Continuous Optical Phase Modulation in a Copolymer Network

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Continuous optical phase modulation in a copolymer network nematic liquid crystal Alexander Lorenz, Larissa Braun, and Valeria Kolosova ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.6b00072 • Publication Date (Web): 02 Jun 2016 Downloaded from http://pubs.acs.org on June 3, 2016

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Figure 1. Schematic of optical setup (monochromatized light). Transmittance of a copolymer network LC test cell (two ITO coated glass plates and copolymer stabilized LC layer) between crossed polarizers. (a) Initial state. (b) Applying a voltage to the test cell will decrease the effective birefringence inside the LC layer and a signal of 12 V amplitude was found to yield 100% transmittance for a green wavelength. The optical phase shift ΓV inside the test cells is indicated. 59x42mm (300 x 300 DPI)

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Figure 2. Polarized optical micrographs recorded in a copolymer network LC test cell at 22 °C with white light (a) and with a green wavelength (b) at various voltages (indicated). The polarizers were crossed and the sample was aligned in 45° position. 47x28mm (300 x 300 DPI)


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Figure 3. Optical retardation measured with Berek tilting compensator (white light) at 22 °C. A halfwavelength change of optical retardation for a blue and red wavelength are indicated, respectively. 57x39mm (300 x 300 DPI)


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Figure 5. Response times ton (a) and toff (b) extracted from the recorded transmission curves at 25 °C (black symbols) and 30 °C (blue symbols). Data in (b) was fitted with a linear trend, respectively. 65x50mm (300 x 300 DPI)


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Figure 6. Phase shift vs. time obtained from transmittance data recorded at 25 °C with addressing voltages from 6 to 20 V, selected voltages are shown. The addressing signals were first switched on (a) and then switched off (b) after a few seconds had passed, respectively. 59x41mm (300 x 300 DPI)


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Figure 7. Response times ton,ph (a) and toff,ph (b) extracted from the recorded transmission curves at 25 °C (black symbols) and 30 °C (blue symbols). Data in (b) was fitted with a linear trend, respectively. 65x50mm (300 x 300 DPI)


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Figure 8. Transmittance vs. time (top curve) for a 250 Hz triangular addressing signal (lower curve) recorded at 30 °C and a probe wavelength of 545 nm. 67x53mm (300 x 300 DPI)


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Figure 9. Scattering loss vs. applied voltage recorded with three different color filters, respectively, in a copolymer network LC (test cell with 2.5 µm cell gap). 37x17mm (300 x 300 DPI)


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Figure 10. Optical retardation vs. electric addressing field recorded in LC test cells (4 µm cell gap) filled with both copolymer network (samples A to D) and samples with polymerized RM257 (samples E to H). 55x38mm (300 x 300 DPI)


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Continuous optical phase modulation in a copolymer network nematic liquid crystal Alexander Lorenz,*,† Larissa Braun,‡ and Valeria Kolosova‡ † ‡

Department of Chemistry, Paderborn University, Warburger Str. 100, 33098 Paderborn, Germany Stranski-Laboratory, TU Berlin, Str. des 17. Juni 124, 10623, Berlin, Germany

Polymer network liquid crystal, liquid crystal, photonics, polymer, phase modulation ABSTRACT A nematic liquid crystal (LC) was doped with a reactive mixture of a mesogenic diacrylate-monomer, a monofunctional acrylate monomer, and photoinitiator. This mixture was filled in transparent test cells with planar electrodes and uniform director alignment. Photopolymerization of the monomers was excited by exposure with ultraviolet light. After photo-exposure, the test cells were electrically addressed and their electro-optic response properties were studied by polarized optical microscopy. The voltage dependence of the optical retardation was investigated with a Berek-tilting compensator. By applying voltages in the range of 4 – 20 V, the optical retardation was continuously tuned. Importantly, the induced optical phase shift had a smooth onset, which is desirable in electro-optic modulators. By applying various voltages, the optical retardation was continuously tuned. Already low driving voltages < 15 V were sufficient to induce a half wavelength retardation throughout the visible spectral range. The time dependence of the transmittance was investigated with monochromatized and converted to optical phase shift data. Square wave signals (2 kHz) of various amplitudes were switched on and subsequently switched off, respectively. The response times were extracted from the measured data and sums (ton+toff) < 3 ms were found at driving voltages 12 V – 20 V. The copolymer network LC was suitable for 500 Hz modulations. The electro-optic properties of samples with and without added acrylate monomer were compared.

