A fast-response and scattering-free polymer network liquid crystal (PNLC) light modulator is demonstrated at lambda = 1.06 mu m wavelength. A decay time of 117 mu s for 2 pi phase modulation is obtained at 70 degrees C, which is similar to 650 x faster than that of the host nematic LCs. The major tradeoff is the increased operating voltage. Potential applications include spatial light modulators and adaptive optics

Chen, Yuan

Sun, Jie

Wu, Shin-Tson

Xianyu, Haiqing

English

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University of Central Florida STARS Faculty Bibliography 2010s Faculty Bibliography 1-1-2011 Submillisecond-response polymer network liquid crystal phase modulators at 1.06-mu m wavelength Jie Sun University of Central Florida Haiqing Xianyu University of Central Florida Yuan Chen University of Central Florida Shin-Tson Wu University of Central Florida Find similar works at: https://stars.library.ucf.edu/facultybib2010 University of Central Florida Libraries http://library.ucf.edu This Article is brought to you for free and open access by the Faculty Bibliography at STARS. It has been accepted for inclusion in Faculty Bibliography 2010s by an authorized administrator of STARS. For more information, please contact STARS@ucf.edu. Recommended Citation Sun, Jie; Xianyu, Haiqing; Chen, Yuan; and Wu, Shin-Tson, "Submillisecond-response polymer network liquid crystal phase modulators at 1.06-mu m wavelength" (2011). Faculty Bibliography 2010s. 1963. https://stars.library.ucf.edu/facultybib2010/1963 Appl. Phys. Lett. 99, 021106 (2011); https://doi.org/10.1063/1.3611031 99, 021106© 2011 American Institute of Physics.Submillisecond-response polymer networkliquid crystal phase modulators at 1.06-μmwavelengthCite as: Appl. Phys. Lett. 99, 021106 (2011); https://doi.org/10.1063/1.3611031Submitted: 28 April 2011 . Accepted: 26 June 2011 . Published Online: 15 July 2011Jie Sun, Haiqing Xianyu, Yuan Chen, and Shin-Tson WuARTICLES YOU MAY BE INTERESTED INFast-response and scattering-free polymer network liquid crystals for infrared lightmodulatorsApplied Physics Letters 84, 1233 (2004); https://doi.org/10.1063/1.1649816A low voltage submillisecond-response polymer network liquid crystal spatial light modulatorApplied Physics Letters 104, 023305 (2014); https://doi.org/10.1063/1.4862474Fast-switching initially-transparent liquid crystal light shutter with crossed patternedelectrodesAIP Advances 5, 047118 (2015); https://doi.org/10.1063/1.4918277Submillisecond-response polymer network liquid crystal phase modulatorsat 1.06-lmwavelengthJie Sun, Haiqing Xianyu, Yuan Chen, and Shin-Tson Wua)College of Optics and Photonics, University of Central Florida, Orlando, Florida 32816, USA(Received 28 April 2011; accepted 26 June 2011; published online 15 July 2011)A fast-response and scattering-free polymer network liquid crystal (PNLC) light modulatoris demonstrated at k¼ 1.06 lm wavelength. A decay time of 117 ls for 2p phase modulation isobtained at 70 C, which is  650 faster than that of the host nematic LCs. The major tradeoffis the increased operating voltage. Potential applications include spatial light modulators andadaptive optics.VC 2011 American Institute of Physics. [doi:10.1063/1.3611031]Fast-response liquid crystal (LC) phase modulators areuseful for adaptive optics,1 spatial light modulators,2 andswitchable displays.3 The response time of a LC device ismainly determined by the cell gap, visco-elastic constant,and operating voltage.4 For nematic LCs, the turn-on processcan be accelerated by a high driving voltage while the turn-off process is governed by the restoring elastic torque whichis relatively slow. Dual frequency LCs allow both rise anddecay times to be accelerated by driving voltage; however, acomplex driving process is required.5–7 By introducing poly-mer into