Article pubs.acs.org/JPCC
Updatable Holographic Diffraction of Monolithic CarbazoleAzobenzene Compound in Poly(methyl methacrylate) Matrix Naoto Tsutsumi,*,†,‡ Kenji Kinashi,†,‡ Kanako Ogo,‡ Takahiro Fukami,‡ Yuuki Yabuhara,‡ Yutaka Kawabe,§ Kazuhiro Tada,§ Kodai Fukuzawa,§ Masuki Kawamoto,∥ Takafumi Sassa,∥ Takashi Fujihara,∥ Takeo Sasaki,⊥ and Yumiko Naka⊥ †
Faculty of Materials Science and Engineering, and ‡Department of Macromolecular Science & Engineering, Graduate School of Science and Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo, Kyoto 606-8585, Japan § Department of Bio- and Material Photonics, Chitose Institute of Science and Technology, Chitose, Hokkaido 066-8655, Japan ∥ Riken, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan ⊥ Department of Chemistry, Tokyo University of Science, Kagurazaka, Shinjyuku, Tokyo 162-8601, Japan S Supporting Information *
ABSTRACT: The comprehensive mechanism of updatable holographic diffraction is presented for a monolithic compound of 3-[(4-nitrophenyl)azo]-9H-carbazole-9-ethanol (NACzE) dispersed poly(methyl methacrylate) film device. The maximum sensitivity occurs at 561 nm, which well coincides with an isosbestic point between cis-trans isomers of NACzE molecule. The holographic grating is ascribed to the absorption grating and the following refractive index grating due to the photo-orientation of NACzE molecules. The response and decay times for the diffraction are governed by the glass transition temperature of the matrix.
photo-orientation have been studied.10,11 The photorefractive response for the monolithic compound consisted of carbazole and azobenzene moieties have been reported under an external electric field,15 in high Tg nonpoled polymer system,16 and without applying external electric field.17,18 Other types of electric field free photorefractive-like polymers have been reported.19−23 Furthermore, the fully functionalized photorefractive polymers and molecular glasses have also been investigated.24−30 Recently, we have successfully demonstrated an updatable hologram using a monolithic compound consisted of carbazole and azobenzene moieties in the polymer matrix.31,32 An updatable holographic stereogram consisted of 100 elemental holograms was stored in long time and quickly over-recorded with other elemental holograms.32 It is interesting that these holograms are quickly recorded and simultaneously reconstructed in a bright environment. However, the comprehensive mechanism has not been proposed although the monolithic compound has the high potential for holographic display device. In this report, we present and discuss the mechanism of the diffraction of a monolithic compound of 3-[(4-nitrophenyl)azo]-9H-carbazole-
1. INTRODUCTION Real time three-dimensional (3D) imaging using an updatable holographic display is one of the promised technologies for the next generation display system. Holography invented by Gabor1 is a unique technique to reconstruct the amplitude and the phase of the object in a space. Conventional holograms are permanently recorded in the media of silver halides, photopolymers, and dichromated gelatin2 and reconstructed with a coherent light such as a laser or incoherent LED light sources. However, these media lack the capability of imageupdating, resulting in the limitation of use. Using photorefractive polymers, updatable dynamic holographic imaging3−5 and 3D holographic display6,7 have successfully been demonstrated. A recent review article summarizes the photorefractive polymers for holographic applications.8 Photorefractive polymers require the application of a high electric field, which induces the risk of a dielectric breakdown. Whereas, without applying electric field, holographic diffraction has been reported using azobenzene chromophore in the liquid crystalline polymer.9 Since then photochromic azobenzene compounds have extensively been studied because of many interesting phenomena of the reversible photo-orientation and photoalignment as well as photoinduced motion of azobenzene moieties in the polymer and the liquid crystalline polymers:10−14 The effects of the polymer structure and glass transition temperature (Tg) on the © XXXX American Chemical Society
Received: May 21, 2015 Revised: July 8, 2015
A
DOI: 10.1021/acs.jpcc.5b04869 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 1. Series of pictures of hologram image of a relief object taken every 0.5 and 1.0 s.
9-ethanol (NACzE) in poly(methyl methacrylate) (PMMA) matrix.
