We describe the application of two-photon processes and fluorescence to 3D optical data storage. We also report the results of two-photon absorption (TPA) cross section measurement of several organic molecules in solution. The data show that the nonlinear transmission method without consideration of other nonlinear effects results in erroneous values for the TPA cross sections. We also find that the cross sections measured by excited state methods, namely two-photon induced fluorescence and a new excited state method, which is based on transient absorption following two-photon excitation, are in good agreement with the accepted values. Therefore one needs to be cognizant of these facts when using the transmission method.

To develop novel nonlinear dyes for photonic applications, we synthesized a series of transition metal-containing phenylacetylene oligomers. Theoretical properties of these compounds were measured by UV/Vis absorption, photoluminescence, and nanosecond flash photolysis experiments. It was found that as the number of oligomer units increased ,the transition energies decreased without saturation. The low ground state absorption and UV absorption edge gives rise to solutions that are nearly water clear. A very broad triplet state absorption extending from the absorption edge to the limits of our spectrometer is demonstrated to also be intense. These results enhance the understanding of these materials when used for nonlinear absorption applications and enable the prediction properties for materials extending this class of dyes.

Stilbazolium derivatives are very attractive nonlinear optical (NLO) materials for photonics and biophotonics applications due to their low linear absorption at low incident intensities in most of the visible spectral range and potentially very strong nonlinear absorption at high intensities. In this work, we investigated the nonlinear absorption of five stilbazolium derivatives, trans-4-[2-(pyrryl)vinyl]-1-methylpyridinium iodide (PVPI), trans-4-[2-(1-ferrocenyl)vinyl]-1-methylpyridinium iodide (FcVPI), trans-4-[2-(1-ferrocenyl)styryl]-1-methylpyridinium iodide (FcSPI), trans-4-[4-(dimethylamino)styryl]-1-methylpyridinium iodide (DASPI) and trans-4-(4-aminostyryl)-1-methylpyridinium iodide (ASPI) using 6 ns and 40 ps laser pulses at 532 nm. These compounds exhibit different nonlinear absorption behavior for nanosecond and picosecond laser pulses. In the nanosecond time regime, they all show reverse saturable absorption, with PVPI exhibiting the best result. With 90% linear transmission in a 2-mm cell, the transmittance of a chloroform solution of PVPI drops to 5% when the incident fluence is increased to 7 J/cm². The nonlinear absorption behavior of these compounds is influenced dramatically by the nature of the electron donating group, with reverse saturable absorption decreasing in the order of PVPI>FcVPI>FcSPI>ASPI>DASPI. In contrast, for picosecond laser pulses, only PVPI exhibit slight reverse saturable absorption, while DASPI, FcVPI and FcSPI show saturable absorption, and ASPI shows no nonlinear absorption. The different nonlinear absorption for ns and ps laser pulses may be due to the relative contributions from triplet excited state absorption and singlet excited state absorption.

Strong three-photon absorption (3PA) and the frequency up- conversion fluorescence induced by 3PA in two novel stilbazolium-like dyes containing triple conjugated bridge have been observed, pumped with 35 ps laser at 1064 nm. The maximum fluorescence peaks are located in the 550, 610 and 700 nm respectively. The emission intensity dependence for the visible emissions on the 1064 nm excitation obeys the cubic law. A local emission (LE) of the donor moiety, excited intramolecular charge transfer state (ICT) and twisted intramolecular charge transfer state (TICT) are response for the multiple fluorescence emissions. The nonlinear transmitted intensity dependence on the laser incident intensity has been demonstrated as a main result referred to three-photon absorption in a 1-cm-path sample quartz cell with solute dissolved in dimethyl sulfoxide (DMSO) at 1064 nm, from which the calculated nonlinear absorption coefficient inferred from three-photon absorption fitted curve are (alpha) ₃ equals 4.3 and 3.2 X 10^-21 cm³/W² and the corresponding molecular three-photon absorption cross sections are (sigma) ₃ equals 1.9 and 1.6 X 10^-76 cm⁶(DOT)s², respectively. The optical limiting performances induced by 3 PA have been also performed and discussed for both dyes in DMSO.

A comparison is made of two, laser-induced-damage assessment techniques. The first technique monitors the sample for changes in linear transmission after high-energy laser illumination. With the second technique, an image is transmitted through the sample, after high-energy laser illumination, at the position of incidence. Both single and multiple shot data are considered. Results show the imaging technique to be an efficient method by which to unambiguously discern the onset of image-degrading laser- damage, regardless of detector noise, shot-to-shot variations and sample inhomogeneities. Practically speaking, the imaging technique is relatively easy to incorporate into a laser-based experimental system and is particularly relevant to the assessment of optical systems for imaging.

