Electro-optic response of liquid crystals in mainstream display applications exhibits a millisecond switching of optical retardance on the order of one micrometer. We demonstrate that a similarly large optical retardance can be switched much faster, within 10-100 nanoseconds, by using multiple passes of light through a cell filled with the nematic liquid crystal. The fast response is based on the so-called nanosecond electric modification of order parameters (NEMOP) effect. The described approach can be used to develop ultrafast optical shutters and modulators.
© 2016 Optical Society of America
Dielectric and optical anisotropy of nematic liquid crystals make them materials of choice in various electro-optic applications, most prominent of which are informational displays . The anisotropy axis of the nematic, the so-called director , defines its optic axis. Electro-optical applications exploit the so-called Frederiks effect, i.e., reorientation of in a low-frequency electric field caused by anisotropy of dielectric susceptibility ; and are the permittivities measured parallel to and perpendicular to it, respectively. A very strong field can realign a nematic rather quickly, within 100 ns . However, when the field is switched off, the relaxation of the nematic to its original ground state orientation is slow, in the range of milliseconds . Although the microsecond switching speeds are acceptable in displays, they limit development of liquid crystals for other applications.
Recently, a new effect, called nanosecond electric modification of order parameters (NEMOP), has been demonstrated to speed up the liquid crystal response by orders of magnitude [4–6]. The NEMOP effect is rooted in the electrically induced changes of the scalar part of the order parameter ; it does not imply realignment of . Since the NEMOP effect is based on microscopic molecular scale interactions, it is extraordinary fast, on the order of nanoseconds and tens of nanoseconds [4–6]. An important advantage over the Frederiks effect is that the nanosecond switching time is the same for the field-on and field-off sub-cycles. The problem to overcome is a relatively low value of optical retardance. The field-induced change of birefringence in a typical NEMOP setting is ; with the cell thickness , the switchable retardance is only , i.e. less than a half-wavelength of the visible light. In principle, the phase retardance can be enlarged by making the cells thicker, but since the NEMOP effect operates at high fields, increasing the cell thickness makes the needed voltages too high for practical purposes. In this report, we demonstrate that NEMOP switching can be achieved in a cell with total internal reflection and multiple passages of light through the cell. We demonstrate switching of optical retardance in the range (335-1050) nm within (30-74) ns under the applied voltages up to 900 V in relatively thin cells, .
2.1 Experimental materials and setup
We used three nematic mixtures, HNG715600-100 (,), HNG705800 −100 (,), both from Jiangsu Hecheng Display Technology, and MLC-2080 (,) from Merck & Co., Inc. The birefringence data correspond to 25°C and the wavelength of 633 nm; the dielectric anisotropy corresponds to the same temperature and the frequency 1kHz.
The cells were constructed from two parallel glass plates of the thickness 1.1 mm and of surface area 25 mm by 50 mm each. The inner surfaces of plates contain indium tin oxide (ITO) electrodes of resistivity () with a rectangular working area A polyimide film PI-2555 (HD MicroSystems) is used for planar alignment. The rubbing directions at the plates are parallel to each other in order to minimize the effects of nonzero pretilt of molecular orientation. The cell thicknesses is in the range , fixed by silica spacers in the Norland optical adhesive used to seal the cells. The square voltage pulses of ~400 ns duration are applied using a pulse generator HV 1000 (Direct Energy) that provides sharp rise and fall edges with a characteristic time of one nanosecond. The applied voltage pulses and photodetector signals (TIA-525 Terahertz Technologies, response time < 1 ns) were measured with a digital oscilloscope TDS2014 (Tektronix), with the sampling rate 1G samples/s. The design allowed the laser beam to experience multiple reflections at the interfaces between the air and glass substrates, Fig. 1(b); we have chosen the geometry in which the beam travelled through the slab 15 times.
