The quantum chemical design of new molecular materials for non-linear applications requires a fundamental understanding of electronic structure and properties. Targeted synthesis of candidates greatly reduces the costs and timescales of an empirical search and this is aided by prior calculation of excited state energies, energy relaxation and transfer rates, molecule-environment interactions and excited state chemistry.

Essentially, the problems encountered in the routine application of standard quantum chemical methods are caused by the large size of the molecules of interest. This necessitates either the design of ultra-fast computers or numerical methods which facilitate the application of ‘exact’ techniques or the development of less resource intensive approximate methods with proven accuracy.

We shall outline the theories used in the calculation of optical properties and review their computational implementation. Calculations on annellated tetraazaporphyrazines will be presented in illustration.

A numerical simulation of the nonlinear optical and thermal response of a material is described. The code is divided into preprocessor, simulation and postprocessing functions. The simulation function is a transient, three dimensional, finite difference algorithm which describes the thermal response and the optical phase change that occurs due to 3rd order nonlinearity and thermal effects. Irradiance changes due to linear and nonlinear absorption mechanisms are included in the code. In the postprocessing function a Gaussian beam decomposition technique is used to calculate the irradiance and transmitted power at a far field location and the temperature time response of the material. Examples of using the code to simulate Z-scan experiments are presented and compared to those reported in the literature. Results are presented that demonstrate that the code can be used as a tool to investigate using thermal transport mechanisms to modify the nonlinear optical response of the material. Limitations of the code are discussed.

Measurements on the reverse saturable absorption (RSA) properties of some polyene dyes are presented. For a series of linear dyes, a correlation is observed between the symmetry of a molecule and its ability to exhibit RSA. To our knowledge, this is the first structure-RSA relationship discovered to date.

Certain of these materials have been studied with semi-empirical quantum chemical techniques and the observed structure-RSA relationship is rationalised in terms of the calculated electronic structures. In this paper, we report primarily on some studies we have carried out on the linear dye systems.

Series of molecules and polymeric systems are being evaluated as optical limiters for the design of a high second molecular hyperpolarizability γ. For example, results for a series of tolanes are discussed, as well as those of semi-flexible, stiff-chain polymers, including substituted biphenyl (X, Y = H, CH3, NO2, or a halogen atom) and benzo-bisthiazole moieties. The non-linear optical response was calculated using a finite field approach and compared for various derivatives.

Semiempirical computational chemistry techniques were used to investigate the absorption spectra of 1′-isopropyl-3′,3′-dimethyl-indoline-spiro-6-nitro-8-methoxy-benzopyran. The spectra of both the closed (spiropyran) and open (merocyanine) forms were calculated using the INDO/CI method. Spectroscopic calculations showed the effects, both electronic and steric, that substituents such as hydrogen, methoxy and thiophene have on the absorption spectrum. Knowledge of the absorption spectra as a function of the choice and placement of substituents and molecular geometry will potentially allow optical properties to be optimized for specific applications.

The performance of optical limiters depends on the ratio of the absorptive cross section for the transition from an excited state, σe, to that for transition from the ground state, σg. Both these factors depend on wavelength. It is appropriate to define two bandwidths, one for high energy pulse suppression, depending on σe, and the other for linear transmission, depending on σg. These bandwidths are affected differently by the use of molecular mixtures.

We present studies of an optical power limiter that uses a combination of the photoconductive and linear electro-optic effects. The electro-optic power limiter (EOPL), first demonstrated using CdTe in the near IR, possesses many desirable qualities including broadband wavelength responsivity, self-actuation, and low threshold power. One unique capability of EOPL, not found in any other nonlinear optical power limiter, is the fact that any light - coherent or incoherent - can be used. Our studies in CdTe, GaAs, and BSO crystals resulted in extending the wavelength range from the visible to MWIR wavelengths.

In the interest of finding useful materials systems for optical limiting, we have sought to combine the attractive third order nonlinear optical properties of polydiacetylenes (PDA) with the desirable ballistic and mechanical properties of polycarbonates in our study of two different systems. Composites of Lexan polycarbonate with the PDA from 1,6-di-N-carbazolyl-2,4-hexadiyne have attractive laser power transmission characteristics. Macromonomers derived from the chloroformates of bis -phenols A and F and diacetylene diols were synthesized, their spectral, molecular weight, and thermal properties were studied, and the crosslinking of the macromonomers to PDA structures was investigated.

