- Email: [email protected]
TiO2 nanocomposite films
Optics Communications 318 (2014) 1–6
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Polarization-dependent and rewritable holographic gratings in Ag/TiO2 nanocomposite films Shencheng Fu a,b, Shiyu Sun a, Xintong Zhang a,n, Xiuli Wang c, Yichun Liu a,n a
Center for Advanced Optoelectronic Functional Materials Research, Northeast Normal University, No. 5268, Renmin Street, Changchun 130024, China School of Science, Changchun University of Science and Technology, No. 7089, Weixing Road, Changchun 130022, China c School of Life Science, Northeast Normal University, No. 5268, Renmin Street, Changchun 130024, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 9 November 2013 Received in revised form 5 December 2013 Accepted 19 December 2013 Available online 1 January 2014
TiO2 nanoporous films loaded with Ag nanoparticles exhibited distinctive photochromism and photoanisotropy under the visible linearly polarized irradiation. Based on such properties, a pure polarization holographic grating was recorded in the photochromic film using two orthogonal circularly polarized green beams (532 nm) and reconstructed with a red beam (632.8 nm). The diffraction efficiency of the holographic grating and the brightness of the reconstruction image were strongly dependent on the polarization state of the probe beam. The hologram can be erased simply by the irradiation of single green beam. This recording–erasing process can be repeated with little loss, which may be benefited from the reciprocating mobility of Ag þ ions, reversible deformation and re-growth of Ag nanoparticles under the alternate irradiation of linearly and circularly polarized light. The novel nanocomposite system with photoanisotropy makes a new range of applications in the field of high-density optical memory media. & 2013 Elsevier B.V. All rights reserved.
Keywords: Rewritable Polarization dependent Circular polarization holography Ag nanoparticles TiO2 nanoporous films
1. Introduction In the future highly information-oriented society, all-optical devices will play a very important role in photonic applications [1–3]. Polarization holographic gratings, in which optical anisotropy is periodically modulated, have attracted considerable attention because of their various unique optical properties, including polarization selectivity of the diffraction efficiency and polarization conversion in the diffraction process [4–7]. In order to record polarization holograms, polarization-sensitive materials, in which dichroism or birefringence may be induced by the polarized irradiation, are necessary [8–10]. Many kinds of recording media have been applied to polarization holographic recording by means of molecular orientation [11], photorefractive effects [12], and photochemical reaction [13]. However, little attention has been given to the application of noble metal-nanoparticles (NPs)/semiconductor composite system in polarization holography. Ag NPs with different morphologies absorb and scatter light at different wavelengths due to localized surface plasmon resonance (LSPR) [14–18]. In particular, oriented anisotropic Ag NPs respond selectively to light polarized in a specific angle [15,16]. Recently, we reported the photo-anisotropy of Ag NPs/TiO2 nanoporous films induced by the irradiation of linearly polarized 532 nm laser [9]. The phenomenon could be explained by anisotropic n
Corresponding authors. Tel./fax: þ86 431 85099772. E-mail addresses: [email protected] (X. Zhang), [email protected] (Y. Liu).
0030-4018/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2013.12.040
photo-dissolution of Ag NPs. At the direction of laser polarization state, the laser-stimulated dissolution of Ag NPs into silver cations easily occurs. Electrons transfer from resonant Ag NPs to TiO2 under light of resonant wavelength, resulting in oxidative dissolution of Ag to Ag þ in the interfacial region between Ag and TiO2 [19,20] and the change of Ag NPs morphology [21]. Ag NPs/TiO2 composite films could be used as rewritable holographic storage media. The reversibility of holographic recording and erasing could be realized by the alternate action of visible irradiation and heat treatment on the film [22–24], or alternating UV/Vis light irradiation [25,26]. However, the multi-source system and UV environmental pollution make the impediment to apply the Ag/TiO2 film in rewritable holographic storage. In this paper, we investigate polarization dependence and rewritable property of circularly polarization holographic gratings in TiO2 nanoporous films loaded with Ag NPs. A pure and more efficient polarization hologram was recorded and erased repeatedly in Ag/TiO2 nanocomposite films only using Nd:YAG lasers (532 nm). The corresponding microscopic origin was also discussed.
