Organic second-order nonlinear optical crystals are expected to be more useful than inorganic crystals such as LiNbO3 or KTiOPO4 (KTP) because oftheir larger nonlinear susceptibility (2))• A long interaction length in the phase-matched direction is required to obtain devices with high wavelength conversion efficiency. We have devised an indirect laser-heated pedestal growth (ILHPG) method in which the growth direction oforganic crystal can be controlled. We have successfully applied the method to grow organic nonlinear optical crystal 2-adamanthylamino-5-nitropyridine(AANP). The grown AANP has been characterized in terms ofits linear and nonlinear optical properties. The results show that Type II angle-tuned phasematched wavelength conversions, such as second harmonic generation (SHG) and optical parametric generation (OPG), are possible in the wavelength region from 1.2 to 1.7 tim. This means, it is possible for AANP to provide Type II angle-tuned phase-matched wavelength conversion with high efficiency foroptical communications systems. As an example, the AANP was applied for optical sampling measurements. In addition, reducing absorption in the AANP crystal by using a deuterated denvertive ofAANP increases the SHG efficiency at 1.55 .tm.

The solvatochromic behaviour of several nonlinear optical (nb) chrornophores, synthesised following the discovery of the novel reaction of triethylamine with tetracyano-p-quinodiniethane (TCNQ)" 2 is investigated. An alternative method of calculating the static first hyperpolarisability, J3 is proposed for molecules that exhibit anomalous solvatochromic behaviour, where it is difficult to apply standard solvatochromic models. Values of f3 are estimated from this data using both spherical and ellipsoidal local field models, where difficulties in the calculations are highlighted. The relative merits of these models are discussed with particular attention paid to the calculation of the cavity radius.

We describe a new approach to second-order nonlinear optical materials, namely quadrupoling. This approach is valid in the regime of Kleinman (full permutation) symmetry breaking, and thus requires a two- or three dimensional microscopic nonlinearity at wavelengths away from material resonances. This "quadrupolar" nonlinearity arises from the second rank pseudotensor of the rotationally invariant representation of the second-order nonlinear optical tensor. We have experimentally investigated candidate molecules comprised of chiral camphorquinone derivatives by measuring the scalar invariant associated with the rank two pseudotensor using hyper-Rayleigh scattering. We have found sizable scalar figures of merit for several compounds using light for which the second harmonic wavelengths are greater than 100 nm longer than the absorption peak location. At these wavelengths, the quadrupolar scalar is as large as the polar (EFISH) scalar of p-nitroaniline. Prospects for applications are discussed.

Mono- and dinuclear sesquifulvalene-type complexes [{LnM(l5C5H4)}Z{17C7H6)MLtn}mXm+i (m =0, 1 ; X = BF4, PF6) have been synthesized, particularly with regards to their nonlinear optical properties. Z =-: LM = CpFe, M'L' = -, la; LM = CpFe; M'L' = Cr(CO)3, ib; LM = CpFe, ML',, = RuCp, ic; LM = CpFe, L'M' = RUCP*, id; LM = CpRu; M'L' = -, le; LM =CpRu, M'L' RuCp, if, LM = CpRu, M'L' = RuCp*, ig - Z = C2: LM = CpFe, M'L' = Cr(CO)3, 2 - Z = C2H2:LM = CpFe, ML' = -, 3a; LM = CpFe, M'L' = Cr(CO)3, 3b; LM = CpFe, L'M' = RuCp, 3c; LM = Cp*Fe, M'L' = Cr(CO)3, 3d; LM = (Ph4C4)Co, M'L' = - 5; Z = thiophene-1,5-diyl (C4H2S): LM = CpFe, M'L' = -, 4a; LM = CpFe, M'L' = RuCp) (Cp = C5H5, Cp* C5Me5, Ph = C6H5). The ferrocenyl containing complexes reveal UV/vis spectra, showing long wave absorption bands beyond 550 nm which are assigned to a charge transfer (CT) transition between the cyclo-C5 and cyclo-C-, moieties. The corresponding transitions for the ruthenocenyl compounds if and ig are found below 500 nm. The CT transitions exhibit pronounced negative solvatochromism. Cyclic voltammetry studies and structural data of some of these compounds confirm the strong electronic coupling between the cyclo-C5 and the cyclo-C7 moieties. Hyper Rayleigh scattering (HRS) investigations of these mono- and dinuclear sesquifulvalene derivaties to determine the first hyperpolarizability 13 show several different important features: i) the measured 13 values of compounds with an additional spacer Z are the highest ever obtained for sandwich-type NLO chromophores; ii) the B values of the dinuclear sequifulvalene complexes surpass the 13 values of the mononuclear derivatives markedly; iii) the exchange of a monocationic electron accepting group (Cr(CO)3), with a dicationic one (RuCp) enhances 13 considerably, iv) the use of the (cyclobutadiene)(cyclopentadienyl)Co unit reveals a surprisingly large B value although this compound is mononuclear. The large experimental 13 values are in part assigned to resonance enhancement.

