Among the many physical properties of organic nonlinear optical (NLO) chromophores that require optimization, their generally inadequate thermal stability has recently attracted significant attention. Here we describe protocols for the evaluation of the thermal stability of chromophores that may be employed prior to their incorporation into a device environment. These protocols involve a variety of conventional analytical techniques including thermal analysis (TGA, DSC) and electrochemistry (CV) that have led to the identification of the most facile thermal decomposition process of the widely utilized family of 4-amino-4'- nitroazobenzene NLO dyes. Knowledge of the decomposition mechanism has facilitated implementation of structural modifications in the amine donor of these dyes that deliver enhanced thermal stability. It is anticipated that these protocols and structure modifications are of general relevance.

Molecules with a large second order electrical polarizability, (beta) , should have both strong electronic acentricity and intense charge-transfer transitions. We report values of (beta) for a number of mixed valency metal complexes in which charge transfer occurs between two redox active metal ions within the same molecule. Because these species are ions the EFISH experiment is inapplicable, so our measurements rely on Hyper-Raleigh Scattering. The compounds discussed here contain metal ions linked by a cyanide bridge. Charge asymmetry in the bridge is maintained by stabilizing the lower oxidation state with (pi) -acceptor ligands and the higher oxidation state with (sigma) -donor ligands. Results are reported for five homo and hetero-bimetallic ions in a variety of solvents. (beta) values are about 10^-28 e.s.u., at 1.06 micrometers , and are comparable with those of organic compounds with larger donor-acceptor separations.

A wide choice of push-pull polyenes and carotenoids of increasing length (up to 30 angstrom) and bearing various donor and acceptor end groups has been synthesized in order to investigate the chain-length dependence of their quadratic hyperpolarizability (beta) . (beta) measurements have been performed using the electric field induced second harmonic generation (EFISH) technique. In each series of homologous compounds, the lengthening of the conjugation path results in a pronounced increase in (mu) (beta) values. This behavior leads to very large static (mu) (beta) (0) values for the longest molecules and can be modeled by (mu) (beta) (0) equals kn^a relationships with respect to the number n of double bonds in the polyenic chain. The exponent value (a) was found to depend on the end groups. The replacement of a double bond by a triple bond in the middle of the polyenic chain results in a blue shift of the charge transfer absorption band and leads to smaller (mu) (beta) (0) values. However, this phenomenon is modulated by the end groups.

We have developed several classes of thermally stable nonlinear optical chromophores based on the combination of efficient thiophene conjugating units and novel electron-donating and electron-accepting functional groups. The incorporation of these chromophores into high temperature polymers produces high linear electro-optic coefficients and long-term thermal stability at elevated temperatures. Active Mach-Zehnder interferometers with a V(pi) phase shift of 50 volts and low attenuation (< 2 dB/cm) were fabricated. Thermal baking of the poled device demonstrated no change in activity after 30 minutes at 230 degree(s)C.

The hyper Rayleigh scattering (HRS) method was applied to determine the absolute value of second-order hyperpolarizabilities ((beta) ) of stilbazolium p-toluenesulfonates and related ionic species in methanol. In general, the (beta) values of stilbazolium derivatives become large with increasing wavelength of absorption maximum and are 1.1 X 10^-28 - 3.9 X 10^-28 esu, i.e., 3 - 10 times larger than (beta) of p-nitroaniline. By way of exception, halogenated stilbazoliums showed rather larger (beta) compared with the unsubstituted stilbazolium having almost the same absorption cutoff and maximum. The calculated d value of hydroxystilbazolium p-toluenesulfonate obtained using the present (beta) and the known crystal structure agreed well with the experimental d value by the Maker fringe method.

An experimental method enabling a simultaneous determination of the linear and quadratic electro-optic coefficients in thin film is described. The technique is based on the interference between electric field induced modulation of transverse electric (TE) and transfer magnetic (TM) fields propagating through a stratified medium consisting on a substrate, semitransparent electrode, and the measured thin film. It allows us to determine both the real and the imaginary parts of corresponding electro-optic coefficients as well as their dispersion. The use of thin film enables both electro-optic coefficients measurements in the thin film absorption band allowing nonlinear spectroscopy studies. The technique is applied to a poled functionalized polymer thin film and the results are interpreted in terms of a two level model. This allows a determination of microscopic parameters such as: excited and fundamental state permanent dipole moment difference and transition moment between fundamental and first excited state. A presence of an excited state in the bottom of the absorption band is revealed.