2. Experimental 2.1. Preparation of the photochromic films TiO2 nanoporous films (3 μm thick) were prepared on glass slides by dip-coating from a solution of TiO2 NPs (STS-01, 0.4 mol/L, Ishihara Sangyo) and PEO20-PPO70-PPO20 block copolymer (20 g/L)
2
S. Fu et al. / Optics Communications 318 (2014) 1–6
in equivolume water-ethanol mixture solvent, and annealing at 450 1C to remove the polymer. The obtained porous TiO2 film has high transparency and provides a perfect growth environment of Ag NPs. An aqueous solution containing 0.005 M silver nitrate (AgNO3) (50 mL) was mixed with ethanol (1 mL). The TiO2 film was immersed in the solution and irradiated with UV light for 5 min. Ag NPs were deposited in the porous film by photocatalytic reduction of Ag þ . During the UV irradiation, the sample turned brownish-gray due to surface plasmon absorption of the deposited Ag NPs. Subsequently, the UV–Vis spectra and morphological changes of Ag NPs were measured and observed, respectively. 2.2. Optical setup For the dichroism measurement, two laser beams were nearly parallel incident to the sample, as shown in Fig. 1(a). The pumping beam is always s-polarized with the power of 3 mW. The probe beam with the power of 0.1 mW can be adjusted as s- or ppolarized by a half-wave plate. Probe-beam transmittance was measured with a photodiode interfaced with a computer. Fig. 1 (b) schematically shows the experimental arrangement for polarization holographic recording and the observation of reconstructed images. The beam from the two double frequency Nd: YAG laser (532 nm, both 3 mW) was divided into right and left circularly polarized beams (RCP and LCP) by adding quarter wave plates. One of the two writing beams was expanded by a beam expander, collimated to pass through an image mask (a letter “F”), and then focused onto the nanocomposite film. The other beam (reference beam) was superimposed on the same spot and interfered with the object beam. The intersecting angle between the writing beams was 101. The polarization hologram was reconstructed and characterized by a low-power He–Ne laser (632.8 nm 0.1 mW) as a probe beam, which is normal incidence to the sample surface. A white screen was temporarily put on the opposite side of the input reconstruction beam (632.8 nm) so that the diffractive images could be clearly identified and recorded with a digital
Fig. 1. Experimental configuration for dichroism (a) and holographic grating recording (b) in Ag/TiO2 nanocomposite films. M, mirror; BS, beam splitter; P, polarizer; RP, retardation plate (λ=2 or λ=4).
camera. Alternately, after removing the mask, the first-order diffractive signal of the He–Ne beam was also registered on a photodiode interfaced with a computer to analyze the grating growth kinetics. All the measurements were performed at the temperature of 300 K and the relative humidity of 40%.
3. Results and discussions 3.1. Photo-induced anisotropy Differential absorption spectra in the UV–Vis–NIR region (350– 1200 nm) of the obtained brownish-gray Ag/TiO2 film irradiated with linearly polarized green light from Nd:YAG lasers (532 nm, 0.5 mW) is presented in Fig. 2. The absorption “hole” is spitted in visible region. A possible reason for this is the enhanced quadrupole or higher multipole resonance in anisotropic-photodissolved Ag NPs by the linearly polarized light in the porous TiO2 film [27]. Besides, an additional accumulated absorption peak at 1000 nm gradually appears. To prove the assumption of anisotropic-photodissolution, the morphological and size-distribution changes of Ag NPs were observed by transmission electron microscopy (TEM). The original Ag NPs in the nanocomposite film shows a spherical shape, as presented in Fig. 3(a). After the linearly polarized irradiation, some of Ag NPs become smaller while the other Ag NPs become larger and anisotropic, as shown in Fig. 3(b). Comparison in cumulative volume fraction (Fig. 3c and d) shows the ultrasmall Ag NPs (o2.5 nm) occupied a considerable volume fraction of 33.1% after the irradiation while that is only 6.5% before the irradiation. The large and nearly-ellipsoidal Ag NPs (over 10 nm) is almost nonexistent before the irradiation; however, those occupied a volume fraction of 9.8% after the irradiation, which may be resulted from the photomobility and photoreduction of the Ag þ ions in a humid atmosphere [9,20]. Correlating the spectra with TEM observation suggest the increases of the volume fraction of large NPs and the aspect ratio of the ellipsoidal NPs contribute the resonance enhancement in near-infrared region. The linearly polarized light induced anisotropy could also be verified in the photo-dichroism of the Ag/TiO2 film. Fig. 4 shows the probe transmittance with different polarization states versus excitation time. The transmittance under horizontal excitation (Ts-pol) is always higher than that under vertical excitation (Tp-pol). And the difference between them increases with excitation time. Under the linearly s-polarized irridiation, for the simple case, the original spherical Ag NPs were gradually dissolved into ellipsoidal NPs (Fig. 4, inserted). Therefore, the mass loss of Ag NP parallel to the
Fig. 2. Differential absorption spectra of the Ag/TiO2 film irradiated with linearly polarized light from Nd:YAG laser (532 nm, 0.5 mW ) for different periods.
S. Fu et al. / Optics Communications 318 (2014) 1–6
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Fig. 3. TEM photographs of Ag NPs in the nano-composite film (a) before and (b) after linearly polarized irradiation, TiO2 NPs in the films were dissolved with HF solution to remove interference. (c) and (d) below show the size distribution histograms and cumulative percentage of volume fraction of Ag NPs derived from TEM photographs before and after irradiation, respectively.
