Large, ultrafast second and third order nonlinear optical properties are widely observed in a large number of organic and polymer structures. These systems can be formed as organizable designed structures possessing a number of special primary properties due to their nonlinear electronic polarization whose microscopic origin resides in highly charge correlated virtual excitations. These ultrafast excitations occur on individual molecular, or polymer chain, sites in condensed assemblies that are described by their orientational distribution functions. These and related issues are addressed based on recently completed studies.

Time-resolved absorption, gain, and laser spectra were measured for molecules (4- diethylamino-4'-nitrostilbene (DEANS), and 4-dimethylamino-4'-nitrostilbene (DMANS)) with large dipole moments. Tuning ranges of DEANS laser are 612-650 nm, 647-705 nm, 653-708 nm, and 680-705 nm in benzene-acetone mixed solution with volume ratios of 1:0, 5:1, 4:1, and 3:1, respectively. The DMANS laser has a short-cavity, resonatortransient configuration. No spectral shift was observed during laser oscillation. This is consistent with the result of the analysis of lasing by simplified rate equations.

A semi-empirical CNDOVSB computer program has been developed to calculate the second-order nonlinear optical polarisabilities of molecules. The program has been parameterised by comparison of calculated and experimental values of molecular properties over a large wavelength range. The use of the program is described, both in the evaluation of the potential of specific compounds and also to study trends in series of related molecules. In particular, the effect of conjugation length on the nonlinear optical properties of polyphenyls and polyenes is described.

Previous nonlinear optical measurements J.G. Bergman, G.R. Crane and B.F. Levine, Appl. Phys. Lett. 20.21 (1972)] show 5- nitrourecil(5NU) to be phase matchable with a coefficient for second-harmonic generation equal approximately to that for Li103 [d36(5NU) = (1.0 ± 0.1)(133( Li103)]. The X-ray crystal structure determination shows 5NU to crystallize in the orthorhombic space group P212121 containing 4 molecules per unit cell of dimensions a = 9.936 ± 0.001 Å, b = 10.355 t 0.001 Å and c = 5.462 ± 0.002 Å. The origin of the excellent structural properties of 5NU is an extensive hydrogen bonding network in the crystal. The orientations of the 4 molecules in the unit cell are such that the unit cell has no net dipole moment. The magnitudes of the ground state dipole moments calculated for isolated 5NU are 6.09 D (INDO-SC1), 6.21 D (INDO-SDC1) and 6.30 D initio-4-31G). The 1.ππ and charge-transfer (CT) excited states of 5NU are calculated to define the second-order nonlinear optical properties of the molecule. The l mu* states of 5NU are calculated to contribute very little to these properties. The level of treatment of electron correlation in the molecular orbital ( MO) procedures is found to have a marked effect on the calculations of the excited electronic state and second-order nonlinear optical properties of 5NU. The INDO-SDCI MO procedure, with its extensive treatment of electron correlation, calculates a value of d36 1.2)1:133( Li103) for isolated 5NU that is very close to the measured value cited above. A preliminary bonding electron density distribution map for 5NU is presented.

Using picosecond and femtosecond laser spectroscopy, the third order optical nonlinearity, relaxation kinetics and ultrafast optical logic circuits of 4E3CMU polydiacetylene in polymethylmethacrylate matrix films and CH₂C₁₂ solutions have been measured. Our efforts will lead to the future generation of photonic and computer instrumentation and devices with operational times in the picosecond and femtosecond regimes.

Measurements of the first x⁽¹⁾ and the third order optical susceptibilities X⁽³⁾ in the spectral region below the band edge of polyacetylene are reviewed. The absorption spectrum in trans-(CH)_x has an intense peak due to the onset of the strong interband transition near 2 eV. Inside the semiconducting gap the absorption has an exponential band edge and the transmission losses are dominated by scattering effects. The magnitude of the electronic contribution to the X⁽³⁾ parallel to the polymer chains in trans-(CH)x is in excess of 10-9 esu for wavelengths larger than 1.3 μ.m, and is one of the largest values for the electronic X(3) inside the gap of a semiconductor. The spectrum of X⁽³⁾ in trans-(CH)_x has a sharp two-photon resonance that has been attributed to the presence of the 21Ag excited state near band edge. It is suggested that through appropriate control of morphology, noncrystalline, processable conjugated polymers can be useful in optoelectronic applications.