Nematic liquid crystals (LCs) are highly responsive, optically uniaxial materials. Because of their electro-optic properties, nematic LCs are ideal to tune the properties of photonic structures1 and they are widely applied in electro-optic displays, microdisplays, and optical phase modulators2-9. However, driving modes, where the optical phase shift can be continuously tuned (continuous optical phase modulation), remain a challenge2,3. In general, nematic LCs suffer from slow electrooptic response times. Although switching a high electric addressing field on will yield fast response times ton, their response times toff (200 – 300 ms) are independent of the addressing field strengths. In intensity modulators, such as in LC displays6, the performance of nematic LCs can be pushed by sophisticated confinement conditions (pretilted director fields), or by use of structured electrodes and fringe electric fields. However, inside photonic microstructures, pretilted director fields are especially hard to obtain. In optical phase modulators planar electrodes are in general desirable. Polymer network liquid crystals4,8-10 are candidates to solve these challenges. These hybrid materials can be created by photo curing of a nematic LC, doped with up to 10% of a reactive monomer mixture (reactive mesogens and photoinitiator). Usually, the reactive mixture consisted of crosslinking mesogenic acrylates like RM2574-6 that have a mesogenic core and two acrylate functional groups, and ≈1% of photoinitiator. The doped LC was filled in a test device (for example a LC test cell) and subsequently exposed with ultraviolet (UV) light in order to excite photopolymerization. A phase separated, crosslinked polymer network was thus created within the LC layer. Such network structures were phenomenologically modeled as

additional alignment field10 for the LC director, which enhances the switching time toff. Various polymer network LCs with doping concentrations of 3% to 10% were reviewed10: The response times toff were in the range of 10 ms to 6.7 ms, two orders of magnitude faster than in the neat LCs. In order to stabilize a liquid crystal, also non-mesogenic acrylates were included in the reactive mixtures (in addition to a reactive mesogens and photoinitiator). Use of such in-situ generated copolymers was especially successful in order to enhance the temperature stability of the liquid crystalline blue phase11,12 and has since been studied in various chiral LCs5,1216 . In these experiments, the purpose of the phase separated polymer network was to structurally stabilize a LC phase or texture5,16, respectively. In the presented experiments, a copolymer network nematic LC was investigated. The reactive mixture consisted of reactive mesogen RM257, a non-mesogenic acrylate (2ethylhexylacrylate, EHA), and added photoinitiator (Irgacure 819). The nematic LC E7 was doped with 10% of reactive mixture and filled in thin (2.5 µm cell gap) LC test cells with uniform director alignment and photo cured to yield a copolymer LC film confined in-between planar, transparent electrodes. The electro-optic response properties of the cured test cells were investigated with polarized optical microscopy and continuous phase modulation with desirable response properties was found. A reactive mixture (A*) was prepared and test cells were filled in a yellow-light environment in order to avoid any unwanted photo exposure. The method was analogue to previous work