Fig. 4. Transmitting intensity (at 532 nm, 0.1 mW) versus excitation (at 532 nm, 3 mW) time of the Ag/TiO2 film for different detection polarizations. s-polarized detection: detection beam has the same polarization as excitation one. p-polarized detection: excitation and detection beams have orthogonal polarizations. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
direction of the pumping laser polarization is less than that perpendicular to the direction of the laser polarization. 3.2. Circular polarization holography The effective formation of photo-induced anisotropy in the Ag/TiO2 film provides the possibility for a pure polarization holographic recording. Fig. 5 presents the kinetics of circular polarization holographic recording and erasing in the Ag/TiO2 film. After the two circular polarized writing lights were switched on, the diffractive intensity of the 632.8 nm beam increase gradually to the maximum value. Then the left-hand circularly polarized green beam was turned off. The diffractive intensity
Fig. 5. Time dependence of the first-order diffraction efficiency in (RCPþ LCP) recording configuration during three record and erase cycles in the Ag/TiO2 nanocomposite film.
decrease sharply almost to zero, indicating the complete erasure of the polarization hologram. The writing and erasing processes can be repeated with little loss. The proper explanations are as follows: The interaction of RCP and LCP waves forms a unique polarization interference pattern, as shown in Table 1. Its intensity is a constant, while the oscillation direction of the resulting electric field of the optical waves varies periodically, creating a series of linearly polarized regions. Based on the assumption of the anisotropic photo-dissolution in the Ag/TiO2 film, the Ag NPs are transformed to Ag þ ions along the direction of the resulting electric field. The morphology of Ag NPs at different sites is modified differently. Besides, the spatial electric field component parallel to the grating wave-vector decreases gradually from x ¼0 to x¼ Λ/2, producing an electric field gradient force. The additional
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Table 1 Periodic polarization modulation of the light used for the grating recording for RCP LCP polarization geometry.
Fig. 6. Left: Sketch of periodic distributions of Ag NPs and Ag þ ion at wring, erasing and re-writing stages;and Right: the corresponding differential absorbance spectra under the alternate irradiation of linearly and circularly polarized green light from Nd:YAG laser (532 nm, 1 mW). (a) Writing, (b) erasing and (c) rewriting. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
electric field may induce the migration of Ag þ ions, producing non-uniform distribution of Ag þ ions, i.e., Ag þ ion concentration gradient to resist the former one. At the erasing stage, the resulting electric field becomes the uniform circular polarized state. The electric field gradient force disappears and the concentration gradient force tends to bring the Ag þ ion to the original sites, as shown in Fig. 6. After excitation, some electrons may transfer from TiO2 to dissolved oxygen [28], and the other must recombine with Ag þ released from the resonant Ag NPs. The
recombination takes place preferably on non-resonant Ag NPs, resulting in regrowth of Ag NPs [21]. The reciprocating mobility of Ag þ ions promotes the reduction and growth of Ag þ ion at every site, which help re-writing and re-erasing of the hologram. It was also noticed that a slight accumulation of the diffraction efficiency after each erasure. In fact, the mobility of Ag þ ions may influence on photo-reduction probability of Ag þ ions. That is, the resident diffraction efficiency may come from the periodical re-growth of Ag NPs.
S. Fu et al. / Optics Communications 318 (2014) 1–6
According to our previous report [9], Ag NPs become anisotropic and isotropic under the linearly and circularly polarized irradiations, respectively; and the microscopic properties of Ag NPs are closely related with the absorption spectra of Ag/TiO2 films. So the reversible morphology change of Ag NPs can be characterized
5
by differential absorbance spectra under the alternate irradiation of linearly and circularly polarization light. As shown in Fig. 6 (right), the splitted and broaden absorption “hole” at 532 nm becomes more smooth and narrow by switching from linearly to circularly polarized irradiation, and can be recovered to the original spectral shape under subsequent linearly polarized excitation. Diffraction efficiency of a pure polarization holographic grating relies on amplitude of the refractive index change, described as Eq. (1). ηðtÞ ¼ J 21 ð2π dΔn=λÞ ð1Þ where J 1 is the first-order Bessel function, d is the nanocomposite film thickness, Δn are the maximum refractive index modulations, and λ is the wavelength of the monitoring beam. Hence, the rewritable characteristics of the polarization holographic grating should come from the reversible refractive index change of the Ag/TiO2 film, as well as the reduction and re-growth of Ag NPs which help to maintain the stability of the amount of the resonant Ag NPs. 3.3. Polarization dependence
Fig. 7. Dependence of the diffraction efficiency on the angle between the fast axis of the λ=4 plate and the polarization direction of the readout beam.
In the case of a pure polarization grating, one of the diffraction efficiencies become zero (corresponding to the right or left circular polarization of the incident probe); while the other one is at its maximum. To verify that the recorded gratings were pure polarization ones, the corresponding diffraction efficiencies of the diffracted beams were examined for different polarization states of the probe beam.
