Printer Friendly

Electrically Tunable Diffraction Grating Based on Liquid Crystals.

1. Introduction

Liquid crystal (LC) phase grating has numerous applications, such as laser beam steering, beam shaping, fiber-optic communications, highly efficient projection displays, wide-viewing and direct-view displays, and other modifications of light intensity or phase [1-9].

For nematic liquid crystals (NLCs) based phase grating, because the operation voltage changing the orientation of the molecular director is usually lower, depending on the dielectric anisotropy ([DELTA][epsilon]) of LCs, an effective refractive index ([DELTA]n) of the medium is experienced by a propagating beam and has become an ideal candidate as an electrically controllable device due to the large optical birefringence [3, 10-15].

There are several ways to fabricate the liquid crystals diffraction gratings. Gibbons et al. have shown liquid crystal grating by exposing a dye-doped polymer layer to an ultraviolet interference pattern. Chen et al. have shown a hybrid liquid crystal configuration to create a double-rubbed polyimide layer for polarization-independent grating [16-20]. Honma et al. have demonstrated multidomain alignment regions liquid crystal gratings through a microrubbing technique [12, 13]. Wen et al. have shown a dual-domain polarization grating which consists of right- and left-handed twisted regions created from scribing a polyimide layer [5].

In this paper, we propose a simple phase grating with tunable diffraction efficiency by applying a small voltage. The grating-like patterning of the electric field for the alignment of liquid crystals is easy to achieve and results in the periodic distribution of refractive index. The physical mechanisms for the first-order diffraction efficiency versus polarization angles of the probed beam are also discussed.

2. Experiment

We studied the liquid crystals phase grating in 25 [micro]m-thick LC cell containing homogenously aligned E7. The cell is with grating-like electrode width w = 15 [micro] m. An LC mixture (E7) with [DELTA][epsilon] = 14.4 (dielectric constants at 1kHz) and An=0.218 (refractive indices difference at [lambda]=633 nm and T=20[degrees] C) was injected into the cell via capillary flow. Figure 1(a) shows the experimental setup for characterizing the liquid crystals phase grating. He-Ne laser ([lambda] = 633 nm) was used as the probe beam. The transmission axis of the linear polarizer was set in the x-direction. An AC power supply (~1 KHz) applied voltage to the grating-like electrode cell and produced the periodic electric field on the cell and resulted in the periodic distribution of the refractive index of liquid crystals, as shown in Figure 1(b). The measured diffraction signals for the zeroth and the first order were detected by the photodetectors and recorded by the computer.

3. Results and Discussions

First, the diffraction intensities of the liquid crystal gratings in the zeroth and the first orders were recorded as a function of time, with an applied voltage of ~4.0V, as shown in Figure 2. The x- linearly polarized light from a He-Ne laser light source is normally incident on the sample; the zeroth-order diffraction rapidly decayed and the first-order diffraction was raised as the voltage was in "on" state. The diffraction intensities rise or decay to a stable level at t=250 ms. Next, the voltage was in "off" state at t=3250 ms; the zerothorder diffraction was gradually increased and the first-order diffraction was gradually raised to an initial state at t=~3800 ms.

Figure 3(a) plots the first-order diffraction intensity versus voltage for a liquid crystal phase grating formed from the periodic grating-like electrodes cell. The threshold voltage occurred at 2.0 V. After 2.0 V, the diffraction intensity gradually grew. The shape of the first-order diffraction versus voltage curve was indicated; as the voltage is increased to 4.0 V, the liquid crystal director orientation may tend to be homeotropically aligned in the electrical field, and the linearly polarized probe beam in the x-direction experienced the difference in refractive index modulation between electrode and nonelectrode stripes zones, which was significant. From 4.0 V to 10.0 V, the diffraction intensity gradually decays. The phenomenon is speculated to be the result of the disordering of the alignment of liquid crystals in the nonelectrode domains due to the edge effect of the electric field, reducing the difference in the refractive index modulation. After 10.0V, the first-order diffraction decays to a stable level. Figure 3(b) shows the images of the diffraction signals taken at 0, 2, 4, 10, and 20 V. At 0 V, only the zeroth-order diffraction signal exists; at 2 V, the first-order diffraction appears and the high order diffraction signals (4th-order diffraction) can be observed; at 4 V, the first-order diffraction becomes the brightest and the 6th-order diffraction signals can be observed; at 10.0 V and 20.0V, the first-order diffraction signal decays, and 8th-order diffraction signals can be observed.

