Calculations are reported on the structure of donor-acceptor phenylazobenzenes, phenylazonaphthalenes, phenylazoanthracenes, and phenylazoheterocycles using the AM1 molecular orbital method. The spectra and hyperpolarizabilities of these systems have been calculated using a modified version of the sum-over-states CNDOVSB method specifically parameterized for electro-optic modulation. The replacement of one phenyl ring of the phenylazobenzenes by naphthalene or anthracene results in a modest increase in the hyperpolarizability but the value obtained is highly dependent on the position of the donor group and the tautomeric form. Enhanced values are obtained when a heterocyclic ring is incorporated into the azoarene with the largest value obtained for a donor-acceptor naphthylazothiophene. Substantial resonance enhancement effects occur for those derivatives which absorb in the red or infrared region of the spectrum.

New organic dyes containing as well a stilbene as an azomethine unit have been synthesized. Some of the compounds show liquid crystalline behavior with interesting phase sequences. Second Harmonic Generation (SHG) Experiments on polymer films consisting of a mixture of poly(methylmetacrylate) (PMMA) with 1% dye show values for the nonlinear optical coefficient in the range of 0.17 - 0.27 pm/V.

We recently reported that there is an optimal combination of donor and acceptor strengths for a given molecular length and bridge structure that maximizes (beta) . For this combination, there is the correct degree of bond length alternation and asymmetry in the molecule. Our recent findings suggest that molecules that can be viewed as asymmetric cyanines with relatively small amounts of bond length alternation are nearly optimal. In this manner, we have identified molecules with nonlinearities many times that of conventional chromophores for a given length. In this paper, we will present a new computational analysis that allows the correlation of bond length alternation with hyperpolarizabilities and will present EFISH data on simple donor-acceptor polyene chromophores.

We report the synthesis, electronic absorption and molecular second order nonlinear optical properties of a novel series of donor-acceptor substituted conjugated compounds with five- membered heteroaromatic rings as conjugating units. Our experimental studies demonstrate that the replacement of benzenoid rings in 4-dimethylamino-4'-nitrostilbene (DANS) and 4- dialkylamino-4'-dicyanovinylstilbene (DADVS) compounds with thiophene moieties significantly enhances their charge-transfer properties and molecular second order nonlinear optical properties.

Metal dithiolenes (Fig. 1) consist of conjugated organo sulphur ligands bound to a central metal atom and exhibit a range of interesting electrical and optical properties. Until recently this type of material was primarily of interest owing to the ability of some derivatives to act as u-electron donors in "organic metal" charge transfer salts1, though a resonant enhanced nonlinearity has been measured in the derivative BDN. 2 Electron delocalisation involving interaction through the metal leads to materials having low energy excited states. Strong absorption bands can occur throughout the visible and near infra-red regions depending on the exact nature of substitution. Tuning the absorption band near to the laser fundamental can result in the observation of a high nonlinear refractive index (n2 = 5 x 1011cm2/kW) coupled with low loss (cx = O.l6cm') in dilute solutions of selected derivatives.

Polymeric composites containing microcrystals of 1:1 salts of tetracyano-p-quinodimethane, TCNQ, have been prepared. Re-crystallization of these organic one-dimensional semiconductors in-situ by either heat treatment or exposure to vapor yields uniform dispersions of crystallites of sub-optical dimensions. We report a strong dependence of the lowest energy electronic absorption band in the lithium and potassium TCNQ salt with crystallite size. The inter-molecular charge transfer band shows a shift of up to 0.5 eV between films where the crystallite size is a few microns and those where crystals cannot be observed optically. The non-linear refraction of a heat treated film containing TEA^{(direct sum})(TCNQ) (TEA^{(direct sum}) - triethylammonium) is reported. (chi) ⁽³) at 712 nm is 2.69 X 10^-10 esu and is found to be a bleaching resonant non-linearity.

We summarize some of our past work in the field on optimizing molecules for second order and third order nonlinear optical applications. We also present some previously unpublished results suggesting a particular optimization of the popular cyano- and nitrovinyl acceptor groups. In addition we provide some new quadratic electro-optic results which serve to further verify our choice of a restricted three-level model suitable for optimizing third order nonlinearities in molecules. Finally we present a new squarylium dye with a large third order optical nonlinearity (-9.5 X 10^-34 cm⁷/esu²; EFISH (gamma) at 1906 nm).