Fig. 8. Read out of the stored image with different probe polarization state (a) right-hand circular polarization; (b) linearly s polarization; (c) left-hand circular polarization.
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Utilizing the high stability of the recorded gratings in Ag/TiO2 nanocomposite films, these analyses can be expediently performed after recording. With the help of a quarter-wave plate, the polarization state of the probe beam can be set to linear, elliptical and circular polarizations without changing its intensity. It was found that when the probe beam was linearly polarized, the 71 order diffraction efficiencies had almost the same values. When the probe-beam was right-hand circularly polarized, only the þ1 order diffracted beam, with the left-hand circularly polarization state, was visible; and vice versa. The dependence of diffraction efficiency on the intersection angle between the polarization direction of the readout beam and the fast axis of λ=4 plate is shown in Fig. 7. The stable and high-efficient storage of the polarization holographic grating provides possibilities for image storage. Fig. 8 shows the 71 order diffracted images from the reconstruction beam with three different polarization states: (a) RCP, (b) linear s polarization, and (c) LCP. Obviously, the þ1 order diffractive signal is the same as the object beam, while the 1 order diffractive signal is a conjugate image to the object beam. It was noticed that the brightness of the diffractive image was not uniform because the mask “F” was illuminated by the inhomogeneous green light. For a RCP irradiation, the þ1 order diffractive signal was brightest and the 1 order diffractive signal was invisible. For a LCP irradiation, the case was the opposite. Additionally, for a linearly polarized one, the brightness of 71 order diffracted images was redistributed equally. The results indicated that variation of the reconstruction polarization state can control the brightness of the diffracted images. The stored image can be erased by either linearly or circularly polarized beams of 532 nm. The rewritable process was verified to be repeated for more than ten times in the orthogonal circular polarization interference pattern. However, the reversible writingerasing process by single-beam could not be accomplished in other polarization interference patterns, such as (s–s), (p–p), (s–p), and (LCP–LCP), which all contain scalar grating components. Further investigation is in process. 4. Conclusions Linearly polarized beam induced photochromism and photoanisotropy in Ag/TiO2 nanocomposite films comes from the changes of morphology and size distribution of Ag NPs, which is assisted-proved by the TEM observation. A pure polarization holographic grating in the Ag/TiO2 films were obtained using two orthogonal circularly polarized green beams and reconstructed with a red beam. The hologram could be erased and rewritten for many times without obvious losses, which may be benefited from the reversible morphology change of Ag NPs, the reciprocating mobility of Ag þ ions and re-growth of Ag NPs under
the co-action of both the electric gradient force and silver ion concentration gradient force. The brightness of the image stored in the polarization hologram was easily controlled by variation of the reconstruction polarization state. The Ag/TiO2 nanocomposite film may be a candidate for the ideal optical memory medium.
Acknowledgements This work was supported by the National Natural Science Foundation of China (10974027, 61007006, 51072032 and 31271442), the National Basic Research Program (2012CB933700), the Program for New Century Excellent Talents in University (NECT-10-0320), the Fundamental Research Funds for the Central Universities (12SSXM001) and the 111 project (B13013).
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D.E. Chang, A.S. Sørensen, E.A. Demler, M.D. Lukin, Nat. Phys. 11 (2007) 807. S. Zhang, H Xu, ACS Nano 6 (2012) 8128. H. Liu, B. He, J. Zhou, J. Dong, Y. Wei, Q. Lou, Opt. Lett. 37 (2012) 3885. Y.J. Liu, Y.C. Su, Y.J. Hsu, V.K.S. Hsiao, J. Mater. Chem. 22 (2012) 14191. A. Ryabchun, A. Bobrovsky, A. Sobolewska, V. Shibaev, J. Stumpe, J. Mater. Chem. 22 (2012) 6245. U. Ruiz, P. Pagliusi, C. Provenzano, K. Volke-Sepúlveda, G. Cipparrone, Opt Express 21 (2013) 7505. B. Kilosanidze, G. Kakauridze, I. Chaganava, Y. Mshvenieradze, Appl. Opt. 52 (2013) 1006. A. Ogiwara, M. Watanabe, Appl. Opt. 51 (2012) 5168. S. Fu, X. Zhang, R. Han, S. Sun, L. Wang, Y. Liu, Appl. Opt. 51 (2012) 3357. U. Ruiz, C. Provenzano, P. Pagliusi, G. Cipparrone, Opt. Lett. 37 (2012) 311. N. Kawatsuki, A. Yamashita, M. Kondo, T. Matsumoto, T. Shioda, A. Emoto, H. Ono, Polymer 51 (2010) 2849. W.C. Su, C.C. Sun, N. Kukhtarev, A.E.T. Chiou, Opt. Eng. 42 (2003) 9. S. Fu, Y. Liu, Z. Lu, L. Dong, W. Hu, M. Xie, Opt. Commun. 