Optical bistability is the quantum optical realization of a first order phase transition far from equilibrium. A resonant nonlinear optical response of it-electron excitations in low dimensional structures provides the nonlinearity essential to the onset of bistability. Saturable absorption studies of glassy polymer films consisting of quasi two-dimensional conjugated disc-like structures of silicon naphthalocyanine demonstrate that on-resonance the system behaves as an optical Bloch system with an intensity dependent refractive index n₂ of 1x10^-4 cm²/kW in the wavelength range of standard laser diodes. Based on this result, electronic absorptive optical bistability is observed on a nanosecond time scale in a nonlinear Fabry-Perot interferometer employing the saturably absorbing naphthalocyanine film as the nonlinear optical medium.

Picosecond pump-probe spectroscopy has been used to investigate excitons in polydiacetylene PTS. At temperatures below 195 K, neighboring chains are inequivalent due to a structural phase transition and their excitons differ in energy by 50 meV. Pumping either band forms excitons that decay in a few picoseconds with no significant interchain transport on the timescale of the exciton lifetime. We attribute a novel long-lived modulation of the low temperature exciton absorption, and the corresponding shifts in oscillator strength, to changes in the orientation of the PTS side-groups.

A new quadratic electrooptic ( QEO ) measuring method is used to measure polymer thin film solutions of dye compounds. The molecular |Reγ| , the real part of the microscopic electronic third order nonlinear susceptibility, is reported for several previously unmeasured molecules. Data analysis is attempted using a two-level model.

A new Green's function method of analyzing Maker fringes from second-harmonic generation experiments was used to determine the nonlinear properties of corona-poled polymer films. Electro-optic properties were determined by fashioning the polymers into planar waveguides on silicon substrates and measuring the polarization rotation induced by the electro-optic effect. The two measurements were compared to test the applicability of the two-level model in the polymer system. Waveguide-loss measurements were performed on the same substrates by launching a guided wave via the prism method and monitoring the scattered light with a CCD video camera. Both guest-host and side-chain polymers were studied in this way. The advantages of the side-chain polymer over the guest-host polymer mixture are higher molar concentration of the nonlinear species, decreased mobility and greater overall film homogeneity.

Cubic susceptibility of poly(butyl)thiophene has been studied by optical third harmonic generation (THG) in solution and in solution cast thin film as a function of fundamental wavelength. The thin film data have been obtained in the polymer absorption range for harmonic wave and show existence of a large three photon resonance. For polymer solutions the THG measurements have been done at 1.064 and 1.907 μm fundamental wavelength. At 1.907 μm the cubic susceptibility is complex with negative real part whereas at 1.064 μm it is real and positive. The observed large three photon resonance is interpreted as a consequence of inhomogenous one photon absorption broadening.

Compounds having large second-order nonlinear optical (NLO) response are those composed of suitable donor and acceptor groups in a delocalized π-system. In addition to the relative strengths of the donor/acceptor groups, the performance of a NLO compound depends also on the nature of the conjugated π-system, the molecular conformation and other geometric factors. To design better NLO materials, it is essential to have a good understanding of these factors. These molecular properties are addressed from the theoretical point of view. Their relation to NLO response will also be discussed.

We report D. C. electric field induced second harmonic generation (EFISH) and third harmonic generation (THG) measurement results on the intrinsic molecular hyperpolarizabilities of benzenes, stilbenes, and other arene derivatives. Structure-property relations, as revealed by a comprehensive set of systematic measurements, are discussed. Issues concerning donor-acceptor strength; charge-transfer; transparency trade-off; conjugation planarity, length, and aromaticity; and heteroatom and side-group substitution effects are included.

A novel type of second-order nonlinear optical (NLO) polymer has been prepared by the reaction between tetrafunctional nitro-substituted aromatic diamines and difunctional epoxy monomers. In these materials the NLO-active moieties are incorporated covalently into the network of a crosslinked polymer. For comparison the corresponding linear (not crosslinked) polymers have also been prepared. The noncentrosymmetry was induced by poling under strong electric fields either with electrodes in liquid crystal display type cells or by corona discharge poling of spin-coated films. The resulting nonlinear optical susceptibilities were evaluated from second harmonic generation experiments. The linear polymer prepared from 4-nitroaniline and bisphenol-A diglycidylether initially exhibits a nonlinear coefficient d₃₃ of 31 pm/V measured at 532 nm. However, 28 days after the poling, d₃₃ decayed to 22 pm/V. This relaxation is comparable to that of a NLO polymer containing a similar chromophore as a side group that we investigated earlier. A significant improvment of the stability of the nonlinear optical coefficients was achieved with the crosslinking epoxy NLO polymers. These materials can be easily melt- or solution-processed in the form of soluble, low molecular weight prepolymcrs. Subsequently, the prepolyiners are simultaneously poled and cured to give crosslinked polymers with high glass transition temperatures. For a crosslinked polymer synthe-sized from 4-nitro 1,2-phenylenediamine and bisphenol-A diglycidylether, a d₃₃ value of 14 pm/V was found. This high nonlinear optical coefficient remained unchanged under ambient conditions for at least 500h. Even at 85°C no signs of relaxation were detected.

