Two-photon absorbing materials such as conjugated polyenes show promise as nonlinear optical materials. Prediction of two-photon absorption frequencies and cross-sections has been limited by the high level of ab initio calculations that must be carried out in order to accurately calculate excited state energies and transition dipole moments, by the size of many of the compounds of interest, and by the difficulty of handling condensed phase effects in the calculations. We have carried out geometry optimizations at the multi-configurational selfconsistent field level on a small polyene, hexatriene, both in the gas-phase and in solution, with the solvent effects being modeled by the effective fragment potential. The excited-state energies have been calculated by the multiconfigurational quasidegenerate perturbation theory. Transition dipole moment calculations have also been carried out, from which the two-photon absorption cross-section can be estimated. The results indicate that just one or two solvent molecules can have a large effect on the nonlinear optical properties of two-photon absorbing materials.

Spangler and He [1,2] have shown that dithienyl polyenes form extremely stable bipolaronic dications when oxidatively doped in solution. Previous theoretical studies applied empirical methods to predict bipolaronic enhancement of hyperpolarizabilities for simple polyenes [3,4]. Here, we employ density functional theory to optimize the gas-phase molecular conformations of neutral, cationic, and dicationic forms of a series of dithienyl polyenes, where the number of ethene units, N, is varied from 1–5. Ab initio Hartree-Fock, generalized valence bond, configuration interaction, and Møller-Plesset calculations demonstrate that the dications are farily well described with a closed shell and therefore have little biradicaloid character. Second hyperpolarizabilities, γ, are subsequently calculated using ab initio Hartree-Fock theory and a finite field methodology. As expected, γ increases with the number of ethene units for a given molecular charge. The cations also show the largest increase in γ with N. For a given value of N, the cations display the largest γ values. However, if we treat the dication as a triplet, which might be present in solution, then it displays the largest γ.

Electronic structure calculations are carried out to predict properties of reverse saturable absorption (RSA) materials that are important in nonlinear optical (NLO) applications. The RSA dyes under investigation are zinc porphyrins with a combination of halogen and phenyl substituents. The computed data include electronic spectra for the ground and triplet excited states as well as ionization potentials. These properties are important in predicting the performance of NLO materials. Since comprehensive data are not available, we begin with the basic porphyrin unit, then systematically replace the peripheral hydrogens of porphin with halogen and phenyl groups to study the substituent effects. The results of halogenated porphyrins and halogenated tetraphenylporphyrins provide insightful accounts of effects of phenyl, fluoro, chloro, and bromo substituents on the spectra of these systems. The computed spectra and IPs are in excellent agreement with available experimental data.

During the absorption of a laser pulse of moderate length, the leading edge can experience excited state absorption out of the first singlet state. The measurement of this excited state absorption spectrum can only be accurately probed using short pulse pump-probe techniques. Specifically, we examine the excited state absorption of AF-380 in THF using ultrafast transient white light absorption spectroscopy (TWLA). This material has been the focus of several investigations due to it's purported large two-photon absorption cross-section, the discrepancies between long and short pulse measurements, and it's use in holographic twophoton induced photopolymerization. It is believed that a substantial excited state absorbance can account for the difference in two-photon cross section measurements. It is also possible that this excited state exhibits coherence for time scales that can affect further absorption of longer pump pulses. We examine the transient absorption of this species, as well as polarization and free carrier effects and discuss the possible implications with regards to measurement techniques.

Over the past five years there has been a renaissance in design studies of chromophores with the possibility of enhanced two-photon absorption (TPA). While two-photon absorption has been described for molecules such as laser dyes in solution for a number of years1, it has only been recently that researchers have attempted detailed structure-property relationships to elucidate how new chromophores with greatly enhanced two-photon cross-sections might be designed. Since the intrinsic cross-sections are related to the Im component of the third order optical nonlinearity, it should come as no surprise that much of the previous work in the literature which focused on structure-property relationships for molecules with enhanced NLO response might be applicable to the design of new TPA chromophores. In this presentation we will review our recent studies in this area, and our rationale for the applicability of dendritic strctures based on photonic-active repeat units for enhancment of two-photon absorption, particularly in the area of optical power limiting applications.

We have developed methods of measuring absorption of transient species utilizing both rapid scan and stepped-scan Fourier transform interferometery that allows a combination of broad spectral coverage (10,000 – 15,000 cm-1 per spectrum), good spectral resolution, and the ability to cover temporal ranges from minutes to nanoseconds. A wide range of pump sources are used including continuous, modulated, ns pulsed, and ultrafast lasers. The probe is provided by a broadband, incoherent source providing large spectral information in each experiment. This capability enables investigation of phenomena such as excited state absorption, charge transfer, formation of metastable species, and others. Extension of the technique to the ps realm is underway and will be discussed.