LC host,8 polymer network liquid crystal (PNLC)exhibits a dramatically improved response time because ofthe increased elastic constant. At elevated temperatures,PNLCs exhibit even faster response time due to the reducedviscosity. However, very few studies on the temperatureeffect of PNLCs have been reported.In this letter, we demonstrated a submillisecond-responseand scattering-free PNLC phase modulator at k¼ 1.06 lm.The measured response time is 117 ls for a 2p phase changeat  70 C with an operating voltage of V2p¼ 90 V. Tounderstand the underlying physical mechanisms, we meas-ured the temperature effects on birefringence (Dn), dielectricanisotropy (De), splay elastic constant (K11), and rotationalviscosity (c1) of the PNLC. Our results indicate that the splayelastic constant of PNLC is increased by  950which con-tributes to fast response time but also to the increased operat-ing voltage.To fabricate PNLC modulators, we first prepared a hostnematic LC mixture by mixing 60% Merck E44 and 40%Merck E48 in order to obtain high birefringence and goodmiscibility with monomers. Light scattering is a seriousproblem for PNLCs. To suppress scattering, the domain sizeof PNLC should be smaller than the wavelength. Increasingpolymer concentration would shrink the domain size whichis favourable for suppressing scattering, but the operatingvoltage will increase dramatically. In our experiment, wehave tried several monomers and different concentrationsand found that following combination led to tolerable scat-tering (<1%) and minimum voltage: 7 wt. % bisphenol-A-dimethacrylate (M1) and 5 wt. % RM-257 (Merck). The rea-son for using M1 is because it helps to reduce the operatingvoltage and improve solubility.We then filled the LC/monomer mixture into an emptycell with homogenous alignment and cured it with a UVlamp. The cell gap was controlled at 11.8 lm in order toobtain more than 2p phase change while keeping the operat-ing voltage below 100 V. High UV exposure intensity helpsto reduce domain sizes of polymer network. In our experi-ment, we controlled the UV intensity at  300 mW/cm2 andexposure time for 1 h.To characterize the electro-optical properties of thePNLC cell, we measured its voltage-dependent transmittance(VT) with a laser beam at k¼ 1.06 lm. The PNLC cell wassandwiched between two crossed polarizers, with the rub-bing direction oriented at 45  to the polarizer’s transmissionaxis. The driving frequency is 5 kHz. The clearing point ofPNLC was measured to be Tc¼ 92.0 C, which is slightlylower than that of the LC host (Tc¼ 96.2 C).Figure 1 depicts the measured VT curves of the PNLCcell (solid line) and the host LC cell (dashed lines) atk¼ 1.06 lm. The total phase retardation of the PNLC cell is3.0p and its corresponding voltage for d¼ 2p occurs atV2p¼ 90 Vrms. The threshold voltage of the PNLC cell isVth¼ 25.1 Vrms while it is 0.72 Vrms for the host LC cell.The increased threshold voltage is attributed to the increasedK11 because the LC domains are tightly anchored by thepolymer networks. The peak transmittance of the PNLC cellis comparable to that of the host nematic LC cell, indicatingthat light scattering is negligible at this wavelength.Decay time of the PNLC cell at 70 C which was ini-tially biased at V2p¼ 90 V was measured. The biasedFIG. 1. (Color online) Measured VT curve of host nematic LC (dash lines)and PNLC (solid line) cells. The cell gap for both cells is 11.8 lm. Wave-length k ¼ 1.06 lm and T¼ 70 C.a)Electronic mail: swu@mail.ucf.edu.0003-6951/2011/99(2)/021106/3/$30.00 VC 2011 American Institute of Physics99, 021106-1APPLIED PHYSICS LETTERS 99, 021106 (2011)voltage was removed abruptly at t¼ 0 and then the transientrelaxation process recorded by a photodiode. The experimen-tal data fit well (not shown) with following exponentialphase decay equation: dðtÞ ¼ 2p expðt=sÞ, where s is thedecay time constant.9 From fitting, we obtain s¼ 50 ls. Ifthe decay time is defined from 100 ! 