Broadband optical switch based on liquid crystal dynamic scattering

This work demonstrates a novel broadband optical switch, based on dynamic-scattering effect in liquid crystals (LCs). Dynamic-scatteringmode technology was developed for display applications over four decades ago, but was displaced in favor of the twisted-nematic LCs. However, we show that with the recent development of more stable LCs, dynamic scattering provides significant advantages over other technologies for optical switching. We demonstrate broadband polarization-insensitive rejection of incoming laser irradiation by 4 to 5 orders of magnitude with switching times from clear to blocking state of 150 s. Further improvements in the switching times are possible with device engineering. No degradation of devices is found after hundreds of switching cycles. The light-rejection mechanism is due to scattering, induced by disruption of LC director orientation with dopant ion motion in the applied electric field. Angular dependence of scattering is characterized as a function of bias voltage. The dynamic-scattering mode technology should enable novel applications for laser light rejection and fast optical switching with performance better than the capabilities of current devices based on electrochromic, photochromic or twisted-nematic technology. © 2016 Optical Society of America OCIS codes: (230.3720) Liquid Crystal Devices; (230.2030) Dispersion; References and links 1. S. Heusing and M. A. Aegerter, “Sol-gel coatings for electrochromic devices,” in Sol-Gel Processing for Conventional and Alternative Energy, M. Aparicio, A. Jitianu and L. C. Klein, ed. (Springer Science Business Media, 2012). 2. M. W. Geis, T. M. Lyszczarz, R. M. Osgood, and B. R. Kimball, “30 to 50 ns liquid-crystal optical switches,” Opt. Express 18(18), 18886-18893 (2010). 3. G. H. Heilmeier, L. A. Zanoni, and L. A. Barton, “Dynamic scattering: a new electrooptic effect in certain classes of nematic liquid crystal,” P. IEEE 56(7), 1162-1171, (1968). 4. G. H. Heilmeier, L. A. Zanoni, and L. A. Barton, “Further studies of the dynamic scattering mode in nematic liquid crystals, IEEE T. Electron Dev. Ed-17(1), 22-26 (1970). 5. H. Kawamoto, “The history of liquid-crystal displays,” P. IEEE 90(4), 460-500 (2002). 6. H. S. Lim, J. D. Margerum, and A. Graube, “Electrochemical properties of dopants and the D-C dynamic scattering of a nematic liquid crystal,” Elec. Soc. S. 124(9), 1389-1394 (1977). 7. S. Kai, K. Yamaguchi, and K. Hirakawa, “Observation of flow figures on nematic liquid crystal MBBA,” J. Appl. Phys. 14(11), 1653-1658 (1975). 8. R. Herino, “Further investigations and comments on the dynamic scattering mode in nematic liquid crystals,” J. Appl. Phys. 52(5), 3690-3692 (1981). 9. W. Helfrich, “Conduction-induced alignment of nematic liquid crystals: basic model and stability considerations,” J. Chem. Phys. 51(9), 4092-4105 (1969). 10. B. Bahadur, “Dynamic scattering mode LCDs,” in Liquid Crystals Applications and Uses, Vol. 1, B. Birendra, ed. (World Scientific Publishing Co. Pte. Ltd., 1995). 11. L.M. Blinov and V.G. Chigrinov, “Modulated and nonuniform structures in nematic liquid crystals,” in Electrooptic Effects in Liquid Crystal Materials, L. M. Blinov and V.G. Chigrinov, ed. (Springer-Verlag New York, Inc., 1994). 12. S. Kai and W. Zimmermann, “Pattern dynamics in the electrodynamics on nematic liquid crystals – Defects Patterns, Transition to Turbulence and Magnetic Field Effects –,” Prog. Theor. Phys. Supplement 99, 458-491 (1989). 13. M. Schadt and C. von Planta, “Conductivity relaxation in positive dielectric liquid crystals,” J. Chem. Phys. 63(10), 4379-4383 (1979). 14. “Physical properties of liquid crystals,” in Springer Handbook of Condensed Matter and Materials Data, Werner Martienssen, and Hans Warlimont, ed. (Springer Science & Business Media, 2006). 15. B.-X. Li1, V. Borshch1, S. V. Shiyanovskii1, S.-B. Liu, and O. D. Lavrentovich, “Electro-optic switching of dielectrically negative nematic through nanosecond electric modification of order parameter,” Appl. Phys. Lett. 104, 201105 (2014). 16. S. Barret, F. Gaspard, R. Herino, and F. Mondon, “Dynamic scattering in nematic liquid crystals under dc conditions. I. Monitoring of electrode processes and lifetime investigation,” J. Appl. Phys. 47(6), 2375-2377 (1976). 17. S. Barret, F. Gaspard, R. Herino, and F. Mondon, “Dynamic scattering in nematic liquid crystals under dc conditions. II. Monitoring of electrode processes and lifetime investigation,” J. Appl. Phys. 47(6), 2378-2381 (1976). 18. H. V. Ivashchenko and V. G. Rumyantsev, “Molecular Crystals and Liquid Crystals Dyes in Liquid Crystals,” Vol. 150A G. H. Dienes, ed. (Gordon and Breach Science Publishers S.A., 1987). 19. T. Shimomura and S. Kobayashi, “Color contrast criteria in a guest-host mode liquid crystal display,” Appl. Opt. 20(4) 819-822 (1981). 20. L. T. Creagh, A. R. Kmetz, and R. A. Reynolds, “Performance characteristics of nematic liquid crystal display devices,” IEEE Trans. Electron Dev. ED-18(9), 672-679 (1971). 21. S. R. Nersisyan and N. V. Tabiryan, “Optical axis gratings in liquid crystals and their use for polarization insensitive optical switching,” J. Nonlinear Opt. Phys., 18(1), 1–47 (2009).