242 (2004) 115. E. Kazuma, T. Tatsuma, Chem. Commun. 48 (2012) 1733. Y. Sakai, I. Tanabe, T. Tatsuma, Nanoscale 3 (2011) 4101. K. Matsubara, K.L. Kelly, N. Sakai, T. Tatsuma, J. Mater. Chem. 19 (2009) 5526. E. Kazuma, K. Matsubara, K.L. Kelly, N. Sakai, T. Tatsuma, J. Phys. Chem. C 113 (2009) 4758. T. Nagahiro, K. Ishibashi, Y. Kimura, M. Niwano, T. Hayashi, Y. Ikezoe, M. Hara, T. Tatsuma, K. Tamada, Nanoscale 2 (2010) 107. Y. Tian, T. Tatsuma, Chem. Commun. 16 (2004) 1810. K. Matsubara, T. Tatsuma, Adv. Mater. 19 (2007) 2802. Y. Takahashi, T. Tatsuma, Nanoscale 2 (2010) 1494. G. Kawamura, S. Sato, T. Kogure, Y. Daiko, H. Muto, M. Sakai, A. Matsuda, Phys. Chem. Chem. Phys. 12 (2010) 6859. G. Kawamura, S. Sato, H. Muto, M. Sakai, P.B. Lim, K. Watanabe, M. Inoue, A. Matsuda, Mater. Lett. 64 (2010) 2648. G. Kawamura, Y. Tsurumi, H. Muto, M. Sakai, M. Inoue, A. Matsuda, Mater. Chem. Phys. 130 (2011) 264. Q. Qiao, X. Zhang, Z. Lu, L. Wang, Y. Liu, Appl. Phys. Lett. 94 (2009) 074104. J. Liu, M. Li, J. Zhou, C. Ye, J. Wang, L. Jiang, Y. Song, Appl. Phys. Lett. 98 (2011) 023110. K. Naoi, Y. Ohko, T. Tatsuma, J. Am. Chem. Soc. 126 (2004) 3664. R. Han, X. Zhang, L. Wang, R. Dai, Y. Liu, Appl. Phys. Lett. 98 (2011) 221905.
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Polarization-dependent and rewritable holographic gratings in Ag/TiO2 nanocomposite films Shencheng Fu a,b, Shiyu Sun a, Xintong Zhang a,n, Xiuli Wang c, Yichun Liu a,n a
Center for Advanced Optoelectronic Functional Materials Research, Northeast Normal University, No. 5268, Renmin Street, Changchun 130024, China School of Science, Changchun University of Science and Technology, No. 7089, Weixing Road, Changchun 130022, China c School of Life Science, Northeast Normal University, No. 5268, Renmin Street, Changchun 130024, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 9 November 2013 Received in revised form 5 December 2013 Accepted 19 December 2013 Available online 1 January 2014
TiO2 nanoporous films loaded with Ag nanoparticles exhibited distinctive photochromism and photoanisotropy under the visible linearly polarized irradiation. Based on such properties, a pure polarization holographic grating was recorded in the photochromic film using two orthogonal circularly polarized green beams (532 nm) and reconstructed with a red beam (632.8 nm). The diffraction efficiency of the holographic grating and the brightness of the reconstruction image were strongly dependent on the polarization state of the probe beam. The hologram can be erased simply by the irradiation of single green beam. This recording–erasing process can be repeated with little loss, which may be benefited from the reciprocating mobility of Ag þ ions, reversible deformation and re-growth of Ag nanoparticles under the alternate irradiation of linearly and circularly polarized light. The novel nanocomposite system with photoanisotropy makes a new range of applications in the field of high-density optical memory media. & 2013 Elsevier B.V. All rights reserved.
Keywords: Rewritable Polarization dependent Circular polarization holography Ag nanoparticles TiO2 nanoporous films
1. Introduction In the future highly information-oriented society, all-optical devices will play a very important role in photonic applications [1–3]. Polarization holographic gratings, in which optical anisotropy is periodically modulated, have attracted considerable attention because of their various unique optical properties, including polarization selectivity of the diffraction efficiency and polarization conversion in the diffraction process [4–7]. In order to record polarization holograms, polarization-sensitive materials, in which dichroism or birefringence may be induced by the polarized irradiation, are necessary [8–10]. Many kinds of recording media have been applied to polarization holographic recording by means of molecular orientation [11], photorefractive effects [12], and photochemical reaction [13]. However, little attention has been given to the application of noble metal-nanoparticles (NPs)/semiconductor composite system in polarization holography. Ag NPs with different morphologies absorb and scatter light at different wavelengths due to localized surface plasmon resonance (LSPR) [14–18]. In particular, oriented anisotropic Ag NPs respond selectively to light polarized in a specific angle [15,16]. Recently, we reported the photo-anisotropy of Ag NPs/TiO2 nanoporous films induced by the irradiation of linearly polarized 532 nm laser [9]. The phenomenon could be explained by anisotropic n
Corresponding authors. Tel./fax: þ86 431 85099772. E-mail addresses: [email protected] (X. Zhang), [email protected] (Y. Liu).