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with chiral nematic5,16 and blue phase6 LCs: Importantly, the present mixtures contained no chiral dopant. First, E7 (178 mg, CAS#: 63748-28-7) was doped with RM257 (10 mg, CAS#: 174063-87-7) and EHA (10 mg, CAS#: 103-11-7) in a brown glass vial. Irgacure 819 (0.2 mg, which corresponds to 1% with respect to the monomer concentration, CAS#: 162881-26-7) was added. The glass vial was closed, placed in a fitting metal tube and kept on a heat plate at a temperature of 60 °C, in order to allow for thorough thermal mixing in the isotropic phase. The reactive mixture was kept on the heat plate for at least 45 minutes. Subsequently, the doped LC was filled in liquid crystal test cells (EHC, 2.5 µm cell gap, planar indium tin oxide electrodes, polyimide alignment layers, antiparallel rubbing,). The test cells were placed on a heat plate (60 °C, to allow for filling within the isotropic phase of the mixtures) and a few minutes were allowed for thermal equilibration. The test cells were filled via capillary action by placing a droplet of doped LC at their open edges. The test cells were then kept on the heat plate for 20 minutes and homogenous filling was obtained. Subsequently, a test cell was placed on the sample holder of a microscope hot stage (0.1 °C temperature precision) at a temperature of 30.0 °C. The hot stage was placed in a UV-proof cabinet and exposed for 15 minutes with a Winger high power UV LED (400 nm wavelength, 1.2 W irradiance power) placed at a distance of ≈ 4 cm from the samples. The cured test cells were electrically contacted and investigated with a polarized optical microscope (Leitz Laborlux 12 POL S, crossed polarizers) fitted with electrically stabilized tungsten halogen white light source (driven by a Toellner TCE 8751 stabilized DC power source) and microscope hot stage (0.1 °C temperature precision). In the no-field state, the test cells showed uniaxial director alignment. Upon applying a voltage, the LC director was distorted and the effective birefringence inside the copolymer network LC layer was reduced. A schematic of the electro-optic setup is shown (Figure 1). Micrographs of a test cell were recoded with a CMOS color camera (the imaging source DFK MKU130-10x22) and electrically addressed with square wave signals (2 kHz) of various amplitudes (generated with a Toellner TCE 7711 A signal synthesizer and in-house assembled amplifier). Signal amplitudes were measured with a multimeter scanner (Keithley Model 199). If the test cells were studied with white light (Figure 2a), characteristic Michel-Levy birefringence colors were observed. Upon applying a voltage, a color change from violet (0V) to purple (6V), orange (8V) and light ochre (20 V) was observed, which is characteristic for a decrease in birefringence. The test cells were also studied with monochromatized light (Schott interference filter of variable pitch). The test cells showed an optically extinct state for green wavelengths at 0 V. If so, the transmittance can be described7 by T = sin(Γv/2)2, where Γv is the voltage dependent phase retardation. Upon applying a voltage, the transmittance first increased with increasing voltage (5 – 12 V) and decreased again at higher voltages, which qualitatively indicated high optical phase shifts Γv > π at voltages > 12V. The voltage dependence of the optical retardation was investigated with a Berek-tilting-compensator (Leitz Compensator M). The sample was aligned in compensation direction and investigated with white light. The measured data are shown (Figure 3). As expected from qualitative investigations, the optical retardation inside the copolymer network LC decreased with increasing applied voltages. The transparent sample