Introduction
There currently exists a great need for optical switches to block intense radiation.The ideal device should have a high on/off contrast, be broadband, polarization-insensitive, sufficiently fast, and durable.
Current light switching technologies are based on photochromic or electrochromic materials and twisted nematic liquid crystals.These technologies are used in devices that detect the light and activate the switching mechanism.All these devices have major shortcomings.Photochromic and electrochromic phenomena have an intrinsically slow response, on the scale of seconds [1].Twisted-nematic devices are polarization-sensitive, which significantly decreases their transmission in the "transparent" state [2].
This article reports on a liquid crystal (LC) technology, dynamic scattering mode (DSM), for use in light blocking.DSM was initially developed as a display technology in the late 1960s [3][4][5].However, in the 1970s a different LC technology, based on twisted nematics, [5] was shown to be superior to DSM for display applications.Research on displays shifted from DSM, and, by the 2000's, twisted nematic LC technology or an extension of it had been implement for all flat panel displays.The recent development of new, more-stable materials has prompted us to revisit DSM technology for a different application -optical switching.By manipulating light scattering into high angles, DSM optical switches have the advantage of nearly-100% transmission in the clear state and 10 4 -to-10 5 rejection of on-axis light from visible to near-IR wavelengths in the blocking state with switching times of ~ 150 s.
Schematic diagram of DSM operation is depicted in Figure 1.DSM cell can be constructed in two ways using a nematic LC.In the first case, the principal orientation of the LC molecules, the director, is perpendicular to the cell windows, Fig. 1a, referred to as the homeotropic alignment.In the second case, the LC director is parallel to the windows, Fig. 1 b, forming a homogeneous alignment.When a voltage is applied to the conductive indiumtin-oxide, ITO, film on the windows, ionized dopants in the LC move under the electric field, disrupting the uniformly oriented LC into randomly oriented domains, Fig. 1c.The clear cell now scatters light and appears as frosted glass.When the voltage is removed the cell becomes clear again.Figure 2 shows an example of a DSM cell in its clear (Fig. 2a) and scattering (Fig. 2b) states when illuminated with a 633-nm laser.Detailed theory of operation of such DSM devices, as developed for display applications, has been described previously [9][10][11][12].The DC dielectric constants, optical densities, and the ion conductivity, parallel and perpendicular to the LC director, are all important parameters for DSM operation.The dielectric constant component normal to the director must be larger than the parallel component, and the ion conductivity component must be higher along the director axis.Under these conditions, two opposing effects on the LC director orientation will occur: the c.