We report an experimental investigation of optical limiting with bulk ZnSe1−xTex crystals at wavelengths just below the band edge, using nanosecond laser pulses. The limiting threshold as low as a few mJ/cm2 has been demonstrated. The generation of free charge carriers by singlephoton absorption, and subsequent free-carrier nonlinearities (nonlinear absorption and selfdefocusing) are responsible for the limiting behavior.

The oxides of vanadium, VO2 and V2O3, are attractive materials for optical switching applications due to their large excursions in both electrical and IR optical properties. These materials can be prepared in thin film form by a number of deposition techniques, making them compatible with optical and electrical device applications. The performance of these films is a sensitive function of microstructure and processing conditions. The effects of these variables are discussed.

To develop optical power limiting thin films prepared from water-soluble materials, we have prepared chromophore-doped gelatin thin films. Thin films of gelatin were prepared by spin coating followed by annealing. We also prepared thin films doped with the water-soluble chromophore copper phthalocyanine sodium tetrasulfonate. The films were characterized by film profilometry and optical absorption spectroscopy. Optical power limiting measurements of these films, as well as comparisons with aqueous solution and gels were performed.

A number of materials are available which exhibit thermochromic properties. These include the superionic materials (eg silver sulphide), phase transition materials (eg vanadium dioxide) and mixed valence materials (eg SmS). Such materials can be exploited as optical thin films and when incorporated into multilayer structures exhibit non-linear properties, particularly at wavelengths in the infra-red (1–12μm). The response of such devices is dependant on the degree of temperature rise produced in the active material, which in turn is dependant on the optical properties of the material and the heat sinking characteristics of the substrate. A summary is presented highlighting techniques for film deposition, multilayer device design and temporal response characteristics measured under conditions of pulsed irradiation.

Broadband infrared filters with uniform spectral transmittance are used for spectrometer calibration and other applications. However, commercially available neutral-density filters with optical density (GD) greater than 2 exhibit significant variations in OD over the wavelength region from 2 μm to 25 μ m. In this work, we found a single-layer alloy film that, for appropriate thicknesses, yields a flat transmittance for OD near 3 and 4. The transmittance and reflectance of the filters were measured using a Fourier-transform infrared spectrometer. The optical constants of the alloy films were obtained from transmittance and reflectance measurements, which can be used for future design optimization.

Extensively conjugated polymer films with excellent morphology, homogeneity, robustness, and nonlinear optical coefficients have been synthesized in the flowing afterglow of a lowpressure argon plasma. Polymers of benzene, thiophene, furan, and napthalene are among the systems studied. The polymers, which grow by accretion of positive ions through a low-voltage sheath, have bulk and surface textures which scale systematically with film thickness. The hopes and perils of employing these new materials as active elements in gradient-index reflectors and optical switches will be described, as will the link between morphology and deposition mechanism.

We report an investigation of excited-state absorption in cubane-like transition metal clusters. The fluence-dependent transmittances of the clusters have been measured with using 8-ns laser pulses. Time-resolved transmission measurements show that the triplet-triplet absorption occurs within a few nanoseconds. We have also developed a five-level (two singlet states, two triplet states, and one ionized state) model to simulate the excited-state absorption in these cluster compounds. The comparison between the model and the experimental measurements indicates that the population of the triplet states is created mainly by an ionization-recombination process and the triplet-triplet transitions are responsible for the observed nonlinear absorption.

We consider the application of photorefractive beam fanning as an optical limiter. Results indicate that by focusing the incident laser light into the photorefractive crystal and by applying electric fields that limiting is possible at the 10μJ range with an optical density or O. D. of 3. These results are independent of the incident optical pulse width.

