DOI: 10.1007/s00340-008-2963-0

Appl. Phys. B 91, 17–20 (2008)

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nLasers and OpticsApplied Physics B

z. zheng1,2

f. guo1,2

y. liu1

l. xuan1,�

Low threshold and high contrast polymerdispersed liquid crystal grating basedon twisted nematic polarization modulator1 State Key Laboratory of Applied Optics, Changchun Institute of Optics, Fine Mechanics

and Physics, Chinese Academy of Science, Changchun 130033, P.R. China2 Graduate School of Chinese Academy of Science, Beijing 100039, P.R. China

Received: 13 November 2007/Revised version: 27 January 2008Published online: 27 February 2008 • © Springer-Verlag 2008

ABSTRACT An electrically tunable mode is proposed to overcome the bottleneck prob-lem of a conventional polymer dispersed liquid crystal (PDLC) grating and realize itslow threshold and high contrast through an alignment-controlled PDLC. The resultsindicate that the threshold voltage VPP of the grating is as low as 0.75–0.8 V, the sat-uration voltage Vpp is 7 V and the contrast ratio of the grating reaches 245, which isabout three times larger than the conventional PDLC grating. Stability testing indicatesa stable state of contrast during a long time.

PACS 42.40.Eq; 42.70.Df; 42.79.Kr

1 Introduction

The diffractive transmissiongrating based on polymer dispersed li-quid crystal (PDLC) is an attractivenew electro-optical device. Due to itselectrically tunable characteristic, sucha grating exhibits a bright prospect inapplications of integrated optics, opti-cal communication, laser techniques,optical data storage, etc. [1–4]. The con-ventional PDLC grating is always pre-pared with a mixture of liquid crystalsand photosensitive monomers. How-ever, because of monomer mixing, thethreshold voltage of such a grating ismuch higher than the other liquid crys-tal devices, which limits its applica-tion. In addition, because of the highthreshold (usually 10 V/µm [5, 8, 9])and poor tunable performance, the con-trast ratio of a PDLC grating is very low.The value of the contrast ratio alwayslies in the range of 10–30 [6–16]. Toimprove the shortcomings mentionedabove, some researchers adopted kindsof surfactant monomers, such as oc-tanoic acid [17], high-dielectric HRL-410 [18] or fluorine monomers [19], to

� Fax: +86-431-85681994, E-mail: zhigang1982@sina.com

decrease the anchoring energy betweenthe liquid crystal layer and the polymerlayer. The threshold voltage decreaseswith the decreasing of the anchoring en-ergy accompanied by a different-levelimprovement of electro-optical (E-O)properties. These works are helpful formaterials development in the PDLCgrating field. However, it is difficult torealize the improvement in practical ap-plication by modifying materials prop-erties only. Therefore, there is an urgentneed to develop a low threshold voltagePDLC grating with high contrast ratiofrom other aspects, such as device struc-ture, drive mode or external conditions.

In this work, we design an elec-trically tunable mode of PDLC grat-ing based on a twisted nematic (TN)cell (provided by North Liquid Crys-tal Display Research and Develop-ment Center, Changchun, China. Thetested cell gap is 6.67 µm) positioned infront of an alignment-controlled poly-mer dispersed liquid crystal (ACPDLC)grating, and we realize a much lowerthreshold voltage and a higher con-trast compared with conventional PDLCgratings.

2 Details of the design anddiscussion

The principle diagram isshown in Fig. 1. A polarizer is insertedbetween the He–Ne laser and the TNcell to produce a polarized incidentbeam, and its polarization directionturns through 90 degrees when the beamis transmitted through the TN cell, be-cause of the optical rotation propertyof the TN cell without an applied volt-age. In the case that a voltage over thethreshold is applied to the TN cell, thetwisting effect disappears. Therefore,the polarization direction of the incidentbeam is unaltered. That is to say, the TNcell is a polarization-direction modula-tor. Because of the optical anisotropyof liquid crystal molecules containedin the PDLC grating, the refractive in-dex of the LC layer is changeable withthe polarization direction of the incidentbeam. Thus, driving the TN modulatorwith a very low voltage changes the li-quid crystal’s refractive index, and con-sequently changes the refractive-indexmodulation (∆n) of the PDLC grat-ing, and, finally, a much better electro-optical property could be realized. Thepolarization directions of the incidentbeam in the cases of no applied voltageand a drive voltage on the TN modulatorare labeled in Fig. 1; the black arrow isthe case of the former and the red arrowis the latter.