The nonlinear susceptibility, (chi) ⁽³), and nonlinear refractive index, n₂, of a range of dithiolenes: metal-sulfur ligand complexes bearing simple H, alkyl and aryl substituents have been measured by degenerate four wave mixing (DFWM) and self focussing/defocussing (z-scan) at 1.064 micrometers using 100 ps pulses. Materials with the largest coefficients consistent with low linear and two photon absorption, for example bis[1-ethyl-2- phenylethene-1,2-dithiolate(2-)-S,S'] nickel have been modified to be soluble as guests in PMMA host polymers, n₂'s as high as 10^-8 cm²/kW have been found but intermolecular interaction causes increased two photon absorption at the high loading necessary for this n₂. Some simple chemical modifications which may improve figures of merit are suggested. Power requirements for simple waveguide devices using these materials are estimated and it is concluded that improved materials are still needed. Dithiolenes have sufficient variability in structure to suggest that this may be achievable with new derivatives of the same family.

Different types of poled polymer thin films (guest-host, grafted and crosslinked) are studied. The optical birefringence is related to electrooptical coefficients. Those coefficients are measured in the absorption range. Their value off resonance is equal to 10 pm/V. Loss and time relaxation experiments are also presented.

We report loss measurements in polymer-bound Disperse Red I slab and photodelineated channel waveguides. Losses resulting from electronic charge-transfer and vibrational carbon- hydrogen stretch overtone absorptions, trans to cis isomerization, exposure to visible or ultraviolet (UV) light and changes in dye pendant group number density are investigated. A waveguide absorption spectrometer is described which can measured waveguide losses (alpha) ((lambda) ) from 600 - 1800 nm. Absorption losses are compared to the wavelength dependent electro-optic coefficient r₃₃((lambda) ) and a figure-of-merit r₃₃((lambda) )/(alpha) ((lambda) ) is determined for the material.

Cooperative processes in polar ordered polymers lead to departures from gas law behavior during thermo-poling. Two non-liquid crystalline dipolar polymer systems are presented to exemplify these ideas. In the first system, 'rigid V'-shaped polymer segments containing dipolar groups may be required to re-orient concertedly in a chain to achieve lower dipolar symmetry. In the second system, a cooperative reorientation process between guest molecules and an amorphous electret polymer host is facilitated through hydrogen bonding interactions. This latter system displays orientational stability comparable to some covalent bonded side- chain polymers.

Third-order nonlinear optical properties, (chi) ⁽³), of novel polymeric materials having large (chi) ⁽³) with processabilities are discussed. These polymers reported here have a potential to fabricate optical switching devices.

The Electro-Optical coefficients in test samples of two different second-order nonlinear optical polymers (with T_g's of 140 degree(s)C and 175 degree(s)C) have been measured between crossed polarizers (r₃₃-r₁₃) and Fabry-Perot interferometrically (r₁₃), both during poling and after poling. In the polymer with T_g 140 degree(s)C, the dependence of the coefficients on the strength of the poling field, and the ratio r₃₃/r₁₃ have been investigated and compared with theory. A quasi-instantaneous loss of field induced Electro- Optical activity is observed when the poling field is switched off after poling. The influence of annealing on the relaxation behavior is discussed. Results of relaxation experiments in the two polymers at different temperatures are presented.

Several classes of organic compounds have been synthesized at IBM's Almaden Research Center, and their nonlinear optical properties measured. Within each class, substitution of different donor and acceptor groups has led to definitive conclusions regarding the relative merits of particular moieties.

In this paper we show that Hyper-Rayleigh Scattering (HRS) yields hyperpolarizabilities of organic molecules in solution which are consistent with EFISHG-data. For molecules where dipole moment and (beta) are non-collinear marked differences are noted. We show also the applicability of HRS to obtain (beta) -values for ionic and octopolar compounds. Information obtained from depolarization studies is discussed and preliminary results on dispersion are presented.

Molecular second-order nonlinear optical properties of a series of interesting nitropyridine derivatives, namely 2-cyclooctylamino-5-nitropyridine (COANP), 2-docosylamino-5- nitropyridine (DCANP), and 2-(N-prolinol)-5-nitropyridine (PNP) were investigated with two different methods: electric field-induced second-harmonic generation (EFISH) and a solvatochromic method. Both techniques yield the same values within the experimental errors, showing that the main contribution of the microscopic nonlinearity arises from a single intramolecular charge transfer transition. The measured values were shown to follow the theoretical two-level dispersion. A comparison of the hyperpolarizabilities and the macroscopic nonlinear optical coefficients shows a good quantitative agreement. This illustrates that intramolecular contributions are the main source for the large nonlinear optical responses observed in these crystals.

