Photochromic command surface induced switching of liquid crystal optical waveguide structures Harald Knobloch and Horst Orendi Mar-Planck-Institut fr Research Saitama 3P51-01, Japan rogram, The Institute Polymerforschung, of Physical and Chemical Ackermannweg 1R0, 55128 Main& Germany esearch (RIKEN), 2-I-Hirosawa, Wako, and Frontier Max-Planck-Institut Michael Biichel fiir Polymer$orschung, Ackermannweg 10, 55128 Mainz, Germany Takahiro Seki National Institute of Materials and Chemical Research, I-1 Higashi, Tsukuba, Ibaraki 305, Japan Shinzaburo Ito Division Japan of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Sakyo, Kyoto 606, Wolfgang Max-Planck-Institut Knoll Research Program, The Institute fr Polymetforschung, Ackermannweg IO, 55128 Mainz, Germany (RIKEN), 2-I-Hirosawa, Wako, and Frontier Saitama 351-01, Japan of Physical and Chemical Research (Received 14 October 1993; accepted for publication 4 October 1994) We report on optical waveguide structures containing a thin liquid crystal (LC) film held between two photochromic command surfaces. The command surfaces consist of three monomolecular layers of a polymer with azobenzene side chains deposited according to the Langmuir-Blodgett- Kuhn technique. When exposed to light of appropriate wavelength, the command surfaces undergo a trans-cis photoisomerization process that induces a reversible change in the liquid crystalline orientation. Such an orientation change of the LC alters the optical properties of the optical waveguide. We present experiments on the dynamics of the LC orientation process.-The transition is shown to be continuous, with the degree of orientation dependent on the ratio of the cis-trans chromophore concentration ratio in the command surface. 0 1995 American Institute of Physics. I. INTRODUCTION magnetic (TM)- and transverse electric (TE)-polarized modes can be excited, the whole set of refractive indices of Optical switching phenomena will play a very important an anisotropic thin film structure, such as of a thin liquid role in future data processing technologies. Of particular in- crystalline film, can be determined. The alignment of a nem- terest in this context are optical waveguide structures, where atic LC depends on its interactions with the command sur- the light propagation properties can be altered under the in- face, and therefore measurements of the dynamics of the fluence of light. In this paper we present an optical wave- switching process allow for an analysis of the switching guide structure with guided modes excited in a nematic liq- mechanism uid crystal (LC) layer sandwiched between two photochromic command surfaces [see Fig. l(a)]. As a com- II. EXPERIMENTAL mand surface we used a Langmuir-Blodgett-Kuhn (LBK) A. Sample preparation film of a polymer with azobenzene side chains.1-3 By exposing the azobenzene side chains to light of ap- The samples used in our experiments were prepared as propriate wavelength a transwcis conformational change is shown in Fig. 1: Suitable waveguide structure were obtained by evaporating a thin silver film (35 nm) onto a glass slide induced: Irradiation at a wavelength of 360 nm induces a (BK 7). The silver film was covered by a thin SiO, film trans+cis transition; at 450 nm the back transition (thickness: 7 nm) in order to prevent quenching effects (cis-+trans) is induced.“.5 On the other hand, the alignment (transfer of excitation energy to the metal acceptor states). of a LC is determined by the nature of the substrate,6 for On top of this structure, we deposited the command layers of example, a LBK film,7 and changes in the surface properties azobenzene side chain polymers according to the LBK will induce a change in the LC alignment. In this context, it technique.9~‘0 is well known that the change in surface properties due to the The material we used is a polyvinyl alcohol (PVA) main transt-tcis transition in the azobenzene command layer in- chain polymer with azobenzene side chains [see Fig. 3(a)]. duces changes in the alignment (homeotropic++parallel) of a Before thin film preparation, the azobenzene polymer was nematic liquid crystal*12d*8 (see Fig. 2). dissolved in CHC13 (0.5 mg/ml). The solution was exposed The refractive index of a liquid crystal depends on its to ultraviolet (UV) light (360 nm) for about 10 min in order molecular orientation. Therefore,, changes in the alignment to obtain a mixture of cis and trans chromophores. This so- alter the light propagation properties of a liquid crystalline lution then was spread onto the water surface of the Lang- optical waveguide. Since in optical waveguides transverse muir trough (KSV 5000) and, after the solvent evaporated J. Appl. Phys. 77 (2), 15 January 1995 0021-8979/95/77(2)/481/7/$6.00 Q 1995 American Institute of Physics 481 Downloaded 30 Apr 2002 to 141.14.233.236. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/japo/japcr.jsp
