Semiconductor clusters with sizes of a few to hundreds of Å, covering the entire range from molecular to bulk, have been prepared within polymer films. For PbS, the band-gap shifts to higher energy as the cluster size decreases and eventually converges to the transition energy of the first excited state of PbS molecule. The observed dependence of band-gap on cluster size can be explained by a tightbinding model that takes into account the effects of band nonparabolicity. These semiconductor-doped polymers represent a new class of nonlinear optical materials. Here we present the nonlinear optical properties of polymer films doped with 50 A CdS and discuss the possible mechanisms.

Conducting polymers have been found to be rather good photoconductors in the sub-nansecond time regime. The TP measurement is an effective probe for determining carrier photogeneration quantum yield as well as for investigating transport mechanisms. Here, we present a short survey of our results for a few interesting polymeric systems. We start with the trans-(CH)x grown by the Shirakawa method. We then compared the TP results to those obtained from PDA-TS which currently is the only conjugated polymer in single crystal form. From this comparison, we are able to gauge the improvements in the material quality as was demonstrated by recent TP in trans-(CH)x grown by the Naarmann method

The promise of conducting polymers as fast response nonlinear optical materials has been recently emphasized [1,3]. Polymers such as polyacetylene, polythiophene and the soluble (and processible) poly(3-alkylthienylenes) contain a high density of x-electrons, and they are known to exhibit photoinduced absorption and photoinduced bleaching, indicating major shifts of oscillator strength upon photoexcitation [2,4]. For polyacetylene, these nonlinear effects have been studied in detail in the picosecond [5a,b] and sub-picosecond [5c] time regime and correlated with the photoproduction of charge carriers through fast photoconductivity measurements [6]. The data have demonstrated ultra-fast response with nonlinear shifts in oscillator strength occurring at times of the order of 10-13 seconds. These resonant nonlinear optical properties are intrinsic; they originate from the nonlinearity of the self-localized photoexcitations [7] which characterize this class of polymers: solitons, polarons and bipolarons [4].

We discuss two important aspects of the nonlinear optical response of polyacetylene, the prototype conjugated polymer. In the subband gap region, χ(3) of polyacetylene has a rich spectral response and its magnitude is found to be the largest among semiconductors. In the superband gap region, absorption of photons leads to direct photogeneration of charged solitons on a femtosecond-time-scale. These results are discussed in the context of a one-dimensional, coupled electronlattice system, and are generalized to other conjugated polymers.

We compute analytic expressions for the nonlinear optical response functions for the Peierls model of a conducting polymer by using the Keldysh non-equilibrium Green's function methods. The only parameter in the model is the gap, which can be determined independently from the linear optical response. Our results for 3rd harmonic generation are in excellent agreement with experiments on polyacetylene. This constitutes strong evidence that polyacetylene is well described by a model of weakly interacting quasi-particles. Further nonlinear optical effects, which should be experimentally observable, are predicted.

We present results of ab initio calculations on the electric response of oligomers representative of conjugated polymers of interest. We mainly focus on : (i) polyacetylenes, polyynes, polydiacetylenes, and polycumulenes; (ii) polyaromatic systems in both undoped and doped states; and (iii) polyaromatic vinylene compounds. Our aim is to find trends among related series of conjugated polymers in order to assess their potentialities for nonlinear optics applications and to help in designing materials with optimal characteristics.

A convenient method of measuring the nonlinear optical properties of molecular compounds is described. The method involves measuring the quadratic electro-optical coefficient of a polymer composite containing a variable concentration of the candidate NLO material. The X(3) (ω) value obtained by this low-frequency Kerr measurement, after local-field corrections, can be compared to the nonresonant third-order susceptibility measured by degenerate-four-wave-mixing technique on selective samples. We find that the choice of the polymer matrix dictates the contribution of second-order susceptibility to the Kerr coefficient. Therefore, our method can also be extended to the measurement of second-order susceptibility, analogous to the technique of field-induced second-harmonic-generation.

Processable polymeric nonlinear optical materials can be synthesized by functionalizing a glassy macromolecule with chromophores having large quadratic hyperpolarizabilities, followed by poling in an electric field. In the present case, the functionalization of polystyrene with 4- (4-nitrophenylaza)(N-ethyl)(2-hydroxyethyl))aniline, 4-(4-N,N-dimethylaminostyryl) pyridine, and N-(4-nitrophenyl)-L-prolinol is described. Particularly noteworthy is the high level of chromophore units that can be incorporated into transparent films of these materials, the high second harmonic coefficients that can be achieved (as high as d33 – 11 × 10-9 esu at 1064 nm), and the long-term temporal stability of the second harmonic generation capacity.

This paper presents some recent theoretical and experimental work carried out in the author's laboratory on the nonlinear optical properties of organic polymers. The ab initio SCF theory has been used with the finite field method to calculate the third order nonlinearity of conjugated structures in order to understand the effect of conjugation and the role of substituents. Experimental studies of third order resonant nonlinearlty in polythiophenes and cumylphenoxy phthalocyanine, the latter in the form of Langmuir-Blodgett films, are presented using femtosecond degenerate four wave mixing.

