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Photochemical modulation of alignment of liquid crystals and photonic applications
Current Opinion in Solid State and Materials Science 6 (2002) 563–568
Photochemical modulation of alignment of liquid crystals and photonic applications Osamu Tsutsumi, Tomiki Ikeda* Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226 -8503, Japan Received 20 February 2003; received in revised form 24 February 2003; accepted 24 February 2003
Abstract The recent highlights in the photoinduced modulation of the liquid-crystalline (LC) alignment and their applications to photonics are reviewed. The molecular alignment of LCs can be controlled by either the photochemical process or the photophysical process and we can change the birefringence as well as refractive index of the material by the modulation of the LC alignment. The photoresponsive LCs show very quick response to stimulus light: the response time is |200 ns. Furthermore, the extremely large refractive index modulation (Dn5|10 21 ) can be also induced. The photoresponsive LCs, therefore, are potential materials for holographic recording and other photonic device. 2003 Elsevier Science Ltd. All rights reserved.
1. Introduction Liquid crystals (LCs) are suitable for photonic materials, because they exhibit a large optical anisotropy and one can control this anisotropy by changing the alignment of LC molecules with external fields applied as a stimulus. LCs, therefore, are well-known materials in electro-optic devices. For example, when an electric field is applied to the LCs placed between a pair of electrodes, the direction of the alignment of the LC molecules is changed, and the intensity of the light transmitted through two crossed polarizers, with the LC cell between them, is altered concomitantly by the change in the molecular alignment. This is the working principle of liquid crystal displays. When the alignment of the LC molecules is changed by the stimulating light, the intensity of the light carrying the optical information is also modulated by the stimulating irradiation. In other words, LCs act as an optical switch driven by light. An LC Spatial Light Modulator (SLM, usually called an ‘LC light valve’) based on a photoconductive cell is one such device, and it is now in use. The LC SLM is driven with an electric field in response to a stimulus of light. *Corresponding author. Tel.: 181-45-924-5240; fax: 181-45-9245275. E-mail address: [email protected] (T. Ikeda).
Linearly polarized light can induce the change in alignment of the LCs with photochromic molecules without application of an electric field [1]. For example, the photoinduced alignment of the photochromic azobenzenes takes place by irradiation with linearly polarized light. In LC phases, isomerization of azobenzenes affects the alignment of mesogens; the photoinduced alignment of the azobenzenes causes realignment of mesogens. Consequently, the direction of the LC alignment can be manipulated with the linearly polarized light. In this process, molecules remain in the LC phase while they change the direction of the alignment (order–order change). With the photochemically controlled LC alignment, one can control the refractive index and birefringence of the materials. Thus, this phenomenon has also attracted great interest from a practical viewpoint. The other approach to manipulate the LC alignment by light is based on the phenomenon known as photochemical phase transition [2–5]. When photochromic compounds are irradiated in LCs to cause the photochemical reaction, the LC-I phase transition is induced photochemically (order– disorder change). This process is reversible: when the back-reaction of the photochromic molecules is induced either photochemically or thermally, the initial LC phase is restored. Since LCs are self-organizing materials, the birefringence and refractive index of LCs controlled by light is
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larger than that of amorphous polymers: they are |10 21 in the LCs, while they are 10 23 |10 22 in the amorphous polymers and inorganic materials. Thus, LCs are advantageous for photonic materials [6]. In this review, we will present some of the recent highlights in the photoinduced modulation of the LC alignment in both modes and their applications to photonics.
2. Photochemically induced order–disorder change in alignment of LCs Photoinduced nematic (N)-to-I phase transition of LCs can be induced by photoisomerization of photochromic compounds. The working principle of these systems is an isothermal phase transition of LCs triggered by photochemical reactions of the photochromic molecules. For example, azobenzene derivatives stabilize the N phase in the trans form since their molecular shape (rod-like shape) is similar to that of LC molecules, while the cis isomers destabilize the LC phase because their molecular shape is bent. In other words, trans–cis photoisomerization of azobenzene derivatives in an LC phase disorganizes the molecular alignment and causes the photochemical phase transition [7*,8*]. Linearly polarized light at 633 nm can be transmitted through a pair of crossed polarizers, with the azobenzene film between them, because of birefringence of the azobenzene LCs. Thus, we can observe the dynamics of the
Fig. 1. Dynamics of the photochemical phase transition of an azobenzene LC induced by irradiation with a laser pulse at 355 nm (25 ps, fwhm). The molecular structure of the azobenzene LC used in this experiment is indicated at the top of the figure.
photochemical phase transition. When azobenzene LCs, in which the azobenzene moiety acts as both a mesogen and a photoresponsive chromophore, are irradiated with a laser pulse at 355 nm in the N phase, the transmittance of the probe light decays immediately. When photoirradiation is ceased, the transmittance of the probe light recovers. Since the mesogenic trans-azobenzene restores thermally, the initial N phase recovers when the irradiated film is kept in the dark. Thus, the photochemical phase transition is induced repeatedly. Our recent research has revealed that the photochemical phase transition of azobenzene LCs occurs on the nanosecond timescale (|200 ns) by irradiation with a single shot of a laser pulse (25 ps, fwhm) (Fig. 1). This response is faster by six orders of magnitude than the response of typical LCs to an electric field.