a> ? CH3-W~.j&=&N=N~W-i2~C-O-CH \\CH, ‘_7”,: HO-CH \\CH, r-;_x A GAzn-WA n=jo SiOn 6AzlO-PVA (x-0.24) spacer SiO2 As b) R,-&-$Cl~OR2 DON-103 T 1‘ 1‘ I‘ ,uxv,;:::~~ RG. 3. Materials: (a) azobenzene side chain polymer, and (b) the liquid crystal used in our experiments. dipping direction > HG. 1. Schematic drawings (a) of the command layer controlled liquid crystalline optical waveguide structure and (b) the geometry of the com- mand layer surface. The whole assembly was placed in a special hotder and fixed by screws. While tightening the screws of the holder, we observed the interference pattern of light falling through the sample in order to ensure a uniform distance between the two command surfaces. When the screws were tight, the real spacing of the two glass slides was below the thickness of the spacer foil. The prepared cell was filled with the nematic liquid crystal (DON-103, RODIC) as active waveguide me- dium. Finally, for optical waveguide excitation, a glass prism (BK 7) was attached on top of the LC cell. The prism was placed onto the cell such that the excited optical waveguide modes propagate along the dipping direction of the LBK film [x direction, see Fig. l(b)]. B. Optical waveguide technique Optical waveguide mddes can be excited in transparent thin film structures, if the propagating light fulfills the well- known mode condition”-‘3 j3,,+P1+mrr=kzd (1) (10 min), the f&n was compressed to a lateral pressure of rr= 13 mN/m. The monolayers then were transferred by dip- ping the prepared substrates through the water-air interface (dipping speed: 5 mm/min) while keeping the surface pres- sure constant. We deposited three monolayers onto each sub- strate. The whole deposition process was done under red light conditions in order to prevent back switching induced by room light. The glass slides were attached face to face with the dip- ping direction of the two glass slides being parallel. As a spacer between the two command surfaces, we used a thin foil of polyethylene terephthalate (Goodfellow, d = 3 pm). with k, being the wave vector of the mode in the z direction, d the film thickness, m the mode order, and 2pi the phase shifts occurring during reflection of the propagating light at the film boundaries. As the wave vector of the propagating light k,, is larger than that of the incident laser light k,, it is necessary to use a coupling arrangement, such as a prism, to match the wave vectors:14 k, = kLnptism sin Q, = kwG . 0) The excitation of the modes of different order can be ob- o--u served by recording the reflected intensity of the setup in Fig. -z----y LC molecules 1 as a function of the angle of incidence 0. Narrow dips in o---o, the reflection curve indicate the excitation of optical wave- azobenzene guide modes. Since optical waveguide modes can be excited LBK film either by TE- or TM-polarized light, the whole set of refrac- tive indices can be determined. Therefore, experimental re- flectivity curves are analyzed by applying Fresnel cis trans calculations,‘5 allowing for the determination of thickness PIG. 2. Illustration of the LC alignment changes induced by a photochromic and refractive indices of the liquid crystal film: n,, the re- command surface consisting of an azobenzene side chain polymer. fractive index in propagation direction; n,,, the in-plane re- 482 J. Appl. Phys., Vol. 77, No. 2, 15 January 1995 Knobloch ef al. homeotropic parakl Downloaded 30 Apr 2002 to 141.14.233.236. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/japo/japcr.jsp
fractive index perpendicular to the dipping direction; and n, the refractive index perpendicular to the plane of the film [see Fig. l(b)]. The light source we used was a polarized He-Ne laser (632.8 nmj; the plane of polariztition could be rotated by a Fresnel rhomb. The laser power was reduced to a few nW in order to prevent any heating of the sample due to absorption of the laser light. For optical waveguide excitation we uSed a 90” glass prism (BK 7); the whole sample was placed on a O-20 goniometer, controlling the angle of incidence and that of the detection optics. The reflected beam was focused onto a photodiode and detected using a.lock-in amplifier (EG&G, Model 5210). C. Sample exposure For inducing the transttcis conformational changes in the azo dye, the sample has to be exposed to either UV light of 360 nm (trans+cis) or to visible light of 450 nm (cis+trans). The light source we used was a high-pressure mercury lamp (Oriel, 200 W) with a glass filter (UG 11, Schott) for UV exposure and an interference filter for qxpo- sures at 450 nm. At these wavelengths the lamp power could be adjusted to values between 0.5 and 8 mW/cm’. As room light influences the switching process, the experiments were carried out in the dark. 1. ’ 1 ‘. ’ - ’ = I. ‘1 40 - ( 1 . 1 . . ,, ‘ !I’=50 80 70 80 90 Angle [deg] - 1. 1 1. 