Figure 4 shows the measured diffraction efficiency of the first-order beams with various polarization of the probe beam with applied voltage ~4.0V. The first-order diffraction efficiency [[eta].sub.1] is defined as the ratio between the intensity of 1st diffracted order and the total intensity at V = 0, described by

[[eta].sub.1] = [[I.sub.1]/[I.sub.0]] (1)

The polarization of the probed beam is adjusted by rotating a polarizer from 0[degrees] to 90[degrees] corresponding to the adjustment of the polarization of the probe beam from x polarized to y polarized. The intensity of the probe beam maintains a constant value as the polarization is adjusted. The polarization angles of the probed beam are 0[degrees], 15[degrees], 30[degrees], 45[degrees], 60[degrees], 75[degrees], and 90[degrees], respectively. When the polarization angle is at 0[degrees], the probe beam experiences ordinary ([n.sub.o]) and extraordinary ([n.sub.e]) refractive indices in the regions with and without the grating-like electrode stripes of the liquid crystal grating, respectively. The modulation of the refractive indices [DELTA]n=[n.sub.e] - [n.sub.o] causes the phase grating effect in liquid crystals. The first diffraction efficiency [[eta].sub.1] is ~12.0% In considering the polarization angle other than the x polarization (0[degrees]), the effective refractive index experienced by the probed beam is a function of the polarization angle, according to [21]

[mathematical expression not reproducible] (2)

where [theta] is the angle between the x-axis and the polarization of the probe beam. As the polarization angle is adjusted from 0[degrees] to 90[degrees], the effective refractive index [n.sub.eff] experienced by the beam in the regions without the electrode stripe is reduced; the refractive index is constant in the regions with the electrode stripe. The modulation of the refractive index influences the phase grating. Therefore, the decrease in the anisotropy of the refractive index weakens the phase grating effect. The phase grating effect gradually decays as the polarization angle is increased. When the probe beam is linearly polarized in the y direction (90[degrees]) it experiences almost the same ordinary refractive index in the regions with and without the electrode stripe. The first diffraction efficiency [[eta].sub.1]is ~1.1% The results reveal that the phase grating effect is directly influenced by the polarization of the probe beam and can be switchable by adjusting the polarization of the probe beam.

4. Conclusions

In summary, the reorientation of liquid crystals can be obtained by applying external voltage in the grating-like electrodes cell; the linearly polarized probe beam experienced periodic distribution of refractive index, resulting in the liquid crystal phase grating. The zeroth-order and the first-order diffraction intensities were probed by a He-Ne laser. The diffraction grating can be switched on by applying a small voltage (~2.0 V). The optimal first-order diffraction efficiency is about 12%. The first-order diffraction efficiency can also be tuned from 12.0% to 1.1% by adjusting the polarization of the probe beam.

Data Availability

The data used to support the findings of this study are available from the corresponding author upon request.

https://doi.org/10.1155/2018/7849529

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This work was supported by the Ministry of Science and Technology (MOST) of Taiwan under Grant MOST 106-2221-E-035-072-MY2. The authors also appreciate the Precision Instrument Support Center of Feng Chia University for providing the measurement facilities.

References

[1] D. J. Field, "Relations between the statistics of natural images and the response properties of cortical cells," Journal of the Optical Society of America A: Optics, Image Science & Vision, vol. 4, no. 12, p. 2379, 1987.

[2] H. Murai, "Electro-optic properties of liquid crystal phase gratings and their simulation using a homogeneous alignment model," Liquid Crystals, vol. 15, pp. 627-642, 1993.

[3] J. Chen, P. J. Bos, H. Vithana, and D. L. Johnson, "Electrooptically controlled liquid crystal diffraction grating," Applied Physics Letters, vol. 67, no. 18, pp. 2588-2590, 1995.

[4] J. N. Eakin, Y. Xie, R. A. Pelcovits, M. D. Radcliffe, and G. P. Crawford, "Zero voltage Freedericksz transition in periodically aligned liquid crystals," Applied Physics Letters, vol. 85, no. 10, pp. 1671-1673, 2004.

[5] B. Wen, R. G. Petschek, and C. Rosenblatt, "Nematic liquid-crystal polarization gratings by modification of surface alignment," Applied Optics, vol. 41, no. 7, p. 1246, 2002.

[6] L. Komitov, G. P. Bryan-Brown, E. L. Wood, and A. B. Smout, "Alignment of cholesteric liquid crystals using periodic anchoring," Journal of Applied Physics, vol. 86, no. 7, pp. 3508-3511, 1999.

[7] C. Rosenblatt, "Nanostructured surfaces: scientific and optical device applications," Molecular Crystals and Liquid Crystals, vol. 412, no. 1, pp. 117-134, 2004.

[8] T. Shioda, B. Wen, and C. Rosenblatt, "Step-wise Freedericksz transition in a nematic liquid crystal," Journal of Applied Physics, vol. 94, no. 12, p. 7502, 2003.

[9] G. P. Sinha, C. Rosenblatt, and L. V Mirantsev, "Disruption of surface-induced smectic order by periodic surface corrugations," Physical Review E: Statistical, Nonlinear, and Soft Matter Physics, vol. 65, no. 4, Article ID 041718, 2002.

[10] S. Varghese, G. P. Crawford, C. W. Bastiaansen, D. K. de Boer, and D. J. Broer, "Microrubbing technique to produce high pretilt multidomain liquid crystal alignment," Applied Physics Letters, vol. 85, no. 2, pp. 230-232, 2004.

