Article pubs.acs.org/JPCB
Photoassisted Holography in Azo Dye Doped Polymer Films Anouar Rahmouni,† Yahya Bougdid,†,‡ Sara Moujdi,†,‡ Dmitry V. Nesterenko,† and Zouheir Sekkat*,†,‡,§ †
Optics and Photonics Center, Moroccan Foundation for Advanced Science, Innovation and Research, 10100 Rabat, Morocco Department of Chemistry, Faculty of Sciences, Mohammed V University, 10000 Rabat, Morocco § Photonics Advanced Research Center, Graduate School of Engineering, Osaka University, Suita, Osaka 585-0871, Japan ‡
ABSTRACT: Holographic storage is one of the most important applications in the field of optics, especially for recording and retrieving data, and information storage by interference patterns in photosensitive materials are no exception in this regard. In this work, we give evidence that holograms recorded by interference of two coherent laser beams in azo dye doped polymer films can be controlled by a third incoherent assisting laser beam. We show that light diffraction can be increased or decreased by an assisting beam depending on the respective orientation of the polarizations of the recording and the assisting beams. We also found that photomanipulation of polarization holograms, prepared by polarization modulation, does not depend on the polarization of the assisting beam, whereas, photomanipulation of holograms prepared by intensity modulation strongly depends on the polarization of the assisting beam. Photoselection is shown to play a major role in the photoassisted diffraction process.
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nonlinear optics.14−17 Strictly speaking, photoisomerization of azobenzene derivatives is an intrinsic nonlinear optical effect, and the Weigert effect in azo-polymers is due to photoselection and photo-orientation. Light can manipulate the orientation of azo dyes by photoisomerization via polarized transitions, so the film’s centrosymmetry and isotropy are alleviated, and anisotropy18−21 and quadratic22−24 and cubic25 optical nonlinearities are induced far below the polymer’s glass transition temperature, Tg. Disperse red 1 (DR1) is perhaps the most studied azo dye, i.e., azobenzene derivative, in polymers, because it combines the virtues of photoisomerization and photo-orientation and nonlinear response to a strong optical field.25 The importance of the polarization sensitivity of the photoisomerization of DR1 was initially illustrated in two experiments.18 In a first experiment, a film of DR1 doped poly methyl-methacrylate (PMMA), denoted by DR1/PMMA, was used as a waveguide supporting both transverse electric (TE) and transverse magnetic (TM) guided modes, and it was irradiated by a 514 nm Ar+ laser beam linearly polarized first parallel and then perpendicular to the polarization of the TE mode. The index variation associated with the photoisomerization of DR1 was calculated from the shift of the angular position of the TE mode in an attenuated total reflection geometry. The mode shift was large when the irradiating light polarization was parallel to the mode’s electric field (∥) and much smaller for a polarization perpendicular (⊥) to it. The observed mode’s shift is proportional to the variation of the index of refraction
INTRODUCTION Optical sciences have been significantly advanced in the past century by holography owing to the use of the full information of light, i.e., phase and amplitude,1 and the principle of holography expanded application possibilities since the initial inscription of interference patterns in photographic plates.2−5 The invention of the lasers in the early 1960s allowed for the technology of holographic recording to be established. The current development in nanofabrication technology may create a revolution in holography, and new possibilities in holographic devices are being opened by the development of, for example, subwavelength structured gratings6 and metamaterials and metasurfaces7−9 owing to their ability of controlling the amplitude and phase and polarization of light at subwavelength scales. One of the major applications of holography is the possibility of storing and retrieving information by the recording of interference patterns in a three-dimentional medium.10 In this article, we revisit the inscription of interference patterns in polarization sensitive media to demonstrate an effect of “command laser”, referred to in the text as assisting laser, which enhances or erases the recorded pattern depending on polarization. The Weigert effect, which is the photoinduction of anisotropy in polarization sensitive materials, has been known since the beginning of the past century,11 a feature which was first exploited, by Kakichashvili,12 for recording polarization holograms into silver emulsions by interfering two light waves of orthogonal polarizations. Todorov et al. built on Kakichashvili’s work and studied polarization holography in azo dye doped polymers;13 since then intensive research has been carried out in azo dye containing materials with views of application in holographic recording and data storage and © XXXX American Chemical Society
Received: September 1, 2016 Revised: October 10, 2016
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DOI: 10.1021/acs.jpcb.6b08855 J. Phys. Chem. B XXXX, XXX, XXX−XXX
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Figure 1. (a) Chemical structure of the DR1 (guest) and PMMA (host). Time dependence of the diffraction efficiency of the first transmission order (1T) for (b) S polarized and (c) P polarized probe beam diffraction on a hologram grating recorded by P and S polarized beams of various intensities.