In addition, the contrast ratio, theother important parameter which re-flects the tunable performance of thePDLC grating, is worth being consid-ered carefully. The contrast is defined asthe first order diffraction efficiency ratiounder the conditions of no applied volt-age and a drive voltage on the TN cell.

18 Applied Physics B – Lasers and Optics

FIGURE 1 Electrically tunable setup to realize low threshold and high contrast with a TN modulatorand an ACPDLC. Arrows represent polarization direction of the incident beam in the cases of no appliedvoltage (colored black) and a drive voltage (colored red). ‘⊗’ represents the direction perpendicular tothe surface

The diffraction efficiency of the gratingis dependent directly on ∆n; therefore,the larger the variation range of ∆n,the better the tunable property is ob-tained; then, the higher contrast couldbe reached. In order to extend the vari-ation range of ∆n, two points are notedby us. First, the index matching betweenthe LC layer and the polymer layer in thegrating, which means that the ordinaryrefractive index of LC (no) is approxi-mately equal to the polymer refractiveindex (np). Second, the uniform align-ment of liquid crystals in the grating isa vital problem. The most perfect caseis a uniform alignment of LC moleculesalong the grating strips, as schemed inFig. 2. When the polarization directionof the incident beam is parallel to the di-rection of the strips, the refractive indexof the LC layer is ne, and ∆n reachesa maximal value; contrarily, when it isperpendicular to the direction, ∆n ≈ 0because of index matching.

For the purpose of index match-ing between materials in our experi-ments, two kinds of acrylates with betterphoto-reactive activity and solubilityare selected. One is penta-functionaldipentaerythritol hydroxyl pentaacry-late (DPHPA) (supported by Aldrich,nD = 1.49 at 20 ◦C); the other is di-functional phthalic diglycol diacrylate(PDDA) (supported by Eastern AcrylicChem. Tech. Co., Ltd., Beijing, nD =1.5458 at 20 ◦C). They are mixed withthe weight ratio of 1 : 1. To obtaina good morphology of the PDLC grat-ing, a small amount of polymerized flu-oridizer, Actyflon-G04 (supported by

FIGURE 2 Sketch map of alignment-controlledPDLC grating. The directors (n) of LC moleculesare parallel to each other and along the directionof polymer grooves

XEOGIA Fluorine-Silicon Chem. Co.,Ltd., nD = 1.5492 at 20 ◦C) is added assurfactant frequently. In addition, somerose bengal (0.5 wt. %, supported byAldrich) and N-phenylglycine (2 wt. %,supported by Aldrich) are mixed to-gether as photoinitiator and coinitiator.The mixture is stirred at 40 ◦C for 24 h;then, the nematic liquid crystal TEB30A(supported by Slichem Co., Ltd., ShiJia Chuang, China, ∆n = 0.1703, ne =1.6925 at 20 ◦C) is mixed into it withthe monomer/LC ratio of 7 : 3 and re-stirred at 40 ◦C for 12 h. The refractiveindex of the polymer, estimated as theaverage value of monomers containedin the mixture, is 1.520, which is veryclose to the ordinary refractive indexof LC no = 1.522. The index matcheswell between the polymer and the LC.The mixture is injected into the LC cellwith 10-µm cell gap and exposed ina two-beam interference field for about20 min. The intensity of a single beamis 3.6 mW/cm2 and the wavelength is532 nm. Finally, the cell is irradiated

under UV for about 3 min for structurefixing.