1 1’ 1 1 1 ‘I I- 1. B E III. RESULTS AND DISCUSSION A. Exposure by unpolarized light 40 45 50 55 As a first exp+ment, we measured the reflectivity of the LC cell before and after UV light exposure at a wavelength of 360 nm. After irradiating the sample for 10 min, the re- flectivity curve -was recorded under permanent exposure. There is a large difference between the two switching states for TM polarized waveguide modes, as shown in Fig. ,4(a). On the other hand, for TE polarization [Fig. 4(b)] the reflec- tivity curve seems to be unaffected. When comparing the measured reflectivity curves with Fresnel calculations (full curves in Fig. 4), we obtained the data listed in Table I. In general; the experimental data for the refractive indices n, and n, are in good agreement with those given by the manufacturer of the LC for the ordinary (no = 1.479) and the extraordinary (n, = 1.567) beam. Before W exposure, the calculated value of n, corresponds to n,, and that of n, to n, . This indicates that the LC molecules are aligned perpendicular to the command surface (homeotro- pit). After irradiation, the salient of refractive indices have changed: Now n, corresponds to n, and n, to no, indicating that the LC changed its orientation. The LC is now in a parallel alignment with respect to the command layer sur- face. When exposing the cell to visible light of 450 nm the LC is switched back’ to the homeotropic state and the reliec? tivity curve is the same as before, W exposure. For TE-polarized waveguide modes, the reflectivity curve seems to be unaffected. Therefore, we can conclude that switching only takes place in the x-z plane, the plane parallel to the dipping direction. In a latter part of this paper we further address this phenomena. J. Appl. Phys., Vol. 77, No. 2, 15 January 1995 60’ 65 70 Angie [deg] 75 80 85 SO PIG. 4. Reflected intensity for (a) p- and (b) s-polarized light, recorded as a function of the incident angle, before and after exposure of the LC cell to UV light. The sharp resonance dips in the reflectivity correspond to the excitation of (a) TM- and (b) TE-polarized optical waveguide modes. In the case of parallel alignment, the reflectivities were recorded under permanent light exposure. B. Switching dynamics In order to study the switching behavior, we set the in- cident angle to that resonant for the waveguide mode m = 3 of the homeotropic system (0=58.5”) and exposed the cell to UV light or visible light, respectively. During exposure the reflected intensity was recorded as a function of time. Figure 5 shows the switching behavior of our LC cell. The position of the reflectivity dip changes its position under the influence of UV or blue light, respectively. Therefore, when keeping the goniometer at a fixed angle, the reflectivity changes un- TABLE I. Refractive indices and thickness of the nematic liquid crystal film in homeotropic and parallel state. The values are obtained by comparing the measured reflectivities to Fresnel calculations. Polarization TM TM TE TE State homeotropic parallel homeotropic parallel nx 1.473 1.567 - - % nz Thickness (nm) 895 895 895 895 - 1.567 - 1.473 1.473. 9. - 1.473 - Knobloch et al. 483 Downloaded 30 Apr 2002 to 141.14.233.236. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/japo/japcr.jsp
500 1000 1500 Time [s] -.- 0 200 400 Time [s] 600 600 0.8 FIG. 5. Reversible switching behavior of the LC cell. The angle of inci- dence is set to the resonance angle for the m=3 mode (8=58.5”) in the reflectivity curve of the homeotropic cell. The reflectivity is recorded as a function of time for exposure with different wavelengths: 360 mn (cis+trans) and 450 nm (truns+cis). der the influence of light, too. The observed changes are reversible and very reproducible. Therefore, the homeotropic++parallel switching of the LC is reversible. On the other hand, in the dark, the parallel phase is very stable with a decay time of several hours (see Fig. 6). When exposing the cell to UV light for only a few sec- onds, the reflectivity keeps constant at an intermediate level [see Fig. 7(a)] indicating that the switching of the LC is not complete. The same behavior can be observed for the back switching process [see Fig. 7(b)], although here the reflectiv- ity level reached is not as constant as in the UV switching case. For a deeper understanding of this phenomena, we again exposed the ceil to UV light for a few seconds only. We stopped the exposure when the reflectivity reached a certain level and then recorded the reflectivity [see Fig. 8(a)] as a 0.0 1 0 500 t 1000 1500 2000 I Time [s] FIG. 7. Gradual switching behavior of the LC cell observed by setting the angle of incidence to an angle of 8=58.5”, resonant for the TM m=3 mode for the LC in the homeotropic phase. (a) when exposing the cell to UV light and, (b) when illuminating with light of a wavelength of 450 nm. function of the incident angle. When comparing the reflec- tivity curves to Fresnel calculations, we obtain the behavior of the refractive index n, [Fig. 8(b)] and n, [Fig. 8(c)] as a function of exposure time. Using these values for the refrac- tive indices, we could calculate the mean tilt angle, 3, of the LC molecules towards the command layer surface normal [see Fig. 8(d)] by (::i=(~;)(:iz::~)* (3) I * I. 1 I. I. I * 1 8.N. 1 1 .oo t 1 0.80 .g g 0.60 2 0.40 0.20 0 500 1000 1500 2000 7.500 3000 3500 4000 4500 5000 5500 0000 Time [s] HG. 6. Decay of the parallel phase as a function of time. The decay time is in the range of 7.5 h. The curve is recorded under an incident angle of 0=58.5”. 