Holographic Grating Formed by Photochemical Phase Transition of Polymer Azobenzene Liquid Crystal
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favorable for fabrication of phase-type gratings with high diffraction efficiency. To establish the LC materials for photonics in which properties of light are controlled by stimulating light, we have used the photochemical reaction of azobenzene derivatives. For instance, an azobenzene derivative in its trans form is rod-like in shape, which tends to stabilize the phase structure of the LC phase. However, the cis form is bent and destabilizes the phase structure. We have performed a study on the photochemical control of the LC materials containing a variety of azobenzene derivatives (photochemical phase transition).2 5- We already reported optical switching and two-dimensional image by transmission-, 2 reflection-3 and scattering-mode4 analyses. We present here the formation of holographic gratings by diffractionmode analysis.5 EXPERIMENT Materials The chemical structure of PALC is shown in Figure 1. A sample film was prepared by casting the polymer solution (THF, 2 mg/mL) on a polyimide-coated glass substrate. The substrate was rubbed unidirectionally with a clean cloth. The optically transparent and homogeneously-aligned monodomain film was obtained after annealing the film at temperature just below the N to I phase transition temperature. To confirm the alignment of the prepared films, we measured polarizing absorption spectra with a UV-vis spectrometer (Model Ubest V-550, Japan Spectroscopic Co., Tokyo, JAPAN).
-EýC -CH2tn
9=0
/
0-(H 2)6-0 Figure 1
,N -&/
N
C2 H5
Chemical structure of PALC
Mw = 79,000; Mw/Mn = 4.4 G 68 N 1501 Optical Setup for the Grating Formation Figure 2 represents the optics used in this study. We employed an unpolarized 488-nm beam from a Ar' laser as the writing beams. The beam was divided into two beams by a beam splitter at equal intensity and they were crossed on the film at an incident angle of 0.
154
The
"ON"
Ar+ laser X= 488 nm Unpolarized He-Ne laser :_D X= 633 nm Linearly polarized
30 20
Co
10
E (A
+1st order" 0
0 0
Photodiode Figure 2 Schematic diagram of optical setup for the formation of phase-type gratings.
100
"Time (s)
200
300
Figure 3 Typical profile of TI and transmittance as a function of time at 80 *C.
experiments were performed in the Raman-Nath (thin) regime, and multiple diffraction beams were observed. Grating formation was evaluated by real-time monitoring of the changes in the intensity of the first-order diffraction beam of linearly polarized beam at 633 nm from a He-Ne laser.
Diffraction efficiency (TI) was defined as the ratio of the intensity of the first-order
diffraction beam to that of the incident beam. In measurement of the photochemical phase transition behavior, the film was irradiated with the unpolarized beam at 488 nm. The intensity of the linearly polarized beam at 633 nm transmitted through a pair of crossed polarizers, with the film between them, was measured with a photodiode. Polarizine Ontical Microscone (POM) Observation After recording, the interference pattern generated was observed with a POM (BK50, OLYMPUS, Tok
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