One of the major focuses in the study of nonlinear optics in recent years has been the microscopic description of the second and third order π-electron virtual excitation processes in organic and polymeric systems. In particular, the question of correlation in systems of reduced dimensionality has been under intense study. Important evidence for the inadequacy of independent particle theories such as Huckel theory in these systems has been provided by the experimental discovery that below the one-photon 11Bu π-electronic state exists a two-photon 21Ag state in linear chain polyenes, in direct contradiction with these simple theories. It was then demonstrated that a proper account of the electron correlation resulting from electron-electron interactions was required in order to obtain the correct ordering of these two states.

A 'missing states' analysis is introduced and used to identify excited states and dynamics important for the second order susceptibility, x(2), and the third order susceptibility, x(3), using PPP-SCF-CI transition moments and energies and is applied to a group of organic molecules. Examination of charge distributions and bond orders for the relevant states suggests the features important for x(2) and x(3). The applicability of the two-level model and contributions of interstate coupling are also examined. Detailed results are presented for nitrobenzene and p-nitroaniline.

Second and third order nonlinear optical properties of some newly synthesized organic molecules and polymers are reported. Powder second harmonic generation efficiencies of up to 200 times urea have been realized for asymmetric donor-acceptor acetylenes. Third harmonic generation x(3)s have been determined for a series of small conjugated molecules in solution. THG x(3)s have also been determined for a series of soluble conjugated copolymers prepared using ring-openinig metathesis polymerization. The results are discussed in terms of relevant molecular and/or macroscopic structural features of these conjugated organic materials.

An accurate and computationally tractable theoretical procedure for the calculation of the non-resonant, electronic components of y(0;0,0,0), y(-3ω);ω)ω,ω), and y(-2ω);ω,ω,0) can be constructed. This procedure partitions y into a σ-electron component (yσ) and a π-electron component (yπ). The yσ term is evaluated using the bond-additivity approximation; the in term is calculated using the semi-empirical INDO all-valence-electron molecular orbital method combined with full single- and double-excitation configuration interaction (SDCI) of singlet it-electron configurations, and Orr and Ward's sum-over-states expression for y. Calculations of the effect of chain length, conformation, methyl substitution, and polarity on the values of y for π-electron conjugated hydrocarbons indicate that y is most strongly influenced by conjugation chain length.

We evaluate at the ab initio level the electronic and geometric structures and the microscopic electrical polarizabilities of polyarylene vinylene compounds such as polyparaphenylene vinylene, polythiophene vinylene, and polypyrrole vinylene. Related fused-ring structures based on isothianaphthene and thieno[3,4-c]thiophene and their vinylene derivatives, are also investigated.

The dynamics of fluoro-aluminum phthalocyanine polycrystalline thin films is studied using differential transmission spectroscopy with femtosecond laser pulses. Following excitation by 620 nm pulses into the first electronic transition a very rapid energy redistribution of the excitons is directly observed within the 55 fs duration of the pump pulse. Following this process two extremely fast decay routes lead to the recovery of the ground state : exciton-exciton annihilation with an intensity dependent decay of 550 fs at power density of 1.2 GW/cm2 and exciton-phonon coupling with a 4 ps decay time. A complete interpretation of the decay mechanisms based on the Frenkel exciton terminology successfully accounts for the observed results. We have analyzed the nonlinear dynamics of a series of linear alkane molecules in pure liquid state using the time-resolved optical Kerr effect. The results show that the Kerr nonlinearity increases linearly with the number of atoms. However, for those molecules with more than eight carbons, a surprisingly large deviation from the bond additivity approximation was observed. The molecular orientational contribution to the nonlinear response is negligible for these types of molecules. A small component, with less than 8% of the total nonlinear Kerr response, is attributed to translational anisotropy and has a 450 fs lifetime. The nonlinear dynamics and the length dependence of optical Kerr nonlinearities will be discussed.

A new convenient method is proposed to determine the electro-optic (EO) constants and relevant nonlinear susceptibilities. It was applied to polycarbonate films doped with 4-diethylamino-4'-nitrostilbene (DEANS). The real and imaginary parts of the third-order nonlinear susceptibility X(3) (-ω;ω,0,0) of the film with several pincentrations of DEANS were obtained in the visible region. The magnitude of relevant X(3) is proportional to the concentration of DEANS. The mechanism of the EO effect of these samples is discussed, in terms of the restricted orientational motion of doped molecules.