Table 1 summarizes the diffraction efficiency (η) and the response time (τ) at various recording wavelengths with a low
2. EXPERIMENTAL SECTION NACzE and PMMA were dissolved in chloroform for 24 h and then cast on a glass plate. The cast film was dried at 40 °C for 24 h. The dried film was melt-pressed between two glass plates with spacers at 250 °C Ultraviolet and visible absorption spectra were recorded with a PerkinElmer Lambda 1050 UV/vis/NIR spectrophotometer. Optical anisotropy of NACzE/PMMA device was observed under crossed nicols using a polarizing microscope (Nikon Eclipse LV100N POL, Japan). Diffraction efficiency was measured using a nondegenerated four wave mixing (NFWM) with four different writing lasers at 632.8, 594, 561, and 532 nm, and probe (reading) laser at 632.8 nm. Recording (writing) lasers were He−Ne laser at 632.8 nm (10 mW, CVI Melles Griot, USA), diode-pumped solid-state (DPSS) lasers Mambo at 594 nm, Jive at 561 nm, and Samba at 532 nm (25 mW, Cobolt AB, Sweden). To increase the laser intensity per unit area, a single convex lens ( f = 200 mm) was inserted in each beam path before the sample. Probe (reading) laser was a He−Ne laser (10 mW, Lasos Lasertechnik GmbH, Germany) at 632.8 nm. Probe beam intensity was controlled using a neutral density filter. The intensity of the probe beam is weaker than that of the writing beam. The angle between two beams was fixed 15° in air.
Table 1. Dependence of Diffraction Efficiency, Response Time, and Sensitivity on Interference Wavelength low intensity, 0.2−0.6 W cm−2 wavelength (nm) 532 561 594 633
intensity (W cm−2)
η (%)
0.53 0.26 0.39 0.22 high intensity, 1.7−6.2
τ (s)
13 26 75 15 48 192 11 2010 W cm−2
S (cm2 J−1) 0.026 0.22 0.0093 0.00075
wavelength (nm)
intensity (W cm−2)
η (%)
τ (s)
S (cm2 J−1)
532 561 594 633
6.2 2.8 3.5 1.7
6 58 20 23
0.68 0.39 1.8 150
0.058 0.70 0.07 0.0019
intense interference beam between 0.2−0.6 W cm−2 and a high intense interference beam between 1.7−6.2 W cm−2. The resultant sensitivity (S) defined by
S=
η
(1) Iτ is also listed for each illumination wavelength at two kinds of different interference intensity in Table 1. I is an intensity per unit area of an illuminated laser. For both weak and strong intensities, maximum diffraction efficiency and sensitivity as well as minimum response time occurs at an illumination wavelength of 561 nm. As pointed out in our previous report,31 absorption from 550 nm extending to 700 nm plays an important role for the holographic gratings in NACzE/PMMA film device. To figure out the detail of the spectrum, absorption spectrum was analyzed using a principle component analysis (PCA). The details of PCA are shown in the Supporting Information. The spin coated film prepared was illuminated at 436 nm for 120 s under 5 mW cm−2. Spectrum recovery was monitored between 10 and 2700 s after illumination. The resultant change in spectrum is shown in Figure 2a. PCA for spectra in Figure 2a was carried out using Esumi multivariate statistics software working in Microsoft Excel, and two useful components were obtained. The reconstructed spectra of trans and cis isomers using useful two components are shown in Figure 2b. Trans and cis isomers of NACzE are shown in Scheme 1. Spectra with peaks at 430 and 470 nm are ascribed to the spectrum due to trans and cis isomers, respectively. Isosbestic point between trans and cis isomers was appeared at 562 nm. The molecular orbital calculation can simulate the absorption spectrum of trans and cis isomers. Figure 3 shows the absorption spectra of trans and cis isomers for NACzE
3. RESULTS AND DISCUSSION The hologram image of a relief object was recorded with the interference of linearly polarized beams at 532 nm and simultaneously reconstructed using a probe beam at 594 nm. The series of the pictures of hologram images of a relief object taken every 0.5 and 1.0 s are shown in Figure 1. Within 0.5 s (500 ms), a hologram was appeared in the NACzE/PMMA film device. The hologram image was clearly reconstructed within 1 s. The related results have already been reported in our previous paper,22 in which the hologram was reconstructed within a couple of hundreds millisecond in NACzE/PMMA film device. Updatable hologram was observed in the matrices of PMMA, polyethyl methacrylate (PEMA), and poly(methyl acrylate) (PMA). Why does a NACzE give the excellent holographic ability of quick recording and reconstructing of hologram image within 1 s? Parallel study under crossed nicols shows that the birefringence of the sample device has been appeared within 1s and the degree of birefringence increases with time. The hologram image recorded with the interference beam of circularly polarized beams (clockwise and counterclockwise) was reproduced by a clockwise circularly polarized beam. It means that the diffraction is ascribed to the refractive index modulation induced by the interference gratings. B
