Volume Holographic Material for Red Spectral Range Based on Polymer with Anthracene Side Groups

The possibility of volumetric phase recording based on photo-oxidation of the side anthracene groups of a new polymer with a glass transition temperature of ~338 K by oxygen that enters the layer across the boundary with the atmospheric medium, was established experimentally. Volumetric holographic gratings with a maximum diffraction efficiency of 2.5–9.5% were recorded with the radiation from a He–Ne laser (λ = 633 nm) in layers 5–20-μm thick. It was shown that in layers thicker than 10 μm the diffraction efficiency is limited by saturation of growth in the modulation depth of the optical path difference. This effect is due to increase in the radius of the photorefraction sphere around the photosensitizer molecule with the increased photoconversion of the anthracene groups, which lowers the resolution of the holographic grating and its effective thickness.

log , where d is the density of the polymer measured at 20 o C; M is the molecular mass of the repeating unit; i K ′ represents the increments of the atoms and bonds contained in the repeating unit; A is a constant for a specifi c series of polymers. By substituting i K ′ from [7] for a series of polymethacrylates we obtain the required parameters for Eq. (1) and T c = 509 K. In the second version a universal equation linking the glass transition point T c to a series of parameters for the chemical structure of the repeating unit in the macromolecules, which does not require knowledge of the density, was used [8]: ∑ is the van der Waals volume of the repeating unit [9]; A is a constant; N A is the Avogadro number. On the basis of Eq. (2) the T c value of the anthracene-containing homopolymer is determined as 514 K.
In the third version of the calculation increments of the van der Waals volumes of the atoms V i and the a i and b i values [10] characteristic of each atom and each type of interaction were used: Substitution of the necessary values in Eq. (3) gives T c = 517 K for the anthracene-containing homopolymer. The average value of T c = 513 K is signifi cantly higher than the glass transition point for polymethylmethacrylate -the base of reoxane (378 K [6]). A photoneutral comonomer, the homopolymer of which has T c = 293 K, was used to lower the T c value. Its mole fraction of 80 mole % gives the calculated T c = 338 K for the investigated polymeric material.
Methylene blue dye in ratios of 800:1, 1000:1, 1400:1, and 1700:1 by weight was used as sensitizer that generates singlet oxygen. The highest diffraction effi ciency for recording the diffraction grids was obtained with the 1700:1 polymer/ dye ratio, and this was therefore used to study the rate of photooxidation. Photorecording layers with thickness of >2 μm were deposited by pouring a solution of the polymer and dye onto glass substrates. Thinner layers were obtained by centrifuging the solution also on the glass substrates. A light-emitting diode with λ max = 650 nm exposing the sample at ~6 mW/cm 2 was used to determine the effi ciency of photo-oxidation. The phototransformation of the anthracene groups was followed from their electronic absorption spectra in the region of 330-410 nm, recorded on a Specord M40 spectrophotometer. The relative concentration of residual anthracene structures, averaged over the layer thickness, was obtained from the spectral data for various exposure times. Their photo-oxidation rate in layers of material of various thicknesses was characterized by the time t e required for the average concentration to decrease by e times. The photorefractive effect with uniform irradiation by the light-emitting diode was determined by measuring the refractive index of the layer of material n before and after irradiation at λ = 633 nm. For this, a technique based on measuring the waveguide refractive indices of the layer modes with a highly refractive prism pressed against its surface was used [11]. Transmitting holographic gratings with a period of d = 1.15 mm were recorded symmetrically with radiation from a He-Ne laser at λ = 633 nm. The intensity of each of the laser beams during holographic recording was ~0.5 mW/cm 2 . The diffraction effi ciency η was calculated by means of the formula: where I 0 is the zero-order beam intensity and I -1 is the intensity of the diffraction beam behind the photographic plate while the grating was being recorded.
Results and Discussion. Figure 1 shows the absorption spectra of the photosensitive layers for various LED irradiation times. The intensity of the long-wave structured band of the anthracene units for the thin layers decreases during irradiation (Fig. 1a, curves 1-3), tending toward zero values. This corresponds to the following scheme for the photoreaction: which leads to disappearance of the unsaturated bonds in the central ring. For layers of signifi cant thickness the photoreaction reduces the edge absorption (Fig. 1a, curves 4-6). At the same time some decrease in the absorption intensity of the sensitizer is observed (Fig. 1b, curves 1 and 2), indicating that its consumption is insignifi cant compared with the anthracene groups.