The integration of photorefractive liquid crystal beam coupling devices into optical systems is often hampered by the need to tilt the liquid crystal cells to high angles of incidence in order to obtain efficient beam coupling. Owing to poor charge diffusion in most liquid crystal systems, charge migration depends mainly on an externally applied drift field. Conventional cells, with electrodes applied to the surfaces of the windows, therefore need to be tilted with respect to the incident light to enable a component of the applied electric field to appear along the direction of the optical grating k-vector. This paper reports on an alternative design in which the electric field is applied transversely, enabling devices to be presented at normal incidence to the system optical propagation direction. We demonstrate the optical gain from transversely excited homeotropic liquid crystal cells is very similar to that obtainable with conventional homeotropic cells, with the added unexpected advantage of an order of magnitude increase in speed.

The optical transmission processes for a nonlinear optical (NLO) material are influenced by the properties of its environment. NLO properties such as intersystem crossing rates may be altered by characteristics of the host such as polarity. The effects of the host material on the optical transmission of the NLO material is investigated using numerical laser beam propagation modeling (LBPM) techniques. Numerical simulations are reported for the optical transmission for zinc meso-tetra(p-methoxyphenyl)tetrabenzporphyrin (ZnTMPTBP) in liquids of differing polarity such as toluene, tetrahydrofuran (THF), dichloromethane (DCM), acetone, and pyridine. In addition to investigating the effect of the solvent on transmission, these calculations explore the effect on transmission of two different singlet lifetimes which have been reported in the literature. Transmission curves are calculated using separately determined parameters obtained from curve fitting using zscan data. Calculated results are compared with experimental data for all cases. Z-scans are also calculated for several cases and the results compared to experimental data. Numerical simulations provide a valuable tool to study the optical transmission behavior of NLO materials such as ZnTMPTBP.

A strategy for protecting and improving the performance of a nonlinear optical device exposed to a high-energy beam is numerically investigated. In this strategy, a thermally stimulated defocusing material is used in combination with a RSA material. To test this new approach, the ability of a CS₂ cell dyed with a liner absorber material to protect a NLO device is determined using calculated values of beam and aperture transmission and the temperature distribution in the NLO device. The results demonstrate that the strategy provides thermal protection and marginally reduces the aperture transmission. These current calculations suggests that other approaches, such as multi-cell devices, may be more effective at providing thermal protection and reducing beam transmission. However, this current approach needs further investigation at other linear transmissions and in addition might be combined with other approaches, such as multiple layers to provide enhanced protection.

The production of a transparent photorefractive glass composite would offer a useful alternative to bulk crystal materials. We aim to produce such a material by incorporating single domain photorefractive Fe:LiNbO₃ particles into a refractive index matched glass host. This glass host is also required to be chemically compatible with the photorefractive material. This compatibility will ensure that the Fe:LiNbO₃ particles added to the host glass will remain in the intended crystalline phase and not simply dissolve in the glass. Due to the high refractive index of the Fe:LiNbO₃ (n_o equals 2.35 532 nm), producing a chemically compatible and refractive index matched glass host is technically challenging. By examining common Tellurite, Bismuthate, and Gallate glasses as a starting point and then developing new and hybrid glasses, we have succeeded in producing a chemically compatible glass host and also a refractive index matched glass host. We have produced preliminary glass composite samples which contain a large amount of Fe:LiNbO₃. We are currently able to retain nearly 90% of the incorporated Fe:LiNbO₃ in the correct crystalline phase, a substantial improvement over previous work conducted in this area in recent years. In this paper we present our progress and findings in this area.

We constructed a computer controlled high stability wide tunable parametric amplifier, and reported on a (beta) - borium borate (BBO) optical parametric amplifier of injection-seeded with a narrow linewidth pulsed Ti:sapphire laser. The pump source of Ti:sapphire laser is the residual second harmonic (532 nm) in frequency tripling (355 nm) of the Nd:YAG laser. We obtained the linewidth interferograph of Ti:sapphire laser is less than 0.003 nm, and tested six times curves of the output energy as a function of tunable wavelengths in injection seeding more than that of no injection. The narrow linewidth (<0.1 nm) continuous tunable range of 570 - 670 nm is achieved.

