A novel scheme for implementing the joint exploitation of different, somehow complementary mechanisms of nonlinear transmission in an optical limiting device is proposed. As active materials we have chosen the fullerene derivative FULP, as a reverse saturable absorber, and a new heterocyclic quadrupolar dye, PEPEP, with highly efficient multiphoton absorption for nanosecond pulses. The nonlinear absorption properties of PEPEP in solution are extensively investigated for both femtosecond and nanosecond pulses. When Z-scan experiments are performed with nanosecond pulses, much larger effective cross sections are measured than with femtosecond pulses and with remarkably different wavelength dispersion. This is interpreted as due to two-photon absorption followed by one-photon absorption from the excited state. Chemically modified nonlinear molecules are incorporated in a hybrid organic-inorganic sol-gel matrix. Sufficiently high concentrations are achieved to allow the assembling of thin sol-gel disks into a "tandem" limiter with a total thickness smaller than the Rayleigh range of the focused laser beam. Preliminary testing of our limiter is reported and shows encouraging results. The resistance of the FULP-doped sol-gel glass to laser damage is substantially improved and the nonlinear attenuation at high pulse energies is enhanced.

We report here the nonlinear absorption and optical power limiting properties of polyfluorenes induced by two-photon absorption (TPA). Measurements were performed in chloroform between 450 and 650 nm for nanosecond time duration pulses. The nonlinear absorption is attributed to a three-photon absorption process involving a first TPA step followed by an excited state absorption (ESA) process. The TPA cross-sections of small oligomers increases with the square of their lengths. This trend is rationalized in terms of excitonic coupling between monomers within the range of the effective length of the oligomer. The corresponding three-photon absorption coefficient α₃ presents at resonance high values for the longest oligomers (α₃ very much greater than 10000 cm³/GW² for N = 60 at a concentration of 200 g/L), leading to efficient optical power limiting in the whole visible range (the maximum transmitted energy is lower than 10 μJ for an input energy of up to 350 μJ in a F/5 optical geometry).

Novel conjugated chromophores were designed and investigated for optical power limitation based on multiphoton absorption processes. Their design is based on the push-push functionalization of a semi-rigid elongated system derived from the extension of biphenyl cores. Biphenyl moieties with tunable twist angle were examined. Phenylene-vinylene rods were selected as connecting spacers between the core and the electroactive end groups to ensure effective electronic conjugation while maintaining suitable transparency. These derivatives combine wide linear transparency and enhanced nonlinear absorptivities in the visible range. Pump-probe Kerr ellipsometry indicates large excited-state absorption cross-sections (with typical σ_e values of 5 10^-16 cm²) while nanosecond nonlinear transmission measurements and optical limitation experiments reveal very strong nonlinear absorption that can be fitted by a three-photon absorption process (leading to α₃ values up to 18000 cm³ GW^-2). Such behavior results from a sequential multiphoton process involving excited-state absorption subsequent to two-photon excitation (with typical σ₂ values of 5 10^-20 cm⁴ GW^-1). Both the linear transparency, the photostability and the nonlinear absorption spectral characteristics of these derivatives can be tuned by playing on the biphenyl twist angle. As a result, chromophores combining good linear transparency and enhanced nonlinear absorptivities in the visible range have been obtained.

The optical limiting performances for β-octa-octyloxy phthalocyanines with initial linear transmission T_o = 88% to approximately 92% at 532 nm in a 5 mm spectroscopic cell in toluene have been measured with 35 ps pulses, in which β-octa-octyloxy phthalocyanine free-base exhibits the strongest optical limiting ability through a reverse saturable absorption from S₁ → S_n excited singlet states with σ_s1 = 4.0 x 10^-17 cm². The optical limiting thresholds are 50, 130 and 140 mJ/cm² at T/T_o = 0.5 for phthalocyanine free base, nickel phthalocyanine, and lead phthalocyanine respectively. The throughput of the phthalocyanine free base is clamped down to 70 mJ/cm² with the limiting nonlinear transmittance T_lim = 18% as the incident fluence reaching to 400 mJ/cm². The solubility, aggregation behavior and photo-stability for β-octa-octyloxy phthalocyanines have been studied. The solubility of the β-octa-octyloxy phthalocyanines has an obvious improving, which dissolve in non-polar solvent more easily than in polar solvent. The aggregation equilibrium constant k = 5.6 x 10³ in toluene for phthalocyanine free base is lager that in mixed solvent (k' = 1.4 x 10³), which means that the β-octa-octyloxy phthalocyanines aggregate more easily in non-polar solvent than in polar solvent through the π-π interaction. The β-octa-octyloxy lead phthalocyanine appeared a lower photo-stability among them.