showed reasonable changes of the optical retardation. Importantly, the observed curve shows continuous changes and has a smooth onset at 3V, although the test cells were not treated with sophisticated boundary conditions (for example to induce pretilt angles and avoid discontinuities in the onset). A half wavelength change in a transparent sample is suitable for high contrast switching of selected wavelengths. Moreover, it corresponds to full 2π optical phase modulation in a reflective sample of equal cell gap4. The addressing voltages for a halfwavelength change of the optical retardation Vπ are indicated in Figure 3 for a blue wavelength (450 nm, 10 V) and a red wavelength (633 nm, 14.5 V), respectively. The compensator was removed from the microscope, the voltage was adjusted to 0 V, the sample was heated to temperature of 25 °C, and the wavelength was adjusted to 545 nm, which yielded an optically extinct state. The voltage dependence of the transmittance was studied with a photomultiplier tube module (Hamamatsu H5701–02). The signal was recorded with a digital storage oscilloscope (Picotech Picoscope 3206). The relative transmittance T/Tmax vs. time is shown (Figure 4). Various addressing voltages were switched on and (after a few seconds had passed) switched off, again. Upon switching a voltage V < 14 V on, the recorded transmission increased continuously with increasing voltage. If higher voltages were switched on, the transmittance first increased, and decreased again after the maximum transmittance Tmax was reached. These addressing experiments were repeated at a temperature of 30 °C. The wavelength was adjusted to 535 nm to account for the slightly lower birefringence in the sample at this temperature. The response times ton and toff (both for a 0-90% response) were extracted from the recorded data, respectively. A comparison of the response times is shown (Figure 5). The response times ton decreased with increasing voltage from 4 – 5 ms at 6 V to well below 1 ms at voltages > 12 V. The response times toff increased with increasing voltages, which can be described with a linear trend, respectively. The sums of response times (ton + toff ) were well below 3 ms in the range 10 – 17 V. The observed voltage-dependence of the response time toff is different than in an unstabilized nematic LC where constant response times toff are expected. Therefore, the time dependence of the transmittance was converted to optical phase shift data according to7 Γ = 2 · arcsin(T0.5) by using Matlab scripts, respectively. The phase shift in radians is shown vs. time at 25 °C (Figure 6). The samples showed an initial phase shift of ≈ 2π, which was reduced, if a signal was switched on and recovered the initial value quickly if the signal was switched off, again. Response times ton,ph and toff,ph were extracted from the data. A comparison of the response times is shown (Figure 7). The response times ton,ph decreased from 4.3 ms at 6 V to 1 ms at 20 V. The response times toff,ph increased with increasing voltages, which can be described with a linear trend, respectively. At 30 °C, the response times toff,ph were below 2 ms throughout the studied voltage range and up to 0.8 ms lower than at 25 °C. The sums of response times (ton,ph + toff,ph ) were ≈ 3 ms at voltages > 11 V. The conversion to phase shift data (which will reveal the response times of the LC more reliably) showed, that the slope of the linear trends is now lower. In addition to addressing with square wave signals of 2 kHz frequency, the time dependent transmittance of the copolymer network nematic LC was also studied with a 250 Hz triangular wave signal (Figure 8). As expected for a fast-enough electrooptic LC-based modulator, where the response depends on the


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magnitude (but not the sign7) of the addressing voltage, an electro-optic answer with doubled frequency (500 Hz) of the addressing signal was found. As expected, the detected signal had a small offset from the addressing signal. The detected transmittance minima had a 300 µs offset from the zerocrossing of the applied signal. The offset was much smaller (150 µs) for the transmittance maxima (Figure 8). This finding reflects the voltage-dependence of the response times ton: Since the modulation speed follows the change of the addressing voltage, the offset will be higher when the signal has a local minimum and smaller when the signal has a local maximum. Scattering losses of the copolymer enhanced LC layer were investigated with a red, green, and blue color filter, respectively. This technique was chosen in order to avoid perturbations caused by the voltage-dependence of positions of interference fringes in the transmission spectrum of the LC test cell (where the parallel ITO-electrodes form a Fabry-Perot-resonator). The recorded data (Figure 9) show that the scattering loss depends on both color and addressing voltage: The scattering loss is much lower for red and blue light than for green light. The scattering loss first increases with increasing voltage, reaches a maximum at 12 V and then decreases with increasing voltage. Scattering is presumably caused by slightly varying director reorientations in adjacent domains, which is more pronounced if the driving voltage is low. The observed color dependence of the scattering can be useful to further understand the behavior of and potentially reveal the domain sizes inside the samples by studying the losses with a spectrometer. In order to shed more light on the role of EHA as component of the reactive mixture, seven more reactive mixtures with various doping concentrations of either RM257 and EHA (B, C, D) or selectively RM257 (E to H) were prepared. The concentration of the photo-initiator (1%) with respect to the overall sample mass was kept constant throughout these samples. These reactive mixtures were filled in test cells with a cell gap of ≈ 6.8 µm and photo-cured, respectively. These test cells were chosen since a deviation in cell gap of ≈0.4 µm between individual test cells will have less influence on the response properties than in thinner test cells. The optical retardation vs. electric addressing field was investigated (Figure 10). Although sample E with a low doping concentration of 4% RM257 showed non-uniformities of the LC texture, measurement of the optical retardation was still possible. At higher doping concentrations, the optical textures were perfect as in the EHA doped samples. At 10% overall doping concentration sample H (10% RM257) was much less responsive than the samples D (7% RM257 + 3% EHA) and C (5% RM257 + 5% EHA) throughout the investigated voltage range (0 – 44 V). Thus, replacing a small amount of RM257 by EHA can enhance the responsivity of the polymer enhanced LC. In samples C and D the optical retardation at moderate field strength was slightly reduced as compared to samples F (5% RM257) and G (7% RM257) but they show equal optical retardation at field strength > 4 V/µm. The response times in the samples were investigated (Figure 11). Comparison of samples E (6.5 ms) and B (1.2 ms) shows that EHA can lower the response times by a factor ≈5. In addition, samples C and D (with added EHA) also showed lower response times (by a factor ≈2.5) than samples F and G (without EHA), respectively (data listed in Figure 11). From the increase in switching speed one can speculate, that the presence of EHA will decrease the domain size4,8,9,10 in the copolymer network LC. As expected4,10, the driving voltage Vπ (where a phase change of π is induced)