3. Order–order alignment change in polymer LCs with azo chromophores The generation of anisotropy induced by linearly polarized light in polymeric systems containing photochromic dyes has attracted increasing attention due to the potential practical applications such as waveguides and optical storage. Photoinduced in-plane (two-dimensional) reorientation of polymer LCs containing azobenzene groups by linearly polarized light has been extensively studied. The accepted mechanism involves photochemically induced trans–cis–trans isomerization cycles of the azobenzene moieties. The transition moment of transazobenzenes is parallel to the molecular long axis. With polarized light, the azobenzenes that are aligned parallel to the polarization direction of the light have the highest isomerization probability. On the other hand, the azo dye is inert to excitation by polarized light if its alignment is perpendicular to the polarization direction. The thermal cis–trans back-isomerization is spontaneous, and the resulting trans-azobenzenes may fall in any direction. Azobenzenes with any transition component parallel to the polarization direction of the light continuously repeat this trans–cis–trans isomerization cycle. However, the azobenzenes that are perpendicular to the polarization direction of the light are not excited again and remain in this direction. Finally, the materials reach the state with an excess of azobenzenes aligned perpendicular to the polarization direction of the light and this results in reorientation of inert mesogens together with azobenzene moieties due to the cooperative effect [9,10,11*,12*]. When the azobenzene moieties are aligned with the molecular long axis along the propagation direction of irradiation light, the photoisomerization does hardly take place, since the propagation direction of light is always perpendicular to its electric field vector. With unpolarized light, only the propagation direction is perpendicular to the electric field vector. Thus, in principle, we may align
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azobenzene moieties in the propagation direction of irradiation light and manipulate the polymer LCs threedimensionally (3-D) by changing the direction of the incident light [13**]. When the sample film was irradiated with unpolarized light at 436 nm, the transmittance of the probe light through crossed polarizers increased rapidly with time until a saturation value was reached. This means that alignment change was induced in the polymer LC even on irradiation of the unpolarized light at each incident angle. We observed the change in transmittance through aligned films as a function of the probe angle. At the incident angle of 1358 for the irradiation light, the profile was asymmetric. The maximum transmittance appeared at the probe angle of 458, at which the probe light was perpendicular to the irradiation light; the minimum transmittance appeared at the probe angle of around 1358, at which the probe light was parallel to the irradiation light. When the incident angle was changed to 458, an opposite tendency was observed. When the incident angle was 908, a symmetric profile in the transmittance was observed, and the minimum transmittance appeared at the probe angle of 908. These results strongly imply that the azobenzene
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Fig. 3. Plausible alignment of the LC molecules induced by unpolarized light irradiation.
moieties are aligned in the propagation direction of the irradiation light. The results were further confirmed by conoscopic observation (Fig. 2) in which films with 1-mm thickness were used to obtain clear images. The cross-point, which represents the optic axis of the LC phase, appeared on irradiation, and changed its position according to the incident angle of the irradiation light. These results indicate not only that alignment change is generated in the polymer film by use of the unpolarized light but also that the alignment direction is controlled by the propagation direction of the irradiation (Fig. 3). The azobenzene moieties are well aligned in the propagation direction of the irradiation light. The induced alignment was very stable at room temperature and remained unchanged even after .5 months. The induced alignment was also stable on further irradiation of 436-nm unpolarized light at the same incident angle, unlike the results reported in the literature in which the tilt angle was controlled by the amount of exposure energy [14,15]. Erasure of the induced alignment was realized easily by irradiation of unpolarized light at 366 nm due to the occurrence of photochemical phase transition, and the alignment was induced again on irradiation of the unpolarized light at 436 nm.
4. Photophysically induced order–order alignment change
Fig. 2. Conoscopic observation of the alignment of the azobenzene mesogens as a function of the incident angle. The incident angle, u, is indicated in the figure and the molecular structure of the polymer azobenzene LC used in this experiment is indicated at the top of the figure.