484 J. Appl. Phys., Vol. 77, No. 2, 15 January 1995 Our results indicate that the switching of the LC molecules is a continuous process, following commensurate changes in the cis-truns chromophore concentration. Therefore, the alignment of the LC molecules can be controlled and fixed to a certain orientation by setting the illumination time. When varying the intensity of the UV light [see Fig. 91 the cell shows an interesting behavior: The final level that can be reached in the switching process under permanent irradiation and the photostationary state depends on the in- tensity of the switching light. Again, this behavior can be explained by the dynamics of the cis-tram switching of the command layer. When exposing the azodye layer to UV light, there will be two competitive processes: The trans-+cis conformational changes and thermal-induced back switching from cis to trans. Considering only these two processes, the switching Knobloch et al. Downloaded 30 Apr 2002 to 141.14.233.236. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/japo/japcr.jsp
0.8 - a) . 0.6 - 0.4- 60 70 80 so I 0 20 40 60 Angle [deg] 80 L. 100 I. 120 IJ Exposure Time [s] $ 1.54 - 8 E 1.52 - 2 .- 22 1.50 - ‘Ei Exposure Time [s] Exposure Time [s] FIG. 8. Reflectivity curve of TM polarized laser light after illumination with W light, recorded for different exposure times and refractive indices: (a) n, , (b) nz, (c) obtained from these reflectivity curves by Fresnel calculations. Using these refractive indices and the refractive indices of the LC we calculate (d) the tilt angle of the LC molecules to the surface normal of the command surface. In (b), (c), and (d) we added solid lines as a guide to the eyes. should be complete with only the switching time being a function of the lamp power. However, in our experiments, the degree of switching is affected, too. Therefore, as this simple model does not correspond to our experimental data, the photochemical equilibrium is changed, and the mecha- nism of the cis+trans switching process must be more complicated. For high intensities, the switching of the azo groups can be regarded as a collective effect where the mol- ecules have enough space to orient. In the case of low UV light intensities only a few azo groups per area switch. Due to the conformational change of the molecules, this switch- ing process requires space. Since the surrounding molecules do not follow the movement of the switching molecules, switching is hindered due to geometrical constrains. The LC alignment follows the trans-cis concentration of the azo chromophores and the orientation of the LC molecules is not in the parallel phase, but in an intermediate phase. C. Switching with polarized light In the following, we focus on the case of switching the LC celI with polarized light. It is well known that azo mol- ecules tend to orient out of the plane of polarization’6 when switched from tram to cis under the influence of polarized light. For the LC waveguide structure this means that, in the case of illumination with UV light polarized along the x J. Appt. Phys., Vol. 77, No. 2, 15 January 1995 direction [LBK dipping direction, see Fig. l(b)], .the azo molecules and therefore the LC molecules should orient along the y direction. Such an orientational change would have an influence on the in-plane anisotropy of the refractive indices and therefore should affect the reflectivity curves for TM as well as for TE polarization. In order to study the switching under the influence of polarized light, we set our goniometer to the resonance angle for TE (0=45.8”, m=3) or TM mode (0=58.5”, m=3), respectively, and recorded the reflectivity as a function of time, while varying the wavelength and polarization of the switching light. What we see in the experiment is the following: Under the influence of unpolarized light, the reflectivity of the TE mode shows only a very small peak when the UV illumina- tion starts, but there is no drastic change in the reflectivity during the illumination period [see Fig. 10(a)]. In the case of UV light polarized in y direction (perpendicular to the dip- ping direction), the TE-polarized waveguide mode shows no response [see Fig. IO(a), UV perpendicular]. When illumi- nating the sample with UV light polarized in the x direction (parallel to the LBK dipping direction), for TE-polarized waveguide modes we see the following effect: When starting the illumination, the reflectivity increases up to a certain level and then slowly decreases with the sample still under Knobloch et al. 485 Downloaded 30 Apr 2002 to 141.14.233.236. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/japo/japcr.jsp
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