Organic compounds which exhibit optical power limiting exclusively via a two-photon absorption (TPA) mechanism have shown only little promise for providing the limiting activity necessary for the practical protection of eyes and sensors. Unfortunately, there have been few systematic studies of the molecular structure I two-photon absorption property relationships for organics documented in the literature. In order to enable the design and synthesis of new molecules with much larger two-photon absorption cross-sections and improved limiting properties, the synthetic chemist must have access to well defined structure Iproperty data. In an attempt to fill this void, work in our laboratory has centered on the design and synthesis of several new families of aromatic heterocyclic chromophores with systematic variations in their molecular structures. Careful characterization of these new materials in solution at the Photonics Research Laboratory, SUNY Buffalo has produced some well-defined structure Itwo-photon property relationships at 800 nm. In this presentation we will discuss the design and synthesis of these materials with special emphasis of how the flexibility of the synthetic scheme employed enables the incorporation of these chromophores into a wide variety of materials forms. The characterization of the twophoton properties of these materials and the relationship of these results to their optical limiting behavior in solution will also be reviewed.

Thenonlinear absorption of a series of polymethine dyes (PD's) have been investigated, and some of them are identified as potential candidates for optical limiting applications. We have demonstrated that very low optical limiting thresholds can be obtained by using polymethine dyes with a large ratio of the absorption cross section ofthe excited to the ground-state: a ratio as large as 200 has been measured in one of the PD's. This value is significantly higher than any previously reported value, while, at the same time, maintaining a large ground-state cross section. It has been shown that nonlinear absorption of PD's in polyurethane acrylate (PUA) polymer is as large as in ethanol solution. In addition, photostability of the PD's and initial measurements ofoptical limiting in these samples are reported.

The nonlinear absorption of two substituted thiophene compounds containing benzithiazole units was investigated using the Z-scan technique. The experiments were performed using a Nd:YAG laser frequency doubled to 532nm with 24ps laser pulses. The samples were prepared as solutions in THF and as melts by heating above their melting points into the isotropic liquid phase. Both compounds show strong nonlinear absorption under all conditions. Solutions of these materials show little or no linear absorption at the laser wavelength. However, the melts do show significant linear absorption. The nonlinear absorption observed has been analyzed using methods for two-photon absorption and two-photon assisted excited state absorption and two-photon absorption cross-sections are reported.

The absorption and emission spectroscopy and nonlinear optical absorption of a series of a,o-dithienyl polyenes were studied in chloroform and n-octane solutions. Dithienyl polyenes containing butyl substituents were also studied. Steady state fluorescence, time resolved emission, fluorescence lifetime and quantum yield measurements were made. The fluorescence quantum yields were significantly lower than those of the corresponding a,w-diphenyl polyenes. Among the dithienyl polyenes, 3,3'-substituted polyenes exhibited stronger fluorescence than those of the 2,2'-substituted. Nonlinear optical absorption experiments showed evidence of two photon absorption. In some polyenes, two photon absorption was followed by excited state absorption.

The characterization of organic materials for their nonlinear optical properties involves the use of samples in the liquid phase. The most convenient way to prepare samples for such investigations at room temperature is to dissolve the materials in a suitable solvent. However, the influence of the solvent on the measurements may depend on the physical and chemical properties of both the solvent and the solute. If the solvent effect is not properly taken into account in relating measured macroscopic quantities to the microscopic properties of solute molecules, the comparison of the experimental results of hyperpolarizabilities with the theoretical predictions may be invalid. In this study, a set of molecules with different sizes of conjugated ir-electron structure designed in a systematic order was used to investigate the structure-property relationship and its dependence on the environment using solvents of varying polarity. The materials used include vanillin Schiff's base derivatives. Experimental techniques such as electric field induced second harmonic generation, hyper-Rayleigh scattering and solvatochromic method were employed in this study.

Nonlinear optical materials with large intensity-induced changes in refractive index and absorption, which can be characterized by their effective third order susceptibility, are needed for numerous optical device applications. We have reported that Mo(CO)(Ph2PX) complexes exhibit strong, non-resonantly enhanced optical nonlinearities. However, the relationship between the structure of these complexes and their nonlinear optical properties is poorly understood. We have now synthesized Pd(II) and Pt(II) phosphine complexes similar to the Mo(O) complexes and have studied their nonlinear optical properties using Z-scan experiments. The Pd(II) and Pt(II) complexes have a low linear absorption at the working wavelength of 532 nm and exhibit both nonlinear optical refraction and nonlinear optical absorption effects. Our measurements indicate that the nonlinear optical properties of these complexes depend both on the nature of the phosphine ligand and on the nature of the metal center and its coordination geometry. However, this dependence is not as pronounced as we have previously observed in the Mo(CO)(Ph2PX) complexes.