We also investigated the control of the LC alignment by photophysical processes of p-conjugated dye molecules. On application of an electric field, the molecular alignment of LCs is changed due to the dielectric anisotropy of the molecules (Freedericksz transition). The electric field of light can also induce the change in the realignment of the LCs (Optical Freedericksz Transition: OFT) [16]. Recently, it became clear that the threshold light intensity of the OFT is decreased by the addition of a small amount of an organic dye to the LCs. We have developed novel LCs containing an oligothiophene moiety as a new photosensitive LC chromophore
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[17*,18*,19**,20**]. By irradiation of a mixture of the thiophene dye and a non-photoresponsive host LC, realignment of the LC molecules was induced and self-diffraction patterns appeared when the light intensity was above the OFT threshold (Fig. 4). Fig. 5a shows the number of self-diffraction rings as a function of light intensity observed for undoped 4-cyano-49-pentylbiphenyl (5CB) in the self-diffraction measurements. The threshold intensities for the appearance of diffraction rings were about 840 W/ cm 2 for 5CB. On the other hand, in 0.22 mol% TR5doped 5CB, the OFT thresholds were only 6.2 W/ cm 2 (Fig. 5b). That is, at the dye concentration of 0.22 mol%,
Fig. 5. The number of diffraction rings due to optical Freedericksz transition as a function of irradiation intensity observed in the homeotropic cell for the non-doped 5CB (a) and the TR5-doped 5CB (b).
Fig. 4. Structure of the novel photoresponsive LC developed in this study (top), typical diffraction pattern observed in the dye-doped LC (middle), and plausible alignment of LC molecules (bottom).
Fig. 6. Holographic recording by means of a polymer azobenzene LC.
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TR5 can decrease the threshold intensities about 135 times for 5CB. Such large values are among the most effective dye-induced realignment effects disclosed so far. In order to confirm the reorientation direction, Z-scan measurements were performed. The principle of the Z-scan method is that the magnitude of non-linear effects in LCs depends on the light intensity. Therefore, when a Gaussion beam is used as the pumping light to induce the photoinduced realignment, the degree of realignment depends on the intensity distribution of the light and the realignment results in a Gaussion distribution in the refractive index within the irradiation region. When the director realignment increases the refractive index, the LC sample can be regarded as a convex lens, and it modifies the light intensity through the aperture. When the realignment decreases the refractive index, however, the LC sample acts as a diverging lens and modifies the incident light in a different way. In the Z-scan curve observed for TR5-doped 5CB, a typical self-focusing phenomenon was observed. That is, a minimum was observed at a sample position between the convex lens and the focal point; a maximum occurred at a position behind the focal point. The Z-scan result for TR5-doped 5CB indicates a tendency to increase the refractive index upon irradiation. It is known that the refractive index of 5CB increases from n o to n e . Therefore, the observation of the self-focusing phenomenon in TR5doped 5CB indicates that LC molecules tend to reorient parallel to light polarization upon irradiation.
5. Photonic application of the photoinduced alignment change
5.1. Rewritable high-density holographic recording by means of polymer LCs Recently, we have developed holographic materials by means of polymer azobenzene LCs [21**,22,23,24**,25**,26*,27*,28*]. On irradiation of a couple of coherent light beams, a real-time hologram can be created by periodic induction of a photochemical phase transition in polymer azobenzene LCs. The gratings are formed within 200 ns by laser pulse irradiation (355 nm; 25 ps, fwhm). The holographic diffraction in the LC phase was observed repeatedly by turning on and off the writing beams. It is worth mentioning that the magnitude of the modulated refractive index reached the order of 10 21 . Observation of the grating structure in glassy ordered films with a polarizing microscope confirmed that the isotropic phase was arranged with a spatial resolution of ,1 mm. The gratings recorded in the ordered samples with a thickness of about 500 nm showed a high diffraction efficiency of 32% (nearly the theoretical maximum of the diffraction efficiency in those thin films (Raman–Nath regime)). We may expect that a rewritable high-density
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3-dimensional image storage is possible with the polymer azobenzene LCs (Fig. 6).
5.2. Modulation of intensity Photochemical control of the optical properties of polymer / LC composite films was performed by means of the photochemical phase transition induced by the trans– cis photoisomerization of azobenzenes. Reversible photoisomerization of the azobenzenes took place in the polymer matrix environments and the consequent optical effects were induced as a change in transmittance of the composite films. Repeatable changes in transmittance between the two different optical states can be induced efficiently by alternative irradiation with UV and visible lights. Furthermore, the composite films with donor–acceptor azobenzenes showed a rapid thermal recovery to the initial state. When polymers with a long alkyl side chain were used as the matrix, the memory effect was observed. Those materials, therefore, are useful for all-optical switching materials and optical storage materials [29**].