[11] S. Varghese, S. Narayanankutty, C. W. M. Bastiaansen, G. P. Crawford, and D. J. Broer, "Patterned alignment of liquid crystals by [mu]-rubbing," Advanced Material, vol. 16, no. 18, pp. 1600-1605, 2004.

[12] M. Honma and T. Nose, "Liquid-Crystal Reflective Beam Deflector with Microscale Alignment Pattern," Japanese Journal of Applied Physics, vol. 43, no. 12, pp. 8151-8155, 2004.

[13] M. Honma, K. Yamamoto, and T. Nose, "Periodic reverse-twist nematic domains obtained by microrubbing patterns," Journal of Applied Physics, vol. 96, no. 10, pp. 5415-5419, 2004.

[14] D. J. Versteeg, C. W. M. Bastiaansen, and D. J. Broer, "Influence of laser writing of polyimides on the alignment of liquid crystals," Journal of Applied Physics, vol. 91, no. 7, pp. 4191-4195, 2002.

[15] M. Schadt, H. Seiberle, and A. Schuster, "Optical patterning of multi-domain liquid-crystal displays with wide viewing angles," Nature, vol. 381, pp. 212-215, 1996.

[16] W. M. Gibbons, P. J. Shannon, S. T. Sun, and B. J. Swetlin, "Surface-mediated alignment of nematic liquid crystals with polarized laser light," Nature, vol. 351, pp. 49-50, 1991.

[17] B. W. Lee and N. A. Clark, "Alignment of liquid crystals with patterned isotropic surfaces," Science, vol. 291, p. 2576, 2001.

[18] L. Nikolova and T. Todorov, "Diffraction efficiency and selectivity of polarization holographic recording," Optica Acta: International Journal of Optics, vol. 31, no. 5, p. 579, 1984.

[19] S. G. Cloutier, D. A. Peyrot, T. V. Galstian, and R. A. Lessard, "Measurement of permanent vectorial photoinduced anisotropy in azo-dye-doped photoresist using polarization holography," Journal of Optics A: Pure and Applied Optics, vol. 4, p. S228, 2002.

[20] P. S. Ramanujam, "Evanescent polarization holographic recording of sub-200-nm gratings in an azobenzene polyester," Optics Letters, vol. 28, p. 2375, 2003.

[21] C. Kuo, S. Huang, I. Jiang, and M. Tsai, "Multiguide directional coupler using switchable liquid-crystalline optical channels," Journal of Applied Physics, vol. 97, no. 10, p. 103113, 2005.

Chuen-Lin Tien (iD), (1,2) Rong-Ji Lin, (2) Shu-Hui Su, (2) and Chi-Ting Horng (3,4)

(1) Department of Electrical Engineering, Feng Chia University, Taichung 40724, Taiwan

(2) Ph.D. Program of Electrical and Communications Engineering, Feng Chia University, Taichung 40724, Taiwan

(3) Department of Ophthalmology, Fooyin University Hospital, Pingtung, Taiwan

(4) Department of Pharmacy, Tajen University, Pingtung, Taiwan

Correspondence should be addressed to Chi-Ting Horng; chitinghorng@gmail.com

Received 6 February 2018; Accepted 3 April 2018; Published 20 June 2018

Academic Editor: Daniel Ho

Caption: Figure 1: (a) Experimental setup for characterizing the performance of the phase grating. (b) The periodic distribution of refractive index of liquid crystals with an external ac voltage.

Caption: Figure 2: The diffraction intensities of the liquid crystal gratings in zeroth and the first orders were recorded as a function of time, using an applied voltage of ~4.0V.

Caption: Figure 3: (a) The first-order diffraction intensity versus voltage for a liquid crystal phase grating formed from the periodic grating-like electrodes cell. (b) Diffraction patterns at different applied voltages: 0, 2, 4, 10, and 20 V, respectively.

Caption: Figure 4: The diffraction efficiency of the first-order beams versus polarization of the probe beam with applied voltage ~4.0V.
COPYRIGHT 2018 Hindawi Limited
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 2018 Gale, Cengage Learning. All rights reserved.

Article Details
Printer friendly Cite/link Email Feedback
Title Annotation:Research Article
Author:Tien, Chuen-Lin; Lin, Rong-Ji; Su, Shu-Hui; Horng, Chi-Ting
Publication:Advances in Condensed Matter Physics
Article Type:Report
Geographic Code:1USA
Date:Jan 1, 2018
Words:2191
Previous Article:Effect of Silver Nanowire Plasmons on Graphene Oxide Coatings Reduction for Highly Transparent Electrodes.
Next Article:Enhanced Photoluminescence in Gold Nanoparticles Doped Homogeneous Planar Nematic Liquid Crystals.
Topics:

Terms of use | Privacy policy | Copyright © 2021 Farlex, Inc. | Feedback | For webmasters