Uniform alignment of LCs ina PDLC grating is a decisive point fora better E-O property and it is studiedin the experiments. What is interestingis that there is a self-alignment phe-nomenon in the LC layer of the PDLCgrating. It could be expressed clearly ac-cording to Berreman’s groove model,which indicates that LC molecules couldbe anchored by the grooves formed bythe polymer [20]. Therefore, the LCscould self-align without any other con-trolling. The anchoring energy could becalculated as the following equation:

WB = 0.25K A2q3, (1)

where A is the depth of the grooveformed by the polymer after exposure,q = 2π/Λ is the wavenumber of thegrating, Λ is the grating pitch and Kis the Frank elastic constant. By as-suming a simple sinusoidal surface re-lief structure with A = 100 nm, Λ =1 µm and K ∼ 10−12 N. Thus, WB ∼10−3 mN·m−1, which is about threeorders of magnitude smaller than rub-bing induced anchoring energy [21].Although this energy is small, its align-ment effect on the LCs could not beomitted.

To explain the uniformity of LCalignment, the angular transmittanceintensity of a PDLC grating is testedthrough polarized optical microscopy(using an Olympus BX-51 microscope)under crossed polarizers. In testing, thepolarization direction of one polarizeris parallel to the grating grooves andthe other is perpendicular to them; thetwo polarizers are fixed on the micro-scope, the grating is inserted betweenthe two polarizers and then the sampleis turned. Finally, the rotation angle–transmittance dependence is obtained(Fig. 3). It is indicated by the symbols‘∆’ in Fig. 3; a light-to-dark changecould be found in the transmittance–angle dependence. The transmittanceratio of light and dark states is about170, which shows the self-alignmentof the LCs in grating. The E-O prop-erty is tested through the tunable modeof Fig. 1 and the first order diffractionintensity decreases rapidly when theTN cell is driven by the applied volt-age. The contrast ratio tested is 125 as

ZHENG et al. PDLC grating modulated by a TN cell 19

shown by the symbols ‘∆’ in Fig. 5.To improve the E-O tunable perform-ance of the grating further, a more uni-form alignment of LCs is a main aspect.Normally, the simplest way to obtaina uniform alignment of LCs is rub-bing; however, the morphology or thephase separation of the grating mightbe affected by a strong rubbing. It wasnoted that the self-alignment of LCs isinduced by grooves, thus, the unifor-mity of alignment could be enhancedthrough a weak rubbing of substrates

FIGURE 3 Angular transmittanceintensity of self-alignment PDLCgrating (shown with symbols ‘∆’)and alignment-controlled PDLC grat-ing (shown with symbols ‘©’) undercrossed polarizers

FIGURE 4 Polarized IRabsorbance at 2227 cm−1

of ACPDLC grating andself-alignment grating, asa function of rotating an-gles of two samples. Thedouble arrow P is the po-larization direction. Theother two double arrows arethe grating vector G andthe rubbing direction R ofthe ACPDLC before it isrotated

FIGURE 5 Electro-optical pro-perty of the self-alignment PDLCgrating (shown by symbols ‘∆’)and alignment-controlled PDLCgrating (shown by symbols ‘©’).The inset shows the magnifiedscale for comparing the state ofthe applied voltage

before exposure, which does not affectthe formation of the grating, as wellas improves the E-O performance orcontrast. Subsequently, the alignment-controlled PDLC (ACPDLC) grating isprepared; its transmittance–angle de-pendence is shown by the symbols ‘©’in Fig. 3. There arises an evident light-to-dark transformation and the trans-mittance ratio in that case could reach280, which increases notably com-pared with the self-alignment sample.In order to evaluate the alignment ef-