0030-4018/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2013.12.040
photo-dissolution of Ag NPs. At the direction of laser polarization state, the laser-stimulated dissolution of Ag NPs into silver cations easily occurs. Electrons transfer from resonant Ag NPs to TiO2 under light of resonant wavelength, resulting in oxidative dissolution of Ag to Ag þ in the interfacial region between Ag and TiO2 [19,20] and the change of Ag NPs morphology [21]. Ag NPs/TiO2 composite films could be used as rewritable holographic storage media. The reversibility of holographic recording and erasing could be realized by the alternate action of visible irradiation and heat treatment on the film [22–24], or alternating UV/Vis light irradiation [25,26]. However, the multi-source system and UV environmental pollution make the impediment to apply the Ag/TiO2 film in rewritable holographic storage. In this paper, we investigate polarization dependence and rewritable property of circularly polarization holographic gratings in TiO2 nanoporous films loaded with Ag NPs. A pure and more efficient polarization hologram was recorded and erased repeatedly in Ag/TiO2 nanocomposite films only using Nd:YAG lasers (532 nm). The corresponding microscopic origin was also discussed.
2. Experimental 2.1. Preparation of the photochromic films TiO2 nanoporous films (3 μm thick) were prepared on glass slides by dip-coating from a solution of TiO2 NPs (STS-01, 0.4 mol/L, Ishihara Sangyo) and PEO20-PPO70-PPO20 block copolymer (20 g/L)
2
S. Fu et al. / Optics Communications 318 (2014) 1–6
in equivolume water-ethanol mixture solvent, and annealing at 450 1C to remove the polymer. The obtained porous TiO2 film has high transparency and provides a perfect growth environment of Ag NPs. An aqueous solution containing 0.005 M silver nitrate (AgNO3) (50 mL) was mixed with ethanol (1 mL). The TiO2 film was immersed in the solution and irradiated with UV light for 5 min. Ag NPs were deposited in the porous film by photocatalytic reduction of Ag þ . During the UV irradiation, the sample turned brownish-gray due to surface plasmon absorption of the deposited Ag NPs. Subsequently, the UV–Vis spectra and morphological changes of Ag NPs were measured and observed, respectively. 2.2. Optical setup For the dichroism measurement, two laser beams were nearly parallel incident to the sample, as shown in Fig. 1(a). The pumping beam is always s-polarized with the power of 3 mW. The probe beam with the power of 0.1 mW can be adjusted as s- or ppolarized by a half-wave plate. Probe-beam transmittance was measured with a photodiode interfaced with a computer. Fig. 1 (b) schematically shows the experimental arrangement for polarization holographic recording and the observation of reconstructed images. The beam from the two double frequency Nd: YAG laser (532 nm, both 3 mW) was divided into right and left circularly polarized beams (RCP and LCP) by adding quarter wave plates. One of the two writing beams was expanded by a beam expander, collimated to pass through an image mask (a letter “F”), and then focused onto the nanocomposite film. The other beam (reference beam) was superimposed on the same spot and interfered with the object beam. The intersecting angle between the writing beams was 101. The polarization hologram was reconstructed and characterized by a low-power He–Ne laser (632.8 nm 0.1 mW) as a probe beam, which is normal incidence to the sample surface. A white screen was temporarily put on the opposite side of the input reconstruction beam (632.8 nm) so that the diffractive images could be clearly identified and recorded with a digital
Fig. 1. Experimental configuration for dichroism (a) and holographic grating recording (b) in Ag/TiO2 nanocomposite films. M, mirror; BS, beam splitter; P, polarizer; RP, retardation plate (λ=2 or λ=4).
camera. Alternately, after removing the mask, the first-order diffractive signal of the He–Ne beam was also registered on a photodiode interfaced with a computer to analyze the grating growth kinetics. All the measurements were performed at the temperature of 300 K and the relative humidity of 40%.