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increases with decreasing response times. Samples with moderate copolymer content (samples A*, B) already show response times of 0.8 – 1.3 ms, which is a highly desirable range. The responses found in sample C (0.3 ms) were even as fast as in a polymer stabilized blue phase LC5,6 or polymer stabilized hyper twisted chiral nematic LC5,16, which consisted of the same or comparable types of host LCs. High modulation frequencies were observed. A thin test cell (filled with the copolymer network LC sample C) was addressed with sine wave signals of 18 V amplitude and various frequencies. As seen in Figure 11 (inset) 2 kHz modulations with relatively high modulation amplitude were found. However, modulation amplitude will be decreased at a higher (6 kHz) frequency, as expected. In conclusion, a copolymer network LC with highly desirable electro-optic response properties was found. Addition of a non-mesogenic acrylate monomer was found a convenient tool to tune the response properties in a copolymer network LC. The presence of the acrylate monomer will enhance the stability of the polymer network LC and result in faster response times. The required driving voltages Vπ were more than 50% lower than in polymer stabilized blue phase LC6 or uniformly standing helix LC5 based on the same host LC. Continuous optical phase modulation was detected and already a thin (2.5 µm) transparent device showed phase modulation depths >π throughout the visible spectral range. Reactive mixtures with various doping concentrations were studied. The response properties of an electro-optic modulator with 2.5 µm cell gap filled with polymer network LC A* were studied in depth: Only low driving voltages Vπ < 20 V were required. The time dependence of the electro-optic responses was studied with monochromatic light. The response times toff were 3 – 4 times lower than in previous studies10 of nematic LCs stabilized with homopolymer networks. In addition, the response times ton,ph were investigated. It was found that the response times toff and toff,ph can be described with a linear trend. For a polymer network LC (A*) with low driving voltages Vπ, the sums of response times (ton + toff ) were well below 3 ms in the range 10 – 17 V. The sums of response times (ton,ph + toff,ph ) were ≈ 3 ms at voltages > 11 V and thus in a voltage range, where π optical phase modulation (which corresponds to full 2π optical phase modulation in a reflective device) can be achieved throughout the visible spectral range. Due to the low driving voltages required, the presented results have great prospects to push uses of polymer stabilized LCs in photonic structures. Both, the low driving voltages and the desirable response times in copolymer network LCs with 4 – 5 % RM257 and 4 – 5 % EHA reveal the high potential of uses of copolymer network nematic LCs in optical phase modulating devices.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Phone: +49 5251 60 5728. Fax: +49 5251 60 4208.

Author Contributions The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT


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Financial support by the German Research Foundation (GRK 1464) and TU Berlin (TU-PostDoc-Fellowship) are gratefully acknowledged.

ABBREVIATIONS LC, liquid crystal; EHA, 2-ethylhexylacrylate; UV, ultra violet.