We have designed a novel type of NLO-phores based on donor/acceptor (D/A)-substituted peri-arylated naphthalenes 2 with improved efficiency-transparency trade off. In contrast to the conventional one-dimensional conjugated (iD-CT) systems 1 our concept prevents a direct conjugation between the D/A-substituents instead it is based on multidimensional polarizations (e.g. 3D-CT) utilizing through-space interactions. The comparison of iD-CT systems with isomeric compounds of the nonconjugated 3D-CT type reveals much better f3- values for the latter ones. This could be attributed to more than one electronic transitions in the long wavelength region. As a result these chromophores are not limited by the two-level model.

This paper summarizes current research to develop photorefractive polymeric materials with improved speed, material stability, and high beam coupling gain. The recent demonstration of significantly improved two-beam coupling marks the entry into a gain regime which enables the observation of new effects for the first time, such as beam fanning and self-pumped phase-conjugation. These effects have previously been observed only in a few thick high gain inorganic photorefractive crystals. The large beam coupling forces the reinterpretation of such traditional characterization techniques such as the grating translation method for the determination of the spatial phase of the index grating. Our subsequent material study focuses on several compositional variations to investigate the effect on the photorefractive performance of varying the chromophore and charge transporting polymer.

A Mach-Zehnder interferometry is employed to measure the Pockels coefficients in a poled polymer thin film in both the coplanar and the parallel-plate electrode structures. The modulated intensity ofthe Mach-Zehnder interferometer is investigated as a function of the optical bias in the reference arm, the modulation voltage applied to the film, the polarization angle of the incident light, and the angle of incidence on the film, as a complete analysis of the optical characterization of an electro-optic polymer film. The Mach-Zehnder interferometry measurement of the Pockels coefficients has an advantage over the single-beam polarization interferometry in permitting the independent determination of the Pockels tensor components, r13 and r33.

Photocarrier generation properties and an electro-optic response were studied in acceptor-substituted carbazole materials. Two kinds of carbazole systems were investigated: molecularly doped polymer and main-chain polymer. The photoconductive sensitivity of 1.2x1012(S cm1)I(W cm2) was obtained with a field strength of 5x1O5V/cm at a wavelength of 534nm. The electro-optic coefficient 33 of l.lpmJV was obtained with a field strength of 5x1O5V/cm at a wavelength of 532nm. These results show the monolithic photorefractive polymer could be developed by using acceptor-substituted carbazoles.

The optical limiting properties of 17 organic molecular crystals (natural faces) have been characterized with 250-ps pulses at &=532 nm. The best nonlinear absorption coefficients are f3=120 cm/OW and f3=400 cm/GW. Such high nonlinear absorptions are explained by the efficient ciystal packing of molecular two-photon absorptions. The ciystal phase allows high molecule concentration, efficient molecule orientation, and enhanced local field factor. Using the frequency-dependent molecular hyperpolarizability and the oriented-gas model one predicts that these high nonlinearities cover the whole visible spectrum.