The organic crystal 4-nitro-4'-methylbenzylidene aniline was identified as a promising nonlinear material by the powder technique. The material gave a second harmonic intensity 16 times that of urea. Large single crystals of dimensions 5 X 3 X 1 cm³ were grown by temperature lowering a supersaturated ethyl acetate solution. An orthoscopic examination revealed the positions of the principal dielectric axes. The refractive index dispersions were determined to an accuracy of +/- 0.0015 using the minimum deviation technique and Maker fringe spacings. These dispersion curves were fitted with a Sellmeier equation which allowed the indices to be determined to +/- 0.0006. The nonlinear d- coefficients d₁₁, d₃₃, d₃₁ and d₁₃ were evaluated at 1000, 1064 and 1300 nm using the Maker fringe technique. The coefficient d₁₁ was over 200 times larger than KDP d₃₆. In addition, the non-diagonal coefficient d₃₁ was comparable to the phase- matching coefficient in the organic material DAN. Critically phase-matched second harmonic signals were observed at all fundamental wavelengths. There was excellent agreement between the experimentally determined and theoretical phase-matched incidence angles. This agreement is presented. Noncritical phase-matched conditions have been calculated and are reported. Both angle and wavelength noncritical phase-matching is possible with this crystal.

Asymptotic longitudinal polarizabilities per unit cell of infinite periodic systems have been computed at the ab initio level by using uncoupled and coupled Hartree Fock procedures.

A previously proposed model of a molecular spatial light modulator (SLM) based on the steady state solution to a multimolecular level kinetic diagram is being extended to include time dependent phenomena. In particular the model addresses a nonlinear molecular absorber of concentration N where a write light pulse modifies the various level populations and where a read pulse interrogates this new population pattern of length L. For absorbers having a significant triplet level absorption, denoted by its crossection (sigma) ^T_ln (R), the transient contrast ratio achieved is R equals exp[2(sigma) ^T_ln(R) N L], which is the square of the steady state value.

The mechanism of the ultrafast nonlinear optical responses in several conjugated polymers was studied by pump-probe experiment using a femtosecond laser. The temporal response of the optical nonlinearity of conjugated polymers with large third-order susceptibility was studied and mechanisms of coherent and incoherent nonlinear processes were clarified.

Laser induced holographic grating relaxation (HGR), electro-optical (EO) and second harmonic generation (SHG) studies have been carried out in a guest/host system subject to an external electric field with and without the presence of a hole transport agent. The guest is NNI, which has moderate second order nonlinear optical (NLO) coefficient, the host is PMMA and the hole transport agent is DEH. In the presence of DEH, the lifetime of the cis isomer of NNI detected by using the HGR technique is lengthened by the application of an electric field of moderate strength. The field dependence effect is not observed when DEH is absent in the sample. The result is interpreted as due to the stabilization of the cis-isomer by the electric field polarized DEH. The data obtained from SHG and EO measurements appear to corroborate the HGR result.

Electro-optic effects in single crystals of 2-cyclooctylamino-5-mitropyridine and 2-(N- prolinol)-5-nitropyridine have been investigated. The measured wavelength dependence of the electro-optic coefficients can be well explained with the quantum-mechanical two-level model. Enhancements of these coefficients near acoustic phonon resonances of up to a factor of 25 were observed when scanning the modulation frequency of the applied electric field. A comparison between measured second-order polarizabilities and electro-optic coefficients revealed that optic phonons may still contribute considerably to the electro-optic effects in the investigated molecular crystals. A simple expression to estimate the limiting values of electro- optic coefficients is proposed and compared with values of known molecular crystals.

Self-pumped phase conjugate wave has been observed in KNSBN:Co crystal at different argon-ion laser wavelengths. For 514.5 nm of the wavelengths, phase conjugate reflectivity as high as 73.0% is measured. The incident angle and incident position dependence of phase conjugate reflectivity is presented. The formation time is observed to be dependent on the incident intensity. The loss of the crystal is regarded connected with threshold intensity of producing self-pumped phase conjugation.

Second- and third-order nonlinear optical responses of photoconductive materials were studied by optical second and third harmonic generation and femtosecond time-resolved spectroscopy. Four types of photoconductive molecular systems were investigated: vanadylphthalocyanine thin films, polymeric charge-transfer complex, single crystals of 9-hydroxyethylcarbazole derivatives, and carbazole head-to-tail polymers.