The quadratic nonlinear optical (NLO) properties of aliphatic polyester/p-nitroaniline (p-NA) systems have been studied. Even without applying an electric field, the composite films show the second harmonic generation (SHG). The SHG activities depended on the ratio of CH2 and 000 groups in the polymer. ( ranging from 80 to 130 times larger than that of urea at CH2/C00 = 4-6.) Especially, poly(buthylenesebacate) (PBSe)/p-NA systems exhibited the largest values (130 times larger than that of urea). The SHG active systems were found to have a new phase as demonstrated by differential scanning calorimetry (DSC) technique.

The fabrication of second order nonlinear optical materials requires the imposition of noncentrosymmetry on the active moieties in the materials. One of the leading methodologies for inducing noncentrosymmetric alignment of dipolar chromophores used in second order electro-optic materials is to pole them in polymer films. The relationship between the supramolecular conformational structure of assembled chromophores and their susceptibility to poling is of interest for maximizing the bulk alignment achievable in an electro-optic material. We have employed solution phase dielectric constant measurements to investigate possible enhancements in dipolar susceptibility as a function of connectivity and state of aggregation in rationally synthesized chromophore assemblies, including conformationally defined head-to-tail oligomers. Angles of dipole moment vectors determined in solution are in agreement with predictions based on x-ray structures of model compounds. We have also determined that conformationally unrestricted, highly dipolar azo dyes behave as relatively isolated molecules even when present in supersaturated solutions and in close proximity on polymer chains. Representative synthetic methodology and dielectric data will be presented.

A novel concept to obtain the highly aligned p-nitroaniline(p-NA) crystals in poly(oxyethylene)(POE) matrix is briefly reviewed. The measurements of second harmonic coefficient for POE/p-NA system exhibiting efficient SHG activity were made by Maker-fringe method. The value of d₃₃ is 39 x 10^-9 esu at 1.064μm resulting from the alignment of p-NA crystals dispersed in polymer matrix. The structure-property relationship between crystallization process and SHG activity for POE/p-NA system is discussed.

A series of organometallic and organic salts, in which the cation has been designed to have a large molecular hyperpolarizability, has been prepared. Variation of the counterion (anion) in many cases leads to materials with large powder second harmonic generation (SHG) efficiencies, the highest of which is roughly 2000 times that of a urea reference.

A study of crystal growth and characterization of a nonlinear organic material, 4-Br-4'-Methoxychalcone is presented. Its linear and nonlinear optical properties have been investigated. The collinear and the noncollinear phase matched second harmonic generation were demonstrated.

In recent years, there has been a growth of interest in organic materials for applications in the field of nonlinear optics!'2 Materials possessing both large second-order nonlinear susceptibilities and bulk crystallinity are important for highly efficient frequency conversion. Organic molecular compounds are quite promising for achieving such high conversions because their second-order nonlinearity can be enhanced by extended conjugation and by the presence of spectroscopic charge-transfer arising from electron-donating and electron-accepting substituents on an aromatic ring or a similar conjugated system. Crystallographically, for quadratic nonlinearity the crystal unit cell must lack a center of symmetry. 4-Nitroaniline is one of the simplest organic compounds having an electron donor group and an electron acceptor group at the para positions. It has large molecular hyperpolarizabilityβ, but does not show second-order nonlinear optical effects because of the centrosymmetry of the crystal. Therefore, in order to achieve noncentrosymmetric structure, large nonlinear susceptibility and transparency, we have modified 4-nitroaniline system. A new organic nonlinear optical material N-methoxymethy1-4-nitroaniline(MMNA) was synthesized. Second-harmonic generation(SHG) measurements performed in powder form shows high second-order optical nonlinearity. This has also been confirmed by the observation of efficient phase-matched SHG from the single crystal. In this article we report the details of synthesis, crystal growth, crystal structure, phase-matching behavior, and effective bulk nonlinearity of MMNA crystal.

