行政院國家科學委員會專題研究計畫 成果報告
全像光柵鬆弛技術研究染料分子在高分子與液晶中之擴散
計畫類別： 個別型計畫
計畫編號： NSC93-2112-M-110-016-
執行期間： 93 年 08 月 01 日至 94 年 07 月 31 日
執行單位： 國立中山大學物理學系(所)
計畫主持人： 郭啟東
報告類型： 精簡報告
處理方式： 本計畫可公開查詢
中 華 民 國 94 年 10 月 27 日
中文摘要:
本研究使用全像光柵技術探討含偶氮染料的液晶樣品中瞬態光柵的動態行為
與機制。一階繞射訊號隨時間的衰變可以分為兩個部份來討論。衰變時間常數較
短的部份是由於偶氮染料的同素異構化所致，而衰變時間常數較長的部份是由於
受激染料引起的液晶重新定向行為。改變激發光的偏振態將反應出繞射效率的非
均向性。繞射效率及其繞射訊號衰減所對應的鬆馳時間常數隨溫度改變的倚變關
係顯示出光柵形成時存在著熱效應。
關鍵字:瞬態光柵、繞射、同素異構化、重新定向、非均向性
Abstract:
The dynamic behavior of transient gratings (TGs) formed in a planar-aligned
azo-dye- doped liquid crystal film is investigated. The temporal profile of the
diffraction efficiency of TGs reveals two components corresponding to the fast photo
induced isomerization of azo dye molecules and the slow dye-induced reorientation of
liquid crystals. The polarization dependence of diffraction efficiency shows a strong
anisotropy of excitation. The temperature dependence of diffraction efficiency and the
corresponding relaxation time of decay in the slow component of diffraction reveal a
thermal effect on grating formation.
Temperature and Polarization Dependence of Transient Gratings
in Azo-Dye-Doped Liquid Crystals
Chie-Tong KUO, Shuan-Yu HUANG, Ming-Syun KUOand Der-Jun JANG
Department of Physics and Center for Nanoscience and Nanotechnology, National Sun Yat-Sen University, 70 Lien-hai Rd. Kaohsiung 804, Taiwan, R.O.C.
(Received August 24, 2004; accepted January 6, 2005; published May 10, 2005)
The dynamic behavior of transient gratings (TGs) formed in a planar-aligned azo-dye-doped liquid crystal film is investigated. The temporal profile of the diffraction efficiency of TGs reveals two components corresponding to the fast photoinduced isomerization of azo dye molecules and the slow dye-induced reorientation of liquid crystals. The polarization dependence of diffraction efficiency shows a strong anisotropy of excitation. The temperature dependence of diffraction efficiency and the corresponding relaxation time of decay in the slow component of diffraction reveal a thermal effect on grating formation. [DOI: 10.1143/JJAP.44.3111]
KEYWORDS: transient gratings, diffraction, isomerization, reorientation, anisotropy
1. Introduction
There is a rapid growth in the development of holographic and diffractive devices for integrated optics, optical data storage, electro optical switching, and displays. Liquid crystals are very promising materials with a fast dynamic response and a high sensitivity for such applications, and attract enormous scientific and industrial interest. The giant optical nonlinearity of liquid crystals has been extensively studied recently due to the large refractive index anisotropy associated with the collective reorientation of liquid crystal
molecules.1) The photoinduced orientational responses of
nematic liquid crystals can be enhanced by doping these
crystals with appropriate dye molecules.2)Photoexcited dye
molecules were found to exert a torque to reorient the director axis of liquid crystals through intermolecular
interactions with host molecules.3–5) Janossy and Kosa3)
demonstrated that the dye-induced optical torque could significantly exceed the direct optical torque for
anthraqui-none dye molecules. Gibbons et al.4)showed that the
laser-induced alignment of nematic liquid crystals is due to surface-mediated reorientation with a dye-doped photo-sensitive material coated on the layer of liquid-crystal cell.
In azo-dye-doped liquid crystals, the director reorientation was reported to be due to the trans-cis photoisomerization of azo dye molecules under irradiation in the absorption band
of the dopants.6–8) Voloschenko et al.9) showed that the
photoexcitation of a small amount of azo dye (<1%) in bulk liquid crystals induces an easy-orientation axis over an
isotropic polymer-coated surface. Khoo et al.10)observed a
large nonlinear photorefractive effect in a small amount of methyl red-doped nematic liquid crystals (5CB) in the presence of an applied dc electric field, which was attributed to the reorientation of the nematic axis caused by the photoinduced space charge field and the dc bias field.
