Compositions – Liquid crystal compositions – Containing nonsteryl liquid crystalline compound of...
Patent
1993-11-19
1996-08-06
Wu, Shean C.
Compositions
Liquid crystal compositions
Containing nonsteryl liquid crystalline compound of...
25229901, 252582, 560 43, 560 48, C09K 1912, C09K 1952, F21V 900, C07C22900
Patent
active
055430783
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to liquid crystal compounds possessing molecular and supermolecular structure providing large bulk electronic second order nonlinear optical hyperpolarizability X.sup.(2) in easily processible optical quality films. These materials, which are ferroelectric liquid crystals (FLCs), have application in fast optical processing and switching devices.
BACKGROUND OF THE INVENTION
The bulk electrical polarization P of a material in an electric field (or the electric part of an optical field) may be expanded in powers of the field according to equal, where P.sub.S is the spontaneous polarization (i.e. polarization present in the absence of applied field, X.sup.(1) is the linear polarizability, X.sup.(2) is the second order nonlinear hyperpolarizability, or second order nonlinear susceptibility, X.sup.(3) is the third order nonlinear hyperpolarizability or third order nonlinear susceptibility. The subscripts i, j, k etc. correspond to the Cartesian coordinates x, y, or z for the system (Williams, D. J., (1984) Angew. Chem. Int. Ed. Engl. 23:690-703). ##EQU1##
The sum of all terms to the right of P.sub.S in equal give the induced bulk polarization in response to an applied field or fields. The spontaneous polarization P.sub.S is a vector, while the susceptibilities X.sup.(1) etc. are tensors with component values which are dependent upon the frequency of the applied fields. The square of X.sup.(1) driven by a DC or low frequency AC field is proportional to the dielectric constant of the material, while the square of X.sup.(1) driven by an optical frequency AC field is proportional to the refractive index of the material. All materials possess non-zero X.sup.(1) and X.sup.(3). There are certain symmetry requirements for P.sub.s and for .sup.(2)), however. Thus, in order to possess non-zero P.sub.s, the system must have polar symmetry. Furthermore, within the electronic dipolar model, X.sup.(2) is zero unless the system possesses noncentrosymmetric symmetry (acentric). All materials with polar symmetry are acentric, but not all acentric materials are polar. Thus it is possible for a material to possess strictly zero P.sub.s by symmetry, but non-zero X.sup.(2) in the electronic dipolar model.
Materials possessing non-zero X.sup.(2) exhibit many effects of great current and potential utility. These include but are not limited to: 1) Second harmonic generation (SHG); 2) Sum and difference frequency generation; 3) Optical parametric amplification; 4) Optical rectification; and 5) A linear electrooptic effect (Pockel's effect). Effects 1, 2 and 3 depend upon the induction of optical frequency AC polarizations (or charge flow in the material changing in sign or magnitude at optical frequencies) in the material in response to optical frequency AC applied fields, and therefore derive from optical frequency X.sup.(2) values. These values of X.sup.(2) may be termed "ultrafast".
In general, the ultrafast X.sup.(2) is a lower limit, and the induced polarization in response to lower frequency applied fields will in general be larger (i.e. X.sup.(2) generally increases with decreasing driving field frequency, though the increase is not monotonic). Very large increases in X.sup.(2) occur at frequencies where resonant absorption of the driving radiation occurs. For the applications of interest in this invention, however, non-resonant interactions of the material with driving and induced fields are preferred.
Currently X.sup.(2) materials are utilized extensively for frequency conversion (effects 1 and 2 above), and more experimentally in electro-optic modulators (effect 5). Typically these materials are inorganic single crystals (for example single crystals of potassium dihydrogen phosphate (KDP) or lithium niobate (LiNbO.sub.3). For many applications, particularly in the emerging opto-electronics and photonics industry, easily processible thin films possessing X.sup.(2) are of great potential utility. Uses of X.sup.(2) thin films include, for example, electro-optic switching an
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Rego James A.
Ros Maria B.
Sierra Teresa
Walba David M.
University Research Corporation
Wu Shean C.
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