We report a new class of molecules, pyrylium dye aggregates, that possess exceptionally large second-order optical nonlinearity. These dye molecules aggregate spontaneously in a polymer matrix to form a head-to-tail ordered structure. We have observed efficient second harmonic generation in a micron-size thin film containing only 1.5 wt% dye aggregate. This observation suggests the exciting possibility of achieving significant nonlinear optical interaction within one coherence length. Semi-empirical molecular orbital calculations also suggest that these dyes possess large second-order polarizabilities. We suggest that charge-transfer excitons within the aggregates may account for the large observed nonlinearities.

Two different ways have been investigated to obtain side-chain liquid crystalline polymers suitable for second harmonic generation (SHG). The aim of the first one was to obtain host nematic comb-like homopolymers having a small degree of polar association of their side chains, by using a 3-fluoro-4-cyanophenyl benzoate end group. The second way consisted in synthesizing liquid crystalline polymeric systems where a mesogenic monomer and a monomeric molecule bearing a group exhibiting a large molecular hyperpolarizability p , were copolymerized. In this paper, we report on the synthesis and the phase behaviour of these two families of polymers.

Side chain liquid crystalline polymers offer unique advantages as a new class of organic materials with potential for nonlinear optical (nlo) response. Design and synthesis of a series of nitroaromatic side chain liquid crystalline polyesters was carried out employing the concept of having the nitroaromatic species serve concomitantly as both the nonlinear optical chromophore and as the (only) liquid crystalline moiety in the polymer.

The use of organic polymers for nonlinear optical (NLO) processes is gaining increased attention because of the ability to engineer molecular structures that have high laser damage thresholds, high values of both the first and second hyperpolarizabilities, and inherently fast response times. This general class of materials also provides synthesis and processing options that are not feasible with inorganic crystals, such as structural optimization through modification, fiber spinning, film casting and thermoplastic processes. This paper provides the synthesis method for a class of polymers commonly referred to as the PBZ polymers. One of these polymers, namely polyp-phenylene-2,6-benzobisthiazole) (PBT) has been shown to have a high value of the macroscopic third order optical nonlinear susceptibility, X (3)1. Development of the synthesis of these π -electron conjugated systems combined with their evaluation for nonlinear optical responses may be viewed to be the preliminary step in the engineering of optimized materials for use in optical signal processing devices of the future. By describing the process of preparing rigid rod and semi-rigid benzothiazole and benzoxazole polymers in polyphosphoric acid (PPA) at the appropriate concentration to achieve the advantages of highly ordered solution morphologies, we establish the foundation for the preparation of materials with optimized figures of merit for NLO applications.

For several years, delocalized 'if-electron polymers have attracted considerable scientific interest because of electrical conductivity properties which could be associated with solitonic, polaronic, and bipolaronic species. More recently, the observation of third order susceptibilities, x⁽³⁾, in the range 10^-9 to 10^-11 esu has generated interest in these materials for nonlinear optical applications. Electrical conductivity and nonlinear optical properties depend upon electronic characteristics such as "time-independent" and "time-dependent" electron delocalization, upon intra- and intermolecular charge transfer, and upon electron-phonon coupling. We have utilized advanced electron paramagnetic resonance techniques to characterize such electronic properties for the linear polyene polyacetylene and for a number of heteroararatic ladder polymers. The results of these measurements are used to make predictions of nonlinear optical activity.

Commercial interest in nonlinear optical (NLO) materials is driven by the development of fiber optics, laser diodes, and high speed computing. In communications, as well as in information processing, the direction of future technology points unmistakably towards optical systems. Active materials for optical modulation, routing, and amplification are in high demand for this technology. Currently available materials for NLO applications lack many of the critical requirements for true industrial implementation. Therefore, significant programs for the development of improved NLO materials have blossomed throughout the world. Organics have been suggested as improved materials for a variety of applications ranging from conducting polymers to superconductors. For NLO applications, there is a solid foundation of basic science which clearly defines intrinsic advantages of these materials in comparison to those which are currently in use. In addition to basic material constants, organics offer the opportunity to apply molecular level science to engineer materials through application of several principles of design. In this paper, these design concepts will be highlighted in an attempt to define an industrial approach to this materials problem.

A new reversible optical storage technique based on azo dye containing liquid crystalline polymers and copolymers is described. Monodomain films of the polymers were used as storage medium. Digital and holographic information was stored and reconstructed. The stored information can be erased and the writing process is repeatable. The storage process is explained in terms of a transcis isomerization of the mesogen inducing local director field distortions.

