Topographic Confinement of a Ferroelectric Liquid Crystal for Highly Efficient Tunable Electrooptic Effect with Reduced Threshold Jun-Hee Na 1y , Jiyoon Kim 1 , Yoonseuk Choi 2 , and Sin-Doo Lee 1 1 School of Electrical Engineering, Seoul National University, Kwanak P. O. Box 34, Seoul 151-600, Korea 2 Department of Electronic Engineering, Hanbat National University, Daejeon 305-719, Korea E-mail: sidlee@plaza.snu.ac.kr Received January 24, 2013; accepted April 17, 2013; published online May 9, 2013 We report a highly efficient linear electrooptic (EO) effect of a tight-pitch ferroelectric liquid crystal (FLC) in a microchannel architecture where the FLC molecules are vertically aligned on the surrounding surfaces. Due to the topographic confinement in a large surface-to-volume configuration, the competing anchoring forces exerted by the lateral walls and the substrates impose directly the restrictions on the deformations of the smectic layers and the molecular director. The observed EO effect should be directly applicable for diverse optical devices requiring fast response, high optical modulation, and high subthreshold slope at a reduced operation voltage. # 2013 The Japan Society of Applied Physics T he self-organization of variety of soft matters, in- cluding lipid membranes, liquid crystals (LCs), and polymers, under topographic confinement has at- tracted much attention for the discovery of new physical phenomena and the development of a unique platform appli- cable to advanced electrooptic (EO) and nanobio sys- tems. 1–3) It is expected that in addition to the geometrical confinement effects, delicate surface interactions inherent to such soft matters, especially in small volumes, on a molecular or nanometer scale lead to the structural self- organization and/or instability at the macroscopic level. 4,5) It is thus important to investigate how the confinement effect as well as the surface interactions affect the structural organization and resultant EO properties of a smectic liquid crystal that possesses both layering and orientational orders, particularly, a chiral smectic liquid crystal showing ferro- electricity. For nonchiral smectic LCs, it was reported 6,7) that either global constraints imposed by the geometry or the competition between local surface interactions can control the topographic defects resulting from the layer deforma- tions. In the ferroelectric LCs (FLCs), however, the spon- taneous polarization plays a primary role in the modification of the surface interactions at a solid-LC interface and is critical for the appearance of a fast linear EO effect. The linear EO effect resulting from the subtle change in the interfacial structural order associated with the layer and molecular tilt in the FLC confined between topographic walls has not been explored so far. In this paper, we present a highly efficient tunable linear EO effect of a tight-pitch FLC realized by the topographic confinement of the smectic layers and the molecular tilt in a vertically aligned geometry. In the presence of competing anchoring forces in a relatively large surface-to-volume configuration, the restricted deformations of the FLC within a certain characteristic length, comparable to the order of the smectic correlation length, lead directly to high optical modulation with a sharp subthreshold at a reduced operation voltage. The schematic diagram showing the confinement of the FLC in the channel structures is depicted in Fig. 1(a). Note that in our case, a tight-pitch FLC is utilized and aligned vertically on the lateral walls as well as on the substrates. This is because in contrast to a surface-stabilized FLC with bistability, 8,9) a tight-pitch FLC is optically iso- tropic along the helix axis in the field-off state and becomes uniaxial in the field-on state, allowing for a continuous EO effect. Moreover, the vertically aligned FLC is extremely uniform 10,11) compared with the helix-deformed FLC in the planar alignment. 12,13) The microchannel architecture was constructed using a replica molding technique. The replica molding process can easily duplicate the information stored in a master such as the geometrical shape and morphology. 14,15) For the channel fabrication, poly(dimethylsiloxane) (PDMS; Dow Corning) was poured onto a master mold, and subsequently cured at 85  C for 1 h. The cured PDMS was then detached from the master mold and used as an upper substrate with an array of channels. Note that the PDMS material is capable of spontaneously aligning the LC molecules vertically 15) in the anchoring strength 16) of about ð1:0  0:5Þ 10 6 Jm 2 . Each channel has a rectangular cross section of h ¼ 5 m, w ¼ 30 m, and length of 15 mm. For the vertical alignment of the FLC on the bottom glass substrate, a polyimide layer (a) (b) (c) Fig. 1. (a) Schematic diagram of the confined FLC cell. Here, w and h denote the width and height of a channel, respectively. (b) Microscopy image of the confined FLC cell under crossed polarizers. Here, A and P represent the transmission axis of an analyzer and that of a polarizer, respectively. (c) Profiles of the light intensity along the x-axis [along the dotted line a–b of (b)]. y Present address: Department of Polymer Science and Engineering, University of Massachusetts Amherst, Amherst, MA 01003, U.S.A. Applied Physics Express 6 (2013) 054102 054102-1 # 2013 The Japan Society of Applied Physics http://dx.doi.org/10.7567/APEX.6.054102