An electro-optical polymer was synthesized where a diazo dye with a dicyanovinyl group as an electron acceptor and a diethylamino group as a donor is attached to the polymer chain. The electro-optical coefficient (r) reached 30 pm/V. It was found that the edge absorption of the chromophore caused a loss increase in the near infrared region, which indicates that the increase in the r value leads to a propagation loss increase in the material. The loss is around 1.0 dB/cm in a single-mode waveguide fabricated by using oxygen reactive ion etching. The polymer waveguide is applied to two types of devices, a Mach-Zehnder optical modulator and a vertically stacked directional coupler, which both achieve electro-optical modulation. As another application, electro-optical measurement of an electric field in a high-speed circuit device is demonstrated, where the polymer is processed into a chip film probe and patched to an integrated circuit, thus enabling the electric signal to be detected.

Polymeric composite materials constitute a new and very promising class of photorefractive materials. In the design of polymeric photorefractive materials we use multicomponent composites in which necessary functionalities can be independently optimized. The investigated composites consisted of charge transporting polymeric matrix, and optically second-order active molecules. Two different photosensitizers were used to vary the wavelength response. Photorefractive properties of these materials were investigated using erasable volume holography in a non-degenerate four-wave mixing geometry and two-beam coupling techniques. A previously developed model of space-charge field grating formation in photoconductive polymers was used to explain the field dependence of four-wave mixing diffraction efficiency. The model takes into account the field dependence of charge photogeneration quantum yield, carrier field mobility, and electro-optic coefficient. Necessary information about these parameters was obtained from the results of photoconductivity and electro-optic modulation experiments. Special attention was focused on the kinetics of photorefractive response in the composite for optical signal processing and optical storage. It was found that a very effective switching of diffraction efficiency induced by dc electric field occurs in this system. Also, the results of kinetic studies of the index grating writing and its subsequent light-induced erasure, as well as a demonstration of the holographic image recording and retrieval, are reported.

One might say this paper is about something old, something new, something borrowed, something blue -- finding chromophoric brides to be wed to polymeric husbands. Several dyes have been synthesized that are red, magenta, or blue in color and have large ground-state dipole moments. Dyes synthesized in our laboratory were characterized by differential scanning calorimetry and UV-VIS spectroscopy. First molecular hyperpolarizabilities ((beta) _o) of the dyes were calculated using MOPAC (V6). Some of the dyes were dissolved in poly(methyl methacrylate), corona poled above the glass transition temperature, and cooled to room temperature to freeze in the polar order. Second-order nonlinear optical properties of these guest-host films were compared by optical frequency doubling measurements.

The synthesis and characterization of polycarbonates and polyformals is described. Copolymers of bisphenol A (BPA) and either a triphenyloxazole (1) or phenylquinoxaline (2) monomer were prepared. The polycarbonate copolymers were prepared by ring-opening polymerization of macrocyclic prepolymers using a solvent-free process. Preliminary electro- optic measurements have been performed on the polycarbonate copolymers of BPA and 1. Polyformals have also been prepared by a conventional polymerization process involving the reaction of dichloromethane with different feed ratios of BPA and either 1 or 2. The glass transition temperature of either the polyformals or polycarbonates is dramatically increased by increasing mole fractions of monomers 1 and 2.

The electro-optical properties of Ultradel^R 9000D polyimides doped with DCM and DADC, a bis(carbazole) analog of DCM with improved thermal stability, are reported. Cure temperatures were restricted to 240 degree(s)C or less to minimize potential thermal degradation of these dyes. Low poling fields of 30 V/micrometers were used in these experiments and yielded r₁₃ coefficients in the 0.1 - 0.8 pm/V range. Photothermal deflection measurements of dye-doped Ultradel 9000D samples showed low optical absorption losses in systems cured at 175 degree(s)C, but losses exceeded 20 dB/cm in samples cured at 300 degree(s)C.