REFERENCES (1) Kitzerow, H.-S.; Lorenz, A.; Matthias, H. Tuneable Photonic Crystals Obtained by Liquid Crystal Infiltration. Phys. Status Solidi A 2007, 204, 3754–3767. (2) Zhang, Z.; You, Z.; Chu, D. Fundamentals of Phase-Only Liquid Crystal on Silicon (LCOS) Devices. Light: Sci. Appl. 2014, 3, e213. (3) Vettese, D. Microdisplays: Liquid Crystal on Silicon. Nat. Photonics 2010, 4, 752–754. (4) Sun, J.; Wu, S.-T. Recent Advances in Polymer Network Liquid Crystal Spatial Light Modulators. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 183–192. (5) Hyman, R. M.; Lorenz, A.; Wilkinson, T. D. Phase Modulation Using Different Orientations of a Chiral Nematic in Liquid Crystal over Silicon Devices. Liq. Cryst. 2016, 43, 1–8. (6) Hyman, R. M.; Lorenz, A.; Morris, S. M.; Wilkinson, T. D. Polarization-Independent Phase Modulation Using a Blue-Phase Liquid Crystal over Silicon Device. Appl. Opt. 2014, 53, 6925. (7) Lueder, E. Liquid Crystal Displays: Addressing Schemes and Electro-Optical Effects, 2nd Edition, Wiley, 2010. (8) Peng, F.; Chen, H.; Tripathi, S.; Twieg, R. J.; Wu, S.-T. FastResponse Infrared Phase Modulator Based on Polymer Network Liquid Crystal. Opt. Mater. Express 2015, 5, 265. (9) Peng, F.; Xu, D.; Chen, H.; Wu, S.-T. Low Voltage Polymer Network Liquid Crystal for Infrared Spatial Light Modulators. Opt. Express 2015, 23, 2361.

(10) Yang, D.-K.; Cui, Y.; Nemati, H.; Zhou, X.; Moheghi, A. Modeling Aligning Effect of Polymer Network in Polymer Stabilized Nematic Liquid Crystals. J. Appl. Phys. 2013, 114, 243515. (11) Kikuchi, H.; Yokota, M.; Hisakado, Y.; Yang, H.; Kajiyama, T. Polymer-Stabilized Liquid Crystal Blue Phases. Nat. Mater. 2002, 1, 64–68.(12) Nordendorf, G.; Hoischen, A.; Schmidtke, J.; Wilkes, D.; Kitzerow, H.-S. Polymer-Stabilized Blue Phases: Promising Mesophases for a New Generation of Liquid Crystal Displays: POLYMER-STABILIZED BLUE PHASES. Polym. Adv. Technol. 2014, 25, 1195–1207. (13) Yuan, J.; Tan, G.; Xu, D.; Peng, F.; Lorenz, A.; Wu, S.-T. Low-Voltage and Fast-Response Polymer-Stabilized Hyper-Twisted Nematic Liquid Crystal. Opt. Mater. Express 2015, 5, 1339. (14) Yan, J.; Cheng, H.-C.; Gauza, S.; Li, Y.; Jiao, M.; Rao, L.; Wu, S.-T. Extended Kerr Effect of Polymer-Stabilized Blue-Phase Liquid Crystals. Appl. Phys. Lett. 2010, 96, 071105. (15) Nordendorf, G.; Lorenz, A.; Hoischen, A.; Schmidtke, J.; Kitzerow, H.; Wilkes, D.; Wittek, M. Hysteresis and Memory Factor of the Kerr Effect in Blue Phases. J. Appl. Phys. 2013, 114, 173104. (16) Lorenz, A.; Gardiner, D. J.; Morris, S. M.; Castles, F.; Qasim, M. M.; Choi, S. S.; Kim, W.-S.; Coles, H. J.; Wilkinson, T. D. Electrical Addressing of Polymer Stabilized Hyper-Twisted Chiral Nematic Liquid Crystals with Interdigitated Electrodes: Experiment and Model. Appl. Phys. Lett. 2014, 104, 071102.


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Continuous Optical Phase Modulation in a Copolymer Network

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