A model of the response of a suspension to a short, intense laser pulse is described. In the model the particles act as initiators ofthe response, which develops in the liquid. Back propagation ofthe active region toward the laser is the dominate feature ofthe response; it can be used to simplify the description. Novel features ofthe model are the assertion of the existence of a liquid "plasma", i.e., a high density of free carriers in the liquid, and the development ofthe plasma in accordance with the Saha relation. The two aspects of the response are absorption by the liquid plasma and scattering from gas bubbles. The relative role ofthese mechanisms depends on the particulars ofthe suspension and the intensity ofthe laser pulse. A computer code implementing the model is described and the results of simulations are presented. Limiting data and images of irradiated suspensions are compared with the predictions of the model. The images are of emitted and scattered light, as well as shadowgraphs. The implications ofthe model for the design of suspension limiters is discussed.

A new method for the design of reverse saturable absorbing (RSA) dye concentration gradient (CG) limiters is developed and applied to produce three new designs. The radiation transport equation and the divergence behavior cI Gaussian beams are solved together analyticaly for the dye concentration gradient and the fluence distribution in the sample. It is demonstrated that the fluence distribution for a given CG can be found which is independent of the dye kinetics. A new figure ofmerit for gradient RSA limiters is defmed and used to compare designs. These new designs are shown to be significantly better than a constant fluence design. These designs are useful for initiating numerical nonlinear beam propagation studies.

The numerical simulation of the short time response, femtoseocond to picosecond regime, of a saturable absorbing material is described. Transient relationships for the intensity and temperature dependent absorption coefficient and index of refraction are included. The thermal response includes a hyperbolic microscale heat transfer mechanism. Initial results are reported for the calculated transmitted intensity, excited state population and temperature field of a slugglish saturable Kerr medium. This short time model can be used to investigate the onset of thermally stimulated nonlinear optical behavior, the "switch on" time, and other short time effects.

Two-photon absorption in organic dyes can be enhanced by orders of magnitude in particular situations. An original model accounting for an enhanced nonlinear absorption in concentrated interacting dye solutions is presented. The related nonlinear absorption which is induced in the transparency region of the dye solutions has an intensity dependent avalanche behavior. It may reach levels as high as using conventional excited state absorption. This open new possibilities for the practical use oftwo-photon absorption.

As our continuous effort to develop a package for modeling of beam propagation in nonlinear optical devices, we use different means to improve its user-friendliness, availability and capability. We have extended our model to include pulse propagation, i.e. 4-dimensional propagation of an optical beam. Currently, we have developed a few models for intensitydependent and fluence-dependent propagation of nonlinear wave, including various nonlinear absorption and refractive mechanisms such as thermal diffusion and reverse saturation absorption (RSA). These models can provide significant insight into the underlying optical processes which occur in nonlinear optical devices such as optical limiters. Here we will concentrate our discussion on thermal diffusion and reverse saturable absorption. To improve user-friendliness, availability and capability of the package, we have implemented two graphical user interfaces, a Internet version based on Hypertext Markup Language HTML/pen script and a standalone version based on TcIITk script. The two interfaces can be executed in a variety of computers (Macintosh, workstation or PC) while the actual simulation can be performed in a more powerful computer. The two interfaces have their own merits. The Tcl!Fk version can be easily modified and installed in a computer that has no access to the Internet. On the other hand, the web based version makes the package available to more users via world-wide web (WWW). The layouts of the interfaces are almost the same. They generate simulation results in text files for plotting as well as animation sequences which can be viewed with a free software, available from National Center for Supercomputing Applications.

Strong fluctuations oflocal fields may result in huge enhancements ofoptical nonlinearities in metal fractal colloid aggregates. Giant enhancement ofthird order nonlinearities was calculated and obtained experimentally. The real and imaginary parts of the cubic susceptibility of silver colloid aggregates were measured. It was found that the coefficient of nonlinear absorption depends on laser wavelength and intensity strongly. An optical limiting in silver colloid was observed.

We describe a coherence filtering technique in the near infrared (IR) based on degenerate four wave mixing (DFWM) in a Barium Titanate photorefractive crystal and a dye-doped liquid crystal layer. in our experiments, we used a self modelocked Ti:Sapphire laser and a Q-switched alexandrite laser as light sources. This technique can be used to provide instantaneous, single-shot, two-dimensional images ofthe internal structure ofmaterials versus depth.

