P-139: Polymer Network Liquid Crystal Grating Cured with Interfered Visible Light

Abstract

In this paper, we report a polymer network liquid crystal grating fabricated using interfered visible light. Exposed to the periodic light pattern, polymer concentration gradient is achieved as the precursor is cured. The grating is simple to fabricate and has low voltage, sub-millisecond response and high spatial resolution. Author KeywordsPolymer network liquid crystal; grating; fast response BackgroundElectrically tunable liquid crystal (LC) diffraction gratings have advantages of light weight, low cost and low power consumption [1][2][3][4][5]. Their electro-optical properties can be conveniently tuned by external voltages. However, the response time of conventional nematic LC is relatively slow (~10-100 ms) [6]. To shorten the response time of the gratings, polymer-dispersed liquid crystal (PDLC) [7], polymer-stabilized blue phase liquid crystal (PSBPLC) [8] and polymer network liquid crystal (PNLC) have been adopted in the gratings [9][10][11][12]. Both PDLC and PSBPLC can achieve submillisecond switching time, however, due to the optically quasi-isotropic off-states, their phase change is limited to ~1/3 of that of regular nematic LC gratings. Moreover, relatively high operating electric fields are required (~10V/ȝm) [8,17].On the other hand, PNLC gratings owing to the alignment effect and polymer network anchoring, can achieve both large phase change and relatively low operating voltage. There are several methods to form refractive index gradient in PNLC materials such as exposing a homogeneous LC/monomer mixture with an inhomogeneous UV light using a photomask [13][14] and controlling the tilt angle of a homogeneous cell containing LC/monomer mixture [15][16]. Most methods use photomasks to control the light intensity which makes the grating period relatively large (~100ȝm) and the fabrication processes complicated.In this work, we use interfered visible laser light to fabricate PNLC gratings. Using a visible-curable precursor, polymerization process and polymer-concentration-gradient are achieved simultaneously. The grating exhibits sub-millisecond response time and relatively low voltage. Moreover, via visible laser holography, high spatial resolution could be conveniently achieved at low cost. Grating operation principleThe fabrication process and operation principle of the proposed PNLC grating are depicted in figure 1. As shown in Fig. 1(a), a homogeneous cell is filled with LC/monomer mixture. The inner surfaces of the top and bottom glass substrates are over-coated with thin transparent indium-tin-oxide (ITO) electrodes, followed by polyimide alignment layers. Then the cell is exposed to the alternative dark and bright interference light pattern. Because the polymerization preferentially occurs in the bright regions, more monomers are consumed, causing a monomer concentration gradient between bright and dark regions. Thus, the monomers will diffuse from dark regions to adjacent bright regions as shown in Fig. 1(b) [16]. Therefore, a denser polymer network is formed in the bright regions wh...