corresponding to the TE polarization, i.e., the probe polarization, implying Δn∥ ≫ Δn⊥. In a second experiment, a DR1/ PMMA film (directly deposited on a glass slide) was irradiated with two interfering beams from a 514 nm Ar+ laser that produced an index grating, which was observed by diffraction of a red λ = 632.8 nm He−Ne laser beam. The diffraction was observed even when the two interfering beams were orthogonally polarized: in such a case the illumination is uniform but the polarization of the irradiating light is spatially modulated and creates a grating of orientation of the azo dye molecules. When the irradiating light was switched off, the diffracted beam rapidly decreased (∼1 s), but a small diffraction was observable for longer times thereby opening the possibility of writing permanent interference patterns in films of polymers containing azobenzene derivatives. Since then, a significant research targeted DR1, as well as azobenzene derivatives, in polymers for applications in integrated optics,19 nonlinear optics22−24 and optical memories,20,21 as well as photoinduced gratings formation.26,27 In this article we revisit photoinscription of holograms in azo dye doped polymers, and we show an intriguing effect of photomanipulation of the light diffracted by this kind of holographic grating. We will show that beam diffraction can be increased or decreased by an assisting beam depending on the respective polarizations of the recording beams versus the assisting beam. We found that polarization holograms, prepared by polarization modulation, do not depend on the polarization of the assisting beam, whereas holograms prepared by intensity modulation strongly depend on the polarization of the assisting beam.
Figure 2. (a−d) Calculated intensity spatial distribution of two-beam interference for (a, c) S polarized and (b, d) P polarized recording beams under 10.5° angle of incidence to the surface normal at 532.0 nm wavelength. (e) Time dependence of 1T order efficiency for S and P polarized probe beam diffraction on the hologram grating recorded by P polarized beams of IR = 17.41 mW/cm2. The x and y axes are in the plane of the film samples.
shown). The absorption spectrum of, e.g., an all-trans sample is consistent with those of trans-DR1/PMMA reported in the literature.23,28 We combined three green laser beams from two diode pumped frequency doubled lasers of λr = 532 nm wavelength. The two beams used for recording the holographic grating are obtained from the same laser by using a beam splitter, and one, incoherent with the two others, from a different laser, was used as an assisting beam. Since the assisting beam is incoherent to the recording beams, it does not interfere with them and only adds to the photoisomerization cycling and photo-orientation of the azo chromophores in both the bright and dark areas of the grating, thereby changing the contrast of the refractive index of the DR1/PMMA sample between the area corresponding to the bright and dark fringes of the interference pattern of the hologram. The angle between the two recording beams (2θ) is 21°, resulting in a grating period Λ = λr/(2 sin θ) of 1.46 μm. The intensity of each of the recording beams measured at the sample position was varied from 1.78 to 17.41 mW/cm2, and that of the assisting beam was varied from 9.37 to 75.26 mW/cm2. The assisting beam irradiated the sample at normal incidence. The polarizations of both recording and assisting beams were controlled by half-wave plates. To
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EXPERIMENTS AND DISCUSSION We used a commercially available azo dye, i.e., DR1, and polymer, i.e., PMMA. The chemical structures of PMMA and DR1 are shown in Figure 1a. We prepared guest−host films by dissolving in toluene DR1 into PMMA with a 10% w/w ratio relative to PMMA, and spin-coating the DR1/PMMA solution on cleaned glass substrates after filtration using a 0.2 μm filter. These films were then dried on a hot plate at 80 °C for 30 min and then at 120 °C for 2 min. The Tg of PMMA was ∼110 °C, and the typical film thickness was d = 1.62 μm. The UV−visible absorption spectrum of the film sample right after the preparation exhibited an absorption centered at 488 nm (not B
DOI: 10.1021/acs.jpcb.6b08855 J. Phys. Chem. B XXXX, XXX, XXX−XXX
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Figure 3. Time dependence of 1T order efficiency for (a, c) S and (b, d) P polarized probe beam diffraction on the hologram grating recorded by (a, b) P and (c, d) S polarized recording beams (RBs) of IR = 17.41 mW/cm2 with the presence of S and P polarized assisting beam (AB) of IA = 75.26 mW/cm2.