Loss measurements for dye-doped polymer optical fibers are reported. These losses can contribute to the degradation of information in communications systems and limit the effectiveness of optical modulators built from nonlinear polymer optical fibers. Therefore it is essential that loss mechanisms be studied to determine which dyes can be used successfully. rThansverse scattering and successive-cut-and-measure techniques are compared. The benefit of the transverse loss measurement is that it is non-destructive while the successive-cut-and-measure method is simple albeit time consuming. Along with a characterization of linear absorption, the transverse loss technique provides a way to quantitatively describe the inhomogeneities in these fibers by providing the means for calculating the correlation length (to be described in the text) of the fiber that is measured.

We report a method for determining the refractive index profile of polymer optical fiber preforms by direct beam deflection measurements. The method is simple to use, compact, and has good resolution. The profile is obtained from the deflection data by numerically integrating the differential ray equation for a radial refractive index gradient. Refractive index profiles of both graded-index (GRIN) and step-index fiber preforms are reported.

All-optical poling is based on the excitation of nonlinear molecules using dual-frequency beams. The process is optimized at a molecular level when the molecules are resonantly excited. From the point of view of frequency conversion applications, this raises the question of device transparency to the frequency converted output. In order to treat this efficiency transparency tradeoff, we recently developed a simple model accounting for all the observable parameters. The model permits a good prediction of the poling dynamics. Its self-consistency permits the description of all the known all-optical poling schemes. In particular, analysis of the parameters of the model permits the identification of new strategies towards the realization of stable and transparent phase-matched materials for frequency conversion. Some preliminary experimental results are presented in this respect.

We present a novel nondestructive experimental technique for the determination of the lateral distribution of the polar order in second order nonlinear optical (NLO) thin films. The sample, which consists of a poled polymer film, is scanned through the focus of an infrared laser beam in a second harmonic generation (SHG) setup and the second harmonic intensity is monitored stepwise. In combination with a conventional electrooptic (EO) characterization it is possible to create an EO-coefficient map of the sample. The resolution of this mapping technique can be significantly increased by using high numerical aperture (NA) microscope optics for the illumination of the poled polymer. This method, for instance, allows the evaluation ofpoling inhomogeneities due to high field poling and field distortions at the edges ofpoling electrodes.

We use high temperature liquid-contact poling as a method to pole efficiently cladded nonlinear optical polymer films. Poling voltage as high as 400 volts is applied to planar waveguides which have a nonlinear optical film of 1.2 im thick. The lack of a quick method to characterize the poled cladded nonlinear optical films inspires us to devise a new electrooptic measurement method. This method can determine r33 and r13 separately because it uses light of single polarization state to probe the nonlinear optical film. The interference between the modulated light and the unmodulated light in the reflected beam is used to extract electro-optic coefficients. Theoretical analysis of the relationship between the reflected light intensity and the electro-optically modulated signal is consistent with the experimental results. Formulae to calculate electro-optic coefficients are deduced. This method uses an even simpler experimental setup than that of the widely used ellipsometric method.

We show that the chemical and physical environment used to synthesise and process poly(p-phenylenevinylene) (PPV) by the soluble precursor route in the matrix of poly(N-vinylpyrrolidone) (PVP) has a remarkable effect on the effective conjugation length of the resulting polymer, which directly affects the third order nonlinearity. A peculiar bathochromic shift of the position of the PPV absorption maximum, an increase of absorption coefficients and the anisotropy of refractive indices upon conversion have been observed. A large third-order optical nonlinearity (In2I>lO cm2/W) at 800 nm can be achieved in the composites. The nonlinearity of the composites depends on the PPV content and the conversion conditions. The PPV-PVP system is interesting from the point of view of its good waveguiding properties in the thin film planar waveguide structures.

