Letter pubs.acs.org/Langmuir
Biphotonic Photochromic Reaction Results in an Increase in the Efficiency of the Holographic Recording Process in an Azo Polymer Anna Sobolewska,* Joanna Zawada, and Stanislaw Bartkiewicz Institute of Physical and Theoretical Chemistry, Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland ABSTRACT: Holographic grating recording in azobenzene-based polymers using a single coherent light of a specific wavelength (monophotonic recording) is the basis of the majority of the applications of this type of material. Here, we report a new approach to holographic recording that uses two light sources having different wavelengths during the recording process (biphotonic recording). The efficiency of the recording process was improved significantly compared to that of the monophotonic recording. The results, by presenting a simple way to increase the efficiency of the recording process, have important implications for the applications of azobenzene-based materials. They indicate how the performance of the already proposed devices can be improved, and they can open a new way to developing applications of this class of materials.
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INTRODUCTION Azobenzenze-containing polymers are a class of organic materials extensively investigated for over a decade as a result of the wide range of possible applications such as optical information storage and processing,1−3 polarization holography,4,5 photonics,2,6−13 and nanotechnology.14,15 The key is the presence of azobenzene groups that undergo multiple trans−cis isomerization cycles when irradiated by light of an appropriate wavelength.16 Photoisomerization of the side groups of the polymer causes changes in the optical properties of the material. The majority of applications of azo polymers have been realized by means of holographic techniques. During the holographic recording process, the modulation of the absorption coefficient, the refractive index, and the film thickness (the surface relief grating - SRG) are induced into an azo material.17,18 It is worth noting that the surface relief grating (effectively formed in azo-based materials and characterized by high efficiency) is of special interest not only because of the fact that the origin of the photoinduced mass transport is still unclear (despite numerous models already reported in the literature) but also mainly as a result of the variety of potential applications ranging from optical technology (holographic memories, diffractive optical elements, photonic components)2,6−13 to nanotechnology (micropatterning, nanopatterning).14,15 In most holographic experiments,2−15 a single coherent light source emitting a specific wavelength (one color) is used to excite the azo molecules and thus to record a diffraction grating. Sometimes in these experiments an additional light, with a wavelength that azo molecules do not absorb, was applied; however, it was used only as a probe light having no influence on the recording process. Holographic recording can also be done thought the use of coherent red light with simultaneous © XXXX American Chemical Society
illumination with UV or blue, mostly unpolarized, and incoherent light.19−25 The purpose of applying light from the short-wavelength range was to sensitize the azo polymer film so that recording with polarized red light could be performed. This type of the recording is called biphotonic (two color).19−25 It is characterized by much lower efficiency than one-color recording because the grating here was formed mainly by the changes in the bulk of the material (SRG was not formed or it was formed with very low efficiency). The low efficiency of the process of the grating formation in the case of the two-color recording limited the range of possible applications of the azo material. With respect to the above considerations, we propose a completely different approach in biphotonic holographic recording in an azo polymer. Unlike previous reports, we applied during the recording process the additional light source of the wavelength coming from the same spectral range as the recording light. As a result, the azo molecules were simultaneously excited by light of two different wavelengths (from the same range), which led to a substantial increase in the efficiency of the recording process. We compared the efficiency of this type of two-color recording to that of onecolor recording. Taking into account the fact that azo materials have a wide range of possible applications, we found that increasing the efficiency of the recording process can improve the performance of the already-proposed devices and may open new ways to develop the use of this class of materials. Received: November 5, 2013 Revised: December 31, 2013
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Letter
EXPERIMENTAL SECTION
Poly(ester-imide) (PEI) containing two azobenzene moieties as side groups was used in the reported studies. The chemical structure of PEI (Tg = 183 °C) is shown as an inset in Figure 1a. The synthesis and
Figure 2. (a) Diffraction efficiency dynamics obtained during the grating recording in the case in which the recording was performed using light of wavelength 532 nm (curve I, monophotonic recording) and when the recording was done with simultaneous irradiation with light of 405 nm (curve II, biphotonic recording). (Inset) Sketch of the experimental setup: GL1 and GL2, recording beams (532 nm); BL, additional beam (405 nm); P, polymer film; G, silica glass substrate. Diffraction efficiency behavior during the cycles of turning the additional light on and off: (b) the case when the recording process was in the beginning stage, B, and (c) when the recording process was approximately in its middle stage, C. The blue line indicates the range of illumination by 405 nm light.