To understand the properties of light-sensitive compounds used in optical limiters having photoinduced charge transfer mechanisms, we have investigated the photophysics of a series of di(2-thienyl-3,4–butyl)polyenes. Spectroscopic measurements were obtained as a function of the number of double bonds(n = 1–8). From analysis of the bandshape of the emission spectra, we found evidence of vibronic coupling between the S1 and S2 excited states. The degree of coupling increased with solvents of high refractive index. The appearance of the cis band in absorption spectra of dithienyl polyenes having 6–8 double bonds suggested equilibria between scis and s-trans conformors. The cis band intensity increased in lower polarity solvents, implying solvent-induced conformation changes in these dyes. Trends in the data reflected the ordering, energy gap between and mixing of 1Bu* and 1Ag* excited state configurations.

Nonlinear transmission in a dye (stilbene3) exhibiting two-photon absorption (TPA) has been studied has a function of wavelength, in the visible range using nanosecond laser pulses. The results have been analyzed within a three level model: the first excited one is populated by TPA and is excited toward an upper level (excited state absorption, ESA). The TPA and ESA spectra obtained by optical limiting (δ∼10-46 cm4s photon-1molecule-1; δ∼ 10-16 cm2) are in good agreement with those determined by two-photon fluorescence and Kerr ellipsometry. Thus, nonlinear transmission appears as a useful technique to estimate the TPA and ESA contributions in optical limiting.

During the past decade there has been considerable progress in the design of new organic materials for optical power limiting (OPL) applications. Among the more promising of the new material approaches are reverse saturable absorbers (RSAs) which derive their optical limiting capability from the photogeneration of highly absorbing transient excited states. Most of the previous research in this field has focused on excited state absorption from singlet or triplet states whose cross-sections are greater than the original S0 – S1 transition- However, other transient states, such as polaronic radical ions or bipolaronic diions, formed by photo-induced electron transfer, are attractive alternatives. We have initiated studies to determine if these transient charge states can be photo-generated efficiently, and if their excited states absorption and lifetimes are acceptable for current optical power limiting requirements.

During the past five years there has been considerable progress in the design of organic materials for optical power limiting (OPL) applications. One of the more promising and widely studied material approaches involves reverse saturable absorption (RSA) from various excited states. We have recently been examining the efficacy of utilizing highly absorbing photogenerated charge states for RSA. Both polaronic radical cations and bipolaronic dications are possible candidates for this mode of optical limiting. Equally intriguing are new approaches to designing chromophores with large two-photon absorption (TPA) cross-sections. For some pulse durations the effective two-photon cross-section for bis-(diphenylamino)diphenylpolyenes, and dendrimers based on these repeat units, are extraordinaryly large, indicative of excited state absorption. In this presentation we will discuss the possibility of combining these two optical limiting mechanistic paradigms in single structures, which may then be considered as bimechanistic optical power limiters.

Phosphorescence studies of reverse saturable absorption materials (RSA) are discussed. Absorption and emission measurements of an RSA material, specifically Zinc 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetraphenylporphyrin (ZnOBP), are performed in a proprietary host polymer. Spectra are analyzed to estimate the relative energy level of the first triplet state of ZnOBP with respect to the ground state.

Over the past decade there has been considerable progress in the design of new chromophores for potential optical limiting applications. Most of the focus has been centered on molecules that are capable of efficient excited state absorptions whose cross-sections are larger than the original π–π* excitation cross-section, usually referred to as reverse saturable absorbers (RSAs). The vast majority of chromophores having RSA characteristics depend on efficient intersystem crossing from the excited state singlet (S1) to a highly absorbing and relatively long-lived triplet state (T1). However, much less attention has been paid to alternative excited state absorptions, particularly from charge states such as polaronic radical-ions and bipolaronic diions. In this presentation, we propose that monodisperse dendrimers based on dendron coupling to core molecules such as bisphenol A are potential optical limiting materials based on photo-generation of such charge states in the dendron-attached chromophores, such as dithienylpolyenes. The design and syntheses of these dendrons and dendrimers are presented, and the absorption characteristics of the most likely charge states discussed from an optical limiting perspective.

In this report we study nine structurally similar porphyrins and show how the material parameters that control the optical limiting depend on the molecular structure. Optical limiting measurements at 532 nm on the metal-free, zinc and lead analogues of tetraphenyl porphyrin (TPP), tetra(trimethylsilyethynyl) porphyrin (TTMSAP) and tetra(4-n-butylphenylethynyl) porphyrin (TNBBAP) reveal a strong dependence on the molecular structure. A double pump/cw probe technique is used to measure the material parameters that control the optical limiting and the results give a strong insight into how the molecular structure influences the individual parameters. These measurements revealed that one of the materials TTMSAP(Pb) has a σex/σgr ratio of ∼45 at 532 rim, which is one of the largest values ever reported. Finally, broadband measurements with a picosecond white-light source show that the excited state absorption profile for the porphyrins examined is much broader than the ground state features and is centred to the long wavelength side of the ground state Soret band.