3. Results and discussions 3.1. Photo-induced anisotropy Differential absorption spectra in the UV–Vis–NIR region (350– 1200 nm) of the obtained brownish-gray Ag/TiO2 film irradiated with linearly polarized green light from Nd:YAG lasers (532 nm, 0.5 mW) is presented in Fig. 2. The absorption “hole” is spitted in visible region. A possible reason for this is the enhanced quadrupole or higher multipole resonance in anisotropic-photodissolved Ag NPs by the linearly polarized light in the porous TiO2 film [27]. Besides, an additional accumulated absorption peak at 1000 nm gradually appears. To prove the assumption of anisotropic-photodissolution, the morphological and size-distribution changes of Ag NPs were observed by transmission electron microscopy (TEM). The original Ag NPs in the nanocomposite film shows a spherical shape, as presented in Fig. 3(a). After the linearly polarized irradiation, some of Ag NPs become smaller while the other Ag NPs become larger and anisotropic, as shown in Fig. 3(b). Comparison in cumulative volume fraction (Fig. 3c and d) shows the ultrasmall Ag NPs (o2.5 nm) occupied a considerable volume fraction of 33.1% after the irradiation while that is only 6.5% before the irradiation. The large and nearly-ellipsoidal Ag NPs (over 10 nm) is almost nonexistent before the irradiation; however, those occupied a volume fraction of 9.8% after the irradiation, which may be resulted from the photomobility and photoreduction of the Ag þ ions in a humid atmosphere [9,20]. Correlating the spectra with TEM observation suggest the increases of the volume fraction of large NPs and the aspect ratio of the ellipsoidal NPs contribute the resonance enhancement in near-infrared region. The linearly polarized light induced anisotropy could also be verified in the photo-dichroism of the Ag/TiO2 film. Fig. 4 shows the probe transmittance with different polarization states versus excitation time. The transmittance under horizontal excitation (Ts-pol) is always higher than that under vertical excitation (Tp-pol). And the difference between them increases with excitation time. Under the linearly s-polarized irridiation, for the simple case, the original spherical Ag NPs were gradually dissolved into ellipsoidal NPs (Fig. 4, inserted). Therefore, the mass loss of Ag NP parallel to the
Fig. 2. Differential absorption spectra of the Ag/TiO2 film irradiated with linearly polarized light from Nd:YAG laser (532 nm, 0.5 mW ) for different periods.
S. Fu et al. / Optics Communications 318 (2014) 1–6
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Fig. 3. TEM photographs of Ag NPs in the nano-composite film (a) before and (b) after linearly polarized irradiation, TiO2 NPs in the films were dissolved with HF solution to remove interference. (c) and (d) below show the size distribution histograms and cumulative percentage of volume fraction of Ag NPs derived from TEM photographs before and after irradiation, respectively.
Fig. 4. Transmitting intensity (at 532 nm, 0.1 mW) versus excitation (at 532 nm, 3 mW) time of the Ag/TiO2 film for different detection polarizations. s-polarized detection: detection beam has the same polarization as excitation one. p-polarized detection: excitation and detection beams have orthogonal polarizations. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
direction of the pumping laser polarization is less than that perpendicular to the direction of the laser polarization. 3.2. Circular polarization holography The effective formation of photo-induced anisotropy in the Ag/TiO2 film provides the possibility for a pure polarization holographic recording. Fig. 5 presents the kinetics of circular polarization holographic recording and erasing in the Ag/TiO2 film. After the two circular polarized writing lights were switched on, the diffractive intensity of the 632.8 nm beam increase gradually to the maximum value. Then the left-hand circularly polarized green beam was turned off. The diffractive intensity
Fig. 5. Time dependence of the first-order diffraction efficiency in (RCPþ LCP) recording configuration during three record and erase cycles in the Ag/TiO2 nanocomposite film.
decrease sharply almost to zero, indicating the complete erasure of the polarization hologram. The writing and erasing processes can be repeated with little loss. The proper explanations are as follows: The interaction of RCP and LCP waves forms a unique polarization interference pattern, as shown in Table 1. Its intensity is a constant, while the oscillation direction of the resulting electric field of the optical waves varies periodically, creating a series of linearly polarized regions. Based on the assumption of the anisotropic photo-dissolution in the Ag/TiO2 film, the Ag NPs are transformed to Ag þ ions along the direction of the resulting electric field. The morphology of Ag NPs at different sites is modified differently. Besides, the spatial electric field component parallel to the grating wave-vector decreases gradually from x ¼0 to x¼ Λ/2, producing an electric field gradient force. The additional
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Table 1 Periodic polarization modulation of the light used for the grating recording for RCP LCP polarization geometry.
Fig. 6. Left: Sketch of periodic distributions of Ag NPs and Ag þ ion at wring, erasing and re-writing stages;and Right: the corresponding differential absorbance spectra under the alternate irradiation of linearly and circularly polarized green light from Nd:YAG laser (532 nm, 1 mW). (a) Writing, (b) erasing and (c) rewriting. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
electric field may induce the migration of Ag þ ions, producing non-uniform distribution of Ag þ ions, i.e., Ag þ ion concentration gradient to resist the former one. At the erasing stage, the resulting electric field becomes the uniform circular polarized state. The electric field gradient force disappears and the concentration gradient force tends to bring the Ag þ ion to the original sites, as shown in Fig. 6. After excitation, some electrons may transfer from TiO2 to dissolved oxygen [28], and the other must recombine with Ag þ released from the resonant Ag NPs. The
recombination takes place preferably on non-resonant Ag NPs, resulting in regrowth of Ag NPs [21]. The reciprocating mobility of Ag þ ions promotes the reduction and growth of Ag þ ion at every site, which help re-writing and re-erasing of the hologram. It was also noticed that a slight accumulation of the diffraction efficiency after each erasure. In fact, the mobility of Ag þ ions may influence on photo-reduction probability of Ag þ ions. That is, the resident diffraction efficiency may come from the periodical re-growth of Ag NPs.