Figure 1. (a) UV−vis absorption spectrum of the initial film of poly(ester-imide) (PEI) and its changes during irradiation with light of 405 nm. Arrows indicate the wavelengths, 405 nm (BL) and 532 nm (GL), used in the reported studies. (Inset) Chemical structure of PEI. Absorption spectral changes close to the isosbestic point during irradiation with the light of (b) 405 and (c) 532 nm.
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RESULTS AND DISCUSSION The photochemical behavior of the studied azo polymer was examined at the beginning. The photoisomerization process of PEI induced by light of wavelengths 405 nm (BL) and 532 nm (GL), indicated by the arrows in Figure 1a, was investigated. Note that the same wavelengths were used later in the holographic grating recording experiment. The UV−vis absorption spectra of the initial PEI film and after irradiation by light of wavelength 405 nm are shown in Figure 1a. They are characterized by two spectral bands: (i) π → π* with a maximum below 250 nm, assigned mainly to the transition of the trans form of the azobenzene group, and (ii) n → π* with a maximum at ca. 440 nm corresponding mainly to the transition of the cis isomer.28 It can be noticed that in the case of the studied material its chemical structure strongly affects the optical spectrum and the lifetime of the cis form. The high value of the intensity of the n → π* transition band clearly indicates the presence of the metastable cis isomer in the virgin sample next to the thermodynamically stable trans isomer. The equilibrium between the populations of both isomers is going to change when the film is exposed to light. Irradiation at 405 nm (or 532 nm) excites mainly these azo groups that are in the cis form, thereby inducing the cis−trans isomerization. As a result, the intensity of the n → π* transition band decreases, the equilibrium shifts toward the trans isomers, and a new steady state between the trans and cis isomers (richer in the trans form) is established (cf. Figure 1a). Irradiation at 532 nm induces cis−trans isomerization as well (data not shown); however, a different population of molecules is excited in this
characterization of very similar poly(ester-imides) can be found in ref 26. The film was prepared by casting a 2% (w/v) solution of PEI in cyclohexanone onto a clean silica glass and then drying at 100 °C for 30 min. The thickness of the film was ca. 2 μm (±10%). The photoisomerization behavior of the PEI film was observed with a UV− vis absorption spectrometer (Cary 3, Varian Techtron). Holographic grating recording was carried out using a standard DTWM setup.18,27 The simplified scheme of the experimental setup is shown as an inset in Figure 2a. The diffraction grating was recorded with two p-linearly polarized laser beams of the Nd:YAG laser (Coherent) operating at 532 nm (GL1 and GL2 beams in Figure 2a). The angle between the writing beams was fixed at ca. 2.3°, resulting in a grating period of Λ ≅ 13 μm. The intensities of the interfering beams were the same and equal to ca. 640 mW/cm2 (beam diameter 2 mm, beam power 20 mW). The additional source of light came from another solid-state laser (RGBLase, LLC) emitting s-polarized light of wavelength 405 nm (BL in Figure 2a). It was used to illuminate the material in the same range of excitation as the recording beams. The intensity of the beam was ca. 960 mW/cm2 (beam diameter 2 mm, beam power 30 mW). The BL beam was incident on the sample in exactly the same spot where the writing beams crossed each other. The grating formation process was monitored by measuring the intensity of the self-diffracted light as a function of time. The signal was measured with a photodiode (Thorlabs PDA 55) and registered by the oscilloscope (Tektronix TDS 2024B). The diffraction efficiency was determined by the ratio between the intensities of the first-order diffracted beam and the incident beam. The amplitude of the surface relief grating, formed for 30 min, was examined by means of atomic force microscopy (AFM, tapping mode, Dimensional V scanning probe microscope, Veeco). B