beams, and it is shown that the higher the intensity of the recording beams, the faster the increase and the larger the diffraction efficiency, η, of the holograms. Indeed, a fit of the slope of η at the initial stage of the formation of the hologram versus the intensity (IR) shows a linear dependence, and ηpss at the photostationary state increases with the increased IR. For the case of polarization gratings, in which diffraction occurs only in the 0th and ±1st orders, the diffraction efficiency is independent of the polarization of the probe beam, and it strongly depends on the intensity of the recording beams.29 In a different experiment, where the polarizations of the recording beams are parallel, and the intensity is spatially modulated (Figures 2a−2d), e.g., obtained with PP-S or PP-P configurations of the polarizations of the beams, the diffraction efficiency of the PP-P configuration corresponding to η∥ (proportional to Δn∥) is much larger than that corresponding to the PP-S configuration η⊥ (proportional to Δn⊥) (Figure 2e). This behavior can be explained by the coexistence of two mechanisms: orientational hole burning (OHB), due to photoselective isomerization, and orientational redistribution (OR) of the DR1 chromophores. The models of both mechanisms are explained in detail in the literature.30−33 Briefly, the probability of exciting a transition dipole in an isomer is proportional to the cosine squared of the angle between that transition dipole and the polarization of the excitation light. Transition dipoles that lie along the polar-
monitor the dynamics of the formation of the holographic recording, we used a He−Ne laser of λp = 632.8 nm wavelength with a weak power of 0.56 mW at the sample surface, and measured the diffraction efficiency of the first transmission order of diffraction (1T). The beam of the He−Ne laser was linearly polarized, and its polarization was controlled with a half-wave plate as well, and it was incident on, and propagating perpendicular to, the front surface of the sample. Neutral density filters were used to adjust the intensities of the laser beams. We first confirmed the formation of the holographic grating as a function of different polarization combinations of the recording and probe beams, and then we investigated the effect of the assisting beam on the formation of the gratings, depending on its polarization. In what follows, we adopt the following notation for the combinations of polarizations of the beams: the first two letters refer to the recording beams, and the third and fourth letters refer to the probe and assisting beams, respectively. For example, an experiment labeled SS-P-S means that recordings are done with S polarization, and reading and assisting are done with P and S polarizations, respectively. S and P refer to TE and TM polarizations, respectively. Figures 1b and 1c show the buildup of the polarization holograms, e.g., obtained by recording beams that have mutual orthogonal polarizations at +45° and −45° with respect to the S polarization, as a function of the intensity of the recording C
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considerably due to OHB, and that of the bright area, nb, changes only slightly because photoassisting starts at saturation of interference recording, thereby reducing the contrast between nb and nd, and a decrease in the diffraction efficiency is observed. In contrast, when photoassisting is performed with perpendicular polarizations, the contrast between nb and nd is augmented because during the OR process the chromophores are reoriented into the direction, which is perpendicular to the assisting beam polarization, a feature which increases nb parallel (perpendicular) to the recording beam (assisting beam) polarization; and leaves nd slightly unchanged, thereby augmenting the contrast between nd and nb, and increasing the diffraction efficiency. For polarization gratings, prepared by polarization modulation, where the intensity is uniform at the surface of the film and the polarization is modulated along the x-axis, we found that photoassisted diffraction always decreases on the photoassisting beam irradiation independent of its polarization (not shown). The polarization of the assisting beam was successively set to +45° and −45°, and S and P for each photoassisting irradiation experiment. This finding was observed for the recording beam of polarizations of +45° and −45°, as well as S and P. The effect of assisting beam on the formation of surface relief grating (SRG), using interference patterns by fully or partially coherent beams, has been reported in azo-polymers, where the azo dye is chemically attached to the polymer main chain,34−40 in contrast to our study where a guest−host system is considered. Indeed, azo dyes are free to rotate in the guest− host system, and the recorded grating corresponds to a contrast of refractive index between the dark and bright areas of the film, whereas in azo-polymers the photorecorded SRG corresponds to a modulation of the film thickness, a feature that is due to photoinduced mass movement of the polymer from the bright to the dark area of the interference pattern. Even though photoisomerization is at the origin of the formation of the gratings in both systems, the mechanisms of the formation of the gratings are different for the guest−host versus the azopolymer systems.41 So, back to our guest−host system, Figure 4a shows the dependence on the intensity and polarization of the assisting beam of both the photoassisted increase and decrease in the diffraction efficiency for gratings recorded by intensity modulation. The value of the intensity of the assisting beam was varied as 9.37, 14.58, and 75.26 mW/cm2. This figure shows that the higher the intensity of the assisting beam, the larger the increase, for crossed recording and assisting polarizations, and the decrease, for parallel recording and assisting polarizations, of the diffraction efficiency. For a thick, two-beam volume phase grating, the spatial modulation of the refractive index is represented as n = n0 + Δn cos(2πx/Λ), where n0 = (nd + nb)/2 is the average refractive index, and Δn = (nd − nb)/2 is the amplitude of the spatial modulation of the refractive index of the sample. According to the coupled wave theory,42 the diffraction efficiency η for a thick, two-beam volume phase grating can be approximated by
Figure 4. (a) Time dependence of 1T order efficiency for S polarized probe beam diffraction on hologram gratings recorded by S polarized beams of 17.41 mW/cm2 intensity using ABs of intensities I1, I2, and I3 which were equal to 9.37, 14.58, and 75.26 mW/cm2, respectively. (b) Calculated images of the refractive index spatial modulation for the hologram gratings corresponding to the diffraction efficiencies from panel a.