Multilayers of the compound 4-[4-(N-n-dodecyl-N-methylamino)phenylazo]-3-nitrobenzoic acid (DPNA) have been deposited onto glass slides using the Langmuir-Blodgett technique. Both Y-type and Z-type multilayers have been obtained. All these multilayers doubled the frequency of Nd3+:YAG laser radiation (1.064μm). Frequency doubling from the "centrosymmetric" Y-type layers can possibly be ascribed to the effect of the interface between the multilayers and the glass slide. By comparison of the second harippic intensity obtained from Z-type monolayers with that from quartz standards a x / value of 2.2 x 10-20 C3 J-2 was calculated for films of pure acid and a value of 2.5 x 10-20 C3 J-2 was obtained for films containing Cd2+ ions. These correspond to molecular a values of 4.5 x 10-⁴⁸C3J-2m³ and 3.3 x 10-48C3J-2m3 respectively. These high nonlinearities are due to resonant enhancement resulting from the proximity of the second harmonic wavelength to the absorption maxima of the films.

Multilayer Langmuir/Blodgett thin films provide a means of designing thin film optical devices monolayer by monolayer. A single layer of a hemicyanine dye or a mixture of hemicyanine dye and PMMA, as the polymeric host, produce second harmonic generation (SHG). With proper dipping procedure it is possible to deposit such films one monolayer at a time and maintain the noncentrosymmetric structure required for SHG. Addition of more layers of the same material increases the amount of generated second harmonic. This evidence of enhanced second harmonic generation in multilayer Langmuir/Blodgett thin films will be presented. Discussion of microwave detection using the linear electrooptic effect in slab and coated fiber structures will be presented. Some of the theoretical problems and advantages of using optical fibers coated by Langmuir/Blodgett films are discussed. Emphasis is placed on the importance of developing in situ diagnostic tools for use during deposition of the organic films.

Integrated Langmuir Devices ("ILD's") are hybrid microelectronic and microptic devices developed at the Polymer Microdevice Laboratory ("PML") at Case Western Reserve University. These devices are part organic/polymeric and part inorganic, utilizing the more desirable features of each to create multifunctional devices. Langmuir films prepared by the Lang-muir-Blodgett technique have been successfully integrated into the conventional batch processing methodologies of usual semiconductors. ILD's processing techniques and several products from this process technology are described.

Nonlinear organic compounds may have important applications in novel laser-controlled electron beam devices and systems. Films of a few hundred angstrom thickness are vacuum deposited on metal substrates. When illuminated with intense laser light in a vacuum, these coatings can produce high emission current densities. A survey of organic materials evaluated at three laser wavelengths shows that some compounds produce more photocurrent than metals.

Third-order nonlinear optical interactions have been studied for thin films of a number of organic polymeric systems to derive an understanding of these processes. Picosecond and subpicosecond degenerate four wave mixing was used to derive the nature of the nonlinearity and the associated response time. The dominant contribution is found to be derived from the 7-electron conjugation and a subpicosecond response is confirmed. In the case of a polymeric photoconductor, the nonlinearity due to 7-electrons dominates if the structure is conjugated. In the case of a non-conjugated photoconductor, contribution due to charge-carriers seems to dominate and has response times of hundreds of picoseconds. Vibrationally resonance enhanced nonlinearity is also investigated by coherent Raman scattering. The resonance enhancement is three orders of magnitude, and vibrational dephasing is found to be extremely fast. For studies of optical nonlinearity in monolayer and successively-built multilayer Langmuir-Blodgett films, the surface plasmon technique is found to be ideally suited. A power dependence study of the total reflectivity vs the internal ii Bence angle in a prism coupling geometry is used to obtain the magnitude as well as the sign of x (3).

We report our initial results on certain aspects of the fabrication of opto-electronic devices based on nonlinear optical organic polymers. In particular, we report the fabrication of rib waveguides in the high-temperature structural polymer PBI, measurement of the third-order nonlinear optical susceptibility in a side-chain biphenyl liquid-crystal polymer, and the determination that the origin of the response is thermal. In addition, we describe the design of a narrowband electro-optic tunable notch filter and report the results of device modeling calculations to determine the expected performance.

The fundamental mechanisms for optical nonlinearities, and recent experimental studies of nonlinear optical processes such as optical wave mixings, beam amplifications, bistability and self-phase modulation effects are reviewed.

Picosecond nonlinear absorption and nonlinear refraction studies are presented for isotropic phase liquid crystals. These materials exhibit a large two-photon absorption coefficient (f3) at 532 nm, with values as large as 0.6 cm/GW. Nonlinear refraction was also observed and the nonlinear refractive index (n9) was measuv.qd at 1064 nm usiAg an external self-focusing arrangement. Values of n2 range from 6 x 10-13 esu to 17 x 10-13 esu.