Phase matched second harmonic generation has been demonstrated in a periodically poled polymer waveguide. Periodic poling is accomplished by periodic finger electrodes. Finite element calculations of the electric field give the optimum design rules for the electrodes.

Side chain liquid crystalline polymers have considerable potential as nonlinear optical materials. A series of polymers based on the polystyrene backbone with pendant side groups were synthesized and characterised. Some electrical poling measurements were also carried out.

Electron nuclear double resonance studies and theoretical calculations suggest that self-localization occurs in coupled π-electron systems. Since intrinsic (electron-phonon and electron coulombic) interactions determine the extent of electron delocalization (hence derivative properties such as optical nonlinearity), it is not necessary to synthesize polymers possessing long segments of uninterrupted π conjugation. Thus, we have investigated the synthesis of copolymers containing alternating saturated and unsaturated segments. The number of advantages realized with this approach include improved solubility (which translates into improved processibility), higher polymer molecular weights (which translates into films of improved mechanical strength) for polycondensation synthetic procedures and sharper bandedges (bandedge tailing due to a distribution of short polymer lengths is avoided). This last feature results in better discrimination of resonant and non-resonant contributions to third order susceptibility. Finally, we discuss the extension of our development program for nonlinear optical materials to the preparation of organometallic as well as organic materials and the extension to the preparation of composite materials as well as homo and copolymer systems.

The chemistry and physics of highly delocalized electroactive polymers has been an area of great interest over the past decade, but it has recently become apparent that the unique nonlinear optical (NLO) properties of these polymers may be their most important characteristic. Ladder polymers have excellent thermal and chemical stability, and have recently been shown to exhibit high third order nonlinear properties. In this study we would like to report how bipolaronic states can be induced in model ladder structures related to PTL and POL systems, and how these results might indicate future polymer design criteria to control and maximize third order NLO behavior and absorption characteristics.

The polydiacetylenes have received much attention in recent years because of their large non-resonant third-order nonlinear optical response and because of the flexibility they offer in designing tailored molecular structures for specific applications. However, the fabrication of these materials into device-quality thin films has proven to be difficult, largely because of their propensity for forming highly scattering polycrystalline solids (with the exception of the few soluble polydiacetylenes, such as 3- or 4-BCMU, which can be spin-coated by conventional techniques, albeit in the less-active yellow form of the polymer). We have addressed this problem by developing a family of polydiacetylene "alloys": polymers derived from mixtures of similar diacetylene monomers having identical positioning of the diacetylene unit within the hydrocarbon chain, but with different "head" groups that provide varying degrees of affinity to water. The rationale here has been to design mixtures that would form a uniform, quasi-amorphous monolayer on the water surface in Langmuir-Blodgett multilayer fabrication, and thus avoid the scattering losses associated with grain boundaries that form within the monolayer on the water surface. To date, multilayer films up to 0.4 um thick have been fabricated by this approach, and photolithographic techniques have been developed for patterning strips down to 4 um wide in the films, without converting the blue form of the polydiacetylene to the red form in the process. Moreover, high quality films of polydiacetylene have been deposited successfully onto curved surfaces, such as polycarbonate lens blanks, using these compositions.

A large and highly stable internal electric field has been measured in thin films of corona poled copoly(vinylidene fluoride - trifluoroethylene) by analysis of the alignment of coloured organic compounds (chromophores) dissolved as guests at low concentration in the host polymer. These chromophores were designed to have both large electric dipole moments and high microscopic second order optical nonlinearities so that, when aligned in the internal field, they contributed to the small intrinsic nonlinearity of the poled host polymer. The second harmonic coefficient of the mixture was found to be linear with the guest concentration and at 10% by weight to have a value of 2.6x10^-12 m/V at 1.064μm.. The coefficient of the films was found to be stable with time unless precipitation of the guest occured owing to an insufficient solubilty at the selected concentration. This system is compared with thermopoled polymeric systems which have no stable internal electric field.

We report on the properties and synthesis of a class of organic nonlinear optical material and the value of the molecular hyperpolarizability β of these molecules. These compounds have been incorporated into molecules possessing surfactive diacetylenic moieties, and also been modified to exhibit liquid crystallinity with spontaneous homeotropic alignment. Properties of guest-host systems incorporating these compounds have also been investigated.

