In this paper, we will briefly review the present status of work on all-optical photonic devices, and discuss the properties of nonlinear optical materials that control and limit device performance. We conclude with some projections for future progress in this field.

The nonlinear optical properties of organics with unsaturated bonds are compared with those of inorganics including semiconductors and dielectrics. Experimental results of ultrafast nonlinear response of conjugated polymers, especially polydiacetylenes, are described and a model is proposed to explain the relaxation processes of photoexcitations in the conjugated polymers. Applying the model constructed on the basis of the extensive experimental study, we propose polymers to obtain ultrafast resonant optical nonlinearity. This paper also reports the first observation of the excitonic superradiance from CdS micro crystallites. We developed a new method to prepare CdS microcrystallites embedded in polymer films, which shows enhanced band-edge emission due to the direct recombination of an exciton in the microcrystallite.

We report the (chi) ⁽³)(-3(omega) ;(omega) ,(omega) ,(omega) ) of a thin film of polycrystalline C₆₀ on glass measured by third harmonic generation (THG) over the fundamental energy range of 0.52 - 1.18 eV. The dispersion clearly shows a three-photon resonance at $HBAR(omega) equals 0.94 eV, whose third harmonic energy corresponds to the dipole allowed transition at 2.8 eV in the solid C₆₀. We use perturbation theory to model the linear absorption spectrum and the nonlinear spectrum, and achieve consistent values for the fitting parameters.

The nonlinear optical response (NLO) of linear polyenes and fullerenes is studied using an anharmonic oscillator model. The influence of size effects and quantum exciton confinement on the nonlinear optical response is examined, and the time evolution of the excitons is studied. The elementary optical excitations for C₆₀ are shown to be correlated charge density fluctuations. For linear polyenes with finite size, oscillations in the time-resolved degenerated four wave mixing (D4WM) signal due to the coherent excitation of several band states are found within the Huckel model (no Coulomb interaction). For infinite size a photon echo like behavior is obtained. The exciton-exciton interaction changes the lineshape of the D4WM signal and gives rise to a broad delayed signal whose position is given by the dephasing time of the system.

By using the second harmonic generation of the nanosecond and picosecond laser systems, we have studied the degenerate four-wave mixing from the C₆₀ and C₇₀ molecules. In nanosecond regime, the thermal effect contributes greatly to the measured signal, but in picosecond case, the thermal process and the electronic process which both contribute to the signal could be completely temporally separated. From time-resolved four-wave mixing measurement, the third-order optical susceptibility of C₆₀ was estimated to be (3.6 + 1.4)*10^-10 esu at 532 nm which is in agreement with recent measurement results.113b

It is possible to define figures of merit which a nonlinear material must satisfy in order for it to be useful for all-optical switching in waveguides (as well as other nonlinear devices). Here we first discuss the physical basis for these figures of merit. Examples of materials in which these figures of merit both are and are not satisfied will be given. Finally the evolution of nonlinear directional couplers will be reviewed and the impact of figures of merit identified.

This paper presents a study of the nonlinear waves and bistabilities in multilayer waveguide systems and all-optical switch with self-focusing media. In terms of Jacobian elliptic functions, the dispersion relations for TE modes in the nonlinear multilayer waveguide are derived analytically. The analyses show that the bistabilities can be obtained with power thresholds in 4 and 5 layer waveguide structures. The characteristics of the nonlinear waves and power- dependent field profiles in the multilayer waveguide systems are illustrated by numerical calculations and possible applications to all-optical devices are also discussed. Furthermore, with extending these knowledges to the analyses of the Y-branch adiabatic all-optical switch, the device nonreciprocal properties are fully investigated as well as demonstrated by simulations of the finite difference beam propagation method (FD-BPM).

We demonstrate that the Maker fringe technique can be applied to third order frequency mixing experiments designed to measure the electronic (chi) ⁽³)(-(omega) ₃;(omega) ₁,(omega) ₁,-(omega) ₂) dispersion with (omega) ₁ for a poly[3-tetradecylthiophene] thin film. Both the magnitude and the phase of the (chi) ⁽³) were obtained. In the magnitude spectrum there was a strong three-photon resonance where the output frequency ((omega) ₃equals2(omega) ₁-(omega) ₂) corresponds to the energy level of the first one photon state. We also found a peak which could be either a 2(omega) ₁ or a (Delta) (omega) (equals(omega) ₁-(omega) ₂) resonance. Since this method is considered to be more sensitive to two-photon resonances that third harmonic generation, it can be used as a powerful tool to probe two photon states in the case of thin film polymer samples.