5.3. Modulation of the wavelength of selective reflection Cholesteric LCs (CLCs) show a selective reflection due to the helical structure. The reflection wavelength (helical pitch) of CLCs is very sensitive to perturbations: temperature, pressure, impurities, and so on. When we put an azobenzene as a photosensitive chromophore in CLCs, we can change the reflection wavelength of the CLCs by light. We found that the reflection wavelength (color of the materials) was shifted to shorter wavelength by irradiation with UV light, which induces the trans–cis photoisomerization of the azobenzene [30**]. On irradiation with visible light to cause the cis–trans back-isomerization of the azobenzene, the reflection band was restored to the initial wavelength. By optimizing the azobenzene structure, we can control the reflection wavelength from near-IR to blue region.
6. Conclusion The molecular alignment of LCs can be modulated by either the photochemical process or the photophysical process. By the modulation of the LC alignment, we can change the birefringence as well as refractive index of the material. The photoinduced refractive index modulation is extremely large: it is more than 10 21 . Thus, the photoresponsive LCs are suitable to the materials for photonics. In this review, we present several examples of the application to the photonic materials. Since the photoresponsive LCs also show very quick response to stimulus light, those materials can be used for other photonic devices such as optical switches, dynamic holography, and so on.
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References Papers of particular interest, published within the annual period of review, have been highlighted as: * of special interest; ** of outstanding interest. [1] Natansohn A, Rochon P. Photoinduced motions in azo-containing polymers. Chem Rev 2002;102:4139–75. [2] Ikeda T, Kanazawa A. Photochemical modulation of alignment in liquid crystals. Bull Chem Soc Jpn 2000;73:1715–33. [3] Ikeda T, Tsutsumi O. Optical switching and image storage by means of azobenzene liquid-crystal films. Science 1995;268:1873–5. [4] Tsutsumi T, Shiono T, Ikeda T, Galli G. Photochemical phase transition behavior of nematic liquid crystals with azobenzene moieties as both mesogens and photosensitive chromophores. J Phys Chem B 1997;101:1332–7. [5] Shishido A, Tsutsumi O, Kanazawa A, Shiono T, Ikeda T, Tamai N. Rapid optical switching by means of photoinduced change in refractive index of azobenzene liquid crystals detected by reflectionmode analysis. J Am Chem Soc 1997;119:7791–6. [6] Ikeda T. Photochemical modulation of refractive index by means of photosensitive liquid crystals. Mol Cryst Liq Cryst 2001;364:187– 97. [*7] Tsutsumi O, Ikeda T. Rapid photochemical control of phase structure of polymer liquid crystals with hydroxyazobenzene as a photosensitive chromophore. Mol Cryst Liq Cryst 2001;368:411–22. [*8] Sung JH, Hirano S, Tsutsumi O, Kanazawa A, Shiono T, Ikeda T. Dynamics of photochemical phase transition of guest / host liquid crystals with an azobenzene derivative as a photosensitive chromophore. Chem Mater 2002;14:385–91. [9] Wu Y, Mamiya J, Kanazawa A, Shiono T, Ikeda T, Zhang Q. Photoinduced alignment of polymer liquid crystals containing azobenzene moieties in the side chain. 6. Biaxiality and threedimensional reorientation. Macromolecules 1999;32:8829–35. [10] Wu Y, Zhang Q, Kanazawa A, Shiono T, Ikeda T, Nagase Y. Photoinduced alignment of polymer liquid crystals containing azobenzene moieties in the side chain. 5. Effect of the azo contents on alignment behavior and enhanced response. Macromolecules 1999;32:3951–6. [*11] Wu Y, Mamiya J, Tsutsumi O, Kanazawa A, Shiono T, Ikeda T. Photoinduced alignment of polymer liquid crystals containing azobenzene moieties in the side chain. 7. On irradiation by a He–Ne laser beam. Liq Cryst 2000;27:749–53. [*12] Ikeda T, Tsutsumi O, Wu Y. Optical switching and image storage by means of photochromic liquid crystals. Mol Cryst Liq Cryst 2000;347:1–13. [**13] Wu Y, Zhang Q, Ikeda T. Three-dimensional manipulation of azo polymer liquid crystal by unpolarized light. Adv Mater 1999;11:300–2. [14] Kawanishi Y, Tamaki T, Ichimura K. Photoregulation of liquidcrystalline orientation by anisotropic photochromism of surface azobenzenes. ACS Symp Ser 1994;537:453–65. [15] Buechel M, Weichart B, Minx C, Menzel H, Johannsmann D. Role of interfacial entropy in the command-surface effect. Phys Rev E 1997;55:455–63.