The measurement of electro-optic (EO) constants of three groups of materials using samples in a Fabry-Perot (FP) cavity composed of two metal mirrors which are utilized also as electrodes is reported. Firstly, the linear (Pockels) and quadratic (Kerr) EO effects of poled polymers doped with organic molecules were studied. The sample used was 4-diethylamino-4'-nitrostilbene (DEANS) doped polycarbonate (PC) film. The origins of the linear (Pockels) and the quadratic (Kerr) EO effects are discussed. Both the second-order and the third-order susceptibilities corresponding to linear and quadratic EO constants, respectively, were reproduced fairly well by two components, which are due to electronic and molecular orientational contributions. The orientational order of the molecules in the polymer film introduced by the poling was also discussed. Secondly, the quadratic EO effect of a polydiacetylene [PDA3BCMU <poly{4,6-decadiyne-1,10-diol-bis[(n-butoxycarbony1)- methyl]urethane}>] doped in poly(methyl methacrylate) (PMMA) was studied. The change in the extinction coefficient of PDA3BCMU doped in PMMA (3BCMU/PMMA) can be explained in terms of the Stark shift of the transition of the 11111 exciton, and by the conformational change associated with mechanical stress. Thirdly, the transmittance changes of several non-doped polymers, PMMA, poly(vinyl alcohol) (PVA), poly(vinyl acetate) (PVAc), polystyrene (PSt), and poly-N-vinylcarbazole (PVCz) were measured. In the case of PSt, the change in transmittance was concluded to be due mainly to change in the FP cavity length induced by mechanical stress. In other cases, the estimation could not be completed, because of lack of information on the mechanical properties. An experiment to separate changes in the refractive index and film thickness has been devised.

Quadratic electrooptic modulation of thin dye-doped polymer films is used to evaluate the electronic third-order susceptibility, χ(3) . To determine this susceptibility, the other contributing mechanisms, including orientational effects, electrostriction, electrode attraction, trapped charge movement, and heating, must be considered. The two dominant non-electronic effects are the orientational effect and the electric field-induced electrooptic effect. The degree of molecular motion of the dye molecules in the polymer matrix is measured with electric field-induced second-harmonic generation below the glass transition temperature of the composite. The microscopic orientational elastic constants, as determined from these second-harmonic measurements, are used to predict the orientational contributions. The molecular electronic third-order susceptibility of a poly(methyl methacrylate)/azo dye system, determined with the electrooptic measurement, is consistent with values derived from third-harmonic generation measurements.

The nonlinear dye N,N-Dihexyl-4 amino-4-nitrostilbene (DHANS) was incorporated in the polymer matrix of poly(α-methyl-styrene) (Pα MS). Dye concentrations between 1 and 10 weight % were investigated. Bulk samples of DHANS/Pα MS were prepared with a residual solvent content of 5 wt% in an optical cell between electrodes. Solid samples of high viscosity were obtained. Electrooptic measurements were perfomed for DC electric fields up to 30 RV/cm with a HeNe laser. Besides the usual Pockels- and Kerreffect we observed birefringence oscillations. For this experiment the Pα MS matrix has to be in the glass transition region. Parallel to the birefringence oscillations peaks in the DC current were measured. Frequency and amplitude of the oscillations depend on the external electric field. We suggest an interpretation of local discharging due to the anisotropic electrical conductivity of the dye molecules.

In this paper, we are reporting on organic polymeric materials designed for electro-optic waveguide device fabrication and a fabrication method for low loss integrated optical devices. These polymers are methacrylic chain polymers with non-linear optical moieties attached as side chains through variable length spacer groups. Variation of the spacer length allows the tailoring of the phase structure of the polymers as well as the glass transition temperature. For electro-optic device use, the most important material parameter is r_{3 3} the largest tensor element of the electro-optic tensor. We have developed polymeric materials with r_{3 3} values as high as 45 pm/V at 1.3 microns (98 pm/V at 632 nm) using an electrical poling process. Additionally, we have characterized optical and electrical properties of the polymer films. Further, a fabrication process for waveguide electro-optic devices from the polymers has been developed and integrated modulators have been built.

Guest-host poled polymer films have become an active area of research as potential candidates for second-order nonlinear optical and electro-optic devices. This interest derives mainly from their large nonlinear optical coefficients, ease of fabrication, and high optical quality. Progress has been rapid in producing stable, efficient materials, and in building demonstration devices. The second-order nonlinear optical properties arise from the orientational order induced in a collection of highly nonlinear molecules incorporated in a glassy polymer matrix. After reviewing the alignment physics, we present results of nonlinear optical measurements on a methacrylate polymer functionalized with a dicyanovinyl terminated azo dye. Device issues concerning nonlinear optical devices are described with-particular attention to phase-matching. We present data on the demonstration of anomalous-dispersion phase matched second harmonic generation in dye solutions. We also present results on electro-optic modulators.

