Ferroelectric Liquid Crystals for Nonlinear Optics: can we Really do It?
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ABSTRACT Ferroelectric liquid crystals (FLCs) are true liquids with a thermodynamically stable polar structure. Furthermore, in some systems excellent polar stereocontrol has been demonstrated (> 80% of the molecular dipole oriented along the polar axis with infinite "thermal stability"). These properties make FLCs an interesting candidate for second order NLO applications. However, until now the orientation of organic functional arrays with "large P", typified by the disperse red 1 (DRI) chromophore, has not been demonstrated. Herein we report the initial results of a study directed towards providing a solution to this problem. Specifically, a new FLC dopant wherein the DRI chromophore (p-nitro-p'-dialkylaminoazobenzene unit) is oriented along the polar axis in FLC mixtures up to 30% by weight is described. These results suggest that it will be possible to obtain FLCs with good stereocontrolled orientation of large 0 functional arrays. Given that hybrid VLSI silicon/FLC optical chips are under intense investigation as micro-displays, and manufacturing issues for this class of devices are being addressed in the display context, the materials presented here suggest VLSIIFLC chips may be useful as high speed electro-optic modulators as well. INTRODUCTION Chiral smectic C ferroelectric liquid crystals (C* FLCs) seem promising for second order nonlinear optics (NLO) applications since they are nicely processible on semiconductor integrated circuits [2] and possess a thermodynamically stable polar structure [3]. Early measurements of the bulk second order susceptibility X(2) in the classic FLC material decyloxybenzylideneaminomethylbutylcinnamate (DOBAMBC) by second harmonic generation (SHG), however, showed that DOBAMBC has a very small second order susceptibility [4, 5] even though the molecular structure should have a relatively large molecular second order susceptibility 0. The observed very small value of X(2) of DOBAMBC serves to illustrate two key problems inherent in the FLC approach to X(2) materials: 1) For many FLC systems the degree of polar stereocontrol is small; and 2) Large J0 functional arrays, which typically involve donor and acceptor units separated by a relatively long conjugating spacer as indicated for DOBAMBC, tend to orient parallel to the liquid crystal director and normal to the polar axis in the C* phase. Thus, 157 Mat. Res. Soc. Symp. Proc. Vol. 392 ©1995 Materials Research Society
j units are present in the FLC structure, the supermolecular stereocontrol is incorrect for achieving large X(2). even when large
Donor
0
Acceptor
N Cl0 H210 DOBAMBC
FLCS FOR NLO Prior Results Within the past six years FLC systems with very large ferroelectric polarization P (up to 1,100 nC/cm2 or >70% polar excess in the C* phase) have been reported [6]. These results prove that excellent polar stereocontrol is achievable with FLCs--certainly a promising development in the context of FLCs for NLO. Furthermore, by appropriate design the second order susceptibility of known FLCs has increased by a factor of
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