DOI: 10.1021/acs.jpcc.5b04869 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Isosbestic point in the absorption spectra coincides well with the wavelength where the maximum diffraction efficiency, the sensitivity, and the maximum response speed (inverse of response time) are measured. At an isosbestic point, cis-trans cycling are most activated. Fast cycling of cis-trans at 562 nm is followed by the enhanced photo-orientation of NACzE in matrix, which produces the refractive index gratings. As shown in Table 1, the increase of laser intensity per unit area significantly accelerates the holographic response speed. The resulting sensitivity is enhanced by the higher laser illumination. Sensitivity is 0.7 cm2 J−1 at 561 nm at an interference intensity of 2.8 W cm−2, whereas it is 0.22 cm2 J−1 at 0.26 W cm−2. Usually, sensitivity is a kind of normalized value, and it will not be varied when the intensity is changed. In the present case, the acceleration occurred when the illumination intensity is increased. For other wavelength, similar acceleration is observed. One explanation for the enhancement of sensitivity is ascribed to the local heating by the illumination of higher laser intensity. That is, the local heating significantly accelerates the photo-orientation of NACzE in the matrix. A separate study for the surface temperature measurement by a thermography camera (FSV1200, APISTE) gave the increment of the surface temperature, a few degrees, of the sample film by 532 nm laser illumination at 1.3 W cm−2 for a couple of seconds. Namely the temperature of the matrix is temporally increased by a local heating in terms of the illumination of higher laser intensity, which assists the molecular motion of NACzE in the matrix. Local heating on laser illumination assists the fast recording within 1 s, and during the following decaying period the surrounding and device environment are kept at ambient condition. It guarantees the prolonged memory of the recorded holographic images. In the process of over recording hologram image, the local heating assists the simultaneous events of erasing previous image and recording new image. However, more detailed studies for the sensitivity change by the local heating should be needed and will be reported in future. Diffraction responses were measured in PMMA, PEMA, and PMA matrices and obtained diffraction efficiency, response time, and decay time are listed in Table 2. The decay time was
Figure 2. (a) Spectrum change immediately after the illumination off and following prolonged time. (b) Reconstructed spectra of trans and cis isomers of NACzE using PCA method.
Scheme 1. trans and cis Isomers of NACzE
Table 2. Diffraction Efficiency, Response and Decay Times, and Tg in the Matrices of PMMA, PEMA, and PMA matrix
η (%)
τ (s)/β
τdecay (s)/β
Tg (°C)
PMMA PEMA PMA
13.4 8.9 2.4
9.7/0.68 3.7/0.72 0.39/1.0
82.4/0.36 15.2/0.36 2.3/0.34
72.1 47.9 13.1
measured after the illumination was turned off. To estimate a response and decay times, we fitted the time trace of rising and decay for the optical diffraction using the stretched exponential function of Kohlrausch−Williams−Watts (KWW). The decay time is about ten times longer than the response time. The longer decay time supports the prolonged memory of the present device. The glass transition temperature (Tg) of the matrix measured by DSC is also listed in Table 2. Fast recording and fast decaying hologram image are apparently measured in the matrix with a lower glass transition temperature. Furthermore, a diffraction response in PMMA was measured at four different temperatures of 23, 40, 50, and 60 °C. The resultant diffraction efficiency, response time, and
Figure 3. Absorption spectra of trans and cis isomers for NACzE simulated by a molecular orbital calculation (B3LYP/6-31G(d)).