10% phase change,then we find trelax¼ 117 ls. The rise time trise (0 ! 90%)was measured to be 45 ls. For the host nematic LC cell, itstime constant s is  35 ms, and the corresponding decaytime and rise time is  77 ms and  104 ms, respectively.Our PNLC improves the decay time by  658 at theexpense of increasing V2p from 1.42 V to 90 V. Figure 2shows the temperature-dependent decay time of our PNLCcell while keeping phase modulation at 2p. As the operatingtemperature increases, the response time gets faster becauseof the reduced visco-elastic constant.Figure 3 shows the temperature dependent Dn of thePNLC cell. As the temperature increases, the birefringencedecreases gradually. The filled and open circles represent theexperimental data for the PNLC and host LC, respectively,while lines represent the fittings with Haller’s equations:10,11Dn ¼ Dn0  S and S ¼ ð1  T=TcÞb; where Dn0 is the bire-fringence at T¼ 0, b is a material constant and S is the orderparameter.From Fig. 3, the PNLC shows lower Dn0 and b(Dn0 ¼ 0:235 and b ¼ 0.20) than the host LC (Dn0 ¼ 0:325and b¼ 0.25). The Dn0 of PNLC is only 72% (instead of88%) of that of the host LC. This discrepancy is believed toresult from the disturbed LC alignment surrounded by thepolymer networks. Although each substrate surface has analignment layer, the polymer networks may not follow per-fectly the surface rubbing direction. Moreover, the employedM1 polymer is not rod-like so that its formed networks willhave bend structure, which further reduces the birefringenceof the PNLC composite. Since some LC molecules are anch-ored tightly by the surrounding polymer networks, the bire-fringence of PNLC is less sensitive to the temperaturechange than the host LC. This explains why the b valuedecreases from 0.25 for the host LC to 0.20 for the PNLC.We also measured the temperature dependent Vththrough the VT curves and De of PNLC by the conventionaltwo-cell method.9 The De results (not shown) fit withDe ¼ C0  S well, where C0 is a material constant and S is theorder parameter. For PNLC, C0¼ 13.5 and for host nematicLC, C0¼ 22.0. By using the obtained Vth and De, we thencalculate K11 according to the following equation:12Vth ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiK11=e0Dep: (1)At 70 C, the splay elastic constant of our PNLC is K11¼ 1:63  104 pN, which is  920 larger than that of theLC host, K11 ¼ 17:7 pN. This is because the strong polymernetwork anchoring helps to increase the elastic constant ofthe PNLC. On the positive side, a large elastic constant isfavorable for achieving faster response. But on the downside, it increases the operating voltage, as Eq. (1) indicates.Figure 4 depicts K11 of our PNLC against S2. Symbolsstand for the experimental data and dashed lines are the fit-tings according to Maier-Saupe theory:13 K11 ¼ A0  S2,where A0 is a material constant. Through fittings, we getA0 ¼ 68:5 pN for the LC host and A0 ¼ 4:7  104 pN for thePNLC. However, it is found that K11 of PNLC no longer fitswell with the mean field theory because of the involvementof polymer network. Therefore, we propose a modified equa-tion for the K11 of PNLCsK11 ¼ A0  Sa; (2)FIG. 2. (Color online) Temperature-dependent decay time of the PNLCcell. d¼ 11.8 lm. Filled circles are the measured decay time for 2p phasechange.FIG. 3. (Color online) Temperature-dependent birefringence of PNLCs andLC host. Filled circles and solid lines are the experimental and fitting resultsfor PNLC, respectively. Open circles and dashed lines are the experimentaland fitting results for LC host, respectively.FIG. 4. (Color online) Elastic constant (K11) vs. S2. Filled circles and emptycircles are the experimental results for PNLC and LC host, respectively. Thedashed lines