We report the results of the laser damage threshold of an undoped optical grade epoxy elastomer and compare its performance to that of an epoxy glassy thermoset and polymethylmethacrylate (PMMA) thermoplastic. Tests were performed in a tight focus, f/5 geometry, with 7 ns pulses at 532 nm. The elastomer exhibits a damage threshold more than 600 times greater than its glassy thermosetting analog and over 20 times greater than PMMA. We describe the importance that thermomechanical properties have on improving laser damage resistance in organic polymers. Optical limiting experiments are performed with matrices doped with silicon naphthalocyanine and zinc octabromotetraphenyl porphyrin as the chromophores. We demonstrate the advantage of using an elastic matrix to improve the performance of solid-state organic optical limiters.

The optical limiting performance of C60 is shown to be extended to longer wavelengths by derivatization of the fullerene. While the reverse saturable absorption behavior of C60 generally improves at wavelengths longer than 532 nm due to decreased ground state absorption and increased excited state absorption, the peak of the triplet-triplet excited state absorption at 750 nm is inaccessible as a result of the near complete transparency of C60 beyond 650 nm. Partial breaking of the spherical it-conjugation by attached substituents leads to increased ground state absorption cross-sections in the appropriate wavelength range. The peak of the triplet-triplet absorption is shifted to 700 nm in the C60 derivatives, resulting in exceptionally strong optical limiting in this spectral region.

Optical power limiting can be accomplished by a variety ofmaterials whose spectral and temporal utility is determined by a number ofparameters. The most important determinant ofhow a particular material will perform is defmed by the mechanism(s) by which optical power limiting is achieved. This paper discusses the use of complementary materials to obtain the greatest spectral and temporal range. One new class of materials has strongly absorbing charge states (cationic and dicationic) which make them highly desirable for consideration. Several materials are discussed along with their optical characterization. These processes can be carried out in solutions containing various compounds including electron acceptors. Other materials such as two photon absorbing materials exhibit optical power limiting with shorter response times and/or over different spectral regions. Recent progress in the development and characterization oftwo photon absorbing compounds is discussed with particular attention to how they can be used in a complementary fashion with other compounds operating through other mechanisms to deliver broad optical power limiting responses.

Recently observed optical limiting effects of nano- and pico- second laser pulses through a fiber array are analyzed using a model that accounts for various nonlinear and excited state absorption, and subsequent thermal and density effects occurring in the fiber guiding core materials. The nonlinear liquid cored-fiber arrays we have constructed are capable of low optical limiting threshold and clamped output, as well as high resolution image transmission.

Recently some phthalocynine derivatives have been reported to exhibit a strong optical limiting effect which is due to their large reverse saturable absorption. In this report we present another close family of phthalocynine, meso-substituted tetrabenzoporphyrins. We have employed both modified Z-scan techniques which uses a CCD camera as the detector and picosecond transient spectroscopy to study the optical power limiting mechanism of the meso-substituted tetrabenzoporphyrin derivatives. With the aid of a CCD detector, a two dimensional image of the beam, in the far field can be obtained directly in the Z-scan experiments. By integrating different parts of the image, both "closed aperture" and "open aperture" information can be calculated at the same time. In the kinetics measurements, several characteristic (ir,ir*), (,*) and (d, d) transitions have been observed, and the absorption cross sections and the relaxation rates of each transient has been measured.

Due to the extensive use of lasers in aiming devices, range fmders, and in remote sensing, the protection of eyes and sensors from laser beams becomes critically important. It has been a challenging task to provide protection from wave-length tunable lasers without the serious sacrifice of vision capability. In this work, we describe two different approaches which can significantly attenuate the laser beam through the whole visible region. One approach is to use specific geometric configuration with thin film aluminum mirrors"2. The other is to use the properties of reverse saturable absorption of fullerene3'4. The combination of these two approaches can possibly become an effective device for eye protection from high power tunable lasers. The principle and mechanism to achieve eye protection will also be discussed. The goal for eye and sensor protection is to attenuate the amount of incident light below the damage threshold. The ANSI standards indicate that for visible wavelengths and nanosecond pulse durations, the maximum permissible exposure on the surface of the comeas is about 0.2 J. The damage threshold by a nanosecond laser on a rhesus monkey was measured to be of the order of a few microjoules5.

We have fabricated Polycarbonate/ Porphyrin blends. The mixtures were prepared with the use of solvents suitable to both the Porphyrins and the Polycarbonate. Thin films were prepared by evaporation of the blends on a Teflon substrate. The samples exhibit reverse saturable absorption. We present results of optical power limiting experiments for the composites of films sandwiched between Polycarbonate sheets. The composites have high impact strength and remarkable mechanical robustness.