A general enhancement mechanism is discussed for nonlinear optical processes originating from real population of electronic excited states. Enhancement of greater than two orders of magnitude of nonresonant third harmonic generation from an optically pumped s1 electronic excited state is experimentally observed in a conjugated disc-like metallophthalocyanine.

The field of electro-optic (EO) polymer materials for integrated optics has been developing rapidly during the past several years. Recent advances include the formulation of poled crosslinked epoxies and guest-host polyimides exhibiting thermal stability at temperatures significantly higher than that previously achieved with thermoplastic acrylate chemistry. These developments are an essential first step toward achieving practical materials exhibiting stability to manufacture, assembly, and end-use in modern electronic systems applications.

Despite these developments, there is still much basic research to be performed to achieve a practical EO polymer material system. A polymer material system for integrated optics consists of a set of three compatible materials: cladding polymers, passive core polymers, and active core polymers. The three materials should ideally be derived from the same base polymer so that the layers of the entire waveguide structure have nearly identical thermal, mechanical, chemical, electrical, and optical properties. In an ideal syst', the active core material would consist of passive materials highly loaded with aligned nonlinear optical moieties exhibiting large hyperpolarizabilities and suitable size and shape for thermal stability.

This paper provides a detailed review of the material requirements for EO polymer waveguide devices in electronic systems and points to new areas of research required for practical systems.

We discuss our rational approach to incorporate optically nonlinear molecules into polymeric and cross-linked materials through the use of isocyanate-hydroxy coupling chemistry. Thin film fabrication, optical loss, poling, second harmonic generation, and electro-optic properties are discussed.

In this study, we report the synthesis and characterization of a new, systematically designed class of second-order NLO polymers in which the appended chromophoric substituent is modified (hydroxylated) to allow direct chromophore involvement in hydrogen-bonding and crosslinking. Thus, the glassy polymer poly(p-hydroxystyrene) has been functionalized to varying levels with the N-(3-hydroxy-4-nitrophenyl)-(S)-prolinoxy (HNPP) high-β chromophore. The time-dependent second harmonic generation characteristics of thin films of this material have been studied at λ = 1.064 μm as a function of chromophore functionalization level, hydrogen-bonding capacity, and thermal crosslinking. These materials are found to possess high d33 values (up to 34 × 10−9 esu) and long characteristic SHG decay times (longer than a year). Analysis of the decay of the second harmonic signal shows that increasing hydrogen-bonding density substantially increases relaxation times; it is further shown that introduction of crosslinks is yet more effective in imparting SHG temporal stability to these polymeric nonlinear optical materials.

Sub-Ts relaxations in polymethylmethacrylate (PMMA) and in narrow molecular weight polystyrene (PS) thin films with and without second order nonlinear optical dyes have been examined using time domain dielectric spectrometry. Dyes used include 4-[N, N-dimethylamino]-4’-nitrostilbene (DANS) and 4-[N-ethyl, N-hydroxyethylaminoJ-4’-nitroazobenzene (DR1) at a level of 0.19 mole percent. In dye/PS blends, dye relaxations are seen to occur at frequencies similar to the frequencies of the β-relaxation of pure PS below the glass transition temperature. Analysis of the glassy phase dispersion leads us to conclude that, at near room temperature, appreciable dye reorientation occurs on a time scale of a few seconds. This dye reorientation is coupled to the dynamic motions of the localized polystyrene subunits. Experiments using dye in PMMA did not conclusively reveal dye motions because of the intrinsically large dispersion of PMMA itself. The β-relaxation frequency range is higher in the PMMA dye/polymer blend than in dye-containing PS implying dye reorientation would likewise occur more rapidly than in PS. Our results indicate that the randomization of dye orientation observed in a dye/polymer system depends on the rapid, local chain motions which continue at temperatures well below the glass transition temperature, as well as on the long range, slow relaxations associated with the glass-to-rubber transition.

Using polyimide as host in a guest-host electro-optic (EO) thin film a thermally stable poled electro-optic response is demonstrated at temperatures at 150 °C and 300 °C. Electric field poling during curing process including imidization (170 -230 °C) and densification (340 -380 °C) accounts for the highly thermally stable EO response. As a room temperature curing process, chemical imidization is employed as a novel curing process. Dehydration occurring through imidization of the polyamic acid is completed chemically after poling rather than thermally during poling. After thermal aging at 155 °C (above the poling temperature) chemically imidized samples retain over 30% of their original poling induced EO signal while similarly poled samples, which have not been chemically imidized, produce a null EO response. For a class of polyimides possessing aliphatic structure, the thermoplastic behavior of cured guest-host polyimide system allowed the electric field poling at temperatures above the glass transition temperature (Tg). One example of polyimide doped with 10% nonlinear optical molecules exhibits Tg near 200 °C, leading to an excellent thermal stability of the poled EO response with a depoling knee-temperature of 150°C.