Meso-tetrakis{4-[2-(trimethylsilyl)ethynyl]phenyl}porphyrin [TPP(4-CCTMS)_H2] and its complexes with metals Zn(II), Ni(II), Ga(III), In(III) and Sn(IV) were synthesized and characterized. Their optical limiting properties were determined under the irradiation of a 532 nm/5 ns pulse laser at the repetition rate of 20 Hz. The optical limiting performance of the zinc derivative, Zn(II)-meso-tetrakis{4-[2-(trimethylsilyl)ethynyl]phenyl}porphyrin, [TPP(4-CCTMS)_Zn], is better than that of its analogues, Zn(II)-meso-tetraphenylporphyrin, [TPP_Zn], and Zn(II)-meso-tetratolyl-porphyrin, [TTP_Zn], indicating that optical limiting property can be improved via fine molecular modification on the para-position of the meso-phenyl rings. In this way, the Soret band of the porphyrin is not red-shifted thereby high photopic transmission of the samples can be maintained. The optical limiting performance can be further enhanced via insertion of closed-shell metal ions. Particularly, In(III)Cl-meso-tetrakis{4-[2-(trimethylsilyl)ethynyl]phenyl}porphyrin, [TPP(4-CCTMS)_InCl], exhibited optical limiting behavior better than C₆₀ and comparable to the state-of-the-art phthalocyanine dye, Chloro-(tetra-(tert-butyl)phthalocyanato)indium (III), [Pc(t-Bu)_InCl]. Furthermore, the samples with photopic transmissions as high as 75% were tested under the same laser setup and [TPP(4-CCTMS)_InCl] again demonstrated its efficient optical limiting, which is comparable to that of C₆₀ and much better than that of [Pc(t-Bu)_InCl].

With the use of high power lasers in varied applications, search for optical limiting materials has become very important to protect the eyes and other sensors. These materials are based on the principle of reverse saturable absorption (RSA) when absorption cross-section for excited state becomes more than the ground state. In such cases transmittance goes down as we increase the intensity. In this paper, we report the experimental results on two dye doped polymer films - Disperse Orange-25 and Disperse Yellow-7 prepared using hot-press technique. For both the dyes Polymehtylmethacrylate-Methacrylic acid (PMMA-MA) has been chosen as a matrix because of its easy availability and easiness of processing. We have used Z-scan technique with CW laser @ 532 nm to study nonlinear optical properties of these materials. The closed aperture Z-scan shows positive nonlinear refraction for both the materials. While the open aperture Z-scan gives RSA at higher intensities for both the samples, making them candidates for optical limiting.

The principle of mode-cut optical limiting in fibers is reviewed briefly, and a calculation method based on angular spectrum analysis is proposed. Experiments that show high efficiency holographic grating generation and self defocusing in disperse-red-1 (DR1) doped poly(methyl methacrylate) (DR1/PMMA) bulk material suggest that it is a good candidate to be used as a core material in polymer fibers to achieve mode-cut optical limiting. Such fibers are fabricated in our lab and its optical limiting effect is reported.

The elementary processes contributing to the ultrafast change of refractive index were studied for organic molecular system. The ultrafast change was measured by optical heterodyne detected optical Kerr effect with femtosecond laser pulses. The observed responses were separated into the electronic and nuclear nonlinearities by an analytical method based on the Fourier transform. Second hyperpolarizability γ was determined from the separated electronic contribution, and is discussed in terms of the structure-property relationship for four thiophene homologues. The γ value was found to increase systematically, which is explained with change of the energy levels of the excited states affected by the hetero atom in their aromatic rings. The structure-property relationship of the nuclear nonlinearity was also discussed for CS₂ and CCl₄. Their pulse width dependences of the nonlinear optical response were demonstrated, which brings the problems of CS₂ as a standard material of femtosecond χ⁽³⁾ measurement to light. Also accumulated thermal effect, which can be a measurement artifact, is demonstrated for femtosecond closed-aperture Z-scan measurements of dye solution. The thermal effect was found to appear at the laser repetition rate as low as 1 kHz and gave apparent large nonlinearity with negative sign.

A numerical parametric investigation using a 2-level factorial design of experiments was performed to determine the effect of the f-number, sample position, sample thickness and the number of passes through the nonlinear optical sample on the beam transmittance through an optical system and on the threshold intensity. Calculated temperature profiles and excited state population dynamics are also reported. These results describe trends that are used in making decisions during the design process.

The analysis of sculpturing picosecond optical pulses by single-mode semiconductor laser structures, multilayered in a direction of passing those pulses, is developed with the calculation of phase effects. Pulses are reshaped due to the passive mode-locking process in traveling-wave regime. The relations between the pulse parameters and structure properties are chosen in such a way that the process is incoherent in behavior and provides the phase decay of incoming pulses. Reaching the steady state in pulse parameters is described in terms ofdiffusive instability and then is investigated from the viewpoint of nonlinear optical transmission. Dynamics of sculpturing is illustrated via computer simulation. Possible application lies in implementing an all-optical lumped regeneration ofpicosecond bit pulses in fiber links.