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case. It is manifested by the different positions of the isosbestic point for both wavelengths. When the cis−trans isomerization is induced by 405 nm light, the isosbestic point equals λBL = 364 nm (Figure 1b), and it equals λGL = 379 nm (Figure 1c) when the isomerization is caused by 532 nm light. Simultaneous irradiation of the sample by BL and GL causes the shift of the isosbestic point from λBL to λGL, indicating the shifting of the equilibrium. Different parts of the molecules are simultaneously excited in such case, which can lead to an increase in the mobility of the polymer. Two holographic grating recording experiments, noted as I and II, were carried out in two different places on the same sample of PEI. In these places, the thickness of the film was 2 μm with an accuracy of 10%. In the first one (experiment I), only the recording of the grating was performed using two interfering beams of wavelength 532 nm (monophotonic recording), whereas in the second one (experiment II) the recording (experimental conditions the same as in I) was done with simultaneous irradiation at 405 nm (biphotonic recording). The results of both experiments, diffraction efficiency as a function of time, are compared in Figure 2a. It is worth recalling here that the diffraction efficiency measured during the holographic recording process in azo materials results from light diffraction on three coupling phase gratings (forming simultaneously): the reorientation grating, the bulk diffusion grating, and the surface relief grating. The explanation of the origin of formation of these gratings can be found elsewhere.15,16 As can be seen in Figure 2a, the diffraction efficiency increased almost 2-fold when additional irradiation at 405 nm was applied during the recording process; moreover, the recording process occurred faster. Furthermore, AFM studies revealed an increase in the amplitude of the surface relief grating (Figure 3). The amplitude of the SRG obtained in a
one-color recording experiment was around 50 nm (Figure 3a), whereas the amplitude of the grating obtained in a two-color recording experiment was ca. 160 nm (Figure 3b). Thus, the additional irradiation resulted in the formation of the surface relief grating with an amplitude that was 3 times larger (Figure 3c). Both the higher diffraction efficiency and larger amplitude of the surface relief grating obtained in experiment II clearly demonstrate that the process of grating formation is much more efficient when the recording is done with simultaneous irradiation at 405 nm. Two additional experiments, marked as B and C, were performed to check the influence of BL on the efficiency of the recording process, and the results have been depicted in Figure 2b,c, respectively. In the first one (B), the influence of BL was investigated for the case when the recording process was in the beginning stage (cf. Figure 2a). The recording process was started by turning on GL beams, and shortly after that (35 s later), the BL beam was turned on for 10 s and then switched off for the next 20 s while the recording was continued (Figure 2b). The cycle of turning the BL beam on and off was repeated. In the next experiment (C), the effect of BL was examined in approximately the middle of the recording process, when the grating was already partially formed (cf. Figure 2a). As in B, the cycles of turning the BL beam on and off were done here (Figure 2c). One can see that in both cases turning on the 405 nm light resulted in the rapid increase in the diffraction efficiency whereas turning it off brought the signal back to the level of the diffraction efficiency measured for pure recording (Figure 2b,c). These results led us to conclude that using additional BL during the recording process increases the mobility of the polymer. When the sample is subjected only to irradiation by the recording light, which is modulated, the cis− trans isomerization is induced in the bright fringes of 532 nm, which in turn influences the mobility of the polymer in these places. However, when additional 405 nm light is applied, which is uniform in the sample, cis−trans isomerization starts to take place uniformly in space, even in the dark fringes of the recording light, and thus the mobility of the whole polymer is generated. When the