New classes of nonlinear optical molecules were synthesized and characterized. It is shown experimentally that molecules with heteroaromatic π-electron systems exhibit exceptional second order nonlinear optical response and good solubility. Measurement of the nonlinear optical properties of molecules containing heteroaromatic moieties with different length scale reveals a power law dependence of Bμ on the number of π-electrons. Also, the effects of two nitro groups as electron acceptors on the aromatic ring are discussed.

Our approach consists of improving the molecular architecture of π-electron delocalization and orientations of the polymeric backbone of materials for cubic nonlinear optics. First, the syntheses of new polymers such as substituted polyacetylenes or polyphenyleneynylenes were done, in which the stiff conjugated chain and the flexible side-chains are well adapted to the orientation of the backbone. They have been studied for their third order susceptibility and their ability to give processable optical films. We have found liquid crystal properties with the rigid rod derivative of the poly(ethynylarylene). Unlike previous poly(ethynylarylene), we obtained a high molecular weight: , which allows films of good optical and mechanical properties. Second, in order to increase the third order response, we focused our attention on the orientational order of polydiacetylenes, which we obtained by epitaxial growth of the monomer on a single crystal substrate. From the proposed epitaxial model a possible extension of highly oriented thin films can be applied to a large class of materials.

Approximations are applied to the three-level perturbation theory expressions for the third order optical polarizability. We conclude there will be three competing dominant terms, γ ≈ γc + γn + γtp each of which can be optimized at different molecular electronic structures. The two-photon term, γtp, has been commonly ascribed as the most dominant one for γ. However, the other two terms, which optimize at different structures, potentially offer equivalent or larger overall γ. The term, γn, will be optimized at structures with large second order polarizability β, while the term, γC, optimizes at certain centrosymmetric free electron structures with large first order polarization, α Optimization by γc is the preferred route, since this leads to elimination of the canceling terms which limit optimization by γtp or γn. Recent EFISH results have confirmed our earlier speculations that the free electron squarylium dye structures are particularly good for optimization by way of γc. Strategies are presented to selectively optimize to either of the three structure types and to obtain molecules with even larger γ.

Multiphoton excitations of conjugated polymers are related to their optical gap, Eg, which serves as an internal standard. In π-electron models with electron-hole symmetry, the alternation gap Ea is the lowest-energy two-photon absorption (TPA) and increases with the alternation δ of the transfer integrals t(l ± δ) along the backbone. Increasing electron-electron (e-e) correlations shift an intense TPA from 2Eg in the band limit to Eg in the atomic limit. Two-photon spectra and multiphoton resonances in polyenes, polydiacetylenes, and polysilanes are related to interacting electrons in Pariser-Parr-Pople(PPP) models with increasing alternation and either carbon or silicon e-e interactions. Some general features of correlated states of conjugated polymers are discussed in the rigid-lattice limit.

Hartree-Fock ab initio calculations have been used to examine the magnitude of non-polar intermolecular interactions on polarizabilities and second hyperpolarizabilities. In the present case two ethylene molecules were examined in a cofacial interaction. Basis set requirements and correlation effects were considered in deriving the intermolecular potential. Two basis sets were considered for the intermolecular interactions at the Hartree-Fock level, while one set of calculations was performed with corrections for correlation. It is found that intermolecular contacts at the van der Waals distance has little effect on the molecular second hyperpolarizability, but it is not clear what the effect would be for longer oligomers.

The nonlinear refractive index, linear and nonlinear absorption coefficients of a number of nickel dithiolene-doped PMMA polymers have been measured. Nonlinear refractive indices up to about 10−8 cm−2/KW were observed, with figures of merit defined by δnsat/αλ and Reχ(3) / Im χ(3) within acceptable device limits.

The third-order nonlinear optical susceptibilities χ(3)1111(-3ω; ω, ω, ω) of macrocyclic compounds such as phthalocyanines and porphyrins are determined for the vacuum deposited and spin coated thin films by optical third-harmonic generation measurements at a wavelength of 1907 nm. The enhancements of the χ(3) values are discussed in terms of the central metal substitution, molecular stacking, extended ring size and number of χ-electrons. We firstly describe the results of extended porphyrin systems with 26 delocalized π-electrons.

Epoxy based material incorporating 4-(4'-nitrophenylazo) phenylamine has been synthesized and subsequently functionalized with cinnamoyl groups. These materials can be easily solution-processed into thin films by spin-coating. The uncrosslinked polymer has excellent thermal and linear optical properties. Glass transition temperature of 115°C and degradation temperature (air) of 278°C were recorded.