are fitting results using K11 ¼ A0  S2, while the solid line is thefitting using Eq. (2) with a¼ 1.5.021106-2 Sun et al. Appl. Phys. Lett. 99, 021106 (2011)where a is a material constant related to polymer network.For nematics, a¼ 2. But for our PNLC, we found A0 ¼ 3:7 104 pN and a¼ 1.5 (solid line).In order to get the rotational viscosity of PNLCs, we firstobtained visco-elastic constant (c1/K11) by measuring thefree relaxation time of the sample cell and then calculated c1based on the K11 values we already obtained. Figure 5 showsthe temperature dependent rotational viscosity of PNLC(filled circles) and nematic host LCs (open circles). The solidline is the fitting results according to the modified Arrheniusmodel14,15c1 ¼ bS expðEa=kBTÞ; (3)where b is the proportionality constant, Ea is the activationenergy, and KB is the Boltzmann constant. From Fig. 5, themeasured viscosity of PNLC is very close to that of LC host;the small variation results from the minor difference in theirclearing temperatures. The fitting parameters are b ¼ 5:0 104 mPa  s and Ea ¼ 372:0 meV.In conclusion, we have demonstrated a submillisecond-response and scattering-free PNLC phase modulator fork¼ 1.06 lm. A decay time of 117 ls is obtained at 70 C for2p phase modulation. Our results show that the temperaturedependent birefringence, dielectric anisotropy, and rotationalviscosity of PNLCs still follow the models for nematic LCs.The rotational viscosity of PNLC remains basically the sameas that of the LC host. However, since the splay elastic con-stant (K11) is largely dependent on the polymer network,some deviation from nematic LCs is observed.This manuscript has been approved by DARPA for pub-lic release under Distribution Statement “A” (Approved forPublic Release, Distribution Unlimited). The viewsexpressed are those of the author and do not reflect the offi-cial policy or position of the Department of Defense or theU.S. Government.1M. Langlois, C. D. Saunter, C. N. Dunlop, R. Myers, and G. D. Love, Opt.Express 12, 1689 (2004).2U. Efron, Spatial Light Modulator Technology: Materials, Devices, andApplications (Marcel Dekker, New York, 1994).3M. G. H. Hiddink, S. T. de Zwart, O. H. Willemsen, and T. Dekker, SIDSymp. Digest 37, 1142 (2006).4S. T. Wu, Appl. Opt. 28, 48 (1989).5H. K. Bu¨cher, R. T. Klingbiel, and J. P. VanMeter, Appl. Phys. Lett. 25,186 (1974).6M. Schadt, Mol. Cryst. Liq. Cryst. 66, 319 (1981).7J. Sun, H. Xianyu, S. Gauza, and S. T. Wu, Liq. Cryst. 36, 1401 (2009).8Y. H. Fan, Y. H. Lin, H. Ren, S. Gauza, and S. T. Wu, Appl. Phys. Lett.84, 1233 (2004).9S. T. Wu and C. S. Wu, Phys. Rev. A 42, 2219 (1990).10I. Haller, Prog. Solid State Chem. 10, 103 (1975).11S. T. Wu, Phys. Rev. A 33, 1270 (1986).12I. C. Khoo and S. T. Wu, Optics and Nonlinear Optics of Liquid Crystals(World Scientific, Singapore, 1993).13W. Maier and A. Saupe, Z. Naturforsch. Teil A 15, 287 (1960).14W. H. De Jeu, Mol. Cryst. Liq. Cryst. 63, 83 (1981).15S. T. Wu and C. S. Wu, Liq. Cryst. 8, 171 (1990).FIG. 5. (Color online) Rotational viscosity (c1) vs. temperature. Filledcircles and empty circles are the experimental results for PNLC and LChost, respectively. The solid line represents the fitting results with Eq. (3).021106-3 Sun et al. Appl. Phys. Lett. 99, 021106 (2011)

Submillisecond-response polymer network liquid crystal phase modulators at 1.06-mu m wavelength

Jie Sun

Haiqing Xianyu

Yuan Chen

Shin-Tson Wu

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Applied Physics Letters

Submillisecond-response polymer network liquid crystal phase modulators at 1.06-<i>μ</i>m wavelength

https://stars.library.ucf.edu/cgi/viewcontent.cgi?article=2962&amp;context=facultybib2010

Submillisecond-response polymer network liquid crystal phase modulators at 1.06-mu m wavelength

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