mobility of the polymer is higher, the amplitudes of those gratings that are related to the mobility of the polymer (i.e., the bulk diffusion grating and the surface relief grating) increase and the gratings form faster. This explains the higher diffraction efficiency and larger amplitude of the SRG obtained in the case when the recording was performed simultaneously with irradiation at 405 nm. It has been shown that the recording process in the azo polymer is more effective when additional light is applied. The possible explanation of the effect is as follows. In molecular complex systems in which the active molecules are placed in the polymer matrix, there are a large variety of energy states of photochromic molecules. This phenomenon is particularly evident in the case of double azo substitution of the polymer chain with different azobenzene derivatives. As was shown in the spectroscopic measurements, the use of light of different wavelengths causes the excitation of different populations of photochromic molecules; in other words, blue light can excite a set of the molecules that cannot be excited by green light and vice versa. This is shown in Figure 1, where the position of the isosbestic point depends on the wavelength of the excitation light. Because of the overlapping of the absorption bands, the light-initiated reaction can lead to both the cis−trans and trans− cis isomerization. However, because of the low absorption of the trans form in the studied spectral range, only cis−trans
Figure 3. (a) Surface relief gratings (2D AFM scans) inscribed during the grating recording in the case when only the recording was performed, experiment I (monophotonic recording), and (b) when the recording was done with simultaneous irradiation with 405 nm light, II (biphotonic recording). These results correspond to the diffraction efficiency dynamics depicted in Figure 2a, respectively. (c) Comparison of the amplitudes of the surface relief grating obtained in experiments I and II. C
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Azobenzene Polyesters with Linear and Circular Photoanisotropy. Appl. Opt. 1996, 35, 3835−3840. (5) Blanche, P.-A.; Lemaire, Ph. C.; Maertens, C.; Dubois, P.; Jérome, R. Polarization Holography Reveals the Nature of the Grating in Polymers Containing Azo-Dye. Opt. Commun. 2000, 185, 1−12. (6) Paterson, J.; Natansohn, A.; Rochon, P.; Callender, C. L.; Robitaille, L. Optically Inscribed Surface Relief Diffraction Gratings on Azobenzene-Containing Polymers for Coupling Light into Slab Waveguides. Appl. Phys. Lett. 1996, 69, 3318−3320. (7) Rochon, P.; Natansohn, A.; Callender, C. L.; Robitaille, L. Guided Mode Resonance Filters Using Polymer Films. Appl. Phys. Lett. 1997, 71, 1008−1010. (8) Natansohn, A.; Rochon, P. Photoinduced Motions in Azobenzene-Based Amorphous Polymers: Possible Photonic Devices. Adv. Mater. 1999, 11, 1387−1391. (9) Rocha, L.; Dumarcher, V.; Denis, Ch.; Raimond, P.; Fiorini, C.; Nunzi, J.-M. Laser Emission in Periodically Modulated Polymer Films. J. Appl. Phys. 2001, 89, 3067−3069. (10) Matsui, T.; Ozaki, M.; Yoshino, K.; Kajzar, F. Fabrication of Flexible Distributed Feedback Laser Using Photoinduced Surface Relief Grating on Azo-Polymer Film as a Template. Jpn. J. Appl. Phys. 2002, 41, L1386−L1388. (11) Natansohn, A.; Rochon, P. Photoinduced Motions in AzoContaining Polymers. Chem. Rev. 2002, 102, 4139−4175 and references therein. (12) Kang, J.-W.; Kim, M.-J.; Kim, J.-P.; Yoo, S.-J.; Lee, J.-S.; Kim, D. Y.; Kim, J.-J. Polymeric Wavelength Filters Fabricated Using Holographic Surface Relief Gratings on Azobenzene-Containing Polymer Films. Appl. Phys. Lett. 2003, 82, 3823−3825. (13) Ubukata, T.; Isoshima, T.; Hara, M. Wavelength-Programmable Organic Distributed-Feedback Laser Based on a Photoassisted Polymermigration. Adv. Mater. 2005, 17, 1630−1633. (14) Kravchenko, A.; Shevchenko, A.; Ovchinnikov, V.; Priimagi, A.; Kaivola, M. Optical Interference Lithography Using AzobenzeneFunctionalized Polymers for Micro- and Nanopattering of Silicon. Adv. Mater. 