There is considered possibility of formation of shock electromagnetic waves (SHEW) on optical cycle of laser pulse propagating in wide band-gap transparent solid dielectric. Main regularities of SHEW formation are studied on the basis of 1D model of plane-wave propagation in isotropic dielectric with nonlinear optical response and dispersion. Special attention is paid to influence of color dispersion on SEW formation and propagation. It is shown, that SHEW can appear at input intensity close to threshold of bulk damage for tightly focused femtosecond pulses, i.e., 10 - 100 TW/cm² when generation of strongly synchronized higher harmonics can take place. There is considered range of parameters of laser pulse and the material within which SHEW can appear. Results of computational modeling illustrating structure and spectrum of SHEW are presented.

There are presented results of theoretical investigation of nonlinear self-depolarization effect resulting in variation of space distribution of polarization-ellipse parameters of monochromatic high-intensity focused laser beam. Both qualitative consideration of symmetry properties and numerical calculations for Gaussian beams of low order (TE₀₀, TE₀₁, TE₁₀ and TE₁₁) show that linear and circular initial polarizations change and turn into elliptic polarization with inhomogeneous distribution of polarization-ellipse parameters in focal area. Bearing in mind obtained results, we discuss specific symmetry structure of self-depolarization effect allowing experimental checking of described phenomenon. There are analyzed and estimated other contributions to depolarization effect resulting from low-intensity diffraction. Obtained results are generalized for the case of laser pulses and other types of nonlinear optical response of isotropic dielectric.

In this work we present the experimental results of contradirectional two-beam coupling in a bulk crystal and single crystal fibers of iron-doped lithium niobate. Results of a reduction of the grating writing instability, a comparison of the two-beam coupling efficiency of the fibers and bulk crystal as a function of focusing geometry, as well as a comparison of theoretical and experimental results of the two-beam coupling efficiency are presented.

We seek to improve the performance of organic photorefractive (OPR) systems by implementing two different design philosophies. In one strategy polysiloxane-based charge transport polymers are used to explore the requirement of a low glass transition temperature. These polymers allow construction of low Tg composites without plasticizcer and additionally may have higher charge mobility than poly(n-vinyl carbazole).The other strategy entails the use of small-molecule organic glasses composed of covalently attached charge transport and non-linrar optical chromophore moieties. Both classes of materials are characterized by holographic, photoconductive, and ellipsometric methods.

A highlight of the recent advances in the study of fully functionalized organic photorefractive materials based on polymers, oligomers and small organic molecules is presented.

We present a theoretical and experimental study of both photoconductivity and photorefraction (PR) in several PVK- based photorefractive composites. We used the modified Schildkraut and Buettner's model of space-charge formation in photorefractive polymers that includes both deep and shallow traps. The dynamic equations have been solved semi- empirically using independent measurements of photoconductive properties to predict photorefractive dynamics. Dependence of the dynamics on charge generation, mobility, trap density, acceptor density, ionized acceptor density, as well as their associated rates is examined. The magnitude of the fast time constant of photorefractive development is successfully predicted. The model has also been found to qualitatively predict the reduction in speed due to deep trap filling and ionized acceptor growth. By choosing chromophores with different ionization potentials and by varying the chromophore concentrations, we study the influence of the chromophore ionization potential on the photoelectric and PR properties and reveal the nature of deep traps in the composites and their contribution to both photoconductivity and PR dynamics. Effects of plasticizer components were also investigated.

We report on simultaneous characterization of space charge gratings in photorefractive PVK-based polymer films by means of photo-EMF and Two-Wave Mixing (TWM) of periodically phase modulated beams. 100 micron thick samples of a polymer DMNPAA:PVK:ECZ:TNF with chromophore (DMNPAA) concentration of 5 wt% were investigated at (lambda) equals 633 nm in reflectance configuration. The amplitudes of the unshifted (i.e. drift induced) and the shifted (i.e. diffusion or saturation induced) components of the photorefractive space- charge field grating were evaluated directly by detection of the fundamental and the second harmonic of the TWM signal and indirectly from the corresponding harmonics of the photo-EMF current. The unshifted grating component exhibited approximately linear dependence on the externally applied dc field E₀ and had an amplitude close to E₀, which can be interpreted as absence of any remarkable saturation of trapping centers associated with photorefractive recording. Also growing with E₀, the amplitude of the shifted component did not depend on the applied field direction, but was nearly as big as the unshifted component for the external fields of about approximately equals 50 V/micrometers . We interpret these facts as well as an experimentally observed double change of sign fo the fundamental harmonic photo-EMF signal with the external field as a result of dramatic growth of the Einstein ratio D/(mu) (relating diffusion coefficient D and mobility (mu) of the photogenerated carriers) - at least up to 1 V for the external dc field mentioned above. This allows us to address the observed shifted component as an external field enhanced diffusion grating, rather than the result of trapping centers saturation. Additionally, the (mu) (tau) product for dominating photocarriers (holes) was evaluated as approximately equals 0.3*10^{- 10} cm²/V from the photo-EMF measurements.

