A new organic material for polarization holographic recording—azo-dye methyl-orange, introduced in poly-vinyl alcohol (PVA)—is fabricated and investigated. It possesses all the good characteristics of the known polarization-sensitive materials—high sensitivity, reversibility, etc.—but excels them substantially in the magnitude of the photoinduced birefringence: δn > 10−3. This makes possible the recording of reversible polarization gratings with diffraction efficiency over 30%. Depending on the conditions of production and subsequent mechanical and thermal treatments the layers of methyl orange/PVA may also have intrinsic birefringence. In this paper the results of experimental investigations into the properties of this new material are reported.
© 1984 Optical Society of America
Theoretical investigations in the last decade proved that the polarization of light can also be recorded and reconstructed by holographic means. A new branch of holography emerged—polarization holography. The progress of polarization holography at the moment is hampered considerably by the performance of existing recording materials in which optical anisotropy is induced in accordance with the light polarization used. Polarization-sensitive media belong basically to one of the following groups: alkali halide crystals containing anisotropic color centers, , photochromic glasses or emulsions on the basis of silver halides (mainly AgCl) with a marked Weigert effect (photodichroism),, organic systems with metastable triplet levels,, and materials on the basis of dyes, undergoing photo-structural changes, leading to a certain degree of stereoregularity in the new structure., The main shortcoming of all these materials is the relatively weak photoinduced anisotropy, usually in the optical absorption (photodichroism) which is the reason for the low efficiency of the recorded polarization holograms (<1%). Most of the enumerated materials have low sensitivity and not all of them allow multiple use.
In this work results are presented from the investigations on a new polarization-sensitive material—a combination of an azo dye and polymer matrix, on which high-efficiency polarization holograms (>35%) can be recorded at relatively low recording light intensity, and which allows multiple erasure and repeated recording without apparent fatigue. Another feature of this new recording medium is that it can be prepared at will with intrinsic birefringence of the matrix (Δn) independent of the acting light. Producing samples with different Δn helped check experimentally the specific polarization properties of such holograms as predicted in Ref. .
The new anisotropic medium is in the form of layers of methyl orange introduced in a polymer matrix. Methyl orange is an azo dye in which, under the action of linearly polarized light, some degree of ordering of the molecules can be achieved leading to dichroism. Our experiments showed that the molecules of methyl orange can be oriented reversibly and many times without apparent fatigue. The reverse process—destruction of the stereoregularity—depends on the matrix and in darkness it may last from a few seconds to several days, i.e., the material has different times of memory. This thermal process dictates the efficiency of the optically attained stereoregularity and the value of the induced anisotropy. Polyvinyl alcohol (PVA) turned out to be most suitable for our purposes. It is a well-known material in which an optical axis along the direction of the force can be induced through mechanical deformation. Thus, the material becomes optically anisotropic—it acquires birefringence (Δn = ne − no) over the whole visible region, the value of An depending on the regime of the mechanical and thermal treatment. We used samples of methyl orange/PVA in the form of ~100-μm thick films on a glass substrate. Depending on their preparation regime, they had different intrinsic birefringence and the phase difference between ordinary and extraordinary waves Δφ = φa − φe after a 100-μm thick layer varied from 0 (no birefringence) to π (λ/2 plate).
III. Investigations into Photoinduced Anisotropy
Figure 1 shows the absorption spectrum of a 100-μm thick layer of PVA containing 0.06-wt. % methyl orange. Since PVA is transparent in the visible region, absorption is due entirely to the dye whose stable form is its transisomer. In later experiments samples with greater dye concentrations were used and they ensured higher values of photoinduced anisotropy.
On absorbing the light of an Ar laser beam with a wavelength of 488 nm, a large part of the dye molecules undergoes spatial (trans-cis) isomerization. The cis-form of methyl orange is thermally unstable and a reverse thermal process of restoration of the transisomer begins. Also, if the light is linearly polarized the molecules tend to order themselves in such a way that the direction of their optical transition is perpendicular to the polarization direction of the light. Thus, the optical transmission for light, polarized along the polarization direction of the acting light, increases; for light polarized perpendicular to the polarization direction of the acting light, it decreases. Dichroism is induced. The kinetics of the anisotropic change in the optical transmission is shown in Fig. 2. Measurement was accomplished with a low-intensity light beam of the same wavelength. The intensity of the acting Ar laser light was 100 mW/cm2. The moments of turning the Ar beam on and off are marked with arrows. After the light action is removed the sample begins to restore its original state and the ordering of the dye molecules is lost. The degree of restoration after the recording light is switched off and the rate of this process depend on the polymer matrix and in PVA it can be altered by thermal treatment. In the case corresponding to Fig. 2, such treatment was not carried out and the original optical transmission was restored almost completely in ~30 sec.
