Nonlinear optics is concerned with the interactions of electromagnetic fields in various media to produce new fields altered in phase, frequency, amplitude, or other propagation characteristics from the incident fields. In order to gain an insight into the origin of nonlinear optical effects, the polarization P induced in a molecule by a local electric field E can be expressed by Equation 1 EQU P=.alpha.E+.beta.E.sup.2 +.gamma.E.sup.3 ( 1)
where
P is the total induced polarization, PA1 E is the local electric field created by electromagnetic radiation, and PA1 .alpha., .beta., and .gamma. are the first, second and third order polarizabilities, each of which is a function of molecular properties. PA1 P is the total induced polarization, PA1 E is the local electric field created by electromagnetic radiation, and PA1 .chi..sup.(1), .chi..sup.(2), and .chi..sup.(3) are the first, second, and third order polarization susceptibilities of the electromagnetic wave transmission medium.
The molecular level terms of Equation 1 are first order or linear polarization .alpha.E, second order or first nonlinear polarization .beta.E.sup.2, and third order or second nonlinear polarization .gamma.E.sup.3.
On a macromolecular level corresponding relationships can be expressed by Equation 2: EQU P=.chi..sup.(1) E+.chi..sup.(2) E.sup.2 +.chi..sup.(3) E.sup.3( 2)
where
The macromolecular level terms of Equation 2 are first order .chi..sup.(1) E, second order polarization .chi..sup.(2), and third order polarization .chi..sup.3 E.sup.3.
D. J. Williams, "Organic Polymeric and Non-Polymeric Materials with Large Optical Nonlinearities", Angew. Chem. Int. Ed. Engl. 23 (1984) 690-703, and Zyss "Nonlinear Organic Materials for Integrated Optics", Journal of Molecular Electronics, vol. 1, pp. 25-45, 1985, disclose a variety of nonlinear optical end uses that can be served by utilizing .chi..sup.(2) or .chi..sup.(3) properties of a propagation medium.
Interest in nonlinear optical devices has particularly centered on devices relying on second order polarization susceptibilities. To achieve on a macromolecular level second order polarization (.chi..sup.(2) E.sup.2) of any significant magnitude, it is essential that the transmission medium exhibit high (herein employed to mean greater than 10.sup.-9 electrostatic units) second order polarization susceptibilities. To realize such values of .chi..sup.(2) it is necessary that the second polarizability .beta. be greater than 10.sup.-30 electrostatic units (esu).
A significant difficulty encountered in finding suitable molecular dipoles for second order polarization effects lies in the molecular requirements that must be satisfied to achieve usefully large values of .beta.. For a molecule to exhibit values of .beta. greater than zero, it is necessary that the molecule be asymmetrical about its center--that is, non centrosymmetric. Further, the molecule must be capable of oscillating (i.e., resonating) between an excited state and a ground state differing in polarity. It has been observed experimentally and explained by theory that large .beta. values are the result of large differences between ground and excited state dipole moments as well as large oscillator strengths (i.e., large charge transfer resonance efficiencies).
For .chi..sup.(2) to exhibit a usefully large value it is not only necessary that .beta. be large, but, in addition, the molecular dipoles must be aligned so as to lack inversion symmetry. The largest values of .chi..sup.(2) are realized when the molecular dipoles are arranged in polar alignment--e.g., the alignment obtained when molecular are placed in an electric field.
For a number of years the materials employed for achieving second order polarization effects were non-centrosymmetric inorganic crystals, such as potassium dihydrogen phosphate and lithium niobate. Williams postulates mathematically and experimentally corroborates second order polarization susceptibilities in organic molecular dipoles equalling and exceeding those of conventional inorganic dipoles.
A technique that has been found useful in obtaining high .chi..sup.(2) organic molecular dipole layers for nonlinear propagation of electromagnetic radiation is to incorporate the organic dipoles in a polymeric medium having a glass transition well above ambient temperatures. The organic molecular dipoles can form a part of the polymer molecule or simply be blended with the polymer. While the polymeric medium is heated above its glass transition temperature, an electric field gradient is placed across the medium. The organic molecular dipoles align themselves with the electric field gradient. With the electric field applied the polymeric medium is cooled to below its glass transition temperature, thereby locking the organic dipoles in a polar aligned, non-centrosymmetric arrangement essential to achieving high .chi..sup.(2) values. The technique is commonly referred to as poling, and high .chi..sup.(2) layers so generated are commonly referred to as poled polymeric layers. Ulman et al U.S. Pat. No. 4,792,208 and Robello et al U.S. Pat. No. 4,796,971 are cited as illustrative of a number of patents disclosing poled polymeric media.
Metal fluorides and oxides have been employed with optical articles, such as lenses Mathers et al U.S. Pat. No. 2,441,668; Socha U.S. Pat. No. 3,176,575; and Hoffman U.S. Pat. No. 3,883,214 are illustrative.
An art recognized class of nonlinear optical articles are those that modulate reflection, either for the purpose of controlling its intensity or polarization. Devices intended for this purpose, commonly referred to as attenuated total reflection devices or ATR's, are illustrated by
ATR-1: Sincerbox et al U.S. Pat. No. 4,249,796; PA0 ATR-2: McNeill et al U.S. Pat. No. 4,451,123; PA0 ATR-3: Sarid, "Long Range Surface-Plasmon Waves on Very Thin Metal Films", Phys. Rev. Lett., Vol. 47, No. 26, Dec. 1981, pp. 1927-1930; PA0 ATR-4: Persegol et al., "A Novel Type of Light Moduator", SPIE Vol. 864, Advanced Optoelectronic Technology (1987) pp. 42-44; PA0 ATR-5: Schildkraut, "Long Range Surface Plasmon Electrooptic Modulator", Applied Optics, Vol. 27, No. 21, Nov. 1, 1988, pp. 4587-4590.
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