Organic materials which exhibit nonlinear optical properties are currently of great interest. The design of such molecular materials, having specific physical properties, needs two steps since the macroscopic effect rises from the additive contribution of each molecular unit: search of active molecules and elaboration of efficient condensed phases. New trends in the design of highly hyperpolarizable molecules are described. Then, several strategies in getting nonlinear crystals able to frequency conversions are reviewed. In the absence of accurate predicting methods of crystal packing, the aim is to control the non-centrosymmetry in the solid state: chirality, intermolecular hydrogen-bonding, vanishing ground state dipole moment, extra charge-transfer polar group proved satisfying to get some of the most efficient crystals for quadratic nonlinear optics.

Large (7x4x4 cm3) crystals of the organic, nonlinear optical material MBA-NP have been prepared by growth from solution, in an optically and structurally highly perfect state. Oriented, cut and polished specimens were examined by the Maker fringe technique. Analysis of the data yielded a coherence length, 122 of 1.8 μm (cf MNA 111 = 0.7μm and quartz 111 = 20μm). Type I, phase-matched second harmonic generation (efficiency 50% at 117 MWCm was observed for 1.06μm radiation incident at 28.8° to the y-axis on the y-z incident plane. There was no evidence of type II phase matching. d22 = 30x10-12 m.V-1 and d14 is between 1 and 10x10-12 m.V-1.

The linear and nonlinear optical properties of 2-(α-methylbenzylamino)-5-nitropyridine (MBANP) have been investigated. The hyperpolarizability of MBANP molecule is as large as those of other para-nitroaniline-like molecules while the charge-transfer absorption band is appreciably shifted to shorter wavelengths. MBANP crystal has an absorption cut-off around 0.45μm. The nonlinear optical coefficient d22 of MBANP is 126 times larger than d11 of quartz and is 1.46 times larger than d33 of LiNb03.

A study of crystal growth and characterization of a promising organic material, 4-aminobenzophenone, is presented. Phase-matched, efficient, second harmonic generation of 1.064-μm radiation is also reported.

The second-order nonlinear optical properties of donor and acceptor substituted benzoxadiazole derivatives were investigated on the basis of their photophysical properties and crystal structure. Second-order molecular hyperpolarizabilities were calculated using photophysical parameters determined by absorption and fluorescence spectra measurements. They were not commensurate with the intensity of the second harmonic radiation by the powder technique. The crystal structure determination of 4-chloro-7- nitro-2,1,3-benzoxadiazole (NBD-C1), which has a phase-matched and exceptionally large second harmonic generation (SHG) among the BFZ derivatives studied, has revealed that it belongs to the monoclinic system with space group P21 and there are two independent molecules in the unit cell arranged in a noncentrosym-metric manner. The charge-transfer angles of the two independent molecules to the crystallographic symmetry axis are 48.66° and 68.79°, whose values are both very close to the optimum value of 54.74°. The existence of two independent molecules with a favorable crystal packing as well as a high crystal density are therefore responsible for the enhancement of the SHG in NBD-Cl. A single crystal of NBD-Cl grown from solution showed remarkable anisotropy in the intensity of the SHG.

Nonlinear optical properties of molecules are currently under intensive investigation. Extensive research has been done on finding inorganic and organic molecules which possess large second order polarizabilities while organometallic molecules remain virtually unexplored. We will present our work on the use of M(PEt3)2(X) (M = Pd, Pt; X = Br, I) as donors in donor-acceptor benzene systems to yield material suitable for second harmonic generation (SHG). We will also discuss our work on new organic compounds suitable for second harmonic generation.

Water soluble organic salts comprise a large class of crystalline materials containing many efficient harmonic generators for producing blue and near ultraviolet light. Among the salts of simple chiral organic acids and bases, about 1 in 10 crystals has a nonlinearity greater than or equal to KDP. The distribution of noncritical wavelengths among these crystals peaks around 500 nm for Type I doubling and 700 nm for Type II. We estimate the number of chiral organic salts which must be examined in a survey in order to assure with high probability that an "optimized" crystal for a given frequency conversion process will be found.

The poling process of polymers doped with chromophors is of great importance for the non linear properties of these materials. So, after having optimized the poling conditions (Ep, Tp), the non linear optical coefficients d33 have been measured for PMMA doped with different percentages of azo dye. We have also synthetized new copolymers in which the dye is linked to the backbone, investigated their behaviour and compared them to the guest host systems.

In this paper we report for the first time in-situ measurements of corona poling induced SHG . We investigated a specially synthesized new amorphous polymer (PPNA) that contains p-nitroaniline as sidegroups covalently fixed to a polyethylene-type backbone. In addition, we did measurements on an amorphous alternating copolymer poly(vinylidenecyanide/vinyl-acetate) (PVDCN/VAc), known for its piezoelectric properties. In order to understand the nonlinear properties and their stability, an extensive characterization of the materials is necessary. Therefore, calorimetry, thermogravimetry, X-ray, dielectric relaxation, waveguide and absorption spectroscopy studies were performed. Fig. 1 shows the chemical structure of the polymers investigated.