Increase of the thickness of the photosensitive layer must hinder diffusion saturation of the layer with oxygen and reduce the average quantum yield of the photoreaction β. Since their optical density in the spectral region of exposure is ≤0.2 while the distribution of the intensity in the thickness is uniform, the layers can be considered optically thin over the whole range of thicknesses (1-60 μm). Under these conditions and with unchanged concentration of oxygen over the whole volume of the material the time t e for decrease of the concentration of anthracene groups by e times is determined by [1]: a e a e e s s e s s where C a (0) and C a (t e ) are the initial and residual (at time t e ) concentrations of anthracene; I e is the intensity of the radiation; σ s and Cs are the absorption section and the concentration of the sensitizer. Inverse proportionality between t e and β and also invariability of the fi rst value with invariability of the second follow from Eq. (5). Figure 2a shows the dependence of t e on the thickness of the layer l, which on the section up to 10-11 μm demonstrates the approximate constancy of t e . With further increase of the thickness of the fi lm there is an increase in the characteristic exposure time. The interpretation of this relationship is fairly obvious -with large thicknesses the diffusion of oxygen through the surface of the layer is not in a state to maintain a constant concentration of dissolved oxygen throughout the thickness, which leads to decrease of β in the depth of the layer and to increase of the averaged values of t e . The refractive indices of the material at λ = 633 nm for the photosensitive layers 4-5-μm thick were determined before (n 0 ) and after uniform irradiation with the LED to ~90% conversion of the anthracene groups (n t ). The values n 0 = 1.545 and n t = 1.525 were identical for polarizations of the probing laser beam in the plane of and perpendicular to the layer (the TE and TM polarization of the waveguide modes of the layer excited during probing). The change of refractive index produced by irradiation was Δn = n t − n 0 = −0.02. For comparison Popov et al. [12] obtained a maximum value Δn ~ −0.005 (λ = 633 nm) for plates of one of the forms of reoxane 1-mm thick and previously saturated with oxygen. The large value of Δn may be due to the higher concentration of anthracene groups in the material (~20 mole %) resulting from their addition to the macromolecules. Evaluation of the sensitivity S by the formula [12]: where H e is the exposure dose during irradiation (H e = I e t e ), gives S = 1.4·10 -3 cm 2 /J, which is substantially higher than for reoxane (1.6·10 -4 cm 2 /J [12]). Possible reasons for the increase of sensitivity under conditions of low oxygen concentration are the large concentration of anthracene groups and the high mobility of the oxygen molecules resulting from the lower glass transition temperature of the material. In addition to the above-mentioned factors the experimentally discovered expansion of the investigated material during irradiation can lead to increased sensitivity. In order to observe it a layer of the photosensitive material was deposited on the surface of a glass substrate with a deposited metal grating (period 70 μm) and irradiated with LED radiation on the substrate side. Figure 2b shows a photograph of the interference pattern in an MII-4 eyepiece with refl ection of light from the irradiated layer on a photo mask. The rise of the interference bands in the dark areas correspond to protrusions on the surface of areas of the layer that were illuminated during exposure. If the picture is interpreted as expansion of the layer resulting from phototransformation and it is considered to be unidimensional (perpendicular to the plane), the relative increase in thickness is then Δl/l = −Δρ/ρ ≈ 0.004. According to the following expression [13]: 2 2 ( 2)( 1 ) 6 n n n n such change of density will lead to change of refractive index Δn ≈ −0.0025, i.e., expansion of the irradiation material under the specifi ed conditions can make an appreciable contribution to the photorefraction effect. However, this contribution cannot explain the increased sensitivity of the material over the sensitivity of reoxane. The reason seems to be the higher rate of the photochemical reaction. Determination of the molecular volumes of the anthracene units and their peroxide by increments [9] gives 0.263 and 0.274 nm 3 . Increase in the molecular volume as a result of the photoreaction implies an increase in its effi ciency with a larger fraction of free volume in the polymeric medium at a lower glass transition temperature. The higher accumulation rate of the photoproduct is also favored by the higher concentration of anthracene units compared with reoxane. Holographic recording was used to determine the optimal proportion of sensitizer in polymer material. Figure 3a shows the kinetics of the diffraction effi ciency in the holographic recording process in layers of material with various polymer/dye mass ratios and demonstrates the advantage of recording in a layer with a selected ratio of 1700:1. The reason for the reduced effi ciency of recording with increased sensitizer concentration is probably its aggregation at high concentrations (Fig. 3b).