fect of the ACPDLC grating further,the order parameter of LCs in thiskind of grating is tested through polar-ized Fourier transform infrared (FTIR)spectroscopy (using a Bio-Red 3000)and compared with the order parame-ter of the self-alignment one. In test-ing, the absorbance of liquid crystals at2227 cm−1 is detected when rotating thesample for a period. The absorbance–rotation angle dependence is shownin Fig. 4. Symbols ‘©’ represent theACPDLC and symbols ‘∆’ are the self-alignment condition. It is evident in thefigure that the maximum absorbance isobtained when the rotation angle is 0◦or 180◦ (i.e. the polarization directionis parallel to the grating grooves) andthe minimum is obtained at 90◦ or 270◦.So, it could be concluded that the aver-age alignment of LCs is parallel to thegrating grooves no matter what types thegratings are. It could also be found that,for the ACPDLC type, Amax = A0/180 =0.512, Amin = A90/270 = 0.087; for theself-alignment type, Amax = A0/180 =0.401, Amin = A90/270 = 0.126. Accord-ing to the equation given by Xuan etal. [22], S = (Amax − Amin)/(Amax +2Amin), the LC order parameter (S) ofthe ACPDLC grating is calculated as0.62, which is 0.2 larger than the self-alignment grating, 0.42. Thus, it is cer-tain that the alignment uniformity ofthe ACPDLC grating is better than theself-alignment grating. From the E-Oproperty shown by the symbols ‘©’in Fig. 5, it is found that the contrastratio reaches 245, two times that of theself-alignment sample. The thresholdvoltage V90 is about 0.75–0.8 V (VPP)and the saturation voltage is 7 V (Vpp),which are much lower than those ofa conventional PDLC grating. The pho-tographs of a first order diffraction spotunder the conditions of no applied volt-age and a 7 V square wave signal (Vpp)

are given in Fig. 6 a and b. It is signifi-cant that the intensity of the first orderdiffraction beam is much more blurry(Fig. 6b) when there is an applied volt-age on the TN cell for the reason ofindex matching of the LC and the poly-mer. However, the diffraction spot isbright in the case of no applied voltage(Fig. 6a), because the refractive indexof the LC is approximatively equal tone in the ACPDLC grating, so that therefractive-index modulation ∆n reachesa maximum; subsequently, the diffrac-

20 Applied Physics B – Lasers and Optics

FIGURE 6 First order diffraction spot of ACPDLC grating switched by TN modulator, no appliedvoltage on the TN modulator (a) and an applied square wave signal with the peak-to-peak value of7 V (b)

FIGURE 7 Stability testing forcontrast of ACPDLC grating duringfive weeks

tion efficiency of the grating in that caseis largest.

Finally, the stability of the ACPDLCgrating is tested. The prepared gratingis placed in a dry and dark box. TheE-O performance and its contrast ratioare tested and recorded once a week.Results during five weeks are shownin Fig. 7. There is no evident change.Some little fluctuation might be ascribedto the error caused by testing. Such re-sults indicate that the LC alignmentin the ACPDLC grating is stable, andthe anchoring energy for alignment issufficient.

3 Conclusions

We have prepared anACPDLC grating with a uniform LC

alignment and index matching, whichcould be electrically switched easily byan excellent tunable mode with a TNmodulator under a much lower appliedvoltage. The high contrast ratio is ob-tained through that tunable mode. Theexperimental data indicates that thethreshold voltage and saturation volt-age are 0.75–0.8 V (VPP) and 7 V (Vpp),respectively. The contrast ratio of theACPDLC grating is 245, which is threetimes higher than that of a conventionalPDLC grating. Moreover, stability test-ing shows that the alignment of theLCs in the ACPDLC is fine and stable,which ensures the perfect E-O propertyand high contrast of the device. Sucha kind of tunable mode contained inthe ACPDLC and TN cell overcomestwo main problems (high threshold volt-

age and low contrast), and realizes itspracticability.

ACKNOWLEDGEMENTS The au-thors would like to thank the support from the Nat-ural Science Foundation (Grant Nos. 60578035and 50473040) and the Science Foundation ofJilin Province, China (Grant Nos. 20050520 and20050321-2).

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