In this paper, a review of some different experimental methods employed for measuring the electrooptic coefficients (Pockels effect and Kerr effect) are presented, together with the most recent technique proposed by Teng and Man. For this last method, the limit of validity of a simplified method is established. A rigorous derivation is also given for studying such polymer films when the wavelength of the light beam is close to the absorption band.

The first demonstration of Surface Acoustic Wave (SAW) propagation in a polar organic crystal, (-)-2-(alpha) -methylbenzylamino-5-nitropyridine (MBANP) is reported. A SAW velocity of 1416 ms^-1 in the [010] direction, in the (001) crystal plane was measured, which is about half that of many inorganic substrates. In addition 3 of the 8 piezoelectric coefficients, d₂₁, d₂₂, d₂₃ have been measured and indicate that performance comparable with lithium niobate may be achieved.

The behavior of the polar orientational order parameter of two nonlinear optical (NLO) polymers is investigated using the second harmonic generation (SHG) and the refractive index/waveguide apparatus. The two NLO systems consist of a MNA/PMMA guest/host and a BCSC according main chain polymer. The MNA/PMMA results show the inadequacy of the independent dipole orientation model for the description of the electric field poling. The BCSC system illustrates the birefringence in the refractive index is not a necessary criterion for the second order nonlinear effect such as SHG.

Using a planar, linear, waveguide second-harmonic generation was observed by nonlinear evanescent field interaction. In this class of devices phase-matching could be achieved with a periodically structured polymer adsorbate.

Langmuir-Blodgett (LB) films have been fabricated from a preformed acrylate amphiphilic copolymer with nonlinear optical (NLO) active chromophores covalently attached to the polymer backbone. By alternate deposition of this NLO active polymer with an NLO passive amphiphilic polymer, high quality multilayer films have been obtained with sufficient thickness to allow waveguiding in the LB films. The observed optical attenuation is between 1 and 3 dB cm^-1. The linear and nonlinear optical characteristics of these films have been determined, both in transmission (or reflection) and in waveguide format. The high degree of order in LB films makes this type of supramolecular structure attractive for NLO applications. Based on the observed parameters (dispersion of the refractive indices, birefringence, tilt angle of the chromophore, hyperpolarizability), different phase-matching schemes can result in efficient second harmonic generation.

We report for the first time phase-matched second-harmonic generation by mode conversion in nonlinear optically active Langmuir-Blodgett (LB) film waveguides. Phase-matched frequency- doubling was observed in LB films of 2-docosylamino-5-nitropyridine (DCANP) by conversion of a fundamental TE₀-mode into a second-harmonic TE₁-mode. The experiments were done in four-layer optical waveguides. For DCANP this four-layer configuration improves the theoretical conversion efficiency more than 100 times compared to a conventional three-layer waveguide. In the preliminary experiment infrared to blue light conversion efficiencies up to 0.5% W^-1 cm^-2 (0.02% W^-1) were measured.

Carotinoids exhibit large nonlinear optical properties due to their extended (pi) -electron system. Compared to other polyenes which show a broad distribution of conjugation lengths, carotinoids exhibit a well defined molecular structure, i.e. a well defined conjugation length. Therefore the carotinoid molecules can serve as model compounds to study the relationship between structure and nonlinear optical properties. In this paper the synthesis of four astaxanthins with C-numbers ranging from 30 to 60, their preparation into thin films, wavelength dispersive Third Harmonic Generation (THG) measurements and some molecular modelling calculations will be presented. Resonant (chi) ⁽³) values reach 1.2(DOT)10^-10 esu for C60 astaxanthin. In the nonresonant regime a figure of merit (chi) ⁽³)/(alpha) of several 10^-13 esu-cm is demonstrated.

The performance and processing of passive and active polymeric integrated optic devices are described. Results obtained with several processed batches are compared, to evaluate the reproducibility of devices realized using polymer technology. The wafer-to-wafer differences in electro-optic switching properties were less than 10% for routinely fabricated devices, despite of the partially manual production process. Also, the stability of electro-optic polymers has been evaluated, both intrinsically and in devices. Using new polymers, with higher glass- transition temperatures, devices with long term stabilities up to at least 100 degree(s)C are expected in due time.

