Polydiacetylenes are a class of compounds which have been of some interest in both electro-optic and all-optical applications. Although their inherent large third order non- linear optic response is very attractive, it has yet to be optimized in a simple way. Of course the crystalline state will possess the highest values because of the high degree of orientation. Problems, however, exist in the preparation of such crystals with high optical qualities and also processabilities. It is therefore advantageous to study polydiacetylenes in a thin film morphology. Research at NASA has developed a way of preparing high optical quality thin films from monomer solution on to virtually any substrate with great ease. It is the focus of this paper to discuss and present preliminary data on efforts to make highly aligned thin films using this process. We look into the concept of incorporating azo moieties onto the side chains of the polydiacetylene backbone. These groups, when exposed to polarized light at moderate temperatures, go through a cis- trans isomerization which forces it and the other side chains to orient perpendicular to the plane of the polarized light. We discuss the synthesis of such molecules and the preparation of their films to create highly ordered polydiacetylene thin films with a large third order NLO response.

New developments in the semiconductor industry are driven by two trends: reducing the device dimensions and further increase of the switching speeds or electrical bandwidths. The electronics industry average feature sizes of integrated circuits (ICs) will be of the order of 100 nm by the year 2010. For instance, currently produced MOS field-effect transistors support electrical fields between the source and the drain that are greater than 10⁵ V/micrometer with switching speeds of 10 - 100 psec. Techniques which would resolve such electrical fields, with the appropriate resolutions in time and in space, are of paramount interest both at the industrial level and in basic research. Initial experiments performed on samples consisting of two metallic electrodes deposited on fused silica substrates covered by thin polymer films show that with only 1 (mu) W of average optical power, a second harmonic signal triggered by an AC/DC field could easily be detected with a spatial resolution of less than 1 micrometer. We anticipate electrical field detection sensitivity of less than 1 mV/micrometer with our technique with 100 nm resolution spatially and less than 1 psec resolution in time.

The formation of thin-solid J-aggregated films Pseudoisocyanine prepared by spin-coating method is described. The process of dye aggregation was stimulated by addition of anions B₁₀H₁₀^-2 and B₁₂H₁₂^-2. It is shown that only addition of decahydro-closo-decaborate dianion (B₁₀H₁₀^-2) to initial solution of dye give rise the J-aggregation with high efficiency. Obtained films have long storage time and suitable for non-linear optical measurements. The third-order susceptibility close to J-peak of PIC aggregates in films was studied by Z-scan method. Prepared films have a large magnitude of (chi) ⁽³) to approximately 10^-5 esu. The thermal contribution to the optical nonlinear response of dye films was estimated by use Kramers-Kronig relationship.

The investigations on the multi-cascade processes of four-wave mixing (FWM) in liquid cored optical fiber is reported. The core material is ethylbenzene and the wavelength of its first order. Stokes is 562 nm by the pumping light of 532 nm. The dye laser is tunable and so are its FWMs. The frequencies of FWMs and their cascade FWM among the frequency difference of the first order Stokes line and the dye laser, (omega) _1s - (omega) _d, and the dye laser; or it and the second order Stokes line; or it and the third order Stokes line; or it and the pumping light are (omega) _d -((omega) _1s - (omega) _d), (omega) _d -2((omega) _1s - (omega) _d), and (omega) _d -3((omega) _1s - (omega) _d); or (omega) _2s - ((omega) _1s - (omega) _d) and (omega) _2s - 2((omega) _1s - (omega) _d); or (omega) _3s - ((omega) _1s - (omega) _d) and (omega) _3s - 2((omega) _1s - (omega) _d); or (omega) _p -((omega) _1s - (omega) _d), (omega) _p -2((omega) _1s - (omega) _d), and (omega) _p -3((omega) _1s - (omega) _d), respectively. In the output of liquid cored optical fiber there are Stokes lines, the broadening of each line, the dye laser, the pumping light, the FWMs and their cascade FWMs. We point out that the FWM process is not only to initiate those obtained FWMs and their cascades, but also to contribute to the stimulated Raman scattering and the stimulated Rayleigh wing scattering for generating higher orders of Stokes and anti-Stokes line and for expanding the spectrum widely in the red-shifted broadening and the blue-shifted broadening in liquid cored optical fiber.