We describe our recent studies of metal-insulator nanocluster composites made by ion implantation in such substrates as glass and sapphire. The metal clusters have diameters ranging from 3 to 30 nm. The composites exhibit an electronic nonlinear optical response which is fast on the picosecond time scale. In addition to their evident possibilities for technological application, these materials also offer an unprecedented way of studying unusual properties of composite materials, such as the quantum confinement of conduction-band electrons and the transverse relaxation time T₂ as a function of cluster size.

Sensitive and easy in operation met hod based on nonlinear polarizat ion transformation at two-beam paramet ric interact ion in macroscopically isot ropic media is analyzed theoretically and demonst rated experimentally for characterizat ion of response and relaxation of the third-order nonlinearity in biopolymer materials like bacteriorhodopsin. The method is shown to be an effective tool for obtaining rat her complete information about nonlinear parameters of biopolymers: mnit ude and sign of real and imaginary parts of nonlinearity tensor components, time of nonlinearity relaxation and others.

We discuss the characterization of nonlinear optical processes in materials associated with loss and index changes using the Z-scan. The primary problem addresses is, given a new material, how do you determine the dominant nonlinearities present and their physical mechanisms. In particular, in extensive studies of a wide variety of materials we have found that there is seldom a single nonlinear process occurring. Often several processes occur simultaneously sometimes in unison, sometimes competing. Distinguishing and separating these processes is important for understanding and modeling the interaction. There are a variety of methods and techniques for determining the nonlinear optical response, each with its own weaknesses and advantages. In general it is advisable to use as many complementary techniques as possible and vary as many experimental parameters as possible in order to unambiguously determine the active nonlinearities. Here we show as examples the two cases of semiconductors and reverse saturable absorbing dyes. We concentrate on the use of the Z-scan in determining the responses, but utilize knowledge from other experiments to help in the interpretation.

We demonstrate a new method for investigating the orientational distribution of the rod-like poly[bis(m-butoxyphenyl) silane] molecules in Langmuir-Blodgett (LB) films. The first two non-trivial in-plane orientational order parameters C₂ and C₄ could be easily deduced by measuring the independent tensor components of (chi) ⁽³)(-(omega) ₃;(omega) ₁,(omega) ₁,(omega) ₂). Using this method, we show that the annealing process improves the molecular alignment along the dipping direction. We were also able to get information about the domain morphology of the film from the off-diagonal components of (chi) ⁽³).

As one of the important elements in natural and artificial electron transfer and energy transfer processes, porphyrin and its derivatives have received much attention in photoelectronics and photoelectronic materials. As our first attempt to relate the (pi) -(pi) electronic couplings between porphyrin macrocycles to apparent third order nonlinear susceptibilities, we measured (chi) ⁽³) for several porphyrin and chlorophyll a derivatives, including dimers with different configurations. Our preliminary results show that the dimers have enhanced (chi) ⁽³) compared to those of the monomer. This enhancement is related to the relative orientations between the two macrocycles in the dimers. The parallel dimers with close face- to-face distances seem to have the highest enhancement in (chi) ⁽³). More model compounds and related theoretical works are in progress before reaching a final conclusion.

We discuss some fundamental limits of electro-optic polymer materials, process, and devices based upon the intrinsic features of these promising optoelectronic materials.

The current status of electric field poled polymeric materials for second harmonic generation (SHG) and optoelectronic devices is discussed. For SHG applications polymeric materials have still not significantly overcome the trade-off between high nonlinearity and absorption of the frequency doubled light. On the other hand, the nonlinear properties of polymeric systems appear to be large enough to consider the use of these materials in electrooptic devices such as amplitude modulators and switches. Here the problem is maintaining the poled orientational ordering of the chromophores within the polymer throughout the device fabrication process, which can involve brief excursions to high temperatures. An empirical relationship is described relating the decay of the poled order to the guest-host system's glass transition temperature. This relationship's extension to side chain polymer systems is also examined.

Two kinds of practical efficient acoustooptically phase-matched noncollinear second harmonic generation (SHG) interactions are predicted in Tellurium for the incident optical wave at 10.6µm .They are produced by two anisotropic acoustooptic. (AO)interactions in which the normal to the AO interaction plane is offset of 10° with respect to the optical (Z)a.xis . For X-propagating slow shear acoustic wave, a high AO figure of merit is predicted, presenting a low optic absorption in Tellurium. The first SHG wave is interpreted as being due to mixing of the incident optical ave and the acoustooptically scattered wave. The other SHG is observed due to mixing of the two acoustooptically scattered waves. It is shovm from absolute SHG calculations and corresponding SHG measurements previously reported in Te, that the nonlinear optical coefficient which governs these high efficiency SHG interactions is d = 65x10-11m/V . 11 Tbe corresponding SHG efficiencies of these anisotropic diffraction geometries are about x6 and x 5 times respectively larger than the previously reported AO phase-matched noncollinear SHG in Te, for the same input and acoustic power. A comparison is made with the next best ex<2 materials like the cbalcogenide compounds (Tl.3Asse3), proustite (AgGaSe3); Gallium .Arsenic (GaAs) and CdGeAs2.

