Voltage-controllable liquid crystal waveguide

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ARTICLE IN PRESS Optik xxx (2015) xxx–xxx

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Voltage-controllable liquid crystal waveguide Chia-Chi Shih ∗ Department of Interior Design, Tung Fang Design Institute, Kaohsiung City, Taiwan

a r t i c l e

i n f o

Article history: Received 15 September 2014 Accepted 29 August 2015 Available online xxx Keywords: Nematic liquid crystal Optical switch Voltage controllable

a b s t r a c t This study presents an optical waveguide formed by a self-focusing pump beam in a nematic liquid crystal thin film. The pump beam is a polarized high power Gaussian laser beam. Applying a voltage near the Freedericksz transition threshold enhances the optical field of pump beam to reorientate liquid crystal molecules. The reorientation of LC molecules forms the refractive index as a Gaussian-like distribution in the pump beam region. By the assistance of applying electric field, the pump beam can form an optic waveguide by self-focusing effect. A polarized lower power probe beam near the waveguide can be coupled to the waveguide. The coupling efficiency can be controlled by applying voltage. © 2015 Published by Elsevier GmbH.

1. Introduction An adjustable device for controlling a variety of operations is important in an optical communication network [1–3]. The device can be flexibly modulated by applying an external field to modify the refractive index and the propagation constant. In fiber-optical system, various devices have been designed for signal routing, switching, coupling, and modulating the phase of beams [4,5]. Transferring a signal to another waveguide is a crucial operation in an optical communication system. Numerous coupling switches are investigated by the linear electro-optic effects [6–8]. Most optical switch devices are manufactured by the complex and expensive semiconductor process. Liquid crystal is another suitable material for fabricating flexible optical modules. The nematic liquid crystal (NLC) exhibits electro-optical nonlinearity due to its anisotropic dipole molecules. An external electric field can easily alter the mean orientation of the director of NLCs. The effective refractive index depends significantly on the molecular orientation, and can be modulated by applying voltage. The unique feature of NLC has led to its extensive used in electrically controllable devices, including flat displays, optical retards, and switches [9–15]. This work investigates the optical waveguide formed by a selffocusing pump beam in a nematic liquid crystal thin film. Applying a voltage near the Freedericksz transition threshold enhances the optical field of pump beam to reorientate liquid crystal molecules. The reorientation of LC molecules forms the refractive index as a Gaussian-like distributed in the pump beam region. By the assistance of applying electric field, the pump beam forms an optic

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waveguide by self-focusing effect. A polarized lower power probe beam inject into the waveguide obliquely can be coupled to the waveguide easily. The coupling efficiency can be controlled by applying voltage. The waveguide is formed by a polarized beam, and the signal beam will be coupled to the waveguide within an incident angle. 2. Experimental This work presents an optical self-waveguide in a NLC film. Applying a proper external electric field, an incident polarized laser pump beam can be formed an optical self-waveguide by selffocusing effect in the NLC film. A probe beam can couple to this self-waveguide easily. The coupling efficiency can be controlled by the external voltage. The NLC cell is composed of two indium-tinoxide (ITO)-coated glass plates. Two Mylar spacers were inserted between the glass plates to maintain a cell thickness of 15 ␮m. The ITO glass plates were spin-coated with a thin layer of polyimide and rubbed in opposite directions to align the LC molecules homogeneously. The NLC material used in this experiment was E7, which was purchased from E. Merck. The extraordinary and ordinary refractive indices (ne and no) were 1.7354 and 1.5175 (measured at a wavelength  = 546.1 nm). The NLC was injected into the sandwich-like structure cell. A covered slide was glued perpendicularly to the planar glass plates to prevent distortion of the impinging optical wavefront. Fig. 1(a) and (b) shows the side and top views of the experimental setup. The cell was placed horizontally on a stage. A pump beam from the Nd-YAG laser ( = 532 nm) was collimated to a waist of approximately 5 ␮m using a lens and launched into the side of the cell. The focal point was inside the LC ∼150 ␮m from the vertical cover glass. A probe beam from the He–Ne laser ( = 532 nm)

http://dx.doi.org/10.1016/j.ijleo.2015.08.229 0030-4026/© 2015 Published by Elsevier GmbH.