The promise of conjugated polymers as fast response nonlinear optical materials has been emphasized, since the nonresonant third order nonlinear susceptibilities of these π-electron polymers are among the largest measured in any material. For example, third harmonic generation (THG) experiments have shown that for trans-polyacetylene, χ║(3)(3ω:ωω,ω) = (4± 2) × 10-10 esu, where χ║(3) refers to that component of the third order susceptibility tensor with all indices parallel to the chain direction [1]. The large nonlinear response is anisotropic and arises from the π-electrons of the conjugated polymer. The magnitude and anisotropy are comparable to results obtained from single crystals of polydiacetylene-(toluenesulfonate). Given these large χ║(3) values, it is important to develop methods for fabricating chain-aligned (oriented) conjugated polymer films which have optical quality.

The synthesis of three types of functionalized benzazole structures is described. A centrosymmetric dihydroxy-benzobisthiazole has been synthesized for incorporation in flexible polymers having rigid, conjugated, third order nlo active units in the main chain. Benzazole structures terminated with an N,N-dimethylamino group and a carboxyl group have been synthesized for use as second order nlo pendants with flexible polymers. Benzazole structures terminated with an N,N-dimethylamino group and an amino group have been synthesized as precursors to second order nlo guest molecules.

A general scheme for the preparation of soluble electroactive polymers is reviewed with emphasis that polymers so prepared permit investigation of the effects of π-electron delocalization and lattice charge upon nonlinear optical activity. Particular attention is focused upon six membered ring polymer derivatized in 1,4 positions with vinylamine substituents. Such substituents influence rates of polymerization reactions and electronic properties as well as solubility. Preliminary measurement of χ(3) for as-synthesized polymers is effected by DFWM.

Polyaniline is a family of polymers whose electronic and optical properties can be controlled through variation of the number of electrons and protons on the polymer chain. For example, the emeraldine base form of the polymer (EB) is insulating while the emeraldine salt form (ES) is metallic. We present here results for photoinduced absorption spectroscopy of the emeraldine base form of polyaniline. Unlike earlier studied polymers, the spectrum of emeraldine base contains both subgap photoinduced bleaching (at 1.8 eV) and photoinduced absorption (at 0.9, 1.4, and 3.0 eV). The existence of the bleaching peak is consistent with a model of optically excited localized molecular excitons in the emeraldine base polymer. The energies of the three photoinduced absorptions peaks and an associated interband bleaching (at energies greater than ∼3.5 eV) are in agreement with the photogeneration of polaron pairs. Laser intensity studies support that the excitons decay monomolecularly while the polarons decay bimolecularly. Picosecond time-resolved spectroscopy of the exciton decay demonstrates the rapid response of this system.

Two pentaammine malonitrile ruthenium dyes that show sharp and intense absorption maxima near 800 nm have been synthesized for use as nonlinear optical (NLO) dyes. Preliminary powder measurements of crystals of these dyes for second harmonic generation (SHG) are reported for incident 1064 nm light. The anomalous dispersion due to the location of the one-photon absorption maximum has interesting consequences for phase matching, and calculations for inherently phase-matched single-mode waveguides are presented.

We have reported previously that metal poly-ynes have large thirdorder optical susceptibilities. Based on the four-wave mixing studies reported here, we conclude that strong two-photon absorptions in metal poly-ynes contribute significantly to the observed third-order nonlinearities of these materials. Hyperpolarizability/r.u. increases only slightly with chain length for tetrahydrofuran solutions of the metal poly-ynes. The possible influence of conformers on this behavior is discussed.

A series of chloronitroaniline compounds were investigated as bulk crystals and in thin film form as potential nonlinear optical materials for use in electro-optic devices. Crystal structure determination and computational modelling were completed on 5- chloro-2-nitroaniline and electro-optic measurements were performed on crystals grown by physical vapor deposition. Second harmonic generation measurements were carried out on poled guest/host systems of 3-chloro-4-nitroaniline in poly(methylmethacrylate).

Molecular doping of Poly(oxyethylene) by p-nitroaniline, followed by a crystallization under electric field, yields a material which exhibit strong second harmonic generation intensity such as 91 times that of urea.

This strong activity arise from the new crystals possessing a noncentrosymmetric structure formed by the electric field crystallization.

This paper describes a new organic nonlinear material, piperoniridene ethyl cyano acetate which shows short cut-off wavelength, good nonlinearity (better than POM in powder) and good crystallinity. These features of this compound encourage its application in second harmonic generation (SHG) coupled with semiconductor laser.

Novel polymers are synthesized by attaching amphiphilic dye groups along a hydrophilic polyether backbone. These polymers are fabricated into noncentrosymmetric films by the Langmuir-Blodgett (L/B) technique. Stacks of molecular bi-layers are assembled by γ-type deposition, interleaving two different dye-substituted polymers. One polymer has the dye's molecular dipole pointing toward the backbone, while in the other polymer, the dye points in the opposite direction. Light from a Nd:YAG laser is passed through these films and the second harmonic is detected.

Second harmonic generation was observed from composites consisting of 3-nitroaniline or 3-methyl-4-nitroaniline grown within a polymer matrix. In the amorphous polymers, polystyrene and poly(methyl methacrylate), the crystals formed as fine grains or needles. The intensity of the second harmonic, from Nd-YAG light at 1.06 μm, was measured as a function of the composition from 10–90 wt% of nitroaniline. The angular distribution of the light output was very dependent on the morphology of the precipitate, with a strong concentration of the output in the forward direction for samples containing locally aligned, needle-like crystals. The behaviour in SHG and in light scattering was compared.

Composites with crystalline polymers were prepared as unoriented or oriented films. Nitroanilines and potassiun dihydrogen orthophosphate were crystallised from a number of polymers. The composite structure is governed particularly by the extent of liquid phase miscibility. The potential for optical applications of these materials are discussed.