Enhancement of the nonresonant second order molecular hyperpolarizabilities y were observed in stacked macrocyclic molecular systems, previously in a t-oxo silicon phthalocyanme (SiPcO) monomer, dimer and trimer series, and now in bacteriochiorophyll a (BChla) arrays oflight harvesting (LH) proteins. Compared to monomeric BChla in a tetrahydrofuran (TifF) solution, the <γ> for each macrocycle was enhanced in naturally occurring stacked macrocyclic molecular systems in the bacterial photosynthetic LH proteins where BChla 's are arranged in tilted face-to-face arrays. In addition, the γ enhancement is more significant in B875 of LH1 than in B850 in LH2. Theoretical modeling of the nonresonant γ enhancement using simplified molecular orbitals for model SiPcO indicated that the energy level of the two photon state is crucial to the γ enhancement when a two photon process is involved. Additionally, charge transfer between the monomers may be important if this produces states which are close to one-photon resonance. The calculated results can be extended to γ enhancement in B875 and B850 arrays, suggesting that BChla in B875 are more strongly coupled than in B850. In addition, a 50-160 fold increase in <γ> for the S1 excited state ofrelative to S0 ofbacteriochlorophyll in vivo was observed which provides an alternative method for probing excited state dynamics and a potential application for molecular switching.

The two dimensional π-conjugated electron system in phthalocyanines allows the tailoring of chemical and physical properties over a very wide range of structural modifications by incorporating many different metal atoms into the ring and by substituting various functional groups at peripheral sites. In this study, a series of I ,4 ,8, 1 1 ,15,18,22 ,25-octa- butoxy/decyloxy metallophthalocyanines containing various central metal atoms such as zinc, copper, palladium, cobolt and nickel were synthesized. They have significantly altered properties compared to the unsubstituted phthalocyanines. Spin-coated films of these materials were fabricated and characterized using third harmonic generation process at the wavelength of 1907 nm. The preliminary results of the effects of metal atoms and peripheral substituents on the third order nonlinearity are presented.

Polymeric materials made of polyalkynes [-CC-] have long been known to exhibit delocalized t-e1ectron systems. However, these materials normally are insoluble and hence of little practical use as nonlinear optical materials. We have prepared a series of polyalkynes which incorporate side chains, typically attached to a metal atom such as Pt. causing the polymer to be readily soluble. The rigid-rod geometry of ordinary polyalkynes is preserved in this arrangement due to the square planar coordination of the metal atom. The thermal stability (decomposition temperature and Tv,) measurements and the third-order nonlinear susceptibilities determined at 1064 nm are presented.

A first molecular, morphological and thermo-optical characterisation of a new type of Polymer Dispersed Liquid Crystal (PDLC) is presented. The chemical and structural properties are investigated by transmission FT4R spectra. The photopolymerisation process is analysed with optical monitoring, in order to obtain the best curing conditions. An experimental study on the transmithvity as a function of the temperature shows a seff-transparency effect, which occurs near the phase transition. An interpretation of the experimental results in tenns of a nonlinearity of thermal origin is given, using the theory of nonlinear scattering in heterogeneous media. The temperature switch, obtained in such a condition, suggests the possibility of employing such a material to realise a new type of all-optical temperature sensors and open the horizon to a new class ofthermo-optical devices.

The origin of the large third-order nonresonant nonlinear optical response of conjugated polymers is explored using single configuration-interaction (S-Cl) theory to solve the Pariser-Parr-Pople (PPP) Hamiltonian of polyacetylene. Both the strength of electron-electron interactions and the degree of bond alternation are treated as free parameters. For a fixed bond alternation, increasing the strength of electron-electron interactions from zero to 150% of that typically used in PPP theory decreases the hyperpolarizability by nearly two orders of magnitude. However, this decrease arises primarily from an increase in the optical gap. When the bond alternation is adjusted such that the optical gap remains fixed, increasing the strength of electron-electron interactions changes the hyperpolarizability by less than 30%. For a wide range of parameters, there is good agreement between Hückel theory and PPP theory, provided the Hückel gap is parameterized to the PPP optical gap. In PPP theory, the hyperpolarizability is nearly proportional to the optical gap raised to the -5.5 power, as compared to the -6 power for Hückel theory. These results suggest that electron-electron interactions need not be explicitly included in a model of the nonresonant response of polymers, but can instead be absorbed into the effective parameters of an independent electron model.