The volumetric holographic gratings were recorded with laser beams polarized in the vertical plane in layers 5-20-μm thick. Figure 4a shows a series of kinetics of the diffraction effi ciency for layers with various thicknesses and a polymer/dye ratio of 1700:1. The increase in the maximum values of the diffraction effi ciency becomes slower with increasing thickness and becomes saturated when the thickness approaches 20 μm. The difference in optical thicknesses at the maxima and minima Δ = 2Δn 1 l 0 created in the structure of holographic gratings was calculated by means of the expression for the diffraction effi ciency of volume phase holographic gratings: 2 1 0 sin , cos where Δn 1 is the amplitude of modulation of the refractive index; l 0 is the layer thickness; θ is the angle if incidence of the recording beams on the surface of the layer. The dependence of Δ on l 0 (Fig. 4b, curve 1) increases up to ~10 μm, and it is then practically horizontal. There is no reason to consider that increase of the layer thickness to >10 μm changed its properties so that the amplitude Δn 1 began to change in inverse proportion to the thickness. A more probable assumption is that a holographic grating with an effective thickness smaller than the geometric thickness is formed in the thicker layers. The front boundary of the lattice is formed by the photooxidation front [14], while the appearance of the back boundary at a layer thickness of >10 μm can be attributed to increase in the root mean square displacement of the singlet oxygen molecules with increase in the photoconversion to values comparable with the lattice constant. A refraction sphere with a radius R SP in the order of the root mean square displacement of singlet oxygen molecules, created during photo-oxidation around the sensitizer molecule, is regarded as a "grain" of the phase image in reoxane [1]. The radius was estimated by means of the equation: τ must increase, raising the radius of the photoreaction sphere. On the whole, this can lead to degradation of the holographic grating in the surface layer, which is most highly enriched with oxygen. As a result, the most effective grating exists in a region of limited thickness, moving into the depth of the recording layer during writing. Degradation of the lattice near the surface begins in layers with a thickness of ~10 μm, which, within the framework of our qualitative model, corresponds to the effective lattice thickness in thicker layers (Fig. 4b, curve 1). If we divide the obtained values of Δ into doubled geometric thicknesses of the layers with l 0 < 10 μm, and with l 0 > 10 μm by 20 μm, we obtain the amplitudes of modulation of the refractive index Δn 1 , which are represented in Fig. 4b by a number of points close to 0.006 (curve 2). The calculated Δn 1 value is approximately constant for the recording layers over the entire investigated range of thicknesses, and this provides a qualitative model for explaining the experimental dependence of the obtained diffraction effi ciency on the thickness of the recording layer. The value of 2Δn 1 ≈ 0.012 amounts to 60% of the maximum possible total degree of modulation of the refractive index.
Conclusions. Possible volume phase recording by photo-oxidation of the anthracene side groups of polymer molecules by oxygen that enters the layer through the boundary with air was confi rmed experimentally. It is shown that photo-oxidation under the action of light-emitting diode radiation with λ max = 650 nm at 6 mW/cm 2 in new polymeric material with addition of methylene blue as photosensitizer gives an almost constant quantum yield if the layer thickness does not exceed 10 μm. As a result of ~90% photoconversion of the anthracene groups the refractive index of the material is reduced by 0.02. The phototransformations are accompanied by increase of layer thickness as a result of increase in the volume of the molecular structure during photo-oxidation, and this can contribute to change of n at a level of 0.002-0.003. Holographic gratings with a period of 1.15 mm, recorded in layers 5-20-μm thick, had maximum diffraction effi ciency approaching 10%. The kinetics of holographic recording was characterized by diffraction effi ciency to saturation until maximum modulation depth of refractive index is reached long before overmodulation of the grating. The effect is explained by increase in the size of the photorefractive sphere with depth of photoconversion as a result of decreased probability of loss of singlet oxygen on the anthracene groups. This leads to a decrease of the modulation depth n, gradual erasure of the part of the grating closest to the surface of the layer during recording, and its effective thickness e less than the geometric thickness of the recording layer.