´ [16] Janossy I. Molecular interpretation of the absorption-induced optical reorientation of nematic liquid crystals. Phys Rev E 1994;49:2957– 63. [*17] Zhang H, Shiino S, Kanazawa A, Tsutsumi O, Shiono T, Ikeda T, Nagase Y. X-ray measurements of an oligothiophene containing acetylene linkage. Synth Met 2002;126:11–8. [*18] Zhang H, Shiino S, Tsutsumi O, Kanazawa A, Shiono T, Ikeda T. Photoinduced reorientation of liquid crystals doped with a mesogenic oligothiophene. Mol Cryst Liq Cryst 2001;368:369–76. [**19] Zhang H, Shiino S, Shishido A, Kanazawa A, Tsutsumi O, Shiono T, Ikeda T. A thiophene liquid crystal as a novel p-conjugated dye for photo-manipulation of molecular alignment. Adv Mater 2000;12:1336–9. [**20] Zhang H, Shiino S, Kanazawa A, Tsutsumi O, Shiono T, Ikeda T. Photoinduced reorientation and thermal effects in an oligothiophenedoped liquid crystal system. J Appl Phys 2002;91:5558–63. [**21] Yoneyama S, Yamamoto T, Tsutsumi O, Kanazawa A, Shiono T, Ikeda T. High-performance material for holographic gratings by means of a photoresponsive polymer liquid crystal containing a tolane moiety with high birefringence. Macromolecules 2002;35:8751–8. [22] Ikeda T, Yoneyama S, Yamamoto T, Hasegawa M. Refractive-index modulation by means of photosensitive liquid crystals. Mol Cryst Liq Cryst 2002;375:45–60. [23] Ikeda T, Yoneyama S, Yamamoto T, Hasegawa M. Holographic grating by means of polymer liquid crystals. J Inform Disp 2001;2:6–12. [**24] Yamamoto T, Ohashi A, Yoneyama S, Hasegawa M, Tsutsumi O, Kanazawa A, Shiono T, Ikeda T. Phase-type gratings formed by photochemical phase transition of polymer azobenzene liquid crystals: 2. Rapid switching of diffraction beams in thin films. J Phys Chem B 2001;105:2308–31. [**25] Yoneyama S, Yamamoto T, Hasegawa M, Tsutsumi O, Kanazawa A, Shiono T, Ikeda T. Formation of intensity grating in polymer liquid crystals with a side-chain azobenzene moiety by photoinduced alignment change of mesogens. J Mater Chem 2001;11:3008–13. [*26] Yoneyama S, Tsutsumi O, Kanazawa A, Shiono T, Ikeda T. Formation of grating by means of photoinduced alignment change of polymer liquid crystals with azobenzene moieties. Mol Cryst Liq Cryst 2001;368:335–43. [*27] Yamamoto T, Hasagawa M, Kanazawa A, Shiono T, Ikeda T. Holographic gratings and holographic image storage by photochemical phase transition of polymer azobenzene liquid-crystal films. J Mater Chem 2000;10:337–42. [*28] Yamamoto T, Yoneyama S, Tsutsumi O, Kanazawa A, Shiono T, Ikeda T. Holographic gratings in the optically isotropic state of polymer azobenzene liquid-crystal films. J Appl Phys 2000;88:2215–20. [**29] Lee HK, Doi K, Kanazawa A, Shiono T, Ikeda T, Fujisawa T, Aizawa M, Lee B. Light-scattering-mode optical switching and image storage in polymer / liquid crystal composite films by means of photochemical phase transition. Polymer 2000;41:1757–63. [**30] Lee HK, Doi K, Harada H, Tsutsumi O, Kanazawa A, Shiono T, Ikeda T. Photochemical modulation of color and transmittance in chiral nematic liquid crystals containing an azobenzene as a photosensitive chromophore. J Phys Chem B 2000;104:7023–8.
Photochemical modulation of alignment of liquid crystals and photonic applications Osamu Tsutsumi, Tomiki Ikeda* Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226 -8503, Japan Received 20 February 2003; received in revised form 24 February 2003; accepted 24 February 2003
Abstract The recent highlights in the photoinduced modulation of the liquid-crystalline (LC) alignment and their applications to photonics are reviewed. The molecular alignment of LCs can be controlled by either the photochemical process or the photophysical process and we can change the birefringence as well as refractive index of the material by the modulation of the LC alignment. The photoresponsive LCs show very quick response to stimulus light: the response time is |200 ns. Furthermore, the extremely large refractive index modulation (Dn5|10 21 ) can be also induced. The photoresponsive LCs, therefore, are potential materials for holographic recording and other photonic device. 2003 Elsevier Science Ltd. All rights reserved.