This contribution describes two approaches to the construction of polymeric non-linear optical materials with persistent second harmonic generation efficiency. In the first, poly(p-hydroxystyrene) is covalently functionalized with chromophores having large quadratic hyperpolarizabilities. Films of these polymers are poled at fields up to 1.8 MV/cm to yield materials with stable (on the timescale of months) second harmonic coefficients (d33) as high as 18 x 10-9 esu. In situ measurements indicate that field-induced chromophore alignment is rapid at temperatures substan-tially below Tg and that relaxation of the alignment is even more rapid upon removal of the field. Annealing of the films prior to poling enhances the stability of second harmonic generation and allows poling at higher fields. A second approach is to disperse chromophores in an uncured epoxy host and to then simultaneously cure and pole the resulting ensembles. This procedure also stabilizes preferential chromophore alignments for relatively long periods of time.

One of key applications desired for second-order non-linear optical materials is that of an electro-optic phase shifter. Organic materials offer the promise of having large electro-optic coefficients for this application. A number of organic materials were examined as either crystalline or ordered polymeric thin films. The results of absorption, second-harmonic generation, and electro-optic measurements on these materials will be presented. The results on waveguide fabrication and coupling are also discussed.

We report an electro-optic effect in the copolymer, polyvinylidene fluoride-trifluoroethylene (PVF2-TrFE). The electro-optic effect in this material has a hysteresis which is related to its ferroelectric hysteresis. The electro-optic effect in combination with field dependent scattering can be used for electro-optic intensity modulation. We have demonstrated a modulation ratio of 13%. By adjusting the bias voltage, in-phase or out-of-phase modulation can be selected.

We present a detailed theory of thermally induced transverse self-phase modulation and optical limiting with nematic liquid crystal films at CO2 (10.6μm) wavelengths. Our theory accounts for the detailed roles played by the laser parameters (such as intensity, polarization, wavelength, etc.), sample parameters (such as the thermal index gradient, thermal conductivity thickness, etc.) and the geometrical placement of the lens, sample and detector. The experimental results are in good agreement with the theoretical predictions. Our theory and observation enable us to determine optimal configurations and physical parameters for practical application of these processes.

Recent spectroscopic investigations and model calculations concerning the short chain reaction intermediate (oligodiacetylenes) of the solid state polymerization in diacetylene crystals are discussed. The oligodiacetylenes are best described by diradical (DR), asymetric carbene (AC), dicarbene (DC) and stable oligomer (SO) molecules of different chain lengths. Their structures and properties are investigated in detail by electron spin resonance (ESR) and optical absorption and emission spectroscopy. Theoretical model calculations are performed to describe the spectroscopic features of the oligodiacetylenes. The transition from diradicals to dicarbenes observed at a critical chain length of six repeat units is quantitatively explained using a configuration model. Triplet excition-polaron exitations are shown to be closely related to charged polarons and to dipole and quadrupole excitations relevant for nonlinear optical effects.

Thin films of diacetylene derivatives with carbazol substituents have been prepared by epitaxial growth on different organic or inorganic substrates. Both structural aspects and optical properties have been investigated in order to obtain high cubic polarisability x(3) in well oriented assemblies of conjugated polymeric chains. Using thin film evaporation technique under high vacuum, we are able to prepare optical films of high quality, perfectly bioriented on some alkali halide crystals and monooriented on various organic crystalline substrates. Third harmonic generation (THG) measurements on such films confirm the large scale orientation of polydiacetylene crystals locally observed by electron microscopy.

The polymerization of a liquid crystalline diacetylene monomer, 116 bis-[N-4-oxybenzylidene 4'-n-octylaniline) 2,4-hexadiyne, 1-0BOA, is reported both within the crystal and liquid crystal phases. The mono-mer exhibits smectic phase polymorphism at temperatures below the clearing temperature as shown by small angle X-ray scattering and optical microscopy. Polymerization kinetic results show first-order disappearance of monomer as seen by differential scanning calorimetr with a thermal activation energy of 31 kcal/mol. in the liquid crystal phase and ca. 71 kcal/mol. in the crystal phase. Poly(1-0BOA) exhibits a "soft" lamellar-like layer structure reminiscent of the monomer when polymerized within a low temerature liquid crystal phase. Consequently, the topochemicalpolymerization of diacetylene monomers within a liquid crystal phase top give ordered polymer structures is reported for the first time.

Transition metal poly-ynes are shown to have large nonlinear optical susceptibilities according to the results of four-wave mixing and power limiting studies. The fact that the metal poly-ynes also have low absorption in the visible and the near infrared makes them potentially useful for many applications. The presence of intense two-photon absorptions and strong back scattering effects in solutions of metal poly-ynes is described.