We have made single mode polymer optical fiber with embedded electrodes. The electrodes can be used to pole the dye-doped core and electro-optic phase modulation of light in the waveguide has been demonstrated. The method of lead attachment to the electrodes is discussed and the optical/electrical properties of device structures characterized.

Fluorescence from squaraine dye-doped polymer optical fibers (POFs) is utilized to measure the linear absorption coefficient as a function of wavelength. By illuminating dye- doped POFs from the side with an appropriate wavelength, the fluorescence generated propagates down the length of the fiber. The attenuation of the fluorescence can be measured as a function of propagation distance by moving the laser source or the fiber in the propagation direction. From this measurement the linear absorption coefficient can be calculated. The benefits of the side-illumination fluorescence (SIF) method are that it is non-destructive and it accurately measures small values of the linear absorption (off-resonance) for a wide range of wavelengths from a single wavelength source. Accurate measurements of the off-resonance absorption are crucial for modeling communication systems and optical logic devices.

We investigate the contribution of two-photon states to the third-order optical nonlinearity in four squaraine compounds. Our experimental results using third-harmonic spectral dispersion are compared to experimental results obtained by other researchers using quadratic electro-optic spectral dispersion and femtosecond excited state absorption measurements. Different relative contributions from high lying two-photon states are seen using different experimental techniques. We also relate these results to finite-field and sum-over states quantum chemical methods which indicate that there are multiple high-lying states that contribute to the third-order nonlinearity. For one of these dyes, we find that contributions from a mixture of isomers needs to be considered. For this latter dye, we compare our calculated transition moments and third harmonic dispersion curve using quantum mechanical methods with our results from third harmonic spectral dispersion and find substantial agreement between the experimental and theoretical results.

The techniques of closed and open aperture Z-scan as well as that of phase-matched and non-phase-matched degenerate four- wave mixing (DFWM) were used to investigate nonlinear optical properties of two poly(p-phenylenevinylenes) substituted with the phenoxyphenyl and fluorophenyl groups at the vinylene position. The polymers studied were poly[1,4-phenylene-1,2- di(4-phenoxyphenyl)vinylene] (DPOP-PPV) and poly[4,4'-biphenyl-diyl-1,2-di(4- fluorophenyl)vinylene] (DFP-PDPV). Spectroscopic studies showed a blue shift of absorption spectra of both polymers in comparison to that of poly(p-phenylenevinylene) (PPV). Z-scan experiments for solutions of the polymers in 1,4-dioxane and DFWM experiments for thin films of the polymers were carried out at 800 nm with amplified 100 femtosecond pulses. The results of both techniques are in good agreement, indicating that the nonlinear refractive index n₂ is on the order of 10^-14 cm²/W for the substituted polymers. The sign of the real and imaginary parts of n₂ is positive. The polymers possess relatively low values of two-photon absorption merit factors at 800 nm. The nonlinear properties of the substituted polymers were compared to the results obtained on unsubstituted PPV films.

The nonlinear optical properties (NLO) of Phthalocyanines (Pcs) and Subphthalocyanines (SubPcs) studied by Third Harmonic Generation (THG), Electric-Field Induced Second Harmonic Generation (EFISH) and Z-scan are reviewed. Experiments were performed in solution and thin films in order to investigate both, the microscopic and macroscopic nonlinear mechanisms. For the Pcs, the analysis of the dispersive behavior of (chi) ⁽³) in vacuum evaporated films allowed for the identification of two-photon states. In addition, it has been observed at both, microscopic and macroscopic level, that the metal leads to enhancements in the nonlinear response, that are probably due to the presence of two-photon (d-d) levels. Measurements of nonlinear absorption (NLA) in Pcs at 1.064 micrometer have confirmed the contribution of two-photon states to the nonlinear response and also a strong dependence on the presence of the central metal ion. The SubPcs, on the other hand, have also shown second order NLO properties, since they are non-centrosymmetric. From Hyper-Rayleigh Scattering (HRS) experiments, large second-order molecular polarizabilities (beta) have been determined. Second order NLO response is also present at a macroscopic level, as measured in thin films by Second Harmonic Generation (SHG).