Cycling between Clear & Scattering States
ITO coated glass windows electric field will attempt to force the LC director parallel to the windows while the fieldinduced movement of ions will force the director towards the perpendicular orientation.The competition between these two forces randomizes the director, leading to scattering of incoming light.For many LCs, the ratio of parallel-to-perpendicular components of ion conductivity is ~ 1.5 [9,13].The known parameters for the LCs used in this report are shown in Table 1.

Experimental Details
The LC cells consist of conductive indium doped tin oxide (ITO) coated glass, cleaned in H 2 SO 4 and H 2 O 2 , rinsed in deionized water, and dried.For homeotropic LC orientation, the ITO surface was coated with polyimide, Merck SE1211, by spinning a solution of the polyimide on the ITO and baking to fix the polyimide in place.For homogeneous LC orientation, the ITO surface was rubbed with lint-free cloth.The cell was assembled in vacuum with degassed LC heated above its nematic-isotropic transition temperature.After assembly the cell was cooled to room temperature over ~ 1 hr.A 50 μm-thick Mylar sheet was used as a spacer between the cell's windows.The orientation of the liquid crystal in the homeotropic and homogeneous cells was verified using polarized light.Two LCs were characterized in this study: n-(4-methoxybenzyliden)-4-nbutylaniline, MBBA, which is a commonly available material from Sigma-Aldrich, and ZLI-4330 LC, supplied by Merck.MBBA, which has been used in the initial DSM cells since the 1960s [16,17], has sufficient impurities so that no dopants were needed to make it conductive.While known to have impurity and stability problems, MBBA is used here as a comparison standard.On the other hand, ZLI-4330 is a purified material, which is normally insulating.Thus, dopants, (2,4,7-trinitro-9-fluoroenylidene)malononitrile, TFM, and n-butylferrocene, BTF, were added to make the material conductive.TFM is an electron acceptor, while BTF is an electron donor.Both compounds can be cycled through their oxidation-reduction states without significant side reactions and with low electrochemical potentials, minimizing electrochemical decomposition of the LC [6].
Spectroscopic transmission of the cells was performed with a commercial spectrophotometer (Angstrom Sun Technologies, SR500), while optical switching measurements were performed with a 633-nm He-Ne laser.Unless otherwise specified, all transmission measurements were referenced to an empty cell.The transmission was determined by passing laser light through the cell, then onto a 2x2 mm photodiode with a cellphotodiode distance of 45 cm.In this arrangement only on-axis transmitted light is captured, within a solid angle of less than 1 x 10 -5 sr.Scattering of light as a function of angle was measured using a large photodiode, 1 cm in diameter, and measuring on-axis light as a function of bias voltage for several cell-photodiode separations, as shown in Fig. 8.

