Within the past several years, the development of advanced materials with large second and third order nonlinear (X2 and X3) effects has generated interest in developing devices to exploit these properties in military systems of the future. This interest has taken many forms, from the funding of basic research aimed at developing enhanced nonlinear optical effects in materials to the deployment of actual mil-spec devices. Two years ago, at this same forum, we delineated a series of materials performance requirements which, if achieved, would allow device developers to take maximum advantage of these nonlinear effects in such disciplines as optical computing, optoelectronic interconnects and sensor protection. In the interim, DoD requirements for advanced materials and devices have continued to accentuate the need for advanced nonlinear optical materials. This paper will discuss some of the most recent trends in nonlinear optical materials research from a device requirements perspective, including an overview of the results of the recent DARPA forum on nonlinear optical materials and a discussion of future directions. As in the previous paper, we will confine ourselves to three challenging areas: nonlinear optical materials for optical computing applications, materials for computer peripherals and internode communications, and materials for sensor protection.

We report initial experiments on the fabrication of active and passiveintegrated optic device structures based on poled polymer electro-optic buried channel waveguides. The process of channel waveguide definition and fabrication through electric field poling is described and experimental results are presented. The fabrication, theoretical performance modeling and experimental evaluation of several integrated optic device structures, including phase modulators, directional couplers and Y-branch interferometers, are also reported.

The wavelengths at which indanthrone behaves as an efficient reverse saturable absorber can be changed by chemical modification of the indanthrone chromophore. Several derivatives of indanthrone have been synthesized and they have been shown to be more efficient nonlinear optical materials than the parent chromophore. In this paper we will discuss the behavior of these indanthrone derivatives, attempt to correlate the nonlinear optical properties observed in this class of compounds to modifications in the structure of the indanthrone chromophore and suggest how these nonlinear materials may be applied to all-optical switching devices.

The apolicability of nonlinear organic materials in integrated optics is discussed theoretically. We have studied the properties of an optically linear waveguide covered with a nonlinear organic film. Optical switching, power limiting and thresholding in optical logic are potential applications of this structure.

The field distribution and propagation constant are solved exactly. The field in the nonlinear film is written in terms of Jacobian elliptic functions, and a procedure to compute them is presented. The trial functions used here turn out to be flexible and useful because – in contrary to the approaches where hyperbolic functions are used – they lead to a solution for a variety of parameters and in the case of negligible nonlinearity it coincides well with the linear case.

The method is applied to a Si3 N4 waveguide with an organic overlayer. The effect of device parameters like refractive indices, thickness of the layers, incident power and strength of the nonlinearity is studied. The intensity dependence of the wavevector is determined and the lower limit cutoff powers for some structures are calculated.

The utility of the Shear method in obtaining thin single crystal films of polydiacetylenes has been demonstrated previously. In this paper the principles of the Shear method are elucidated with analogy to the well-known L-B method. Both the methods, L-B and Shear, utilize the polar molecular interactions with the substrates as a control parameter for the molecular organization. The major difference between the two is that one (L-B) utilizes a liquid as the subphase while the other (Shear) uses solid substrates. Because of the nature of the interface the L-B approach leads to only one-dimensional organization and poor optical quality for the films while the Shear method provides 3-D organization and excellent optical quality.

Thin single crystal films of a specific polydiacetylene, PTS, are prepared on oxide coated silicon substrates by the Shear method. Waveguiding through the PTS films is demonstrated for the first time. Much progress is also made in the waveguide-structural fabrication of PTS films. The results are quite encouraging in terms of device applications of polydiacetylenes as optical switches and logic gates.

Nonlinear organic materials, in conjunction with integrated optics structures, can potentially be used to perform a variety of all-optical operations on subpicosecond time scales. A number of potential devices are described and the material properties necessary to implement them are summarized. Progress in our laboratory toward these goals is detailed.