In order to measure the retardance change, we use a He-Ne laser beam that passes through two crossed polarizers, the cell, and an optical compensator, Fig. 1(a). The cell is placed between two right-angle prisms. The director is in the incidence plane of light. The probing beam enters the nematic layer at the angle of . Its polarization direction makes an angle of with respect to the incident plane that contains the rubbing direction; the projection of polarization onto the plane of the cell makes with . The angle is chosen because of the following two reasons. First, it allows one to achieve the total internal reflection at the interface between air and glass, since the critical angle of total internal reflection is smaller than ; here 1 and 1.52 are the refractive indices of air and glass, respectively. The second reason is that light propagation at with respect to the director eliminates relatively slow (~100 ns) contributions of director fluctuations  to the optical response, as detailed in [4–6]. The experimental scheme in Fig. 1 allows us to measure the field-induced retardance determined by the number of passages through the cell ( = 15) and by the field-induced modification of the biaxial and uniaxial contributions to birefringence change , calculated as Eq. (30) in .
2.2 Four-point measurement of the optical retardance
The field-induced contribution to the transmitted light intensity changes associated with the uniaxial and biaxial modifications of the optic tensor needs to be separated from other field-induced effects. The measurement method is based on four consecutive measurements of the transmitted intensities (where and 4) under the same electric pulse with four different values of the preselected optical retardance of the Soleil-Babinet compensator:Fig. 2(a), and set four different values of (here is an integer). These four settings of the Soleil-Babinet compensator are chosen to yield four values of : one representing the maximum of (when ), another representing the minimum of () and two being an average value of ( and 3). , and can by determined from :Fig. 2.
Figure 2 demonstrates an example of the four-point measurement of field-induced optical retardance during the action of a 350 V voltage pulse, in a cell of thickness 5.8 filled with HNG715600-100. Using Eq. (2), we calculate , Fig. 2(d), as well as and , Fig. 2(c), from the transmitted intensities , Fig. 2(b), measured for the four settings , shown in Fig. 2(a). One can see that changes of and are about 10% of and should be taken into account for the correct determination of .
3. Experimental results
The three nematics show a relatively large NEMOP effect when placed in regular cells with a single passage of light ; however, the corresponding optical retardance in these cells is below 100 nm , which is too small for applications such as optical shutters and modulators. Multiple passage of the beam allows one to produce optical retardance well above the required minimum of a half-wavelength. Namely, HNG715600-100 provides the largest field-induced optical retardance 1050 nm when driven by 700 V pulses in a cell of thickness 5.8 µm, Fig. 3. In Fig. 3(a), the transmitted light intensity experiences multiple oscillations that result from the optical retardance changing in increments of . The light intensity is more sensitive to higher voltages since the higher voltages yield a higher induced birefringence; thus the data at high voltages appear to be more noisy. The other two materials produce a somewhat smaller field-induced birefringence, but their response is faster than that of HNG715600-100. Namely, HNG7058 produces at 550 V in a cell of thickness 5.9 µm, Fig. 4. Finally, MLC2080 shows at 950V in a cell of thickness 5.6 µm, Fig. 5.
To characterize the response times of the optical retardance changes, we fitted the data for the field-on and field-off processes by the exponential functions ,4–6]. These response times are 72 ns, 42 ns, and 30 ns for HNG715600-100, HNG705800-100, and MLC-2080, respectively. Note that the values of = above are larger than the values measured for the same materials in the geometry of a single passage of light (33 ns, 12 ns, and 6 ns, respectively) . The reason is that in the present experiment, the electrode area is larger than in  (to enable multiple reflections), which implies an increase of capacitance and the RC time of the circuit (the resistance is determined mostly by the bridge that connects the working area to the leads and thus does not decrease much when the working area is increased). One can reduce the response times by using transparent electrodes of a lower resistance. The transmitted beam’s intensity is about 10-15% of the incident intensity. The losses are caused by the refractive index mismatch at the interfaces, absorption in ITO layers and by light scattering in the liquid crystal.
The optical response of MLC-2080 and HNG705800-100 in Figs. 4 and 5, respectively, decreases during the voltage pulse duration. The effect is caused by the decay of the applied voltage illustrated in Fig. 1(c). Since the field-induced birefringence is proportional to the square of the applied field [4–6], the optical effect of a decaying voltage is especially pronounced for MLC-2080 and HNG705800-100 that are characterized by a fast response time.