Results
Figure 3 shows the transmission of ZLI cells containing just TFM, BTF, and a TFM-BTF mixture.Because of the superior results obtained with the 0.1% combination of TFM and BTF in ZLI, this mixture was used in all experiments described below.
Figure 4 shows the on-axis transmission as a function of bias voltage for LCs ZLI and MBBA.After a few hundred voltage cycles the MBBA degraded and became fixed in the scattering state, only being restored to the clear state after several hours without bias voltage.Others have reported polymer formation on the windows when using MBBA [16,17].By contrast, we found no degradation with ZLI even after many hundreds of cycles to 150 V. Fig. 5 shows the comparison of the transmission properties of the homeotropic and homogeneous configurations as a function of bias voltage.For static DC bias voltage, both cells show similar transmission for voltages < 60 V, but at higher voltages the homeotropic cell transmission becomes saturated and eventually increases with increasing voltage.The current through the homeotropic cell also saturates, probably due to the buildup of ionized doping gradients in the cell.These gradients can be minimized by using an AC voltage.The third curve of Figure 5 was obtained using an AC square-wave voltage at ~ 3 Hz, which reduces the formation of doping gradients.When applying the AC voltage, the current for the homeotropic configuration increases and the transmission decreases with increasing voltage.Similar transmission curves were obtained with a 60 Hz sine wave voltage.By contrast to the homeotropic cells, homogeneous cells show no current or transmission saturation and minimal change in transmission between DC and AC bias voltages.AC driven cells, both homeotropic and homogeneous, start to attenuate the on-axis light through the cell at ~ 5 V, while the DC driven cells require higher voltages,  10 V, for the onset of attenuation.Figures 6 show the switching times of the homogeneous and homeotropic cells as a function of voltage.The curves are a power-law fit to the data.According to theory, the switching time should be proportional to inverse voltage squared [10], which is in good agreement with our experimentally-observed trend.Figure 7 shows the transmission as a function of time for a given voltage for the two cells.Similar time-dependence of switching is observed for homogeneous cells containing either MBBA or ZLI LCs.
n II @589 nm 25C 1.774 n  @589 nm 25C   When a voltage is applied to a homogeneous cell only the ions feel the effect, since the LC's director is already parallel to the cell's windows.However, for the homeotropic cell, when a voltage is applied to the LC, the director rotates from perpendicular to parallel

AC Bias Voltage (V)
Cell-photodiode distance (cm) orientation relative to the cell's windows, in addition to the movement of the ions.This combination adds to the disorder of the director.Thus, the homeotropic cell switches faster than the homogeneous cell.Since the LC is nearly transparent to 633 nm light, light blocking is achieved through scattering.Using a 1-cm diameter photodiode, scattering light intensity as a function of bias voltage was obtained for several separations between the cell and the photodiode, Fig. 8.As shown in Fig. 9, at the smallest separation, 0.1 cm, the light intensity reaching the photodiode for a bias voltage of 150 V decreases only to ~ 75 %.In other words, ~ 25 % of the incoming light is scattered out of a 79° on-axis cone.From the data in Fig. 9 the light intensity per steradian was calculated for several bias voltages and is plotted in Fig. 10.For comparison, a black-body radiator, which has the same radiance when viewed from any angle (Lambert's cosine law), represents the maximum scattering limit and is shown in Fig. 10.
A beneficial property of the DSM is its wavelength-independent blocking behavior from 400 to 1700 nm. Figure 11 shows the comparison of an empty cell and a cell filled with ZLI and dopants.Subtracting the empty cell absorption from the measured on-axis light transmission, Fig. 12 was obtained for ZLI and MBBA, showing the transmission of the LC for several bias voltages.Undoped ZLI is transparent, but after the addition of dopants the liquid becomes tinted light yellow from the BTF.The MBBA is naturally light yellow.Other dopants besides BTF could be used to retain the intrinsic ZLI transparency.To summarize, the transmission spectrum is limited by the combination of the ITO, the glass, the chemistry of the LC, and the dopants.

