Abstract
Electrohydrodynamic instabilities (EHDI) in liquid crystals form uniform and continuously variable diffractive structures when subject to certain material and geometry determined conditions. A one-dimensional grating is one such diffractive structure, where the refractive index changes periodically in a direction parallel to the initial liquid crystal director. The period of this structure has been shown previously to vary continuously between the values of the cell gap and half-cell gap approximately, allowing continuous angular modulation of optical beams but with a limited angular range. In this work, the lower pitch limit is shown to also be governed in part by the ratio of the splay and bend elastic constants (k11/k33) of the liquid crystal. A host nematic liquid crystal with standard elastic constant ratios (k11/k33 < 1) is doped with odd-alkyl-spaced dimeric liquid crystal CB7CB, to create a liquid crystal mixture with a far higher elastic constant ratio (k11/k33 > 5) than for those previously used in literature EHDI studies. The EHDI gratings formed in this new mixture exhibit pitch lengths significantly below half-cell gap, allowing up to 50% wider angle continuous steering of light. This improves the potential for application in beamsteering and diffractive optical devices.
Highlights
Given that many pixels are required to create all but the simplest optical features, and the fact that liquid crystal on silicon (LCoS) devices operate in reflection mode only,[5,16] developing alternative methodologies to create high-quality diffractive structures is desirable
MLC 2081 is a commercial nematic liquid crystals (NLCs) mixture provided by Merck Chemicals UK and was chosen as the host liquid crystal because it exhibits a wide nematic temperature range, a typical k11/k33 (0.7–0.8 at 25 1C), high negative dielectric anisotropy (De = À5.5 at 25 1C) and, importantly, a low inherent conductivity (s> o 10 nS mÀ1 at 25 1C) to enable conductivity to be controlled through the addition of dopants
This paper demonstrates the influence of LC elastic constants on the pitch of the grating structures in nematic Electrohydrodynamic instabilities (EHDI)
Summary
Since the 1960s nematic liquid crystals (NLCs) have been used successfully as switchable optical materials in optoelectronic applications, including displays.[1,2] The number of different applications of NLCs has expanded in recent decades to a wider variety of optical devices including spatial light modulators,[3,4,5,6,7,8] lenses,[9,10,11] diffraction gratings[12,13,14,15] and beamsteering devices.[4,16,17,18] An important goal has been to produce entirely arbitrary phase profiles for incident light when passed through the liquid crystal. Liquid crystal on silicon (LCoS) spatial light modulators are used for such purposes.[3,4,5,6,7,8,19,20,21,22] In LCoS devices, individual electrodes, driven by the silicon back-plane, reorient the NLC to induce the required optical phase profile. Fringing fields and defects in the nematic director profile limit pixels to a minimum size of B5 mm.[8,16] Given that many pixels are required to create all but the simplest optical features, and the fact that LCoS devices operate in reflection mode only (with much larger minimum pixel sizes for transmissive devices),[5,16] developing alternative methodologies to create high-quality diffractive structures is desirable. In most NLC devices the electro-optic switching is achieved by utilising an external electric field, in
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