Compounds WCu2OS3(PPh3)4 (I) and MoCu2OS3(PPh3)3 (II) were synthesized by reactions of (NH4)2MO2S2 (M = W, Mo), Cu2S and PPh3 in solid state for nonlinear optical studies. A wide transparent window (λ = 400 - 1000 nm) was observed for both clusters, which makes them attractive for nonlinear optical (NLO) applications. NLO properties of the clusters were studied with a 7-ns pulsed laser at 532 nm. Cluster I exhibits mainly optical self-focusing (n2 = 8 × 10-18 m2 W-1, as measured with an 1.2 × 10-4 M acetonitrile solution) with negligibly small nonlinear absorption. Cluster II exhibits both optical self-focusing (n2 = 5 × 10-17 m2 W-1, as measured with a 7.4 × 10-5 M acetonitrile solution) and nonlinear absorption (α2 = 2.6 × 10-10 m W-1). The third-order NLO susceptibilities (χ(3)) of the two clusters at the above-mentioned concentrations are 2 × 10-11 esu for I and 1.2 × 10-10 esu for II respectively. These nonlinear optical properties of the clusters were compared with those of cubic cage shaped and nest shaped clusters to reveal a qualitative structure/NLO property correlation.

We present numerical beam-propagation simulations of optimized reverse-saturable absorption (RSA) based optical limiters where the depth of focus of the input beam is much smaller than the thickness of the nonlinear material. The optimization is achieved by allowing the molecular concentration to vary along the propagation path, allowing the entire length of the limiter to reach the maximum possible nonlinear absorption before eventual damage to the limiter. We review in detail the analytic model originally derived by Miles [1] to determine the design and performance of such limiters. This model requires the usual 5-level model used in the numerical solution to be approximated by a quasi-three-level system. We show that this effective 3-level excited-state cross section is both pulsewidth and fluence dependent. The numerical propagation output shows that there is considerable diffractive beam distortion, which cannot be accounted for in the analytic model. The end result is that while there is qualitative agreement with numerical results, the magnitude of the limited output can be an order-of-magnitude underestimated. We determine that the fluence level at all parts of the limiter must be at least ten times the saturation fluence to efficiently utilize the nonlinear absorption. We further describe how the optimized distribution of molecular density is the limit of the multi-element tandem limiter for an infinite number of elements. By carefully accounting for saturation over the entire length of each individual element, we show how a multi-element limiter may be designed to closely approach the performance of the optimized distribution for as few as four elements. With current materials technology the damage threshold of solid hosts needed to vary the molecular density is much lower than that of glass cuvettes used for liquid based limiters. Therefore, a multi-element liquid based tandem limiter can be used in full saturation so that better limiter performance should be obtained. Ultimately, however, the operation of all RSA-based limiters involves a strict trade-off between performance and linear transmittance.

The excited state properties of dyes become important under intense illumination of a short laser pulse. The pulse induces a significant population in an excited state which will have different absorptive properties, this causes a change in absorption of the incident pulse within its duration. An increase in absorption arises from molecules in an excited state having an absorption greater than the corresponding ground state.

The z-scan technique has been used to measure the induced absorption of a carbocyanine dye 1,1′,3,3,3′,3′- Hexamethylindtricarbocyanine Iodide (HITCI) for laser pulses of different wavelengths. An optical parametric oscillator pumped by the third harmonic of a Q-switched Nd:YAG laser was used. Z-scans were taken at a number of different wavelengths between 460 and 630 nm. Data is presented which shows the spectral region where induced absorption occurs. An eight level rate equation model is used to predict the non-linear behaviour and to fit to the experimental data, good correlation is found between the model and data.

We have reported high Chi(3) values observed for metallo meso-tetra (substituted phenyl) tetrabenzporphyrins (metallo TPTBP). In order to find the relationship between the molecular structures and the Chi(3) values, we have attempted to find relationships between these values and the electronic spectral properties of the compounds. For a series of Zn TPTBPs, we found that the B and Q band peak wavelengths and the peak height ratio between these two bands are related as one would expect from such a series of porphyrins. The Chi(3) values appear to be directly related to the Q/B peak height ratio. For a series of metallo meso-tetra(p-methoxyphenyl) tetrabenzporphyrins, such a correlation of Chi(3) with the spectral properties of various metal species was not apparent. Strategies to increase the Chi(3) values of metallo TPTBP on the basis of these observations are discussed.