The dynamical responses of dye-doped liquid crystals (DDLC) to a short intensive optical field are rather complex because of the associations between density, flow, temper-ature, and reorientation. The physical properties involved in the associations generally depend on the geometrical con-figuration of nematic-optical interactions. The dynamic
holography technique involving various polarizations of excitation and probe beams has been employed to identify the strong asymmetry of excitation and diffraction in the guest-host system.
In this paper, we report the temporal profiles of diffraction efficiency for transient gratings and their temperature and polarization dependences in azo-dye-doped liquid crystals. The dynamics of molecular reorientation phenomena in transient gratings is investigated by analyzing the buildup time of the diffraction maximum and diffractive decay. The study of the polarization dependence of the first-order diffraction efficiency and its corresponding temperature effect allows us to understand the underlying mechanism of laser-induced transient gratings.
2. Experimental
We investigated pentyl-cyanobiphenyl (5CB) liquid crys-tals doped with an azo dye (DR-1) at a weight concentration of approximately 1.5%; their nematic temperature range is
20.6–31.8C. The dye-doped liquid crystals (DDLCs) are
capillarily injected into the sample cell, which is assembled from pairs of glass plates with transparent indium-tin oxide layers and is spaced by 25-mm-thick Teflon sheets. The homogeneous planar alignment of nematic liquid crystals (NLCs) is achieved by spin-coating a thin layer of polyimide on the inner surface of the front glass substrate, which is mechanically rubbed with a velvet cloth unidirectionally. At room temperature, the azo-dye molecules are in the stable trans form and in the alignment parallel to the director axis of liquid crystals through the guest-host effect. The align-ment of the azo-dye molecules and liquid crystals is confirmed by the polarized absorption spectrum shown in Fig. 1. The absorption peak at 510 nm corresponds to the n–
electronic transition of the trans isomer of DR-1. The
absorption intensity of parallel polarized light is larger than that of perpendicularly polarized light with respect to the director axis of the liquid crystals. The strong anisotropic absorption shows that the azo-dye molecules are well aligned along the director axis of the liquid crystals.
The geometry of the sample cell and laser beams is depicted in the inset of Fig. 1. The cell substrates are in the
x–z plane and the molecular director ^nn of NLC is along the
z-axis. Two simultaneously incident excitation pulses
de-E-mail address: [email protected]
Vol. 44, No. 5A, 2005, pp. 3111–3114
#2005 The Japan Society of Applied Physics
rived from a Q-switched Nd:YAG laser with a BBO frequency doubling crystal are operated at a wavelength of 532 nm with a pulse width of 6 ns. The excitation beams are
unfocused onto the sample with an irradiance of 4.5 mJ/cm2
for each pulse. The crossed excitation beams are arranged with the bisector normal to the sample and the resultant grating spacing is approximately 15 mm. The polarizations of the two excitation beams are adjusted in the same direction as either parallel or perpendicular to the molecular director of the nematic liquid crystals.
An unpolarized cw He–Ne laser at 632.8 nm is used as a normally incident probe beam that passes through a polarizer to overlap with the excitation beams on the sample. The sample cell is placed in a temperature-controlled chamber with glass windows, which enables the excitation and probe beams to be transmitted. The chamber is operated in the range of nematic temperatures of 5CB maintained at an
accuracy within 0:1C. The interference pattern of the
transient grating is recorded on the DDLC sample with a single shot of pulsed laser. The first order of diffraction efficiency is detected with a photodiode and recorded as a function of time with a digitizing storage oscilloscope.
3. Results and Discussion
The dynamics of holographic excitation and the temper-ature dependence of diffraction efficiency are investigated with various polarizations of the excitation and probe beams. The first-order diffraction efficiency in the temperature range
from 29.5C to 32.0C is recorded for the perpendicular
polarization of the excitation beams and the parallel polar-ization of the probe beam relative to the molecular director of the liquid crystals, as shown in Fig. 2. Two components in the temporal profile of diffraction are observed. The fast components of diffraction corresponding to the response time from 0 to 2.5 ms are observed almost simultaneously at various temperatures. During this period, the rise in the diffraction efficiency is related to the laser-induced isomer-ization of the azo dye from the trans isomer to the cis isomer and the decay of diffraction is related to the thermally induced relaxation from the cis isomer back to the trans isomer. The slow components of diffraction corresponding to a longer response time from 2.5 ms to 90 ms vary with
temperature in both the intensity and buildup time to reach the diffraction maximum. The contribution of diffraction to the slow components is attributed to the dye-induced reorientation of the liquid crystals, and the time delay of the diffraction maximum with increasing temperature is related to the temperature dependence of the viscosity and order parameter of the liquid crystals. The viscosity decreases more rapidly than the order parameter as the temperature increases from the clear point. But when the temperature approaches the clear point, the order parameter decreases markedly and gives a dominant contribution to the time delay of the diffraction maximum.