We studied the third-order nonlinear optical properties of five azulenic donor-acceptor compounds. The compounds were studied in solution in Tetrahydrofuran using backward degenerate four-wave mixing and Z-scan with 30 picosecond pulses at 532 manometer wavelength. Extrapolated values for the third-order nonlinear optical susceptibility ((chi) ⁽³)) of the solute molecules were obtained from concentration dependent measurements. The compounds possess large values of (chi) ⁽³) in the range 3 X 10^-10 esu to 3 X 10^-8 esu. The two orders of magnitude change in the magnitude of the susceptibility that we observe for this series of donor-acceptor azulenic compounds makes them interesting candidates for the study of structure property relationships in nonlinear optical chromophores. The compounds also exhibit large solvatochromism and may also prove to be useful in second-order nonlinear optical applications.

The results of coupled perturbed Hartree-Fock (CPHF) ab initio extended basis set calculations on the gepmetric structures, dipole moments, static first-order ((alpha) ), second-order ((beta) ), and third-order polarizabilities ((gamma) ) of a series of fused heterocyclic aromatic compounds based on quinoline are reported. The effects of the presence/absence of the nitrogen atom as well as the introduction of other substituents (OH, NH₂, NO₂) at various positions in the ring system on these molecular properties are described. The effect of the presence of N-oxide is also examined. Suggestions for the design of heterocyclic systems with enhanced polarizabilities are made.

The Pockels coefficient of the chiral nonlinear optical crystal was measured in a longitudinal electro-optic configuration. The presence of the optical activity (the linear optical property) in a chiral system facilitates the measurement of the Pockels coefficients (related to the second order nonlinear optical property) with a low modulation voltage, important in the practical applications.

In recent years bichromophoric compounds have emerged as promising candidates for electro-optics applications. A bichromophoric molecule D-B-A may be defined as a molecule built of two distinguishable molecular units D and A connected by a molecular bridge B. The properties of the bridge determine the flexibility of the whole bichromophoric structure. For a bichromophoric molecule the electronic absorption spectrum can be described by a simple superposition of the absorption spectra of the two chromophores. The bridge serves as a molecular spacer unit which does not influence the basic electronic structure of the two chromophores while preventing intrachromophore interaction in their ground state. However, electronic excitation of either chromophore may lead to intramolecular electronic interactions and to the observation of phenomena such as intramolecular complex formation intramolecular proton transfer, intramolecular electron transfer intramolecular electronic energy transfer. The main problem is to differentiate between all these processes which may act simultaneously and competitively and to be able utilize the various transfer processes for information storage or information transfer, thus performing a switching act on the molecular level as being done in nature in various biological systems. Several examples will be discussed.

Third-order nonlinearity and polarization holographic storage in acceptor-donor azo dye-doped PMMA films are studied by using nonresonant degenerate four-wave mixing with a He-Ne laser. The samples exhibit reasonably large third-order susceptibility and strong polarization sensitivity, and the highest third-order nonlinear susceptibility is up to the order of 10^-3 esu. The relationship between optical nonlinearity and 'push' and 'pull' electron ability in polar azo molecules is studied, and further effect from a push- electron group attached to azobenzene ring on third-order nonlinearity of the samples is explored. Third-order nonlinearity of the sample is found to be proportional to the push-pull electron ability of polar azo molecules. Attaching a side group with push-electron ability to the azobenzene ring increases third-order nonlinearity (dipole moment) of the material. Polarization holographic storage properties in azo dye-doped PMMA films are also studied. The samples show strong polarization sensitivity and real-time information storage properties. The polarization hologram in the sample results from a birefringent grating formed by photoinduced reorientation of polar azo molecules. The optical phase conjugation signal consists of both fast and slow parts when two recording beams are turned off. They respond to the processes of four-wave mixing and holographic grating. It is demonstrated that nonresonant nonlinearity of samples mainly results from photoinduced anisotropy of polar azo chromophores. The advantages of nonresonant nonlinearity and its applications are discussed.