ization of the irradiation light will be excited with the highest probability, and molecules, say the trans isomers, may be isomerized, therefore a hole is burned in the angular distribution of the trans isomers, i.e., OHB, and may fade by reorientation from the direction of the polarization of the irradiation light, i.e., OR. As a consequence, the isomers are eventually oriented into a direction that is perpendicular to the irradiation light polarization. The model of OR predicts Δn∥ = −Δn⊥/2, and that of OHB predicts that a weak pump irradiation intensity induces an index variation (or an optical density variation) three times larger for a probe beam polarized along the polarization of the pump beam than for a probe beam polarized perpendicularly (Δn∥ = 3Δn⊥). For large pump intensities, saturation reduces this ratio, and Δn∥/Δn⊥ tends toward 1. In fact, both OHB and OR contribute to the observed polarization dependent diffraction shown in Figure 2e. Next, we discuss the effect of the assisting beam on the diffraction efficiency. Figure 3 shows the diffraction efficiency of holograms recorded with intensity modulation, i.e., where the polarizations of the recording beams are parallel, in 8 different configurations. It is clear from this figure that when the polarizations of the recording and assisting beams are parallel (perpendicular) to each other, the diffraction efficiency decreases (increases). The time characteristics of the decrease and the increase of the diffraction efficiency are consistent with the OHB and OR mechanisms, respectively (vide inf ra). Indeed, since the observed diffraction efficiency is proportional to the contrast of the refractive index of the holograms between the dark and bright areas, and since the assisting beam illuminates uniformly and simultaneously both dark and bright areas, in the case of photoassisting with parallel polarization, the index of refraction of the film at the dark area, nd, changes
η = sin 2[πdΔn/(λ p cos θ0)]
(1)
where θ0 is the angle of incidence in the medium. This assumption is valid for our experiments since the thickness of the hologram, i.e., 1.62 μm, is larger than the grating period, i.e., 1.46 μm. We approximately estimated the values of the modulation Δn by eq 1 using the values of diffraction efficiency from Figure 4a. Thus, Δn = 0.0025 RIU at t = 1 min without D
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(15) Delaire, J. A.; Nakatani, K. Linear and Nonlinear Optical Properties of Photochromic Molecules and Materials. Chem. Rev. 2000, 100, 1817−1846. (16) Sekkat, Z.; Kawata, S. Laser Nanofabrication in Photoresists and Azopolymers. Laser Photonics Rev. 2014, 8, 1−26. (17) Priimagi, A.; Shevchenko, A. Azopolymer-Based Micro- and Nanopatterning for Photonic Applications. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 163−182. (18) Sekkat, Z.; Dumont, M. Polarization Effects in Photoisomerization of Azo Dyes in Polymeric Films. Appl. Phys. B: Photophys. Laser Chem. 1991, 53, 121−123. (19) Shi, Y.; Steier, W. H.; Yu, L.; Chen, M.; Dalton, L. R. Large Stable Photoinduced Refractive Index Change in a Nonlinear Optical Polyester Polymer with Disperse Red Side Groups. Appl. Phys. Lett. 1991, 58, 1131. (20) Rochon, P.; Gosselin, J.; Natansohn, A.; Xie, S. Optically Induced and Erased Birefringence and Dichroism in Azoaromatic Polymers. Appl. Phys. Lett. 1992, 60, 4. (21) Maeda, M.; Ishitobi, H.; Sekkat, Z.; Kawata, S. Polarization Storage by Nonlinear Orientational Hole Burning in Azo DyeContaining Polymer Films. Appl. Phys. Lett. 2004, 85, 351. (22) Sekkat, Z.; Dumont, M. Photoassisted Poling of Azo Dye Doped Polymeric Films at Room Temperature. Appl. Phys. B: Photophys. Laser Chem. 1992, 54, 486−489. (23) Loucif-Saibi, R.; Nakatani, K.; Delaire, J. A.; Dumont, M.; Sekkat, Z. Photoisomerization and Second Harmonic Generation in Disperse Red One-Doped and -Functionalized Poly(Methyl Methacrylate) Films. Chem. Mater. 