The peak efficiency of the slow component of diffraction as a function of temperature is presented in Fig. 3 for various polarizations of the excitation and probe beams either parallel or perpendicular to the director axis of the liquid crystals. Four different mutual orientations of the excitation and probe beams are investigated and denoted by (?, k), (?, ?), (k, k), and (k, ?). The first symbol in the parentheses represents the polarization of the excitation beams and the second symbol represents the polarization of the probe
350 400 450 500 550 600 650 700 0.0 0.5 1.0 1.5 2.0 2.5 3.0 x y z Pump beam 1 Probe beam
First order diffraction To detection system Pump beam 2 Sample
A A
Absorbance (a.u.)
Wavelength(nm)
Fig. 1. Absorption spectra of nematic liquid crystals (5CB) doped with azo dye DR-1 (1.5%). The symbols Akand A?represent the absorbance of the DDLC system measured by light polarizations parallel and perpendicular to the director of NLC, respectively. The inset shows the geometry of the sample that interacted with light beams. The cell substrate is in the x–z plane and the molecular director is along the z-axis.
0 1 0 10 20 30 40 50 60 70 80 90 100 0 1 0 1 0 1 0 1 0 1 32.0°C 29.5°C 30.0°C 30.5°C 31.0°C 31.5°C Diffraction efficiency (%) Time(ms)
Fig. 2. First-order diffraction efficiency as function of time in the temperature range from 29.5C to 32.0C. The probe beam is polarized parallel and the excitation beams are polarized perpendicular to the director axis of liquid crystals.
29.5 30.0 30.5 31.0 31.5 32.0 32.5 0.0 0.5 1.0 1.5 Peak efficiency (%) Temperature (°C) ( , ) ( , ) ( , ) ( , )
Fig. 3. Peak efficiency of slow component of diffraction as function of temperature for various polarizations of excitation and probe beams. The symbols k and ? represent the parallel polarization of the excitation beams and the perpendicular polarization of the probe beam relative to the director axis of liquid crystals; respectively.
beam. The symbol k represents the beam polarization
parallel to the molecular director ^nn and the symbol ?
represents the beam polarization perpendicular to the
mo-lecular director ^nn in the x–y plane. All peak efficiencies in
the four different configurations of the polarizations increase
with an increase in temperature to 31.0C and thereafter
decrease until they reach zero as temperature approaches the nematic-isotropic phase transition point. In the nematic phase, the growth of diffraction is dominated by the reorientation of liquid crystals driven photochemically by the conformation change of azo-dye molecules, and the decrease in diffraction efficiency is primarily due to distortion in the nematic ordering of liquid crystals. In the isotropic phase, the ordering state of the nematic liquid crystals disappears and the contribution of the dye-induced reorientation of the liquid crystals is negligible.
From the dependences of the polarizations of both the excitation and probe beams on diffraction efficiency, the corresponding refractive-index modulation is stronger for
the polarization of the probe beam parallel to the director ^nn
than for the perpendicular case when the excitation beams
are perpendicularly polarized to the director ^nn. The
corre-sponding refractive-index modulations are almost the same for both the parallel and perpendicular polarizations of the probe beam when the polarization of the excitation beams is parallel to the director axis. Either parallel or perpendicular polarization of the excitation beams can lead to director reorientation and destabilization in the orientational order of the liquid crystals. Comparing the peak efficiency of the slow component of diffraction among the four configurations of polarizations, the excitation beams are more important in determining diffraction efficiency than the probe beam. The polarization dependence reveals that the diffraction efficien-cy of perpendicularly polarized excitation beams is larger than that of parallel polarized excitation beams, even though the absorption of the azo dye DR-1 for the parallel polarized light is stronger than that for the perpendicularly polarized
light. Two mechanisms proposed by Chen and Brady.11)can
explain this observation. The reorientation and cis-trans transformation of azo-dye molecules can induce a significant local disturbance in director orientation. Thus, the ordering state of the nematic liquid crystals is disrupted particularly during strong absorption. This effect results in a refractive-index modulation that is stronger for perpendicularly polarized excitation beams than for parallel polarized excitation beams.
The shift in peak efficiency with temperature is observed in the slow component of diffraction. The rise time of the diffraction maximum as a function of temperature for the four configurations of polarizations is presented in Fig. 4. The rise time of the diffraction maximum increases with increasing temperature, which is primarily due to the decreasing order parameter of the liquid crystals. The rise time of the diffraction maximum is longer for the parallel polarization than for the perpendicular polarization of the excitation beams. This effect could be explained by the destabilized alignment of liquid crystals induced by the strong anisotropic absorption of the dye.