Along with dichroism, light birefringence is induced in the films under the action of Ar+. Photoinduced birefringence, particularly for wavelengths outside the optical transmission, is of greater interest from a holographic point of view, since it leads to higher recording efficiency. We investigated birefringence at λ = 633 nm. For this purpose the samples were placed between two crossed polarizers P and A in the path of a He–Ne laser beam. They were irradiated with an Ar+ laser beam, linearly polarized at ±45° with respect to the directions of transmission of P and A. The optical anisotropy, induced in the sample under the action of the blue light, causes a light signal to appear after the analyzer A. At λ = 633 nm this transmission is due solely to the photoinduced birefringence which is given by
where I is the intensity of the transmitted light after A, I0 is the intensity of the light passing through the pair of parallel polarizers,
d is the thickness of the sample (100 μm), and δn is the birefringence. The kinetic curve of such a process is plotted in Fig. 3. Here again switching the light on and off is marked with arrows.
The values of δn depend on the intensity of the Ar+ beam, dye concentration, and preliminary treatment of the polymer. In optimal conditions δφ of the samples exceeded π/3 (δn > 10−3). Such large values for the birefringence make us think that the polymer matrix does not remain unchanged in the recording. The exact mechanism of this process is not clear but probably the PVA molecules are deformed in accordance with the photoinduced ordering of the dye molecules, and that is the reason for the large values of the induced birefringence.
The thermal erasure of the birefringence reached depends on the sample and may be slowed down substantially by preliminary thermal treatment. The sample, whose photoinduced change is shown in Fig. 3, was heated beforehand to 80°C for 30 min. After such treatment a large part (>0.5) of the photoinduced birefringence is retained for a long time after the acting light is removed. The ability of these layers to store information is several days.
Quite interesting is the induction of anisotropy in the samples with intrinsic birefringence achieved by mechanical tension of the polymer matrix. On studying the photoinduced birefringence these samples were positioned in such a way that their principal dielectric axes were at an angle of ±45° to the directions of transmission of P and A and the Ar+ light was alternately polarized along one or the other axis. In this case the intensity of the light coming out of the analyzer was
where I0 is the intensity of the light transmitted due to the intrinsic birefringence of the layer, and δI is connected with the photoinduced birefringence. The sign depends on which axis is coincident with the Ar+ beam polarization. The values of δφ measured in this case are the same as those in the isotropic samples.
IV. Polarization Holographic Recording
We determined the basic holographic characteristics of the polymer layers described. The polarization holographic recording was accomplished by two plane waves with mutually orthogonal polarization at 488-nm wavelengths (Ar+ laser). In this type of recording the resulting light field is not modulated by intensity but only by polarization. The induced optical anisotropy (dichroism or birefringence) is spatially modulated in accordance with the polarization modulation of the recording light field, i.e., a polarization holographic grating is recorded. As shown previously, simultaneous dichroism and birefringence is induced in the investigated layers. In spite of the relatively strong anisotropy in the optical transmission for λ = 488 nm (Fig. 2), the light with this wavelength diffracted from the recorded polarization grating is weak (diffraction efficiency η < 0.1%), due to the large average optical density of the samples. Much larger is the diffraction efficiency for wavelengths outside the absorption region where strong birefringence is induced under the action of light, i.e., phase holographic recording is accomplished. Readout was done with a He–Ne laser beam at Bragg’s angle and diffraction efficiency of the +1 order was measured.
The curve, illustrating the increase of η during the recording (Fig. 4, curve a) resembles the increase of δn, shown in Fig. 3. After ~20 sec η reaches saturation (in this case ηs = 20%) and remains constant as long as the recording continues.
When the recording light is turned off initially the diffraction efficiency falls to around one-third and remains practically unchanged for more than 24 h (curve b, upper scale). As mentioned earlier, such memory effect is observed only in samples that have undergone preliminary thermal treatment; it is probably connected with residual deformation in the PVA matrix.
If, after reaching saturation of η during recording, only one of the recording beams is turned off, the decrease of is much faster (curve c, lower scale). This is due to a new uniform re-ordering of the methyl orange molecules under the action of the light field, uniform in polarization. The optical erasure is still faster if it is accomplished with a light beam of greater intensity. (In the case corrresponding to Fig. 4, curve c, the erasing light intensity is half of the total recording light intensity.) A new holographic grating with the same diffraction efficiency can be recorded immediately on the same sample and this process of record–erase can be repeated many times with no fatigue.
The saturated value of the diffraction efficiency depends both on dye concentration and recording light intensity. This relationship is plotted in Fig. 5 for a sample with an optimal dye concentration of 0.6 wt. %, treated at 80° C beforehand. The maximum value of η (~35%) is obtained at a total intensity of the two recording waves I = 600–700 mW/cm2.
The methyl orange/PVA layers are of high-efficiency material for polarization holographic recording, allowing multiple use and storage of information for several days. Also, this material makes it possible to obtain layers with intrinsic initial birefringence. Combined with the high diffraction efficiency, the material is suitable for investigating the properties of the various cases of recording polarization holographic gratings, studied theoretically in Ref. . The results of these investigations are reported in the second part of this work.
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