Please cite this article in press as: C.-C. Shih, Voltage-controllable liquid crystal waveguide, Optik - Int. J. Light Electron Opt. (2015), http://dx.doi.org/10.1016/j.ijleo.2015.08.229

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Fig. 1. Schematic drawing of experimental setup: (a) side-view, (b) top-view, P: polarizer, M: mirror, /2: half-wave plate, L: Lens, LC: liquid crystal. BS: beam splitter. MS: microscope. The director of the LC is aligned along the z axis.

was impinged into the waveguide formed by pump beam with a 5 degrees angle. A half-wave plate and a polarizer were placed between the laser beams and the sample cell along the beam route to adjust the intensity and polarization of the incident beam. The molecular alignment was initially parallel to the z axis. Applying a voltage on the cell reorientated the LC molecules toward the direction of the electric field (x-axis). The polarization of pump beam and probe beam were the same as the electric field. The reorientation of LC molecules caused the refractive index to become Gaussian-like distributed in the pump beam cross section. The beam region in the LC film served as a waveguide. A same polarization probe beam entered the waveguide obliquely, as presented in Fig. 1(b), was studied. 3. Results and analysis The pump beam self-focusing effect and the probe beam coupling effect were observed under a microscope using a CCD. Fig. 2(a)–(c) presents the sequential beam self-focusing effect at various applied voltages. The figures are obtained from the top of the planar cell. A pump beam propagates along the z-axis from the left and is focused inside the LC, ∼150 ␮m away from the face on

Fig. 3. Evolution of the probe beam coupling effect with applied voltage of (a) 3.5 V, (b) 4.2 V, and (c) 6.8 V.

which it is incident. The beam is linearly polarized in the x direction, and its power is about 15 ␮W. Fig. 2(a) displays the situation when no voltage is applied to the cell. The pump beam is defocusing in the cell. The optical electric field of pump beam is not strong enough to rotate liquid crystal molecules without the assistance of applying electric field. Increasing the applied voltage near the Freedericksz threshold begins to rotate the LC molecules toward the direction of applied field. With the assistance of applying field, the pump beam can reorientate the LC molecules easily, and then the effective refractive index in LC file will rearrange. The optical field of pump beam is a Gaussian distribution so the effective refractive index is grade decreasing from center to rim of the beam cross section. The arrangement of effective refractive index is similar to a Grin lens so the pump will self-focus along the beam trace. Fig. 2(b) shows the case of applying 3.5 V to LC cell. The pump beam start to self-focus but the efficiency is not very well. Fig. 2(c) displays the situation when 6.8 V are applied to the cell. The pump beam self-focuses very well shown in the figure. The self-focusing beam trace can be served as an optical waveguide. Within 5 degrees incident angle, a probe beam can couple to the waveguide easily. The coupling efficiency can be controlled by applying voltage. Fig. 3(a)–(c) presents the sequential beam coupling effect at various applied voltages. The incident angle is 4 degrees. Fig. 3(a) displays the situation when 6.8 V are applied to the cell. With the assistance of applying electric field, the pump and the probe beam start to self-focus but they propagate along separated path. Fig. 3(b) shows the case of applying 4.2 V to LC cell. The probe beam starts to couple to pump beam can be observed from the figure. Fig. 3(c) displays the situation when 6.8 V are applied to the cell. With the assistance of 6.8 V applying voltage, the path of pump beam forms a refractive index well. A probe beam will be trapped when it is close to the well. Within 4 degrees incident angle, a probe beam can couple to the waveguide easily. 4. Conclusions

Fig. 2. Evolution of the beam self-focusing effect with applied voltage of (a) 0 V, (b) 3.5 V, and (c) 6.8 V.

This study presents a voltage-controllable optical switch. The optical waveguide formed by a self-focusing pump beam in a nematic liquid crystal thin film. With the assistance of applying voltage, a several mW power Gaussian laser beam can self-focus in LC film. The self-focusing laser beam can be served as a waveguide. Within 4 degrees incident angle, a polarized lower power probe beam can coupled to the waveguide easily. The coupling efficiency can be controlled by applying voltage. The device of waveguide formed by a polarized beam can be fabricated very easy. And the signal beam can coupled to the waveguide easily within an incident

Please cite this article in press as: C.-C. Shih, Voltage-controllable liquid crystal waveguide, Optik - Int. J. Light Electron Opt. (2015), http://dx.doi.org/10.1016/j.ijleo.2015.08.229

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Please cite this article in press as: C.-C. Shih, Voltage-controllable liquid crystal waveguide, Optik - Int. J. Light Electron Opt. (2015), http://dx.doi.org/10.1016/j.ijleo.2015.08.229