1. Introduction Liquid crystals (LCs) are suitable for photonic materials, because they exhibit a large optical anisotropy and one can control this anisotropy by changing the alignment of LC molecules with external fields applied as a stimulus. LCs, therefore, are well-known materials in electro-optic devices. For example, when an electric field is applied to the LCs placed between a pair of electrodes, the direction of the alignment of the LC molecules is changed, and the intensity of the light transmitted through two crossed polarizers, with the LC cell between them, is altered concomitantly by the change in the molecular alignment. This is the working principle of liquid crystal displays. When the alignment of the LC molecules is changed by the stimulating light, the intensity of the light carrying the optical information is also modulated by the stimulating irradiation. In other words, LCs act as an optical switch driven by light. An LC Spatial Light Modulator (SLM, usually called an ‘LC light valve’) based on a photoconductive cell is one such device, and it is now in use. The LC SLM is driven with an electric field in response to a stimulus of light. *Corresponding author. Tel.: 181-45-924-5240; fax: 181-45-9245275. E-mail address: [email protected] (T. Ikeda).
Linearly polarized light can induce the change in alignment of the LCs with photochromic molecules without application of an electric field [1]. For example, the photoinduced alignment of the photochromic azobenzenes takes place by irradiation with linearly polarized light. In LC phases, isomerization of azobenzenes affects the alignment of mesogens; the photoinduced alignment of the azobenzenes causes realignment of mesogens. Consequently, the direction of the LC alignment can be manipulated with the linearly polarized light. In this process, molecules remain in the LC phase while they change the direction of the alignment (order–order change). With the photochemically controlled LC alignment, one can control the refractive index and birefringence of the materials. Thus, this phenomenon has also attracted great interest from a practical viewpoint. The other approach to manipulate the LC alignment by light is based on the phenomenon known as photochemical phase transition [2–5]. When photochromic compounds are irradiated in LCs to cause the photochemical reaction, the LC-I phase transition is induced photochemically (order– disorder change). This process is reversible: when the back-reaction of the photochromic molecules is induced either photochemically or thermally, the initial LC phase is restored. Since LCs are self-organizing materials, the birefringence and refractive index of LCs controlled by light is
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larger than that of amorphous polymers: they are |10 21 in the LCs, while they are 10 23 |10 22 in the amorphous polymers and inorganic materials. Thus, LCs are advantageous for photonic materials [6]. In this review, we will present some of the recent highlights in the photoinduced modulation of the LC alignment in both modes and their applications to photonics.
2. Photochemically induced order–disorder change in alignment of LCs Photoinduced nematic (N)-to-I phase transition of LCs can be induced by photoisomerization of photochromic compounds. The working principle of these systems is an isothermal phase transition of LCs triggered by photochemical reactions of the photochromic molecules. For example, azobenzene derivatives stabilize the N phase in the trans form since their molecular shape (rod-like shape) is similar to that of LC molecules, while the cis isomers destabilize the LC phase because their molecular shape is bent. In other words, trans–cis photoisomerization of azobenzene derivatives in an LC phase disorganizes the molecular alignment and causes the photochemical phase transition [7*,8*]. Linearly polarized light at 633 nm can be transmitted through a pair of crossed polarizers, with the azobenzene film between them, because of birefringence of the azobenzene LCs. Thus, we can observe the dynamics of the
Fig. 1. Dynamics of the photochemical phase transition of an azobenzene LC induced by irradiation with a laser pulse at 355 nm (25 ps, fwhm). The molecular structure of the azobenzene LC used in this experiment is indicated at the top of the figure.
photochemical phase transition. When azobenzene LCs, in which the azobenzene moiety acts as both a mesogen and a photoresponsive chromophore, are irradiated with a laser pulse at 355 nm in the N phase, the transmittance of the probe light decays immediately. When photoirradiation is ceased, the transmittance of the probe light recovers. Since the mesogenic trans-azobenzene restores thermally, the initial N phase recovers when the irradiated film is kept in the dark. Thus, the photochemical phase transition is induced repeatedly. Our recent research has revealed that the photochemical phase transition of azobenzene LCs occurs on the nanosecond timescale (|200 ns) by irradiation with a single shot of a laser pulse (25 ps, fwhm) (Fig. 1). This response is faster by six orders of magnitude than the response of typical LCs to an electric field.