An advanced versatile low-cost polymeric waveguide technology has been developed for optoelectronic applications. This technology is based upon new polymeric materials for ultra-low-loss optical interconnection, particularly for the key wavelengths of 0.83, 1 .3, and 1 .55 microns. Development of these materials has required a thorough understanding of fundamental principles of optical absorption due to both vibrational and electronic resonant absorptions. We have thus created materials with measured losses at 830 nm which are in the range ofO.02 dB/cm. At longer wavelengths, the losses can be higher due to the vibrational absorption within the polymer. However through careful selection of chemical structure, polymeric materials with intrinsic loss below 0.08 dB/cm have been demonstrated at 1 .55 micron wavelength. These high-performance organic polymers can be readily made into both multimode and single-mode optical waveguide structures with controlled numerical aperture (NA) and geometry. We will discuss the use of these materials in a variety of passive photonic devices.

We are developing a traveling electro-optic modulator for analog microwave modulated fiber optic links used in radar applications. The modulator is a polymer in-line fiber device that has a rugged and low loss interface to single mode fibers and can be engineered to provide linear modulation over a large dynamic range. In the development of the modulator we take advantage of a variety properties available in polymers. The ability to deposit a conformal electro-optic thin film is used to fill the gaps between high-frequency co-planar electrodes and thus obtain a good overlap between the electric field produced by the the microwave electrodes and the fields in the optical fiber and the electro-optic waveguide. Reactive ion etching of the electro-optic polymer is used to trim the thickness of the polymer waveguide to obtain operation at a specific wavelength. The thermo-optic effect is used to fine tune the operation point of the modulator to obtain a large dynamic range. The geometry of the modulator permits operation close to the absorption peak of the electro-optic polymer and this provides the opportunity to take advantage of the resonant enhancement of the nonlinearity in the vicinity of an absorption band.

We have observed light coupling in dual-core dye-doped polymer optical fibers. The experimentally observed coupling length is consistent with the standard coupled-mode theory for single-mode--singlemode coupling between step-index waveguides [1. We have also observed intensity-dependent coupling in a fixed-length dual-core fiber.

We report on single optical beam splitting into several beams (the optical branching effect) in a single mode slab waveguide made from poly(methyl methacrylate) (PMMA) doped with dye 4-(Dicyanomethylene)-2-methyl-6-(pdimethylaminostyryl) 4H-pyran known as DCM. The effect is associated with permanent refractive index decrease accompanying upconverted dye photobleaching. Unlike the defocusing Kerr effect, the refractive index response to the optical field is nonlocal in time for the index depends on the absorbed energy instead ofthe instant light intensity. The effect therefore takes place at much lower power then nonlinear propagation effects in Kerr media (less than 1 kW/cm2 ).The proposed model of branching uses soliton-like solutions of Shrodinger-type nolmear propagation equation complemented by the rate equation for the refractive index change. Computer simulations based on the model demonstrate all the effects observed experimentally such as beam splitting into two primary side branches followed by their collapse into multiple secondary branches.

Electrooptic Mach-Zehnder and straight channel modulators integrated with high speed microstrip line electrodes have been fabricated with a double-end crosslinked polymer containing amino-sulfone azobenzene chromophores. The poled double-end crosslinked polymer films exhibited not only a long-term molecular alignment stability at 100°C but also good optical power handling capability at 1 .3 jtm wavelengths. The optical power handling capability of these modulators was tested at 250 mW input optical power and the output was more than 40 mW. The optical power level was compatible with commercial analog transmitters. At a CW peak intensity near 106 W/cm2 inside the waveguide, the double-end crosslinked polymer waveguide modulators exhibited neither observable increase in optical loss nor degradation of nonlinearity during our experiment for over 120 hours. The photochemical stability at 633 nm and 543nm wavelengths was also monitored to simulate the exposure from second-harmonic light of an intense infrared fundamental beam. A stable electrooptic response was observed under 633 nm laser exposure.