Side chain polymers, containing hyperpolarizable moieties as pendant groups, have been applied as optically nonlinear materials. Electro-optic properties have been induced via electric field poling of nultilayer structures incorporating the nonlinear polymers. For a poling field strength of 156 Wpm, the obtained i r₃₃ coefficient was equal to 28 pn/V at the wavelength of 1.3 pm, which is close to r₃₃ = 32 of the inorganic single crystal lithium niobate. Low loss mcnomode charnel waveguides (< 1 dB/cm @ 1.3 μm wavelength) have been realized in the optically nonlinear polymer nultilayers. Tunable Fabry-Perot etalons, monamode phase and intensity modulators and monomode Mach-Zehnder interferameters, have been made and tested; their performances will be discussed. Owing to their considerable electro-optic coefficients and attractive processing properties, polymers are becoming serious alternatives for the currently often applied lithium niobate.

Conjugated polymers such as polydiacetylenes possess large third-order optical susceptibilities in their transparent gap regions, making them potentially attractive materials for applications in all-optical signal processing. In order to investigate this potential we have developed techniques for fabricating a variety of integrated optical structures such as channel waveguide directional couplers and grating couplers from thin-films of soluble polydiacetylenes. Experiments carried out at wavelengths of 1.06μm and 1.32μm have identified nonlinear transmission and switching phenomena due to both slow thermal nonlinearities and ultrafast (picosecond) electronic nonlinearities in the directional coupler devices. At 1.06μm, the fast electronic component of the device response is dominated by the imaginary part of the nonlinear index, n₂, and intensity dependent absorption and apparent switching effects are observed. The large imaginary component of n₂ is due to the presence of a two-photon resonance, and a value of 10^-3 cm/MW for the two-photon absorption coefficient at this wavelength is observed. In contrast, at 1.32μm the measured two-photon absorption coefficient is smaller by more than a factor of 10, and we find evidence for the onset of switching due to a real n₂. These results highlight the important role that two-photon effects play in determining the optimum operating conditions for conjugated polymer-based devices.

Noncollinear second harmonic generation (SHG) of Nd:YAG laser in 2- methy1-4-nitroaniline (MNA) was observed. The intensities of the SHG varied depending on the crossing angles of the two incident beams. The maximum SHG tensor component, d₁₁=235X10^-12(m/v), was determined by a new estimation. The photorefractive index change in the MNA crystal was estimated as 0.009 at light intensity increase 18(MW/cm²). The light beam diffraction at the transient grating was observed by the inter-ference of the two incident beams.

Planar lightguides were fabricated from polyimides containing two hexafluoroisopropylidene groups. In multimode lightguides of these fluorinated polyimides loss values below 0.5 dB/cm can be realized using optimized curing conditions. After curing, this material is insoluble in organic solvents so that multilayer films can be prepared. As an example we combined these polyimides with a planar organic photoconductive layer to form an integrated photodetector. In the system poly-(N-vinyl carbazole) (PVK) - trinitrofluorenone (TNF) a photoconductive complex is formed which is sensitive in the visible spectral region. This compound is deposited from a solution of chlorobenzene/tetrahydrofuran on the substrate between thin gold electrodes. The photoconductivity of these planar detector layers is studied using He-Ne-Laser light. An increase of photoconductive sensitivity is observed for high TNF concentrations.

Poly[5,7-dodecadiyn-1,12-diol-bis(n-butoxycarbonyl-methyl-urethane)], or poly-4BCMU, thin films permanently bleach when irradiated with light in the absorption band. The resulting decrease in refractive index has been measured and is sufficient to allow formation of waveguide channels and gratings. The optical properties of the bleached material are discussed. A waveguide grating coupler is fabricated by periodic bleaching and demonstrated.

The third order optical susceptibilities X⁽³⁾1111(-3ω;ω,ω,ω) for the macrocyclic conjugated structural class of annulenes with an 18-28 membered ring size were determined systematically by optical third harmonic generation measurements of molecularly doped polymer films at a wavelength of 1.907 μm. The X⁽³⁾₁₁₁₁/N values are found to possess a power law dependence with increased size of the annulenes.