Although organic crystals may be used to experimentally verify the large nonlinearities and short response times of organics, such crystals are not acceptable for device applications due to significant fabrication difficulties. Further, the bulk material nonlinearity is a function of molecular orientation and symmetry which may not be controlled during the crystallization process.

Nonlinear optical polymers have been synthesized at Hoechst Celanese for which the active NLO unit is attached to the polymer backbone as a pendant side chain. Control of orientation and symmetry of the unit is achieved by poling in an external electric field at elevated temperatures resulting in second order susceptibilities larger than inorganic crystals. The polymers have attractive secondary properties (i.e., optical transparency, high glass transition temperatures which are controlled by adjusting the side chain length and nature of the polymer backbone, low dielectric constants, and flat frequency respose). Further, single mode waveguides may be fabricated by spin coating. Deposition of electrodes on the waveguide permits application of an external field which changes the material's index of refraction due to the linear electrooptical effect. Thus, a host of electrooptical waveguide devices may be constructed which operate at low voltages and very high frequencies.

We introduce neural networks as a new basis for computation. The role optics can play In implementing neural networks is discussed in terms of the requirements for optical systems and devices. A conclusion is drawn that the rapidly evolving knowledge of neural network models argues for a flexible and adaptable device technology. Such a technology is described using the self electro-optic effect device as an example.

Using a recently developed many-electron theory of second and third order nonlinear optical susceptibilities, microscopic descriptions of γxxxx(-3ω;ω,ω,ω) are presented for two conformations of polyenes. The importance of electron correlations and the effects of variation in conformation and chain length are discussed.

Crude pictures relating molecular features and enhancement of nonresonant second-order hyperpolarizability have been known and successfully utilized in selection and preparation of materials for nonlinear-optical applications for over a decade. On the other hand, other than the requirement of “electron delocalization”, such a useful picture does not exist for nonresonant third-order hyperpolarizability. In pursuit of such a picture, and to obtain a better view of the range of hyperpolarizability magnitudes which might be obtainable, simple linear conjugated molecules have been synthesized and characterized. Liquid solutions of these model compounds were studied using recently developed high precision optical third harmonic generation techniques. Results of these characterizations and a new addition to the picture of structure-property relationships are reported.

The non-resonant electronic component of γ(-3ω ω, ω, ω) is calculated for ethylene, all-trans 1,3-butadiene, all-trans 1,3,5-hexatriene, and all-trans 1,3,5,7- octatetraene. The γ(-3ω; ω, ω, ω) is evaluated using the sum-over-states method and the all-valence-electron, semi-empirical INDO-SCF-MO procedure combined with single- and double-excitation configuration interaction (CI). The treatment of electron-electron interactions at the level of only single-excitation CI calculates negative values of γ(-3ω ω, ω, ω), which are in disagreement with measured positive values. The treatment of the interactions at the level of single- and double-excitation CI correctly calculates positive values for γ(-3ω ω, ω, ω). We conclude that the treatment of electron-electron interactions at least at the level of single- and double-excitation CI is needed to properly describe the dynamic thirdorder polarizabilities of linear conjugated polyenes, and related molecules with π-electron bonding networks.

Three polymers containing pyridine N-oxide imine pendant moieties were prepared and examined. Although the appropriate low molar mass model compound is liquid crystalline, the polymers are not mesogenic. Two of them form fibers and clear, transparent glasses. Side reactions involving the N-oxide group during polymerization are postulated as leading to the lack of mesomorphism in these materials although the π-chromophore remains essentially intact for the bulk of the material as judged by UV-Vis and other spectral data. They may have NLO application as poled polymeric glasses.

Molecular second-order nonlinear optical susceptibilities (β) based on EFISH measurements in dimethylsulfoxide of selected donor-acceptor substituted organic compounds are reported. Enhanced values were noted for compounds containing highly ionizable substituents. Compounds possessing the highest susceptibilities were dissolved in polymer films and electric-field poled, leading to corresponding enhancements in the nonlinear optical coefficients. The data are consistent with molecular and thermodynamic models.