We present a novel post-fabrication laser trimming technique to adjust the power splitting ratio of strip waveguide Y-branches made in thermally crosslinked electro-optic polymers. The trimming is based on the irreversible index change due to photobleaching. Our method uses simple equipment and the process takes only a few seconds. Waveguides made by both reactive ion etching and photobleaching are trimmable. An adjustable range of the splitting ratio as wide as is achieved with less than 0.2 dB of excess loss. This in situ trimming technique is effective for both the TE and TM modes of the waveguide and is very suitable for automated device processing.

We have used second-harmonic generation as a technique to examine different types of chiral materials. All materials investigated showed large circular-difference effects in the second-harmonic response, i.e., the efficiency of the process was different for left- and right-hand circularly-polarized light. Study of various chiral materials has shown that a different quantitative and qualitative response can be expected depending on the composition and structure. In addition, we have demonstrated the existence of magnetic-dipole coniributions to the nonlinearity of those materials.

In the course of our research on excited state enhancements of nonlinear optical processes in it conjugated structures1, we discovered new linear electro-optic (EO) effects2 in chiral systems. It has long been known that because of symmetry conditions, the macroscopic susceptibility x7(—W;O, (0) for the linear EO effect as in the case of second harmonic generation vanishes in an isotropic chiral system3'4. We found, however, that if the electronic excited state of the chiral system is optically pumped, then permutation symmetry is broken and the macroscopic linear EO susceptibility is non-zero, given by x(—co;O,w) =EaiyX2 where is an antisymmetric third rank unit tensor. We have further observed that the new EO effect depends on the molecular spatial dimensionality of the chiral system and is non-zero only for three-dimensional structures. This dependence is important to molecular design rules for candidate it-conjugated structures. We have studied and designed several classes of molecular structures possessing the required three dimensional noncentrosymmetry. Included, for example, are donor-acceptor substituted chiral cyclophanes having nonvanishing transition moments in three dimensions. There are several additional advantages associated with our new chiral EO effect. Among these is the fact that no electric filed poling is necessary to achieve the noncentrosymmetry required for observing the linear EO effect. Moreover, the new EO property avoids the long term decay of the EO coefficient commonly observed in poled EO materials. The new EO coefficient can remain thermally stable up to temperatures approaching the material decomposition temperature. Our studies also show that simple structural alignment of otherwise isotropic chiral centers leads to further enhancement of the linear £0 coefficients5. Additionally, our findings are general, extending directly to other second order optical processes such as second harmonic generation (o + (i)= 2o) and parametric rectification (o — o =0). In the present report, we will center our discussion on basic theory of the new chiral EO effect in section II. A many-electron sum-over-states calculation of the microscopic EO susceptibility for a model chiral donor-acceptor cyclophane is presented in section ifi as an illustration of both what a new class of EO chiral organic structures might look like and how to begin formulating design rules for new optimized structures.

We demonstrate, for the first time, that a new ionically self-assembled monolayer (ISAM) technique for thin film deposition can be employed to fabricate materials possessing the noncentrosymmetry that is requisite for a second order, nonlinear optical response. Using two different commercially-available polyelectrolytes, we have produced ISAM nonlinear optical thin films with values comparable to that of quartz. As a result of the ionic attraction between successive layers, the ISAM films self-assemble into a noncentrosymmetric structure that has exhibited no measureable decay of at room temperature over a period of more than four months. The x2 of ISAM films has been examined by second harmonic generation using a fundamental wavelength of 1200 nm. The second harmonic intensity of the films exhibits the expected quadratic dependences on fundamental intensity and film thickness while the polarization dependence is consistent with orientation of the chromophore dipole moment perpendicular to the substrate.