We study the effect of energetic and spatial disorder, anisotropy and sample orientation on the field-dependent mobility of charge carriers using a dynamic Monte Carlo simulation. Our transfer rate is based on a polaronic model of phonon-assisted hopping in an effective diabatic potential (Marcus-theory). We find that our simulations, in contrast to the Gaussian Disorder Model or the Correlated Disorder Model, neither require unphysical model parameters nor correlated disorder to explain experimental data for the field and temperature dependence of mobilities. Our simulations show, that no energetic disorder is necessary to fit experiments. A clear transition from a 3-D diffusion and drift limited mobility to a quasi 1-D drift limited process with increasing external fields in the presence of spatial disorder can be observed. A well-controlled degree of disorder can under certain conditions increase carrier mobility. Simulation of mobilities on a regular lattice are found to strongly depend on the direction of the external field with respect to the lattice in a non-trivial and field-dependent manner. This usually neglected effect is highly sensitive to the choice of the hopping rate and the underlying lattice and can easily modify mobilities by 25% or more.

The polarization and depolarization behavior of electric field-induced polar alignment in a stable photorefractive polymer containing carbazole was measured by thermally stimulated current (TSC) and electro-optic (EO) modulation experiments. A relaxation peak was observed around the glass transition temperature (T_g) for the poled photorefractive polymer in a TSC experiment. The apparent TSC was not observed at room temperature. The polarization calculated from the relaxation peak linearly increased with the strength of the poling electric field. This result was consistent with the EO behavior which showed the linear dependence of the EO coefficients on the poling electric field. The polar alignment induced by the electric field at an elevated temperature was stable at room temperature, which could lead to the long-term stability of the photorefractive responses without an external applied electric field.

We have studied the photorefractive properties of three different polymer types. A first class were (2,4,7-trinitro- 9-fluorenylidene)malononitrile sensitized poly(N- vinylcarbazole)/N-ethylcarbazole polymer composites doped with dyes of varying polarity. The performance was optimized, and at an applied field of 59 Volt/micrometer, we have observed complete internal diffraction and a gain coefficient of 167 cm^-1. A photorefractive two-beam coupling novelty filter was constructed with such a polymer. The response of these composites was compared to that of polymethacrylates functionalized with carbazole, an internal plasticizer and different chromophores. The best results were obtained in a third polymer type, where only a bifunctional chromophore was attached to the polymer backbone.

We have studied photogeneration, transport, trapping and recombination as the governing mechanisms for the saturation field strength and the time response of the photorefractive (PR) effect in PVK-based PR materials, utilizing xerographic discharge and photoconductivity experiments. Both the charge carrier photogeneration efficiency and the photocurrent efficiency were found to be independent of chromophore content, suggesting that the chromophore does not participate in carrier generation and trapping. The photoconductivity gain factor G defined as the number of charge carriers measured in photoconductivity in relation to the number of carriers initially photogenerated as determined by the xerographic experiments is found to be much smaller than unity, which indicates that the mean free path of the photogenerated charge carriers is less than the grating period. Photoconductivity data can be explained over 3 orders of magnitude in field, assuming a field-independent trap density. Based on the photoelectric data, PR response times have been predicted by Yeh's model for the build-up of space or by calculating the time, which is necessary to fill all traps by photogenerated holes. Only the latter model can reasonably well explain the observed field dependence of the PR growth time, suggesting that trap-filling essentially controls the PR onset behavior.

The erasing dynamics of holographic gratings in a low molecular weight photorefractive glass depending on the sample temperature were investigated. Changes in the overall speed of the material by three orders of magnitude over a temperature range of 13 K were observed. We identified two distinct processes below the glass transition temperature T_g, a fast one on time scales of seconds and a slower one with lifetimes around 10³ s. We attribute the fast process to the electro-optic effect and the slower one to orientational diffusion processes of the glass molecules. Above T_g, the fast process vanishes, whereas the diffusional processes accelerate up to time constants in the range of seconds. This study shows, that an accurate temperature control is indispensable when measuring photorefractive dynamics, especially in the temperature range around T_g.

We test poly(4-vinylpyridine) as a new matrix polymer in photorefractive polymer composites. It has excellent compatibility with OH-derivatized electro-optical chromophores due to H-bonding. Due to the slightly higher polarity, the molecular figure-of-merit of the chromophores is enhanced, yielding improved steady-state PR performance compared with PVK-based reference materials. However, because of the H-bonding the orientational mobility of the chromophores is hindered, limiting the dynamics of grating formation.