simulated by a quantum chemistry calculation (B3LYP/631G(d)). Quantum chemistry calculation was performed with the Gaussian 09 program using a B3LYP hybrid density function with a 6-31G(d) basis set. The absorption spectrum of the cis isomer has a peak at 500 nm and that of the trans isomer at 440 nm, and an isosbestic point between trans and cis isomers appeared around at wavelength of 562 nm. C
DOI: 10.1021/acs.jpcc.5b04869 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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η (%)
τ (s)/β
τdecay (s)/β
23 40 50 60
13.4 9.3 6.6 3.4
9.7/0.68 3.6/0.45 2.4/0.50 0.64/0.30
82.4/0.36 13.9/0.38 5.1/0.40 1.9/0.50
In order to compare the diffraction response and relaxation (decay) in the matrices with different glass transition temperatures, the scaling relation of ⎛T − T ⎞ ⎟ τ(T ) = A exp⎜ 0 ⎝ T ⎠
(2)
is given where A is a constant, T0 is the glass transition temperature of matrices (here T0 ≈ Tg), and T is the measurement temperature. Scaling plots are related to the Williams−Ladel−Ferry (WLF) and the Vogel−Tamman− Fultcher (VTF) expressions frequently used to describe in the field of viscoelastic properties of the polymers above Tg. Logarithmic response time (τ) and decay time (τdacay) are plotted as a function of (T0 − T)/T in Figure 4. All results are
Figure 5. Reading laser intensity trace as a function of measurement time in NFWM measurement at 594 nm. Reading probe beam is He− Ne laser at 633 nm. Polarization of recording beam and reading beam is p-polarized, and the diffracted and transmitted reading beam is monitored through p-polarizer (a) and s-polarizer (b).
as a function of measurement time in NFWM measurement at 594 nm. He−Ne laser at 632.8 nm was used as a reading beam. Polarization of a recording beam and a reading beam is ppolarized, and the diffracted and the transmitted reading beams are monitored through p-polarizer (a) and s-polarizer (b). As shown in Figure 5a, within the initial 50 s, significant increase of diffracted beam and the corresponding decrease of the transmitted reading beam to reach 50% diffraction efficiency at 50 s. Beyond 50 s, both beam intensities decrease and level off at the time above 800 s. These decreases of the intensity in p-polarized diffracted and transmitted reading beams suggest the population increase of the perpendicularly photo-oriented azobenzene chromophore. Indeed, as shown in Figure 5b, the diffracted and the transmitted reading beams through spolarizer increase after 50 s and almost level out at the time above 800 s. These results indicate that the significant diffraction has already completed within 50 s, which follows the apparent perpendicular photo-orientation of NACzE at ppolarized recording beams. These results imply that the prolonged polarized laser illumination induced dichroism in NACzE/PMMA device. In the previous paper,34 we have reported that the illumination of a circularly polarized beam into the azobenzene polymer induced the chiral structures.
Figure 4. Logarithmic plots of response and decay times as a function of (T0 − T)/T. (a) Matrix difference and (b) temperature difference.
4. CONCLUSIONS We investigated the holographic diffraction mechanism in the NACzE dispersed PMMA matrix. The diffraction efficiency, the response rate (the inverse of the response time) and the sensitivity have significantly depended on the wavelength of the interference beams. The maximum performance was measured at 561 nm, which corresponds well to the isosbestic point between cis and trans isomers of NACzE. The absorption
well fitted by eq 2. The difference of the molecular motion of NACzE in the matrices significantly relates to the formation and the erasing of the holographic gratings. It is well-known that an azobenzene molecule is photooriented by a UV or a visible light.33 Conventional photoorientation of azobenzene occurs through trans−cis−trans isomerization. Figure 5 shows the reading laser intensity trace D
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grating followed by the refractive index grating due to the photo-orientation assisted by the fast cycling between cis and trans isomers are the main role for the diffraction of the present system. The present system can be used for the holographic digital signage.
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ASSOCIATED CONTENT
* Supporting Information S
Details of PCA. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.jpcc.5b04869.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Fax: +81 75 724 7805. Tel: +81 75 724 7810. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS N.T. acknowledges the program for Strategic Promotion of Innovative Research and Development (S-Innovation), Japan Science and Technology Agency (JST) for the financial support.
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REFERENCES
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