We give the analytical expressions of the nonlinear transmittance of materials through an aperture in the far field with finite aperture size. The result can be used under the conditions of and . Based on this equation, the nonlinear refractive index coefficients of some nanomaterials from both the closed I-scan and Z-scan measurements at 800 nm and 1064 nm were extracted, respectively. The obtained values from I-scan measurements are comparable with those from the Z-scan measurements. The nonlinear absorptions of these materials were also studied by fulfilling the open I-scan and Z-scan measurements. The advantages of I-scan technology were discussed.

The nonlinear transmission of a combination of carbon nanotubes and nonlinear optical (NLO) dye is studied experimentally using nanosecond pulses. Multiwalled carbon nanotubes (CNT) are suspended in a solid matrix. The refractive index of the solid matrix is matched to a solvent/NLO dye solution. Evidence of both nonlinear scattering and nonlinear absorption is reported. Though there is evidence of increased scattering due to the CNT solid matrix, no enhancement in nonlinear response is observed.

Experimental investigations have shown that CdTe semiconductor microcrystals possess large third-order susceptibilities and short response times at both resonant and non-resonant wavelengths (580 nm and 1064 nm). These excellent properties indicate their potential applications in nonlinear photonic devices. In this work, we measured the nonlinear refraction and nonlinear absorption coefficients of CdTe nanocrystals using Z-scan method at 800 nm. Application in optical limiting of the sample was also demonstrated. The samples used were made by ball milling process and then embedded in polymethylmethacrylate (PMMA). The two photon absorption (TPA) and nonlinear refraction were evaluated from the normalized transmittance with open aperture and with closed-aperture, respectively. Optical limiting studies were carried out as a function of input intensity at 800 nm. The input intensities were varied from 5 to 70 kW/cm2. The transmitted power was collected by a photo-detector through a 2-mm diameter aperture. We found that the transmitted power decreased significantly over the input intensity range of 10-20 kw/cm2.

Squeezing of radiation is a purely quantum mechanical phenomenon which has no classical counterpart. This quantum effect is expected to manifest itself in optical processes in which the nonlinear response of the system to the radiation field plays an important role. The concept of squeezing started in mid 1980s. Since then there has been an enormous upsurge of interest in this field owing to its low noise property. In the present paper various aspects of squeezing of radiation are studied and possible means of generation of squeezed light in which the quantum fluctuations are reduced below shot noise limit are investigated. The amplitude of the electric field of a mode of an electromagnetic field is always associated with quantum mechanical fluctuations. Squeezing states are characterised by reduced quantum fluctuations in one quadrature component of the field at the expense of increased fluctuations in the other noncommuting component. A fully quantum mechanical approach is followed. In the present paper we intend to review different aspects of squeezing and higher order squeezing in higher order Raman processes as well as in multiwave mixing processes. A comparative study between different processes are made which will pave a way for easier generation of squeezed light.

Non-critical phase-matching KTP OPO pumped by single-cell-SBS and dual-cell SBS phase-conjugation beam, which is generated by backward stimulated Brillouin scattering (SBS) on Nd:YAG laser as pumping source, is presented together with the maximum output energy 21 .6mJ and the corresponding conversion efficiency 32%. And it has obtained tunable pulse-width ranging from less than ins to 6ns and comparatively high conversion efficiency with a low-power pumping laser. The output character of this OPO is also analyzed with experiment results according to characteristics of long-pulse pumping OPO.

Dendrimers have unique highly branched repeating structures that display intriguing processing and photonic properties. We have recently synthesized several generations of a new dendrimer series based on bis-(diphenylamino)-E-stilbene repeat units which have proven to be highly processible in common organic solvents, and which can be designed and synthesized in either three-arm or four-arm structural motifs. We have measured the dependence of the two-photon absorption (TPA) on excitation wavelength for the G-0, G-1 and G-2 generations of these monodisperse macromolecules, and have shown that the maximum value for the intrinsic TPA cross-section for femtosecond pulses inceases in proportion to the total number of stilbene chromophores, and yields a record high cross-sectrion for the G-2 dendrimer (11,000 GM units). We have now been able to incorporate a variety of electron donor and acceptor substituents in the three-arm dendrimer G-0 system to establish structure-property relatiuonships for the further enhancement of the intrinsic two-photon cross-sections. We have found a dramatic enhancement of the two-photon cross-sections for these dendrimers compared to the parent bis-(diphenylamino)-E-stilbene (BDPAS). In the 3-arm G-0 series, the intrinsic TPA cross-section, measured at the TPA maximum, varies from 1,400 to 1,900 GM units compared to 130 GM units for BDPAS, a more than 10-fold enhancement, while the number of BDPAS repeat units in the G-0 dendrimer is only 3 times the parent BDPAS structure.