1993, 5, 229−236. (24) Charra, F.; Kajzar, F.; Nunzi, J. M.; Raimond, P.; Idiart, E. LightInduced Second-Harmonic Generation in Azo-Dye Polymers. Opt. Lett. 1993, 18, 941. (25) Sekkat, Z.; Knoesen, A.; Lee, V. Y.; Miller, R. D. Observation of Reversible Photochemical Blow out of the Third-Order Molecular Hyperpolarizability of Pushpull Azo Dye in High Glass Transition Temperature Polyimides. J. Phys. Chem. B 1997, 101, 4733−4739. (26) Rochon, P.; Batalla, E.; Natansohn, A. Optically Induced Surface Gratings on Azoaromatic Polymer Films. Appl. Phys. Lett. 1995, 66, 136. (27) Kim, D. Y.; Tripathy, S. K.; Li, L.; Kumar, J. Laser-Induced Holographic Surface Relief Gratings on Nonlinear Optical Polymer Films. Appl. Phys. Lett. 1995, 66, 1166. (28) Sekkat, Z.; Morichere, D.; Dumont, M.; Loucif-Saibi, R.; Delaire, J. A. Photoisomerization of Azobenzene Derivatives in Polymeric Thin Films. J. Appl. Phys. 1992, 71, 1543. (29) Nikolova, L.; Todorov, T. Diffraction Efficiency and Selectivity of Polarization Holographic Recording. Opt. Acta 1984, 31, 579−588. (30) Sekkat, Z.; Dumont, M. Photoinduced Orientation of Azo Dyes in Polymeric Films. Characterization of Molecular Angular Mobility. Synth. Met. 1993, 54, 373−381. (31) Sekkat, Z.; Knoll, W. Creation of Second-Order Nonlinear Optical Effects by Photoisomerization of Polar Azo Dyes in Polymeric Films: Theoretical Study of Steady-State and Transient Properties. J. Opt. Soc. Am. B 1995, 12, 1855. (32) Sekkat, Z.; Wood, J.; Knoll, W. Reorientation Mechanism of Azobenzenes within the Trans Replaced by Cis Photoisomerization. J. Phys. Chem. 1995, 99, 17226−17234. (33) Sekkat, Z.; Yasumatsu, D.; Kawata, S. Pure Photoorientation of Azo Dye in Polyurethanes and Quantification of Orientation of Spectrally Overlapping Isomers. J. Phys. Chem. B 2002, 106, 12407. (34) Wu, P.; Wang, L.; Xu, J.; Zou, B.; Gong, X.; Zhang, G.; Tang, G.; Chen, W.; Huang, W. Transient Biphotonic Holographic Grating in Photoisomerizative Azo Materials. Phys. Rev. B: Condens. Matter Mater. Phys. 1998, 57, 3874−3880. (35) Wu, P.; Rao, D. V.; Kimball, B. R.; Nakashima, M.; DeCristofano, B. S. Transient Optical Modulation with a DisperseRed-1-Doped Polymer Film. Appl. Opt. 2000, 39, 814−7. (36) Jäger, C.; Bieringer, T.; Zilker, S. J. Bicolor Surface Reliefs in Azobenzene Side-Chain Polymers. Appl. Opt. 2001, 40, 1776−8.
assisting beam. P polarized assisting beam increases the modulation up to 0.004 RIU, while S polarized assisting beam decreases it down to 0.001 RIU at t = 10 min. The corresponding calculated refractive index distributions are shown in Figure 4b.
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CONCLUSIONS
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AUTHOR INFORMATION
In conclusion, we have demonstrated a method for controlling the inscription of holograms in azo dye doped polymers. The holograms can be completely erased or enhanced depending on the intensity and the polarization of the assisting beam. Such a method may find applications in all-optical signal processing. Work is in progress to theoretically describe in detail the mechanism of photoassisted diffraction reported in this article, and use photosensitive media, such as photosensitive liquidcrystalline type materials, that have the ability to enhance the diffraction efficiency by cooperative light induced molecular reorientation.
Corresponding Author
*Phone: +212661183964. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the Moroccan Ministry for Higher Education and Research in the framework of Priority Research Projects.
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REFERENCES
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