Long range orientational order in a nematic liquid crystal phase is described by the orientation distribution function in the form of ascending Legendre functions <Pn> of even order n that are experimentally determined by measurements of the tensor components of a corresponding nth rank tensor physical property. We report new formalism and experimental results that demonstrate relationships between <P2>, <P4> and the third harmonic suscepti-bility of liquid crystal phases. The experimental system studied i 3) Oe well-known nematic liquid crystal MBBA. The microscopic third order response )0(-3w;w,w,w) of aligned MBBA samples was determined by third harmonic generation using t'Oe,Maker fringe wedge method. The measured polarization and temperature dependence of x°) are in good agreement with theory and with independent results from separate experimental studies of <P2> and <P4>.

A new variation of nonlinear susceptibility measurement in liquids using optical third harmonic generation is described. High precision (-1%) and throughput are advantages which facilitate observations of systematic trends and deviations from idealized behavior. Nonlinearities in several simple series of molecular liquids are presented and discussed.

We measured the linear polarizability along the dipole moment direction using the Kerr effect and find a good correlation with the first hyper-polarizability. We also studied the polymer alloy MNA/PMMA by the Kerr effect and third harmonic generation, and deduce that the MNA molecule is relatively mobile in the PMMA host.

A number of organic materials have been observed to possess large nonlinear optical susceptibilities. The potential applicability to integrated optics is substantial owing to both microscopic and bulk properties of organic and polymeric materials, including potentially fast switching times. Fabrication flexibility and the tailoring of optical properties via material engineering are but two of the advantages of using organic materials. Special requirements of guided-wave nonlinear optic devices include high optical quality, low dielectric constant, and a reproducible fabrication technology. Polymer glasses have been shown in several contexts to possess these properties. In addition, we have recently demonstrated the fabrication of nonlinear optic polymer glasses through the introduction of nonlinear optical molecular dopants oriented by electric-field poling techniques. Polymer glasses are discussed in the context of the requirements of integrated optics. Recent results in the characterization and processing of these materials for use in guided-wave integrated optical devices is also discussed.

Interest in the fabrication of devices compatible with optical fibres has been stimulated by the possibility of introducing non-linear organic materials in and around waveguides. In this paper, the application of electric fields on cylindrical devices with a view to performing all-optical non-linear interactions is examined theoretically. It is shown how a static field in the core of the fibre based device can be enhanced by a suitable choice of surrounding material. We also propose the use of pure liquids, solutions and doped polymeric materials as cores of optical fibres as an alternative to the crystal cored fibre, in order to perform all-optical nonlinear interactions. The isotropy of the such guides can be used to advantage in device design. These waveguides can then exploit third-order processes, to make devices such as, all optical modulators and logic gates. As an example of a parametric process, phase-matched second-harmonic generation in a nitrobenzene waveguide, by modulating the third-order nonlinearity is considered. It is shown that conversion efficiencies in SI units, as high as 0.06P(w fundaTptal) can be expected for a 30 mm optimised device. An order of magnitude increase in x (3) nitrobenzene is all that is required to make such devices competitive with x (2) devices. This should be possible with currently available materials.

During the past few years, new concepts for nonlinear optical signal processing have spurred interest in nonlinear optical devices based on third-order susceptibilities. As a result, there has been an increase in materials fabrication and processing development, particularly in semiconductors. For many applications) ^{1, 2} these semiconductor materials are particularly attractive because of their large nonlinearities and their compatibility with existing opto-electronic technology. However, the resonant-type processes responsible for the large nonlinear response of these materials yield absorption coefficients that cause thermal problems and also limit the speed of response (recovery) of the material to optical signals. Thus the driving force behind some of the nonlinear organic materials development is the need for good optical quality, highly nonlinear optical materials with low absorption coefficients.

Using the Bridgman method, an organic material MNA(2-methyl-4-nitroaniline) was crystallized into a core of an optical fiber. Relatively efficient optical second-harmonic generation was observed and the effect of polarization direction of the fundamental wave was investigated. The Bridgman method is a useful means of crystallizing the organic materials as cores of optical fibers and as guide-layers of thin-film optical waveguides.

Planar optical waveguides were fabricated on glass substrates using soluble polyimides. A fast curing treatment leads to nearly isotropic products with improved waveguiding properties. The stability of these films is discussed. Strip waveguides were recorded in doped polyimide films and the optical constants of the doped material were investigated.