We have developed a new class of main-chain second-order nonlinear optical (NLO) polymers using well-developed condensation polymerization methods. In these polymers, the NLO chromophore dipoles are expected to be randomly arranged along the polymer backbone (i.e., the dipoles can be head-to-tail, head-to-head, or tail-to-tail). For comparison, a side-chain polymer having virtually the same chromophore as a pendant has also been prepared. The effect of variation in polymer structure on the second-order NLO properties and the effect of crosslinking on the stability of poling-induced macroscopic order are studied. Our results demonstrate that the random main-chain, second-order NLO polymers can be efficiently poled, yielding (chi) ⁽²) as high as 300 pm/V. Long-term temporal stability of the poling- induced order (e.g., no significant NLO decay is observed for more than 2000 hours) can be realized by crosslinking the polymer backbone.

With bacteriorhodopsin (BR) it will be demonstrated that biological systems--optimized by nature under evolutionary conditions--can be applied with great success to solve material requirements in optical applications. The possibility of genetic engineering further increases the technical applicability of these systems. The application of BR and its mutants as molecular processing units in optical data processing and storage will be shown in several examples.

We discuss the means for writing and reading information in a volume optical memory induced by two-photon processes. Photochromic materials based on reversible transformation of spiropyran molecules and polyacene photodimers have been used in 3D writing and reading with some success. The spectroscopic and physical properties of these materials are presented.

This paper describes the recent progress of our group in the field of photorefractive polymers. Three separate systems are discussed. The first two consist of polymers with optically nonlinear molecules attached in a pendant side-chain configuration, which are made photoconductive through the addition of a molecular charge-transport agent. The photorefractive properties are investigated using degenerate four-wave mixing (FWM) as well as two-beam coupling (2BC) techniques. The latter technique is essential for confirming the photorefractive origin of the FWM signals through the observation of asymmetric 2BC. The performance of various sensitizing agents used to generate mobile charges at long wavelengths is also compared for such systems. The third system consists of a charge-transporting polymer, which is made nonlinear through the addition of a optically nonlinear guest molecule. The photorefractive properties of this system are found to be superior of those of other polymeric systems, with diffraction efficiencies of 1% in a 125 micrometers thick sample for gratings written with red light (647 and 676 nm), and a 2BC gain coefficient (Gamma) which exceeds the absorption coefficient by 7.2 cm^-1 at 753 nm. These properties are comparable to those of inorganic crystals, and should generate interest in organic photorefractive materials for device applications.

Strong polymer fluorescence is linked to the nature of the lowest single excitation, S₁. The interplay of electro-electron (e-e) correlations and alternation (delta) of transfer integrals t(1 +/- (delta) ) along the backbone leads to a dipole-forbidden S₁ for small (delta) and a dipole-allowed S₁ at large (delta) . We analyze the crossover from band to correlated behavior in alternating Hubbard and Pariser-Parr-Pople (PPP) chains in terms of elementary triplet and singlet excitations, make contact with exact results for regular ((delta) equals 0) and dimer ((delta) equals 1) chains, and show that molecular PPP parameters distinguish naturally between polymers like poly(p-phenylenevinylene), PPV, and polysilanes with a dipole-allowed S₁ and others with dipole-forbidden S₁. The phenyl rings in PPP provide a topological contribution to (delta) . The different energy thresholds of conjugated polymers for triplet, one and two-photon, and charge-carrying excitations arise from e-e correlations in half-filled bands. Correlations also rationalize the different relaxation energies of singlet, triplet, and charged bipolarons confined to trans stilbene or to anthracene. The coincidence of PPV photo- and electroluminescence implies that the relaxation energy of polarons is less than the Coulomb binding and relaxation energy of the single exciton S₁.