The synthesis and first hyperpolarizabilities ((beta) ) of various donor-acceptor substituted thiophene containing compounds with extended conjugation are reported. Molecules incorporating the acceptor group, 3-phenyl-5-isoxazolone, that can gain aromaticity upon charge-separation had larger (beta) than analogous molecules containing a dicyanovinyl acceptor. For both acceptors insertion of a vinyl group between the thiophene bridge and methine carbon of the acceptor enhances the second-order hyperpolarizability but does not lead to a significant decrease in thermal stability. The molecules have been incorporated into poly(methyl methacrylate) and the electro-optic coefficients of these host-guest polymers measured.

The second-order nonlinear optical coefficients d₃₃ (-2(omega) :(omega) ,(omega) ) of polymer films doped with novel disazo dyes, that have a perfluoroalkylsulfonyl group as an electron acceptor and an extended (pi) - electron system, are determined by second-harmonic generation (SHG) measurements at wavelengths of 900 - 1550 nm. Thin polycarbonate (PC) film doped with 4-(4-(4- perfluorobutylsulfonylphenylazo) naphthylazo)-N-ethyl-N-hydroxyethylaniline showed a large d₃₃ value, 67 pm/V, at a wavelength of 1064 nm. The decay of the d₃₃ value was slow due to the stability of the molecular arrangement in a polymer matrix at high temperature. The large d₃₃ values and the stability of polymer films doped with novel disazo dyes in this study are discussed in terms of electron acceptor substitution and an extended (pi) -electron system.

The critical role of two-photon (pi) -electron excited states in third order optical processes in squaraine dyes is described. The discussion is based on recently completed femtosecond measurements of the excited state absorption spectrum of a typical centrosymmetric squaraine. The new experimental data agree well with the earlier theoretical description and quantum many-electron calculations reported for the dye chromophores.

We employ the Pariser-Parr-Pople (PPP) Hamiltonian to calculate the three lowest polarizabilities ((alpha) , (beta) and (gamma) ) of Octatetraene. By applying a strong static electric field, the molecule undergoes a significant structural change, which affects the optical polarizabilities. We find that these polarizabilities can be described using a small number of charge fluctuation modes. We further relate the magnitude and sign of these polarizabilities with some coarse features of the ground state reduced single electron density matrix, which in turn is directly related to the chemical bonding.

The dependence of the second- and third-order polarizabilities ((beta) and (gamma) ) on ground-state polarization was measured for a series of donor-acceptor polyenes using electric field induced second harmonic generation and third harmonic generation, respectively. The changes in ground-state polarization, associated with the donor/acceptor strength or solvent polarity, were probed by x-ray crystallography, ¹H-NMR, electronic absorption, and Raman spectroscopies. The observed behavior of (beta) and (gamma) as a function of ground- state polarization agrees well with theoretical predictions. In particular, positive and negative peaks, as well as sign changes, were observed for both (beta) and (gamma) . The dependences for (beta) and (gamma) are consistent with a derivative relationship between them. In addition, the third-order polarizability of a series of molecules possessing zero bond length alternation was found to be negative, in agreement with predictions based on the relationship between the polarizabilities and ground-state geometry.

Novel two-dimensional conjugated systems have been developed for second- and third-order nonlinear optics. Four types of asymmetrically substituted macrocycles and non-aggregated phthalocyanines were synthesized and their second- and third-order nonlinear optical properties were characterized: meso-monosubstituted octaethyl metalloporphyrins, (beta) - monosubstituted tetraphenyl metalloporphyrins, mono (tert-butyl) vanadyl phthalocyanine, tris (tert-butyl) mononitro metallophthalocyanines, and hexadeca (trifluoroethoxy) metallophthalocyanines. Control of molecular packing in asymmetric vanadyl phthalocyanine was described in terms of the enhancement of the third-order nonlinear optical susceptibility. No evidence of molecular aggregation was observed in optical absorption and electroabsorption spectra for hexadeca (trifluoroethoxy) metal-free and vanadyl phthalocyanines. The effects of the substituents and metallation on the dipole moment for asymmetric porphyrins is discussed. A preliminary investigation of the second-order nonlinear optical responses for Z-type Langmuir-Blodgett films and poled molecularly doped polymer is presented.

