Optical: systems and elements – Optical modulator – Having particular chemical composition or structure
Reexamination Certificate
2000-04-18
2002-02-19
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
Having particular chemical composition or structure
C252S582000, C359S345000
Reexamination Certificate
active
06348992
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to nonlinear optical chromophores and, more particularly, pertains to sterically stabilized second-order nonlinear optical chromophores and devices incorporating the same.
2. Description of the Related Art
Organic second-order nonlinear optical (NLO) materials have received increasing attention for applications involving signal processing and communications. One of the challenges in this field is to design and synthesize second-order NLO chromophores (the active components of second-order nonlinear optical materials) that simultaneously possess large first molecular hyperpolarizabilities (&bgr;), good chemical and thermal stability, and optical transparency at optical communication wavelengths (1.3 and 1.55 &mgr;m). Chromophore intermolecular electrostatic interactions prevent the simple scaling of molecular optical nonlinearity into macroscopic optical nonlinearity. Such interactions strongly attenuate the efficient induction of acentric chromophore order (hence, electro-optic activity) by electric field poling or self-assembly methods. Chromophores with &bgr; values many times those of the well-known Disperse Red 19 dye are thus required to obtain electro-optic coefficients comparable to or higher than those of the leading commercial material crystalline lithium niobate.
The value of &bgr; for a chromophore can be increased by using a diene moiety in place of thiophene in the conventional phenylethenylenethiophene &pgr;-conjugated bridge. Moreover, this enhancement in &bgr; can be accomplished without an increase in the wavelength of the charge-transfer absorption &lgr;
max
. However, the resulting phenylpolyene bridge has poor thermal stability unless the polyene structure is sterically protected. The synthesis of various sterically-protected (ring-locked) phenylpolyene chromophores involves cyclic enones such as isophorone, verbenone and double-ring locked dienone as starting materials and intermediates. The Knovenegal coupling reaction between enones and electron acceptors is the critical step in both backward and forward methods reported. The low reactivity of enone severely limits the choice of acceptor to only a few molecules including malononitrile, isoxazolone, and thiobarbituric acid and therefore has become the bottleneck in the development of ring-locked phenylpolyene-bridged high &bgr; chromophores.
In addition to microscopic and macroscopic nonlinearity, the photochemical stability of second-order NLO material has long been recognized as another major problem which must be solved for successful employment of these materials in commercial devices. Chemical degradation of the NLO chromophore in polymer film can be caused by photoinduced chemical reaction and thermal decomposition. In an oxygen-containing environment (e.g., air), a major cause of chromophore degradation is photoinduced oxidation of the chromophore. Photooxidation changes the chromophore to a new species that is effectively electro-optically inactive.
Researchers have been trying to improve the photochemical stability of NLO chromophores by modifying their chemical structure. The results of their efforts indicate that the stability can be improved by avoiding adjacent alkyl groups on the nitrogen donor of the chromophore [See, U.S. Pat. No. 5,776,375 to Hofstraat, et al.], by using aromatic substituents on the nitrogen donor, by using azo bridge linkage instead of a carbon-carbon double bond, and by attaching a radical scavenger to the chromophore [See, Optics Letters, 2000, Vol. 25, no. 5, 332-334, Andriana Galvan-Gonzalz, et al.]. It has also been found that shorter chromophores generally have higher photochemical stability in air. However, due to the strong dependence of molecular nonlinearity on chromophore conjugation length, short chromophores cannot provide a sufficient degree of nonlinearity for practical applications [Cheng, L.-T.; Tam, W.; Marder, S. R.; Stiegman, A. E.; Rikken, G.; Spangler, C. W.
J. Phys. Chem.
1991, 95, 10643-10652. Marder, S. R.; Cheng, L.-P.; Tiemann, B. G.; Friedli, A. C.; Blanchard-Desce, M.; Perry, J. W.; Skindhøj, J.
Science,
1994, 263, 511-514. Wong, K. Y.; Jen, A. K.-Y.; Rao, V. P.
Phys. Rev.
1994, 49, 3077-3080.].
In addition to the chemical decomposition of the chromophore, light can also cause randomization of electric poling-induced alignment. In this process, although the chromophore chemical structure is not destroyed, the material will still lose all nonlinear activity evantually. To address this aspect of the photo-related problem, one would need to modified the chromophore structure to reduce or eliminate certain structural units that could lead to the unwanted random motion of the chromophore backbone, and to find a more rigid yet processible polymer host to restrict free motion of the chromophore. Since the properties (microscopic nonlinearity, macroscopic, chemical and thermal stability, etc.) of second-order nonlinear optical materials are inter-related, optimization of one property often causes unacceptable amounts of attenuation in other properties. Thus, a systematic approach to addressing both the stability and nonlinearity issues is needed for a balanced improvement in both properties. An approach based on chromophore structure modification has been addressed in the parent U.S. patent application Ser. No. 09/546,930. The present invention provides a solution to the photochemical issue associated with NLO materials, a solution which does not sacrifice requirements of molecular nonlinearity and high poling efficiency.
SUMMARY OF THE INVENTION
A new class of ring-locked aminophenylpolyenal donor-bridges has been developed. These new donor-bridges, according to the present invention, have very high Knovenegal reactivity and have been coupled with a variety of acceptors bearing acidic methyl or methylene groups (including the most desired TCF and TCI type of acceptors shown in
FIG. 11
) to obtain a new class of second-order NLO chromophores. This methodology broadens the scope of polyene-bridged chromophores without significantly sacrificing thermal stability or optical transparency. This synthetic approach leads to the development of device-quality NLO chromophores (shown in
FIG. 1
) possessing &mgr;&bgr; values (where &mgr; is the chromophore dipole moment) of 15,000×10
−48
esu or greater at 1.9 &mgr;m as determined by the electric field induced second harmonic generation (EFISH) technique.
A variety of different molecular structures are possible for the chromophores of the present invention. An exemplary preferred basic chromophore structure according to the present invention includes an electron donor group, an electron acceptor group and a &pgr;-conjugate bridge structure therebetween. The bridge is a polyene structure having a five-, six- or seven-membered ring to lock one carbon-carbon double bond. Uniquely, the bridge contains an unlocked conjugate diene unit to connect the bridge ring and the acceptor (A). In a preferred embodiment, the bridge structure also includes at least one bulky side group.
Another exemplary preferred chromophore according to the present invention includes an electron donor group, an electron acceptor group and a ring-locked bridge structure between the electron donor group and the electron acceptor group. The bridge structure comprises a fused double- or triple-ring structure which functions to lock two or three double bonds. The bridge also contains an unlocked conjugate diene unit to connect the bridge ring and the acceptor (A). In a preferred embodiment, the bridge structure also includes at least one bulky side group.
Another exemplary preferred chromophore according to the present invention includes an electron donor group, an acceptor, and a bridge structure therebetween, wherein the acceptor is a five- or six-membered ring-locked tricyano electron acceptor.
Another exemplary preferred chromophore according to the present invention includes an electron donor group, an electron
Fetterman Harold R.
Michael Joseph
Steier William
Zhang Cheng
Epps Georgia
Henricks Slavin & Holmes LLP
Pacific Wave Industries, Inc.
Spector David N.
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