A method to produce guided wave second harmonic generation (SHG) in coated multimode optical fibers is described and analyzed. The fiber cores are coated with a highly nonlinear hemicyanine dye by the Langmuir/Blodgett technique. This technique allows precise control over the film thickness to one monolayer (~3 nm). Deposition ratio data indicate that the deposition forms a noncentrosymmetric (Z-type) film. The SHG signal increases as a function of the film thickness for up to 80 monolayers (~240 nm) of dye. Analysis for calculating the propagation constants and energy distributions of the fundamental and harmonic modes is used to determine the requirements for phase matching in the composite fiber-film system. For most cases of interest the effective index of refraction for a given film mode in the coated region may be closely approximated by the propagation constant in a planar waveguide of equal thickness. The conditions for phase matching and selective excitation of modes for improved SHG efficiency are proposed. Efficient SHG in coated fibers could prove useful in frequency doubling of lasers. They would be more compatible with fiber optic communications systems than currently used single crystal and channel waveguide doublers. In addition, demonstration of SHG suggests that other second order nonlinear effects (i.e. linear electro-optic effect, parametric amplification) may be produced in a Langmuir/Blodgett (L/B) nonlinear film coated fiber system.

Novel conjugated nonlinear optical polyene molecules, having donor and acceptor substituents and being designed so as to be suitable for Langmuir Blodgett deposition have been synthesised, deposited and characterised. Alternating Y-type multilayer structures have been fabricated, and their nonlinear optical properties have been studied by second harmonic generation experiments. The dependence of the observed second harmonic signal on film thickness and on deposition conditions will be described. The magnitude of the nonlinear optical coefficients obtained from the best films exceeds that of other systems reported in the literature.

Organic and polymeric materials exhibit many important properties as electro-optic materials. Poled polymer films are particularly well suited for integrated optic devices, due to their large electro-optic coefficients, low dielectric constants, and flexible processability. We review our work toward the fabrication of active polymer integrated optic devices based on a newly developed channel waveguide fabrication procedure, and detail some of the properties of the devices.

Organic nonlinear optical materials continue to hold promise for electro-optic applications. However, the requirements of devices such as phase shifters, modulators, and optical switches, place stringent constraints on the magnitude of the electro-optic coefficients (rij), optical loss, processability, and physical robustness of the electro-optic materials. The design criterion for a phase shifter, based on polymeric guest-host systems or thin film organic crystals, are presented. Both channel and slab waveguide configurations are discussed, with their associated coupling and propagation problems.

We have fabricated channel waveguides from an optically nonlinear organic material, using an inverted rib structure. Guides consist of a planar layer of the optically nonlinear polymer spun on a glass substrate in which ion-exchanged channels provide lateral confinement. This paper discusses the design, fabrication, and optical properties of the composite waveguides.

The synthesis of a new class of soluble polydiacetylenes is reported. Large area films of one of this class, 9-SMBU, have been formed by deposition from solution. The non-linearity of films of 9-SMBU has been measured by THG and demonstrates the trade-off between processability and non-linearity. The material was also deposited in channels etched in Si0xNy substrates and we have demonstrated, for the first time, channel optical waveguiding in a non-linear polydiacetylene material. Linear optical characterisation of these channels has been carried out at a laser wavelength of 1.3 μm. These waveguides should allow demonstrations of all-optical non-linear effects in guided wave devices.

Theoretical studies suggest that particular classes of organic molecules possess promising features with respect to second order optically nonlinear effects (frequency doubling, Pockels effect, etc.). These attractive features allow the application of such organic materials in integrated optical and electro-optic devices for manipulation of optical beams in communication or optical processing systems. Different forms of organic media, such as single crystals, Langmuir-Blodgett films, polymers, etc., exist and will be discussed and compared in this paper. In the case of side chain polymers, estimates of the expected second order nonlinear susceptibilities (x(2)ijK) and electro-optical coefficients (rmn) will be made, based on simple theoretical models. Experimental results, obtained with such optically nonlinear polymers, will be presented and compared with the theoretical predictions. The role of liquid crystallinity, in relation to the achievement of optical nonlinearity, is discussed, applying the results of an extended Maier-Saupe model.

Results of measurement of the dispersion of the second-order optical nonlinear coefficient d11 of 2-methyl-4-nitroaniline (MNA) using laser diodes as light sources are presented over a wavelength range of 1.20 to 1.54 μm. The nonlinear optical coefficient strongly depends on wavelength, exhibiting a resonance enhancement. We also report the first observation of multiple-reflection effects in optical second-harmonic generation. The observation has been made possible by the use of a single-mode laser diode as a light source. The experimental results are in excellent agreement with a theoretical analysis which takes account of the multiple reflections of both the fundamental and second-harmonic waves.