The field-induced optical retardance shows a quadratic dependence on the applied electric field, Fig. 6, as expected [4–6]. At higher fields, the response is not as strong as expected from the quadratic dependence, which is probably associated with the saturation of the orientational order of permanent molecular dipoles.
We demonstrate that the nematic cells are capable of nanosecond switching of large values of optical retardance, on the order of 1 micrometer. The mechanism is based on the effect of nanosecond electric modification of order parameters. The amplification of the optical retardance is achieved by using multiple passages of light through the cell. Although the proposed geometry makes the response somewhat slower because of an increased area of the electrodes, the switching times are still on the order of tens of nanoseconds in both the field-on and field-off processes. We demonstrated the feasibility of large and fast switching by using planar cells with the nematic liquid crystals of a negative dielectric anisotropy. We characterized three different materials to illustrate that the performance of the electrically-controlled optical compensator can be adjusted both in terms of the field-induced retardance and in the switching speed. For example, HNG715600-100 yields the largest optical retardance change, but the materials HNG705800-100 and MLC-2080 demonstrate faster switching. One can also use other NEMOP materials, in particular, nematics of positive dielectric anisotropy; the relevant study is in progress. In the experimental design, we limited ourselves by 15 passages of the beam through the cell. Note that the proposed set-up is flexible and makes it possible to increase the optical retardance switching amplitude by simply increasing the length of the active area of electrodes. The limitation is set by intensity losses at interfaces, absorption by ITO and light scattering in the liquid crystal. The RC-time limitation does not appear to be a very serious issue since it is in the range of tens of nanoseconds, thus its increase by few times would still mean a very fast switching of large amounts of retardance, presumably in the range of hundreds of nanoseconds. An individual pulse does not cause any ionic problems because the pulse duration is too short to cause a significant shift of the ions. With the typical mobilities on the order of 10−10 m2/(V s) and applied fields 2108 V/m, the ions shift by only about 10 nm within the duration of the voltage pulse, which is much shorter than the thickness of the cell. To prevent a possible cumulative effect of the successive pulses, these can be designed to be of alternating polarity. From the practical point of view, the reported nanosecond electric field-controlled variation of optical retardance on the order of a wavelength of visible and near-infrared spectrum can be used in ultrafast modulators and shutters.
National Science Foundation (NSF) IIP-1500204.
References and links
1. D.-K. Yang and S.-T. Wu, Fundamentals of Liquid Crystal Devices (John Wiley, 2006).
2. H. Takanashi, J. E. Maclennan, and N. A. Clark, “Sub 100 nanosecond pretilted planar-to-homeotropic reorientation of nematic liquid crystals under high electric field,” Jpn. J. Appl. Phys. 37(1), 2587–2589 (1998). [CrossRef]
3. L. M. Blinov and V. G. Chigrinov, Electrooptic Effects in Liquid Crystal Materials (Springer-Verlag, 1994).
5. V. Borshch, S. V. Shiyanovskii, B.-X. Li, and O. D. Lavrentovich, “Nanosecond electro-optics of a nematic liquid crystal with negative dielectric anisotropy,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 90(6), 062504 (2014). [CrossRef] [PubMed]
6. B.-X. Li, V. Borshch, S. V. Shiyanovskii, 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(20), 201105 (2014). [CrossRef]
7. P. G. de Gennes and J. Prost, The Physics of Liquid Crystals (Oxford University Press, 1995).
8. J. L. Martinand and G. Durand, “Electric field quenching of thermal fluctuations of orientation in a nematic liquid crystal,” Solid State Commun. 10(9), 815–818 (1972). [CrossRef]
9. B.-X. Li, V. Borshch, S. V. Shiyanovskii, S.-B. Liu, and O. D. Lavrentovich, “Kerr effect at high electric field in the isotropic phase of mesogenic materials,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 92(5), 050501 (2015). [CrossRef] [PubMed]