We present results of studies of orientational order of Langmuir-Blodgett monolayer and multilayer films based on second harmonic generation and linear absorption measurements. Two new compounds, a tricyanovinyl aniline and a tricyanovinyl azo dye, both possessing large second order molecular susceptibilities have been synthesized and transferred as monolayers from the air-water interface to solid substrates using the Langmuir-Blodgett technique. The azo dye is a superior Langmuir-Blodgett material, forming films which are chemically and structurally stable. Multilayers of this compound show an enhancement in polar orientational order with thickness, as observed in the dependence of the second harmonic polarization components on number of layers.

Organic materials having aminophenyl and nitrophenyl moieties linked by various atoms or groups (S, O, SO, SO2, CH2) were prepared and their ability to generate a second harmonic was compared within the series and with urea. We find that the compound containing a sulfur bridge, 4-amino-4′-nitrodiphenyl sulfide, exhibits a powder second harmonic generation efficiency which is approximately 10x that of urea. The result is consistent with earlier spectroscopic studies which indicate that sulfur, as opposed to the other bridges studied in this series, is effective in allowing the transmission of electron density between donors and acceptors.

We investigate the applicability of the Genkin-Mednis perturbative approach to the ab initio calculation of the static (hyper)polarizabilities of infinite periodic polymers. Test calculations on molecular hydrogen and polyacetylene chains are presented to assess some of the computational problems inherent to the method.

Factors contributing to the polarizability (αij) and frequencydoubling hyperpolarizability (βijk) of 2-(p-dimethylaminophenyl)-6-(pnitrophenyl) benzo(l,2-d:4,5-d′ )blsthiazole (DNBT) are analyzed via perturbation theory and the PPP-SCF-MECI π-electron model Hamiltonian. While the observable part of β (βvec) is clearly identifiable with a small number of charge transfer excitations along the molecular dipole direction, a (and by inference, the second-order hyperpolarizability γ) is more closely related to the overall size (volume) of the π-electron cloud. As a consequence, βvec is far more sensitive to molecular distortions which affect donor-acceptor charge transfer interactions than is π. The more sensitive frequency dependence of βvec can be understood in terms of the three-photon character of this nonlinearity.

We present femtosecond time-resolved transient absorption data taken on polydiacetylene-pTS single crystals over the entire wavelength region of exciton resonance. Observed optical changes are accounted for by a model which predicts quantitatively the large resonant and nonresonant optical nonlinearities.

We apply the Pariser-Parr-Pople Hamiltonian to study many-electron excitations in polyenes and polyacetylene. The excited singlet states of polyenes, calculated by a multireference double excitation expansion, are classified as quasi-particle excitations, namely as triplet-triplet magnons and particle-hole excitons. From finite polyene spectra we derive approximate dispersion relations for these quasi-particles in the infinite polyene, i.e. polyacetylene.

A two-level expression of the second-order hyperpolarizability (y) is proposed, and shown to contain a negative charge transfer (CT) contribution. Frequency dispersed measurements of the quadratic and cubic susceptibility of N-(4-nitropheny1)-L-prolinol (NPP), N-(4-nitrophenyl)-N-methylaminoacetonitrile (NPAN) and 2-(N-prolinol)-5-nitropyridine (PNP) are reported. The electric field induced second-harmonic generation (EFISH) and third-harmonic generation (THG) techniques in solution are used. A two-level model dispersion of y as from THG measurements is shown to lead to a neglegible electronic contribution to the overall EFISH nonlinearity, as compared to the orientational contribution. The theoretical β-components of these molecules, obtained consistently by finite-field and perturbational approaches, clearly confirm that the β-tensor is one-dimensional along the donor-acceptor CT axis. The off resonance quadratic polarizabilities (β) of these molecules are sequenced as follows : β(NPP)> β(PNP)> β(NPAN). However this increased efficiency is obtained at the expense of the transparency range.