OPTICALLY TUNABLE DIFFRACTION GRATINGS in Java

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11.5. OPTICALLY TUNABLE DIFFRACTION GRATINGS
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index of the LC, no, that is, B1.5. Figure 11.24b shows the POM image of the same grating rotated at B351, with respect to the crossed polarizers to better explain why this large index modulation can be controlled both optically and electrically. At this alignment, oriented LC molecules in nonirradiated areas appear bright by allowing light to pass through, and the remaining azobenzene polymer network is visible as denser zones under the optical microscope. When an electric eld is applied across the cell, LC molecules distant from polymer walls are aligned along the eld direction, that is, perpendicular to the plane of the cell (homeotropic orientation) and thus appear dark under crossed polarizers, which can be seen in Fig. 11.24c. In this case, the probe light sees essentially no and np in the alternating regions, and the sharp decrease in index modulation should result in a drop of the diffraction ef ciency. The electrical switching of index modulation should be reversible, since the initial planer orientation of LC can be recovered when the electric eld is turned off. More interestingly, the index modulation can also be reversibly changed by light using two different wavelengths. Illuminating the grating with a UV light (360 nm), azobenzene groups on the polymer remaining in the LC regions undergo the trans cis photoisomerization that can induce the photochemical nematic isotropic phase transition at room temperature. This can be observed in Fig. 11.24d where the LC stripes appear signi cantly darker because of disordered LC molecules. When this happens, the probe light experiences mainly the average refractive index of the LC, n* = (2no+ne)/3, which should be close to np, leading to a drop of the diffraction ef ciency. While illuminating the grating with a visible light (440 nm), the back cis trans photoisomerization of azobenzene occurs that allows the initial surface-induced LC orientation to be recovered. As already mentioned, the polymer and LC strips forming the grating are actually polymer- and LC-rich regions. Figure 11.25 shows a magni ed POM image of the grating, where azobenzene polymers remained in the LC regions are clearly noticeable. On the basis of the morphology of the azobenzene polymerstabilized LCs revealed on POM (Leclair et al., 2003), the estimated amount of polymer in the LC regions is B3%. Likewise, the polymer regions also con ne LC molecules whose switching at the eld-on state is responsible for the darker appearance of the polymer strips in Fig. 11.24c. Nevertheless, as shown in Section 11.5.2.2, experimental data of both electrical and optical switching of the diffraction grating corroborates quite well the analysis of index modulation changes without taking into account the remaining polymer or LC, suggesting that the residual amount of polymer or LC in the LC or polymer regions, respectively, is minor and does not determine the switching behavior of the grating. 11.5.2.2. Optical and Electrical Switching Behaviors. The grating in Fig. 11.24 was used to investigate the switching behavior of diffraction ef ciency using light at two wavelengths (UV and visible) as well as an electric eld. The setup was the same as described in Section 11.4. The results are shown in Fig. 11.26. In Fig. 11.26a, the multiple-cycle switching of the rst-order (+1)
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CHAPTER 11: TUNABLE DIFFRACTION GRATINGS BASED ON AZO MATERIALS
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Figure 11.25. A magni ed polarizing optical micrograph of the grating in Fig. 11.24, showing the polymer-rich (dark) and liquid crystal-rich regions (clear). A remaining polymer network in liquid crystal regions is visible. The width of the fringes is B40 mm.
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diffraction ef ciency between the high (6.5%) and low (0.5%) state is observed under a square-wave electric eld between 0 V ( eld-off) and 40 V ( eld-on). The low diffraction state is somewhat unstable, which is likely related to some dynamic instability of the LC in the DC electric eld. Under the used conditions, at the eld-on state, the time for the diffraction ef ciency to drop from 90% to 10% of the initial value is B110 ms, whereas at the eld-off state, the diffraction recovery from 10% to 90% takes B1 s. The electrical switching can be repeated many times. At eld-on state, the speed of switching from high to low diffraction ef ciency is determined by the speed at witch LC molecules change from the surface-induced in-plane orientation to the homeotropic (or tilted) orientation. On removal of the eld, the switching from low to high diffraction is governed by the relaxation rate of the LC returning to the initial in-plane orientation. Similar to azobenzene-containing self-assembled LC gels discussed earlier in the chapter, the electrical switching is based on the change in index modulation resulting from electric eld induced reorientation of LC molecules. The appealing feature of the grating is the switching of diffraction ef ciency controlled by light. Figure 11.26b shows the reversible optical switching of diffraction ef ciency using UV and visible lights. In the experiment, the irradiation light was directed to the grating at an angle to the normal of the cell to prevent it from entering the photodetector used to measure the intensity of the diffracted
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