3. Order–order alignment change in polymer LCs with azo chromophores The generation of anisotropy induced by linearly polarized light in polymeric systems containing photochromic dyes has attracted increasing attention due to the potential practical applications such as waveguides and optical storage. Photoinduced in-plane (two-dimensional) reorientation of polymer LCs containing azobenzene groups by linearly polarized light has been extensively studied. The accepted mechanism involves photochemically induced trans–cis–trans isomerization cycles of the azobenzene moieties. The transition moment of transazobenzenes is parallel to the molecular long axis. With polarized light, the azobenzenes that are aligned parallel to the polarization direction of the light have the highest isomerization probability. On the other hand, the azo dye is inert to excitation by polarized light if its alignment is perpendicular to the polarization direction. The thermal cis–trans back-isomerization is spontaneous, and the resulting trans-azobenzenes may fall in any direction. Azobenzenes with any transition component parallel to the polarization direction of the light continuously repeat this trans–cis–trans isomerization cycle. However, the azobenzenes that are perpendicular to the polarization direction of the light are not excited again and remain in this direction. Finally, the materials reach the state with an excess of azobenzenes aligned perpendicular to the polarization direction of the light and this results in reorientation of inert mesogens together with azobenzene moieties due to the cooperative effect [9,10,11*,12*]. When the azobenzene moieties are aligned with the molecular long axis along the propagation direction of irradiation light, the photoisomerization does hardly take place, since the propagation direction of light is always perpendicular to its electric field vector. With unpolarized light, only the propagation direction is perpendicular to the electric field vector. Thus, in principle, we may align
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azobenzene moieties in the propagation direction of irradiation light and manipulate the polymer LCs threedimensionally (3-D) by changing the direction of the incident light [13**]. When the sample film was irradiated with unpolarized light at 436 nm, the transmittance of the probe light through crossed polarizers increased rapidly with time until a saturation value was reached. This means that alignment change was induced in the polymer LC even on irradiation of the unpolarized light at each incident angle. We observed the change in transmittance through aligned films as a function of the probe angle. At the incident angle of 1358 for the irradiation light, the profile was asymmetric. The maximum transmittance appeared at the probe angle of 458, at which the probe light was perpendicular to the irradiation light; the minimum transmittance appeared at the probe angle of around 1358, at which the probe light was parallel to the irradiation light. When the incident angle was changed to 458, an opposite tendency was observed. When the incident angle was 908, a symmetric profile in the transmittance was observed, and the minimum transmittance appeared at the probe angle of 908. These results strongly imply that the azobenzene
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Fig. 3. Plausible alignment of the LC molecules induced by unpolarized light irradiation.
moieties are aligned in the propagation direction of the irradiation light. The results were further confirmed by conoscopic observation (Fig. 2) in which films with 1-mm thickness were used to obtain clear images. The cross-point, which represents the optic axis of the LC phase, appeared on irradiation, and changed its position according to the incident angle of the irradiation light. These results indicate not only that alignment change is generated in the polymer film by use of the unpolarized light but also that the alignment direction is controlled by the propagation direction of the irradiation (Fig. 3). The azobenzene moieties are well aligned in the propagation direction of the irradiation light. The induced alignment was very stable at room temperature and remained unchanged even after .5 months. The induced alignment was also stable on further irradiation of 436-nm unpolarized light at the same incident angle, unlike the results reported in the literature in which the tilt angle was controlled by the amount of exposure energy [14,15]. Erasure of the induced alignment was realized easily by irradiation of unpolarized light at 366 nm due to the occurrence of photochemical phase transition, and the alignment was induced again on irradiation of the unpolarized light at 436 nm.
4. Photophysically induced order–order alignment change
Fig. 2. Conoscopic observation of the alignment of the azobenzene mesogens as a function of the incident angle. The incident angle, u, is indicated in the figure and the molecular structure of the polymer azobenzene LC used in this experiment is indicated at the top of the figure.
We also investigated the control of the LC alignment by photophysical processes of p-conjugated dye molecules. On application of an electric field, the molecular alignment of LCs is changed due to the dielectric anisotropy of the molecules (Freedericksz transition). The electric field of light can also induce the change in the realignment of the LCs (Optical Freedericksz Transition: OFT) [16]. Recently, it became clear that the threshold light intensity of the OFT is decreased by the addition of a small amount of an organic dye to the LCs. We have developed novel LCs containing an oligothiophene moiety as a new photosensitive LC chromophore
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[17*,18*,19**,20**]. By irradiation of a mixture of the thiophene dye and a non-photoresponsive host LC, realignment of the LC molecules was induced and self-diffraction patterns appeared when the light intensity was above the OFT threshold (Fig. 4). Fig. 5a shows the number of self-diffraction rings as a function of light intensity observed for undoped 4-cyano-49-pentylbiphenyl (5CB) in the self-diffraction measurements. The threshold intensities for the appearance of diffraction rings were about 840 W/ cm 2 for 5CB. On the other hand, in 0.22 mol% TR5doped 5CB, the OFT thresholds were only 6.2 W/ cm 2 (Fig. 5b). That is, at the dye concentration of 0.22 mol%,
Fig. 5. The number of diffraction rings due to optical Freedericksz transition as a function of irradiation intensity observed in the homeotropic cell for the non-doped 5CB (a) and the TR5-doped 5CB (b).
