ < 530 nm;

The process of post-exposure warm-up leads to redistribution of concentration of unexposed PQ molecules uniformly in the sample bulk, which ensures enhancement of recorded interference pattern, formed by photoproduct, that is, hologram "development". The sample after fixation becomes non-light-sensitive, the unreacted PQ, distributed uniformly across the sample volume, transforms into non-light-sensitive photoproduct with the same distribution, therefore, the fixation process leaves n1 unchanged. The presence of photoproduct molecules in the medium makes the maximum values of n1 of constructed hologram dependent on recording radiation intensity, spatial frequency of interference

given wavelength.

up (3), after fixation (4).

λ = 430-490 nm.

pattern being recorded and initial PQ concentration.

• warm-up of samples for 50 hrs at temperature 50°С;

The work used samples obtained by means of casting polymerization in the shape of disks with diameter 20-40 mm and thickness 1-4 mm. Fixation of samples under study was affected by radiation of mercury lamp at 436 nm or that of "blue" LED with maximum of radiation band at 470 nm. Schematic diagram of information recording in such medium is shown on Fig. 7b by the example of constructing a hologram-grating.