2011, 23, 4174−4177. (15) Kulikovska, O.; Gharagozloo-Hubmann, K.; Stumpe, J.; Huey, B. D.; Bliznyuk, V. N. Formation of Surface Relief Grating in Polymers with Pendant Azobenzene Chromophores as Studied by AFM/UFM. Nanotechnology 2012, 23, 485309. (16) Delaire, J. A.; Nakatani, K. Linear and Nonlinear Optical Properties of Photochromic Molecules and Materials. Chem. Rev. 2000, 100, 1817−1845. (17) Sobolewska, A.; Bartkiewicz, S. Three Gratings Coupling During the Holographic Grating Recording Process in Azobenzene-Functionalized Polymer. Appl. Phys. Lett. 2008, 92, 253305. (18) Sobolewska, A.; Bartkiewicz, S.; Miniewicz, A.; Schab-Balcerzak, E. Polarization Dependence of Holographic Grating Recording in Azobenzene-Functionalized Polymers Monitored by Visible and Infrared Light. Phys. Chem. B 2010, 114, 9751−9760. (19) Bach, H.; Anderle, K.; Fuhrmann, Th.; Wendorff, J. H. Biphoton-Induced Refractive Index Change in 4-Amino-4′-nitroazobenzene/Polycarbonate. J. Phys. Chem. 1996, 100, 4135−4140. (20) Wu, P.; Zou, B.; Wu, X.; Xu, J.; Gong, X.; Zhang, G.; Tang, G.; Chen, W. Biphotonic Self-Diffraction in Azo-Doped Polymer Film. Appl. Phys. Lett. 1997, 70, 1224−1226. (21) Wu, P.; Wang, L.; Xu, J.; Zou, B.; Gong, X.; Zhang, G.; Tang, G.; Chen, W. Transient Biphotonic Holographic Grating in Photoisomerizative Azo Materials. Phys. Rev. B 1998, 57, 3874−3880. (22) Pengfei, Wu; Rao, D. V. G. L. N.; Kimball, B. R.; Nakashima, M.; DeCristofano, B. S. Nonvolatile Grating in an Azobenzene Polymer with Optimized Molecular Reorientation. Appl. Phys. Lett. 2001, 78, 1189−1191. (23) Sánchez, C.; Alcalá, R.; Hvilsted, S.; Ramanujam, P. S. Biphotonic Holographic Gratings in Azobenzene Polyesters: Surface Relief Phenomena and Polarization Effects. Appl. Phys. Lett. 2000, 77, 1440−1442. (24) Sánchez, C.; Cases, R.; Alcalá, R.; Lopez, A.; Quintanilla, M.; Oriol, L.; Millaruelo, M. Biphotonic Holographic Recording in a
isomerization is effective. Therefore, as a result of blue or green light irradiation cis−trans isomerization occurs. The periodic modulation of green light leads to the formation of the holographic grating in the bulk and on the polymer surface (the surface relief grating). The last process is quite slow because of the spherical limitation, and usually the formation of the SRG takes many minutes. The introduction of the illumination of the material by the additional beam of different wavelength (blue light) leads to the cis−trans isomerization of a new population of the molecules that, in further steps, can return to the cis form or can stay in the trans form. Such a process leads to the addressing of a larger population of the molecules, resulting in an increase in the speed and/or the efficiency of a given unit of time. The applied additional beam acts similarly to a plasticizer placed in a polymer composition;29 however, in this case, the plasticization of the material occurs in the hologram recording area only during illumination. The yield of the biphotonic addressing strongly depends on the state of the additional beam: the wavelength, polarization, intensity, and so forth. The value of these parameters should be selected depending on the photochromic polymer type (for example, a method of chain substitution) and configuration of the experimental setup (the grating period, polarization, and wavelength of the recording beams). Studies on these topics are in progress.
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OUTLOOK We have demonstrated that simply applying an additional light source during the grating recording process in the azo polymer can result in a substantial increase in the efficiency and speed of the recording process (in the studied material, a 2-fold increase in the diffraction efficiency and a 3-fold larger amplitude of the SRG). Such an approach can be used to increase the efficiency and to speed up different processes that involve trans−cis photoisomerization in azo materials. The selection of additional light should be done in a conscious way.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Dr. E. Schab-Balcerzak from the Polish Academy of Science for polymer synthesis and the supply of material. This work was performed under grant no. 2011/01/B/ST8/03317 from the Polish National Science Centre.
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REFERENCES
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