This paper will deal with the electronic and physical effects of purification of a macrocyclic dye, 2,3-napthalocyanine bis(trihexylsiloxide) (SiNC), on the optical limiting performance.

The changes in optical limiting performance of SiNC, a carbocyanine dye (1,1′,3,3,3′,3′,- Hexamethylindocarbocyanine iodide (HITCI) and two metallated porphyrins upon the addition of an electron acceptor material, methyl viologen dichloride (MV) (1,1′-dimethyl-4,4′ bipyridinium dichloride), are also shown and the results discussed.

We report an investigation of optical-limiting behavior in two neutral nickel complexes with multi-sulfur 1,2 dithiolene ligands, [Ni(medt)2] I (medt = 5,6-dihydro-6-methyl-1,4-dithiin-2,3- dithiolate) and [Ni(phdt)2] 2 (phdt = 5,6-dihydro-5-phenyl-l,4-dithiin-2,3-dithiolate) in benzene solution. The fluence-dependent transmission of the complexes was observed with nanosecond and picosecond laser pulses at 532-nm wavelength. The limiting thresholds of the complexes were ˜0.3 J/cm2, when measured with the picosecond pulses. Both picosecond time-resolved pump-probe and Z-scan measurement revealed that the limiting effects should originate from excited-state absorption and refraction. The transparency window (400˜900 nm), observed in the linear absorption spectra of the complexes, indicated that their limiting response should cover a wider range than those of fullerenes and phthalocyanines.

The Mn4+ ion has the same d3 electron configuration as Cr3+, but is less stable and can transform to other valence states. Both ions are known to occupy several sites in garnet hosts, giving rise to rather complicated optical spectra and photo-induced phenomena. Here, the spectra of the transient, photo-induced absorption and the emission of Mn4+,:YAG are analyzed over a broad wavelength region (from 450 to 1200nm) and over a wide range of time scales (from ns to minutes). Very large amplitude and extremely broadband excited state absorption are observed. Additionally, a long lifetime, photo-induced species was detected that is likely formed via charge transfer. The demonstrated behavior of this material shows it has potential applications for optical limiting and optical storage.

We report herein the syntheses and optical limiting characterizations of a series of porphyrin-C60 dyads in which the porphyrin and C60 moieties are held within close proximity of one another by an o-phenylene bridge. The electron-donating ability of the porphyrin moiety in the dyads is systematically varied through chemical modification. This allows us to control the ease of electron transfer from the porphyrin to the C60 moiety in these dyads. Optical limiting measurements at 532 nm using 3-ns pulses show that the limiting performances of the dyads are poorer relative to their model compounds. This could be due to an ultra-short lifetime for the charge-separated species or a depopulation of the triplet states by photoinduced electron transfer. The optical limiting performances of the free-base porphyrin-C60 dyads are also related to the electron- donating abilities of the meso substituents of the porphyrin moieties. 12PorphyC 60 displays better limiting performance at 532 nm (with 3-ns and 15-ns pulses) compared to Val12PorphyC 60 . This trend has been rationalized in terms of the relative energies of the first excited singlet and triplet states of the fullerene moiety and the charge-separated state of the dyads.

We present our recent advances toward the development of high-performance solid-state optical limiting devices using reverse saturable absorption (RSA) dyes doped into optical host materials. Femtosecond transient absorption spectroscopy was employed to determine both the spectral regions of strong RSA, and the singlet-triplet excited-state dynamics. The optical limiting in the visible spectrum in both metallo-phthalocyanines and metallo-porphyrins is due to a combination of singlet and triplet RSA. Optical limiting performance was studied for RSA dyes in dual tandem limiters (both in solution and solid-state). Our best results in the solid-state yielded an attenuation of 400x, and a damage threshold of up to several mJ at f/5 focusing. The optical limiting at f/5 is further enhanced, particularly in the solid-state, by self-defocusing thermal nonlinearities.

This paper examines the goals which a nonlinear limiter must achieve in order to protect an eye against injury by a pulsed laser. The shape and temporal structure of the image presented to the retina are discussed. New bio-optical data describing retinal injury thresholds for largeimage sources are used to estimate the ocular hazard. The issues and trade-offs surrounding the incorporation of a limiter into an optical system are examined, and the suitability of different materials for an effective low-loss limiter are compared.