Electroabsorption spectra have been measured on poly(methyl methacrylate) thin films doped with three kinds of metallophthalocyanines: tetrakis (tert-butyl) metal-free and lead phthalocyanines, and hexadeca (trifluoroethoxy) metal-free phthalocyanine. The third-order nonlinear optical susceptibility, (Chi) ⁽³) (-(omega) ; (omega) , O, O), was determined from the electric field-induced absorption change. The considerable differences in absorption, electroabsorption, and complex (Chi) ⁽³) (-(omega) ; (omega) , O, O) spectral profiles of these metallophthalocyanines are attributable to the difference in molecular aggregation states.

Electro-optic polymer waveguide modulators may be used in parallel external modulation arrays supplied by branching structures providing fanout from a cw laser for a variety of applications, including CATV and data communications. This paper highlights some of the benefits of using EO polymer modulators in arrays for point-to-point digital data communications.

An electro-optic polymer guest-host system has been constructed and demonstrated. The polymer host is a polyimide (PIQ2200) and the guest chromophores are dimethyl (or diethyl) amino alkyl sulfone stilbenes. The alkylated-NLO moieties as guest chromophores have been modified, yielding new alkylated-NLO moieties. The higher content of alkylated-NLO moieties, compared to unmodified NLO moieties, was doped into a polyimide host system due to the improved solubility of new alkylated-NLO moieties. To the 40 wt%, the new alkylated- NLO moiety has been completely dissolved in the preliminary experiment, leading to the increase of refractive index by 0.0016. These polyimide-based guest-host systems exhibited a significant improvement in the thermal stability at high temperatures exceeding 250 degree(s)C. The electro-optic coefficient reported in the present study is 13 pm/V for the 40 wt% DASS-6- doped polymer system poled at the 135 V/micrometers . However, further increase up to 25 pm/V may easily be achieved by increasing the amount of guest moieties and/or the intensity of the poling field. This work presents new materials for photonic switching devices with low operating voltage.

All-optical devices, in which one beam of light controls another, would make possible such applications as fast optical logic, optical amplifiers, and computers with massively parallel architectures. We report on advances in making single mode polymer optical fiber with nonlinear-optical cores, optical loss measurements, beam confinement and imaging studies, and nonlinearity measurements. Aside from all-optical devices, polymer-optical fibers can be used to make a new class of devices such as optical actuators and self-stabilizing positioners. Such devices rely on the fact that the length of a material can change in the presence of high intensity light in direct analogy to light induced refractive index changes. We show that fast fiber length changes can be induced with high power pulsed lasers. Furthermore, by combining photomechanical effects with feedback, we show that smart optical stabilizers can be fabricated. Such a device is demonstrated using the lower thermal effects and a low-power continuous wave laser.

The distribution of the NLO polarization induced in amorphous polymers by poling is generally nonuniform, especially in those with a high glass transition temperature desirable for stability of the polarization. At high temperature and high poling fields, space-charge effects come into play and make the internal field and polarization nonuniform. As the polarization distribution may affect phase matching, knowledge and control of this distribution is desirable for efficient SHG. We have used a laser-induced-thermal-pulse method to obtain polarization- versus-depth profiles over a wide range of poling variables for the amorphous, strongly coupled paraelectric copolymer of vinylidene cyanide and vinyl acetate with superior intrinsic SHG properties and high glass-transition temperature. The stability of the polarization in the glassy state is also documented.

Transient absorption and degenerate four-wave mixing measurements on a thin film Pb- phthalocyanine-urethane-butanediol copolymer show that this material has a strong excited state absorption in the visible and near-IR. The excited state absorption cross section, (sigma) _ex, exceeds that of the ground state, (sigma) _g from 430 to 620 nm and at 1064 nm. DFWM measurements at 590 nm show both an instantaneous response ((tau) < 1 ps) and a long lived response from an excited state with a rise time of less than 1 ps, and a decay time of > 10 ns.