Fig. 4. Structure of the novel photoresponsive LC developed in this study (top), typical diffraction pattern observed in the dye-doped LC (middle), and plausible alignment of LC molecules (bottom).
Fig. 6. Holographic recording by means of a polymer azobenzene LC.
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TR5 can decrease the threshold intensities about 135 times for 5CB. Such large values are among the most effective dye-induced realignment effects disclosed so far. In order to confirm the reorientation direction, Z-scan measurements were performed. The principle of the Z-scan method is that the magnitude of non-linear effects in LCs depends on the light intensity. Therefore, when a Gaussion beam is used as the pumping light to induce the photoinduced realignment, the degree of realignment depends on the intensity distribution of the light and the realignment results in a Gaussion distribution in the refractive index within the irradiation region. When the director realignment increases the refractive index, the LC sample can be regarded as a convex lens, and it modifies the light intensity through the aperture. When the realignment decreases the refractive index, however, the LC sample acts as a diverging lens and modifies the incident light in a different way. In the Z-scan curve observed for TR5-doped 5CB, a typical self-focusing phenomenon was observed. That is, a minimum was observed at a sample position between the convex lens and the focal point; a maximum occurred at a position behind the focal point. The Z-scan result for TR5-doped 5CB indicates a tendency to increase the refractive index upon irradiation. It is known that the refractive index of 5CB increases from n o to n e . Therefore, the observation of the self-focusing phenomenon in TR5doped 5CB indicates that LC molecules tend to reorient parallel to light polarization upon irradiation.
5. Photonic application of the photoinduced alignment change
5.1. Rewritable high-density holographic recording by means of polymer LCs Recently, we have developed holographic materials by means of polymer azobenzene LCs [21**,22,23,24**,25**,26*,27*,28*]. On irradiation of a couple of coherent light beams, a real-time hologram can be created by periodic induction of a photochemical phase transition in polymer azobenzene LCs. The gratings are formed within 200 ns by laser pulse irradiation (355 nm; 25 ps, fwhm). The holographic diffraction in the LC phase was observed repeatedly by turning on and off the writing beams. It is worth mentioning that the magnitude of the modulated refractive index reached the order of 10 21 . Observation of the grating structure in glassy ordered films with a polarizing microscope confirmed that the isotropic phase was arranged with a spatial resolution of ,1 mm. The gratings recorded in the ordered samples with a thickness of about 500 nm showed a high diffraction efficiency of 32% (nearly the theoretical maximum of the diffraction efficiency in those thin films (Raman–Nath regime)). We may expect that a rewritable high-density
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3-dimensional image storage is possible with the polymer azobenzene LCs (Fig. 6).
5.2. Modulation of intensity Photochemical control of the optical properties of polymer / LC composite films was performed by means of the photochemical phase transition induced by the trans– cis photoisomerization of azobenzenes. Reversible photoisomerization of the azobenzenes took place in the polymer matrix environments and the consequent optical effects were induced as a change in transmittance of the composite films. Repeatable changes in transmittance between the two different optical states can be induced efficiently by alternative irradiation with UV and visible lights. Furthermore, the composite films with donor–acceptor azobenzenes showed a rapid thermal recovery to the initial state. When polymers with a long alkyl side chain were used as the matrix, the memory effect was observed. Those materials, therefore, are useful for all-optical switching materials and optical storage materials [29**].
5.3. Modulation of the wavelength of selective reflection Cholesteric LCs (CLCs) show a selective reflection due to the helical structure. The reflection wavelength (helical pitch) of CLCs is very sensitive to perturbations: temperature, pressure, impurities, and so on. When we put an azobenzene as a photosensitive chromophore in CLCs, we can change the reflection wavelength of the CLCs by light. We found that the reflection wavelength (color of the materials) was shifted to shorter wavelength by irradiation with UV light, which induces the trans–cis photoisomerization of the azobenzene [30**]. On irradiation with visible light to cause the cis–trans back-isomerization of the azobenzene, the reflection band was restored to the initial wavelength. By optimizing the azobenzene structure, we can control the reflection wavelength from near-IR to blue region.
6. Conclusion The molecular alignment of LCs can be modulated by either the photochemical process or the photophysical process. By the modulation of the LC alignment, we can change the birefringence as well as refractive index of the material. The photoinduced refractive index modulation is extremely large: it is more than 10 21 . Thus, the photoresponsive LCs are suitable to the materials for photonics. In this review, we present several examples of the application to the photonic materials. Since the photoresponsive LCs also show very quick response to stimulus light, those materials can be used for other photonic devices such as optical switches, dynamic holography, and so on.
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