The analysis of diffraction decay in the slow component could yield information about the dynamics of molecular reorientation in liquid crystals under nanosecond-pulsed
excitation. The time constant of the diffraction decay in the slow component as a function of temperature for the four configurations of polarization is shown in Fig. 5. The time constant of the decay covers from 10 to 320 ms and is associated with the relaxation of molecular reorientation. The decay time exhibits a strong dependence on temperature for the perpendicular polarization of the excitation beams but is less affected for the parallel polarization of the excitation beams. The relaxation time of the diffraction decay increases with increasing temperature for the perpen-dicular polarization of the excitation beams and is mainly attributed to the decreasing elastic constant of the liquid crystals. In contrast to the perpendicular polarization, the relaxation time of the diffraction decay is less affected with increasing temperature for the parallel polarization of the excitation beams; this can be explained by the disruption of the ordering state of the nematic liquid crystals owing to the strong absorption effect. The relaxation of the induced grating governed by the viscoelastic behavior of the nematic fluid can be explained by the temperature dependence of =k, where is the NLC orientational viscosity and k is Franck’s constant. This relaxation dynamics conforms with
the continuum theory of Ericksen and Leslie12)and confirms
29.5 30.0 30.5 31.0 31.5 0 10 20 30 40 50 60 70 80 90 Rise time (ms) Temperature (°C) ( , ) ( , ) ( , ) ( , )
Fig. 4. Rise time of diffraction maximum in slow component as function of temperature for four configurations of polarizations of excitation and probe beams. 29.5 30.0 30.5 31.0 31.5 0 50 100 150 200 250 300 350 ( , ) ( , ) ( , ) ( , ) Decay time (ms) Temperature (°C)
Fig. 5. Time constant of diffraction decay in slow component as function of temperature for four configurations of polarizations of excitation and probe beams.
that the director reorientation of liquid crystals plays an important role in the formation of transient gratings for the perpendicular polarization of the excitation beams.
4. Conclusions
The dynamic behavior of transient gratings formed in the system of azo-dye-doped liquid crystals has been inves-tigated. Two mechanisms are responsible for the formation of the transient gratings. The photoinduced isomerization of azo-dye molecules initially dominates the formation of the first transient grating, and the reorientation of liquid crystals driven by the dye-induced torque then dominates the formation of the second transient grating. The diffraction efficiency of DDLC exhibits a strong dependence on the polarization of the probe beam for perpendicular polar-ization of excitation beams and is less affected for the parallel polarization of excitation beams. The director deformation of liquid crystals is enhanced by the increase in temperature, resulting in the delay of the diffraction maximum. The relaxation time of diffraction decay is dependent on temperature for the perpendicular polarization of excitation beams and is less affected for the parallel polarization of excitation beams. The polarization and temperature dependences of diffraction efficiency reveal a strong anisotropy of excitation and a thermal effect on grating formation, respectively.
Acknowledgment
The authors would like to thank the National Science Council of the Republic of China, Taiwan for financially supporting this research under contract No. NSC 91-2112-M-110-003. Dr. Ming-Shan Tsai of the National Chiayi
University (NCU) is also appreciated for valuable
discussions.
1) I. C. Khoo: Liquid Crystals: Physical Properties and Nonlinear Opticial Phenomena (Wiley Interscience, New York, 1994). 2) F. Simoni and O. Francescangeli: J. Phys.: Condens. Matter 11 (1999)
R439.
3) I. Janossy and T. Kosa: Opt. Lett. 17 (1992) 1183.
4) W. M. Gibbons, P. J. Shannon, S. T. Sun and B. J. Swetlin: Nature (London) 351 (1991) 49.
5) A. G. Chen and D. J. Brady: Opt. Lett. 17 (1992) 1231.
6) O. Francescangeli, S. Slussarenko, F. Simoni, D. Andrienko, V. Reshetnyak and Y. Reznikov: Phys. Rev. Lett. 82 (1999) 1855. 7) B. Saad, T. V. Galstyan, M. M. Denariez-Roberge and M. Dumont:
Opt. Commun. 151 (1998) 235.
8) D. Voloschenko and O. D. Lavrentovich: J. Appl. Phys. 86 (1999) 4843.
9) D. Voloschenko, A. Khykhnyak, Y. Reznikov and V. Reshetnyak: Jpn. J. Appl. Phys. 34 (1995) 566.
10) I. C. Khoo, S. Slussarenko, B. D. Guenther, M.-Y. Shih, P. Chen and W. V. Wood: Opt. Lett. 23 (1998) 253.
11) A. G. Chen and D. J. Brady: Opt. Lett. 17 (1992) 441.
12) H. Hsiung, L. P. Shi and Y. R. Shen: Phys. Rev. A 30 (1984) 1453.