We demonstrate optical limiting in phthalocyanine doped solid host materials such as poly- (methylmethacrylate) and organically modified sol-gels. It is shown that the nanosecond nonlinear absorptive properties and the excited-state properties, such as the triplet-triplet absorption spectrum and the triplet quantum yields, of the metallophthalocyanines in these solid hosts are very similar to those in solutions. The key figures-of-merit for these materials suggest potential for the realization of high-performance solid-state optical limiters based on phthalocyanines.

Passive optical limiters that rely on fluence dependent absorption from excited states of metallo-organics typically provide a 100 - 200 fold suppression of 8 nanosecond pulses in a device with a linear transmission of 70%. In principle, these same materials in an optimized `bottleneck' limiter design should provide a 10⁴-fold suppression. To obtain this performance, the ground state population of the active molecules must be largely depleted, while at the same time the fluence at all points in the limiter must be kept below the material damage level. Design determinants and material figures of merit appropriate for such optimized devices are outlined and discussed.

We use a pump-probe technique to measure the change in optical transmission through a 1 cm C₆₀/benzene (0.58 g/l) solution caused by a 532 nm 25 ps `pump' pulse having fluence up to approximately 10 mJ/cm². Temporal dependence indicates that transmission drops within the pump pulse width and stays at the reduced level up to our maximum delay time (approximately 10 ns). If we use the standard three-level model for the C₆₀ molecule we deduce that either the excited-singlet-to-triplet crossing happens faster than our pulse width or the two states have indistinguishable absorption cross-sections ((sigma) _S approximately equals (sigma) _T) at our laser wavelength. We believe that the latter assumption is more probable, and interpret our data by a simple two-cross-section ((sigma) _O, (sigma) _T) model. We find that the difference (sigma) _T - (sigma) _O is (3.5 +/- 0.2) X 10^-17 cm². We measure the ground state absorption cross-section (sigma) _O to be (3.5 +/- 0.3) X 10^-18 cm² in an independent experiment.

An optically bistable system is described that operates without feedback and is based on intrinsic response of impurity centers in a crystal interacting with laser light. An analysis is presented which demonstrates that bistability is possible in laser fields of 10 W/cm².

The spectral dependences of the nonlinear light absorption by organic dyes have been measured for the first time. To analyze the results quantitatively, we propose to redetermine the nonlinearity parameter I_s (the critical bleaching intensity) from a three-energy-level model of the dye. The resulting nonlinear absorption spectra indicate a monotonic and significant decrease in the region of the 0 - 0 transition compared with the short-wavelength part of the spectrum. A more than tenfold decrease in I_s is recorded in the anti-Stokes region in a polymeric matrix, which is associated with inhomogeneous distribution of the absorption centers in the microheterogeneous medium, and with a decrease in the rate constant for rearranging centers in the ground state.

We give an overview of the current status of research in the field of molecular crystals with respect to frequency-conversion, electro-optic modulation, and photorefractive effects. Applications of organic materials for nonlinear optics will depend on the development of waveguide devices and bulk crystals. Up to now the most advanced integrated optical organic devices use polymers. Thin crystalline films would also be very attractive if suitable techniques for high quality film deposition would become available. Organic molecular beam epitaxy offers new possibilities for fabricating low loss waveguides based on crystalline materials.

The nonlinear absorptive properties of C₆₀ are of significant interest. In this paper we describe spectral emission measurements on C₆₀/toluene solutions and neat toluene in the fluence regime of 3 to 1500 J/cm² excited by nanosecond and picosecond pulses. Ten emission lines between 547 nm and 635 nm are observed in both toluene and C₆₀ solutions. These lines are found to correlate well with published Raman spectra of toluene. In C₆₀ solutions an additional broad double feature is observed with peaks at 692 nm and 732 nm matching the published fluorescence spectra of C₆₀. At the highest fluences a broad new feature appears at 655 nm, grows nonlinearly, and saturates in the C₆₀ solution. In the blue spectral region, at wavelengths shorter than the pump wavelength, a plethora of lines are observed in neat toluene between 340 and 520 nm and found to correlate with molecular disassociation products. No evidence for breakdown, however, is apparent in the C₆₀ solutions. Both neat toluene and C₆₀ solutions show evidence of photochemical reaction.