Langevin Type Alignment in a Smectic Liquid Crystal Mixture Showing V-Shaped Switching As Studied by Optical Second-Harmonic Generation

Abstract

Second-harmonic generation (SHG) was observed in a homogeneously aligned smectic C*-like liquid crystal (LC) cell showing V-shaped switching during the application of a triangular wave. At normal incidence of light, abnormally strong SHG signals were observed at about zero electric field. It was confirmed that this phenomenon is characteristic in LCs showing the V-shaped switching, and is not observable in typical ferroelectric and antiferroelectric LCs. The dynamic response of SHG was successfully simulated using the two-dimensional Langevin type potential model.

Surface molecular alignment by in-plane anchoring in the cell showing the V-shaped switching

“…1͑a͔͒ is one of the major areas of current interest in the field of chiral smectic liquid crystals. [1][2][3][4][5][6][7][8] The reason for interest in them lies in their promise of realizing new types of liquid crystal displays ͑LCDs͒. 9 Since the discovery of V-shaped switching, random 1,2 and the collective models 5,6 were proposed to explain its switching mechanism.…”

“…The liquid crystal samples including the Inui mixture have a helix with a finite pitch in freestanding films. 3,4 Meanwhile, the V-shaped switching cell in which the helix is suppressed, has an average optical axis parallel to the smectic layer normal l for an applied voltage of Vϭ0. This has been confirmed by good extinction under crossed polarizers when one of them is along l. 3,5,6 Recently, Clark et al suggested the electrostatic model to explain the stabilization of molecules like this.…”

“…3,4 Meanwhile, the V-shaped switching cell in which the helix is suppressed, has an average optical axis parallel to the smectic layer normal l for an applied voltage of Vϭ0. This has been confirmed by good extinction under crossed polarizers when one of them is along l. 3,5,6 Recently, Clark et al suggested the electrostatic model to explain the stabilization of molecules like this. 7 According to their model, polarization charge self-interaction becomes important with material having sufficiently large spontaneous polarization P. This makes P in the bulk align uniformly parallel to the aligning surface ( ϭ /2) through P on both surfaces heads for the surface normal ͑ ϭ0 and ͒.…”

“…[2][3][4]8 We used two polyimides, RN1199 and RN1266 ͑Nissan Chemical Industries, Ltd.͒, as the aligning material. They were spin coated on glass substrates with an ITO coating.…”

“…The switching behavior was examined in SmX* phase ͑50°C͒ under an electric field of triangular wave form with frequency of 1 Hz ͓the phase sequence: AF ͑20-43°C: coexistence with SmX*͒ SmX* ͑64°C͒ SmA ͑68.5°C͒ Iso͔. 3 The two aligning materials, RN1199 and RN1266, show the different wavelength dependencies of transmittance given in Fig. 2; RN1199 has transmittance Tϭ0.64 for ϭ514 nm, while RN1266 is almost transparent (Tϭ0.97) for the same wavelength.…”