Discussion
Below, we discuss possible improvements to the DSM cells operation for light blocking.The switching time of ~ 150 μs is limited by a ~ 100 μs dead period where the voltage is applied to the cell, but no reduction in transmission occurs, Fig. 7. Instead of having the director normal to the cell windows, homeotropic, as reported here, a small tilt from normal could eliminate the dead time, potentially reducing the switching time to ~ 50 μs.This can be accomplished by either appropriate chemical treatment of the cell windows or by applying a small AC bias, < 5 V, to the cell.Earlier work has reported homogeneous cell switching faster than homeotropic cells [10], using LCs available in the 1980s.We speculate that ZLI, having a more negative Δε than MBBA, Table 1, is likely responsible for the superior switching time of the homeotropic cells.The optical rejection of light can be increased for the homeotropic cell with the addition of a dye to the LC.In the homeotropic cell, a dye that absorbs in the desired spectral region can be aligned with the LC to have minimal absorption in the clear state.Upon switching to the scattering state the dye would increase overall light rejection by the cell.Similar LC devices, only relying on the dye for light rejection, report clear trans mission of ~ 90 % with blocking transmission of ~ 10 % over a limited optical spectrum [18,19].
The recovery time to the clear state from the scattering state when the scattering bias voltage is stepped to 0 V can be from a few milliseconds to 20 sec.The 50-μm cells discussed here have recovery times from 1 to 20 s depending upon the applied voltage.Thinner cells recover faster with the recovery time decreasing by the square of the cell thickness [20].Our limited experience found that transmission at a bias voltage (blocking state) is not a strong function of cell thickness, but the bias current increased by more than two orders of magnitude for a cell thickness change from 50 to 10 μm.Additionally, by applying a high frequency voltage on the cell a decrease in clear state recovery time has been reported [3,4,10].
Other light switching technologies, electrochromics [1] and LC twisted nematics [2], can only block light over a comparatively narrow spectrum and have a lower rejection of incoming light ~ 10 3 .Twisted nematic cells can switch in ~ 100 ns, but have < 50 % transmission in the clear state due to polarization sensitivity.Electrochromics with near 100 % transmission have switching times of > 40 ms.Other light scattering devices using gratings are being developed [21].At present they have a limited operating spectrum and lower light rejection ratio of < 10 2 .The major advantages of DSM are its high rejection of on-axis light, wide operational spectrum, fast switching times to the light blocking state and inexpensive fabrication.Previously reported operational light blocking lifetimes of ~ 10 3 -10 4 hr before the LC degrades are expected to increase with the development of more stable LCs and appropriate conductive ion doping.In the clear state DSM cell will not degrade if sealed from water and other atmospheric impurities.

Fig. 1
Fig. 1 Schematic diagram of dynamic scattering mode, DSM, cells, based on two clear states: (a) homeotropic, where the LC director is perpendicular to cell windows, and (b) homogeneous, where the LC director is parallel to cell windows.(c) When a voltage of > 10 V is applied across the cell, the ions in the LC, represented by the blue and red spheres, move under the electric field, forming ion channels as shown by the colored arrows.The ion movement disrupts the director, creating microcrystalline regions, which scatter the incoming light.The positive and negative ions are believed to form separate conduction channels between the electrodes [6-8].

Fig. 3
Fig. 3 Transmission of 633-nm light as a function of bias voltage for three dopant concentrations.The dopants by weight are: 0.2% TFM an electron acceptor, 0.2% BTF an electron donor and 0.1% of TFM and BTF in ZLI.

Fig. 4 Fig. 5 .
Fig. 4 Comparison of on-axis optical transmission of 633 nm laser light through homogeneous DSM LC cells of MBBA and ZLI.The insert compares the current through the cell during the measurements

Fig. 6 .
Fig.6.Switching time to reduce transmitted light to 10% of its clear state as a function of the switching voltage step.The curves are a power-law fit to the data, following Ref.[10].

Fig. 7
Optical transmission as a function of time for the highest voltage step used in these studies, 650 V, in a homeotropic cell containing ZLI and a homogeneous cell containing MBBA.

Fig. 8
Fig. 8 Setup to measure scattering angle of light using a 1 cm diameter photodiode.The photodiode response was measured as a function of cell AC bias voltage for several distances between the cell and the photodiode, X.

Fig. 9
Fig. 9 Normalized scattered light intensities (photodiode current) as a function of AC bias voltage for a homeotropic ZLI cell at several cell-photodiode distances.

Fig. 10
Fig. 10 Optical energy density as a function of scattering angle from the cell's optical axis, θ, for several AC bias voltages.Curves are an empirical power law fit to the data.The maximum scatter limit, Lambert cosine law, of a black body is shown for comparison.

Fig. 11
Fig. 11 Transmission of an empty cell and a cell filled with ZLI and dopants, referenced to air.The colored bars on the top of the graphs indicate the visible spectrum.

Fig. 12
Fig. 12 Transmitted spectrum of 50-μm-thick homogeneous LC cells at several bias voltages from 350 to 1700 nm (a) for ZLI with dopants 0.1 % by weight of TFN and BTF and (b) MBBA.The colored bars on the top of the graphs indicate the visible spectrum.