A novel dye laser based on the soluble conducting polymers poly[2- methoxy, 5-(2' ethyl-hexyloxy)-p-phenylenevinylene], MEH-PPV, and [2,5-bis(5,6- dihydrocholestanoxy)-1,4-phenylene vinylene], DC-PPV, has been demonstrated. Laser action has been tested in a transverse cavity configuration where the conducting polymer laser was pumped by light pulses generated from the second harmonic radiation of a Q-switched Nd:YAG laser. The performance of MEH-PPV in solution as a laser dye was compared to that of Rhodamine 6G in solution under identical conditions. The results indicate that the luminescence quantum yield of MEH-PPV and DC-PPV lasers is comparable to that of Rhodamine 6G.

Luminescence spectra, both emission and excitation, and the excitation dependence of the resonance Raman (RR) spectra have been measured for a 1-dimensional charge-density-wave solid, [Pt(L)₂Cl₂][Pt(L)₂](ClO₄)₄; L equals 1,2- diaminoethane. The luminescence experiments support the existence of tail states in the band gap region, which indicate the presence of disorder. In contrast, the RR measurements conclusively demonstrate that the effects of static structural disorder on the vibrational spectroscopy can be neglected. This apparently paradoxical result can be explained by considering the zero-point motion of the lattice. Our experimental results are compared to recent theoretical models.

The fidelity of direct image correlation depends on the properties of the recording material. Results of plane wave interference are shown for oriented samples as well as polymer films. Also diffraction properties of holograms stored in frequency selective recording media are considered.

Experimental evidence of electron transfer from photo-excited conjugated polymers onto C₆₀ is reported. Photoinduced optical absorption, photoinduced electron spin resonance, and quenching of the photoluminescence in conducting polymers interacting with C₆₀ imply that charge transfer from the excited state of the conducting polymer to C₆₀ occurs on a picosecond time scale. The charge-separated state in composite films is metastable at low temperatures. Rectifying bilayer heterojunctions (diodes) have been fabricated using semiconducting polymers and C₆₀; rectification ratios in the current vs. voltage characteristics exceed 10⁴. When illuminated, the devices act as photodiodes and exhibit photovoltaic response as a result of photoinduced charge separation across the heterojunction interface from the semiconducting polymer (donor) onto the C₆₀ (acceptor). The photodiode and photovoltaic responses are characterized. The results are discussed in terms of opportunities for solar energy conversion, for photodiode detector devices, and for a variety of other applications which use photoinduced intermolecular charge separation.

Optical second-harmonic (SH) generation in nonlinear semiconductors can be used to convert infrared radiation from semiconductor laser sources to visible light. The high incident light intensities and spatial modulation of the linear and nonlinear optical properties occurring in Al_xGa_1-xAs vertical cavity heterostructures can lead to SH generation efficiencies many orders of magnitude larger than possible homogeneous GaAs. SH generation in vertical cavity heterostructures is analyzed using a model incorporating multilayer reflections and spatially varying nonlinear sources.

We present a comparative study of the steady state photo-induced absorption of conducting polymers mixed with Buckminsterfullerene, C₆₀. For conjugated polymers with nondegenerate ground state as donors, electron transfer takes place prior to either radiative or nonradiative recombination of (pi) -electron photoexcitations on the conducting polymer. In the case of a conjugated polymer with degenerate ground state as donor, the structural relaxation associated with the formation of charged solitons is faster, and no indications of photoinduced charge transfer are observed.

We present the results of picosecond time-resolved luminescence studies of poly(2-methoxy,5- (2'-ethyl-hexyloxy)-p-phenylene-vinylene),MEH-PPV, in four forms: a pure film, in solution, in an ultra-high molecular weight polyethylene (UHMW-PE) gel, and a MEH-PPV/UHMW- PE blend. The luminescence has a characteristic lifetime of 200 - 300 ps at 293 K and 500 - 700 ps at 80 K. The decay times are approximately the same for the four forms of the material we studied. Calculated lifetimes longer than those measured and the observed temperature dependence imply that the luminescence is quenched by non-radiative processes. Fitting and luminescence versus pump intensity are used to determine possible mechanisms.

Polymethacrylate copolymers, containing non-linear optically active 4'-dialkylamino-4-nitro- azobenzene side groups and crosslinkable 2-butenyl side groups have been studied. Films of these copolymers can be thermally crosslinked at elevated temperatures and photochemically crosslinked by exposure with UV light; both methods allow the control of the crosslinking density. In situ second harmonic generation (SHG) of a Nd-YAG laser (1.064 micrometers ) while poling was used to study films with different degrees of pre-crosslinking. A pre-crosslinking step, prior to the corona poling step, can be applied to optimize alignment and relaxation of the chromophores. Depending on the conditions of pre-crosslinking, the SHG signal intensity and the alignment stability can be maximized.