Three nonlinear optical polymeric systems based on nonlinear optical chromophores attached as side chains to poly(methyl methacrylate) were synthesized and their nonlinear optical properties measured in thin film form by second harmonic generation and by the electro-optic effect. Also, attenuation of light propagation in optical waveguides and photothermal deflection infrared spectra were measured on these films to aid in selecting the best system for device development. Based on these results, a phase modulator was constructed and with sub- carrier multiplexing the signals from five TV channels were placed on an infrared laser beam as sidebands. After a short transmission through space, these signals were then heterodyne detected and displayed on a TV monitor with good signal to noise and with no crosstalk.

The nonlinear and linear optical properties of a group of polymers containing DR19 are reviewed. The thermal setting polymers have demonstrated large NLO coefficient and long term poling stability. Dry processed micro-patterning of the index of refraction and the birefringence has been demonstrated with sub-micron resolution.

Polymers and, more recently, sol-gel glasses are generally regarded as the better hosts for optical devices which rely on organic dye molecules as the active component. Most traditional low melting temperature glasses are too easily attacked by water to be a serious alternative; however, a novel composition family based on SnF₂ eliminates many of the shortcomings of other glasses with similar viscosity properties and they seem to have an affinity for dissolving many organic dye molecules. The properties of these glasses and the ongoing efforts to demonstrate the feasibility of fabricating guided wave structures are described.

The nonlinear optical channel waveguide consisting of the lead phthalocyanine derivative doped polymer layer over the ion-exchanged glass substrate was fabricated. The single-mode propagation was observed in this polymeric channel waveguides. Propagation loss was estimated to be 2 dB/cm at 830 nm. Nonlinear optical properties are investigated by measuring the intensity-dependent refractive index change using a femtosecond interferometric method.

An experimental technique is presented for calibrating the photobleaching process in a highly adsorbing dyed polymer. The interface between the bleached and unbleached layers of the material travels with a constant velocity. By determining the dependence of the velocity on the bleaching intensity, the order of the photobleaching kinetics is determined.

Optical Kerr effect were applied to an all-optical switching device in the form of nonlinear waveguide directional couplers. The nonlinear directional coupler had a nonsymmetrical waveguide whose structure consists of a quartz thin gap between two Corning 7059 guided layers on the pyrex substrate with ion-milled grating and organic thin film as a top layer. The vacuum-deposited polydioacetylene film was used as an organic nonlinear material. Switching phenomena in this nonlinear directional coupler were confirmed for 10 ns pulse Nd:YAG laser and 150 fs pulse duration of mode-locked Ti:Sapphire laser.

Second order nonlinear optically active heterocyclic compounds with extended length-scale were synthesized and their nonlinear optical properties were characterized by EFISH measurements. The results from the study of two series of heterocyclic compounds with different electron acceptors reveal a power-law dependence of (Beta) (mu) on length, and in both series similar power-law exponent of approximately 2 are observed. Results of a theoretical model calculation are in qualitative agreement with the experimental results.

This paper reports on quadratic electroabsorption studies of thin-film solid solutions of squarylium dye molecules in poly(methylmethacrylate) polymer with the aim of understanding the role of electronic and reorientational mechanisms in the third-order nonlinear-optical susceptibility. We present a generalized theory of the quadratic electrooptic response that includes both electronic mechanisms and molecular reorientation and show that the ratio of two independent third-order susceptibility tensor components, namely (chi) ⁽³)₃₃₃₃/(chi) ⁽³)₁₁₃₃, determines the relative contribution of each mechanism. Based on these theoretical results, we have designed and built an experiment that determines this ratio as a function of temperature and wavelength. Results show that at room temperature and near the first electronic transition wavelength, the response is dominated by the electronic mechanism, and that the reorientational contribution dominates when the sample is heated above its glass transition temperature. Furthermore, results show that, off-resonance, the sign of the imaginary part of the third-order susceptibility is positive. Quadratic electroabsorption is thus shown to be a versatile tool for measuring the imaginary part of the third-order nonlinear-optical susceptibility which yields information about the interaction of polymer and dopant molecule.

An amorphous, nonlinear optical polymer, which has a glass transition temperature of 208 degree(s)C, was prepared. It is soluble in common organic solvents, and can be processed by conventional photolithographic processes. Corona-poled thin films of this polymer exhibit second-order nonlinear optical properties. This polymer is comprised of mainchain cinnamamide chromophores in the syndioregic configuration. The rigid U-shaped links which bridge the chromophores in this polymer were designed to geometrically stabilize polar, folded conformations in the backbone.