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According to our previous report [9], Ag NPs become anisotropic and isotropic under the linearly and circularly polarized irradiations, respectively; and the microscopic properties of Ag NPs are closely related with the absorption spectra of Ag/TiO2 films. So the reversible morphology change of Ag NPs can be characterized
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by differential absorbance spectra under the alternate irradiation of linearly and circularly polarization light. As shown in Fig. 6 (right), the splitted and broaden absorption “hole” at 532 nm becomes more smooth and narrow by switching from linearly to circularly polarized irradiation, and can be recovered to the original spectral shape under subsequent linearly polarized excitation. Diffraction efficiency of a pure polarization holographic grating relies on amplitude of the refractive index change, described as Eq. (1). ηðtÞ ¼ J 21 ð2π dΔn=λÞ ð1Þ where J 1 is the first-order Bessel function, d is the nanocomposite film thickness, Δn are the maximum refractive index modulations, and λ is the wavelength of the monitoring beam. Hence, the rewritable characteristics of the polarization holographic grating should come from the reversible refractive index change of the Ag/TiO2 film, as well as the reduction and re-growth of Ag NPs which help to maintain the stability of the amount of the resonant Ag NPs. 3.3. Polarization dependence
Fig. 7. Dependence of the diffraction efficiency on the angle between the fast axis of the λ=4 plate and the polarization direction of the readout beam.
In the case of a pure polarization grating, one of the diffraction efficiencies become zero (corresponding to the right or left circular polarization of the incident probe); while the other one is at its maximum. To verify that the recorded gratings were pure polarization ones, the corresponding diffraction efficiencies of the diffracted beams were examined for different polarization states of the probe beam.
Fig. 8. Read out of the stored image with different probe polarization state (a) right-hand circular polarization; (b) linearly s polarization; (c) left-hand circular polarization.
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Utilizing the high stability of the recorded gratings in Ag/TiO2 nanocomposite films, these analyses can be expediently performed after recording. With the help of a quarter-wave plate, the polarization state of the probe beam can be set to linear, elliptical and circular polarizations without changing its intensity. It was found that when the probe beam was linearly polarized, the 71 order diffraction efficiencies had almost the same values. When the probe-beam was right-hand circularly polarized, only the þ1 order diffracted beam, with the left-hand circularly polarization state, was visible; and vice versa. The dependence of diffraction efficiency on the intersection angle between the polarization direction of the readout beam and the fast axis of λ=4 plate is shown in Fig. 7. The stable and high-efficient storage of the polarization holographic grating provides possibilities for image storage. Fig. 8 shows the 71 order diffracted images from the reconstruction beam with three different polarization states: (a) RCP, (b) linear s polarization, and (c) LCP. Obviously, the þ1 order diffractive signal is the same as the object beam, while the 1 order diffractive signal is a conjugate image to the object beam. It was noticed that the brightness of the diffractive image was not uniform because the mask “F” was illuminated by the inhomogeneous green light. For a RCP irradiation, the þ1 order diffractive signal was brightest and the 1 order diffractive signal was invisible. For a LCP irradiation, the case was the opposite. Additionally, for a linearly polarized one, the brightness of 71 order diffracted images was redistributed equally. The results indicated that variation of the reconstruction polarization state can control the brightness of the diffracted images. The stored image can be erased by either linearly or circularly polarized beams of 532 nm. The rewritable process was verified to be repeated for more than ten times in the orthogonal circular polarization interference pattern. However, the reversible writingerasing process by single-beam could not be accomplished in other polarization interference patterns, such as (s–s), (p–p), (s–p), and (LCP–LCP), which all contain scalar grating components. Further investigation is in process. 4. Conclusions Linearly polarized beam induced photochromism and photoanisotropy in Ag/TiO2 nanocomposite films comes from the changes of morphology and size distribution of Ag NPs, which is assisted-proved by the TEM observation. A pure polarization holographic grating in the Ag/TiO2 films were obtained using two orthogonal circularly polarized green beams and reconstructed with a red beam. The hologram could be erased and rewritten for many times without obvious losses, which may be benefited from the reversible morphology change of Ag NPs, the reciprocating mobility of Ag þ ions and re-growth of Ag NPs under
the co-action of both the electric gradient force and silver ion concentration gradient force. The brightness of the image stored in the polarization hologram was easily controlled by variation of the reconstruction polarization state. The Ag/TiO2 nanocomposite film may be a candidate for the ideal optical memory medium.
Acknowledgements This work was supported by the National Natural Science Foundation of China (10974027, 61007006, 51072032 and 31271442), the National Basic Research Program (2012CB933700), the Program for New Century Excellent Talents in University (NECT-10-0320), the Fundamental Research Funds for the Central Universities (12SSXM001) and the 111 project (B13013).
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