This material exhibits large second order nonlinear optical (NLO) effects subsequent to poling. In a poled film sample, second harmonic generation coefficient, d33, of 34 pm/V was observed at 1.06 μm. Crosslinking is brought about by exposure to UV radiation at 254 nm wavelength. In a poled and crosslinked film sample, a d33 of 22 pm/V was observed. This d33 value is less than the d33 value of the poled and uncrosslinked film sample due to photo-induced cis-trans isomerization and partial degradation of the NLO chromophores. Nevertheless, the poled and crosslinked film shows a much more stable second order response at ambient conditions compared to the poled film which has not been crosslinked.

NLO-active polyurethanes were developed and optically characterized. We synthesized polyurethanes possessing large hyperpolarlzabilities, which are covalently attached or crosslinked to the polymer backbone. At ambient temperature the d33 values for poled polyurethanes were measured shortly after poling and 6 months later. The second-order nonllnearlty of the poled, crosslinked polyurethane is extremely stable decreasing slightly over 6 months. Also, using a polyurethane bearing a tricyanovinyl moiety as the host, we developed a nonlinear optical silver nanoparticle composite system. A third-order susceptibility was measured for the composite and was determined to be larger than that from the sum of its components. A chemical enhancement mechanism is speculated to be contributing to the large optical nonlinearities.

Droplets of l, 6-di(N-carbazolyl)-2, 4 hexadiyne (DCHD) diacetylene were prepared by room temperature evaporation of dilute (0.01 wt %) solution of the monomer in chloroform on amorphous carbon-coated mica substrates. HREM and SAED examination revealed small crystallographically textured droplets (∼1 μm diameter) with cracks parallel to the [001] chain direction. The droplet geometry allowed us to investigate the structure of the polymer near edges both parallel and perpendicular to the chain axis. It was found that the curvature of the droplet edge caused a local bending of the polymer crystal lattice. The direct imaging of the molecular organization near the droplet surface allowed an investigation of the mechanisms of lattice bending via the formation of edge dislocations. An understanding of these defects is important in determining how they relate to the optical properties of poly(diacetylenes).

New poly(vinyl ethers) with pendant groups exhibiting nonlinear optical properties were synthesized: 4-cyano and 4-nitro-4'-(alkoxy) azobenzene substituted poly(vinyl ethyl ether), copolymers and 4-dicyano or 4-cyano-4-carbomethoxyvi-nyl-4'-(alkoxy)benzene substituted poly(vinyl ethyl ether). Films spincoated from these polymers were poled by a positive corona discharge just above their T9 (as measured by DSC). The second order susceptibilities of tne films were derived from full-angle Maker fringe patterns at 1064 nm. Susceptibilities x(2) zzz as high as 23×10−9 esu were measured.

Nonlinear optical materials which are highly transparent and show large second harmonic generation coefficients were fabricated via the sol-gel process. Nonlinear chromophores were directly incorporated into the silica network by polymerizing chromophore containing organo silanes such as N-[3-(Triethoxysilyl)propyl]-2, 4-dinitrophenylamine (TDP) and 3-(4-Formyl phenoxy)propyl trimethoxy silane (FPT).

All-optical switches and all-optical bistabili ties are realized by waveguide structures with vacuum evaporated polydiacetylene(PDA) films. The basic structure of the all-optical switches are prepared in the form of layered waveguide directional coupler with vacuum evaporated PDA top layer. Clearly switchings are observed at 1064nm of pulsed Nd:YAG laser. Furthermore ultra-high speed switching with Ti;Safire 135fs laser are recognized.

Operations of all-optical bistabilities are carried out by nonlinear coupling at grating structures in waveguides with PDA top layers. Hysterisis curves of bistability are affected by coupling parameters of gratings and parameters of waveguides.

The third order nonlinear optical properties of conjugated polymers have been considered promising since the 1970s when Sauteret et al reported large non-resonant values in PTS.[1] Although it is well-understood that the physical origin of the nonlinearities is due to the delocalization of the π-electrons, the details, and how best to calculate them have been the focus of a continuing theoretical dialogue. Until recently, experimental investigations of nonlinearities have been limited to only a few wavelengths. Now third harmonic generation (THG), which accesses only the electronic nonlinearities, can be performed over wide spectral ranges, for example from 500 to 2000 nm. The resulting third harmonic wavelength typically spans the electronic molecular transitions associated with the nonlinearities. By measuring the spectral distribution of both the amplitude and phase of the third harmonic signal, the dominant transitions (between the“essential states”) contributing to the nonlinearity can be identified. Such information is most useful for comparing with theories in which the oscillator strengths (transition dipole moments) for the various molecular transitions are calculated.