We study absolute cross section of simultaneous two-photon absorption (TPA) in a series of porphyrins and tetraazaporphyrins by 100-fs-duration pulses in two ranges of laser wavelength, from 1100 to 1500 nm and from 700 to 800 nm. The cross section in Q transition region is, sigma(2) ~ 1-10 GM, and is explained by partial lifting of the parity prohibition rule. In Soret transition region we find enhancement by about an order of magnitude due to Q transition, which acts as a near-resonance intermediate state, and also due to gerade energy levels, which we identify in this spectral region. Further enhancement (up to sigma(2) ~ 1600 GM) is achieved by symmetrical substitution in tetraazaporphyrins with strong electron acceptor groups.

A series of 2-dimensional two-photon absorbing chromophores and their 1-dimensional analogs were studied. The influence of the solvents on the linear absorption, photoluminescence and two-photon absorption cross-sections were also examined for these chromophores. The stoke's shift increase with increasing solvent polarity, that can be adequately described by Lippert equation. Two-photon absorption cross sections were measured with femtosecond pulses by the two-photon-induced fluorescence technique. It was observed that two-photon cross-sections were also strongly dependent on the solvents, however no simple correlation with solvent polarity was found in this study. Interestingly, a linear relationship was observed in these chromophores between the molar extinction coefficient and the two-photon cross section when plotted in log-log formats. Understanding of the relationship may provide a better insight of the two-photon absorption processes, and potentially will contribute to the design of highly efficient two-photon absorbing chromophores.

Two-photon absorption (TPA), a molecular excitation process by the simultaneous absorption of two photons, has recently attracted growing interest in many photonic and optical applications because of the availability of chromophores that exhibit large, effective TPA cross-sections. Over the past 6-7 years, we have successfully synthesized a family of TPA molecules (designated as AFX) that possess large nonlinear optical properties, including unsymmetrical and multi-branched chromophores comprised of 'electron acceptor-aromatic bridge-electron donor’ structural motif. These chromophores are excitable with useful wavelengths around 800 nm and fluoresce in the 400-450 nm region. However, practical uses of these chromophores depend on developing functionalization chemistry pertinent to specific applications. We are particularly interested in incorporating these TPA active molecules into polymeric or low melting glassy materials that are amenable to easy processing and fabrication. Our recent results in this direction will be discussed in this paper.

We review the progress we have been making in recent years in the application of the real-time real-space higher-order finite-difference method to the calculation of various linear/nonlinear response functions. The method was devised for numerical calculation of electronic properties of large quantum systems, and has so far been applied primarily to the calculation of dielectric functions. However, the introduction of a fast statistical algorithm for intermediate state averaging makes the method promising also for computing nonlinear response functions. With the use of random vector averaging for the intermediate states, the task of evaluating the multi-dimensional time integral is reduced to calculating a number of one-dimensional integrals. Then the CPU time necessary for computing a nonlinear response function scales only linearly both with the number of basis states and with the inverse of the required energy resolution, irrespective of the order of nonlinearity. The effectiveness of the algorithm is demonstrated in the calculation of the TPA spectra of silicon. We discuss future applications in such areas as the investigation of electronic properties of biomolecules and complex systems, and designing materials of large nonlinear optical properties.

An accurate prediction of two-photon absorption (TPA) spectra, especially the position and magnitude of the peak cross-section, as well as the line width, would be of value in the design of TPA molecules for specific applications. The spectrum can be calculated using the sum-over-states (SOS) formalism, which requires accurate values for the energies of the relevant electronic states and for the transition dipole moments between these states. In addition, the form of the line-width function, such as Gaussian or Lorentzian, as well as its value, must be determined. Time-Dependent Density Functional Theory (TDDFT) has been shown to give accurate excitation energies and ground-state transition dipole moments for a wide variety of molecules. It is now possible to use this method to calculate excited-state transition dipole moments and to subsequently calculate TPA cross-sections using the SOS formalism. We report TDDFT calculations of the TPA spectra for trans-octatetraene and trans-stilbene. Gaussian functions have been used to describe the TPA line-broadening, with the width determined from experiment or previous convention. The energy levels, transition dipole moments, and TPA cross-sections calculated by this and other methods are compared to experiment and to other theoretical methods.

Organic materials with large excited state and/or two-photon absorption are desired for numerous device applications, such as optical limiting, two-photon upconversion lasing, three-dimensional data storage, and two-photon photodynamic therapy. Dialkylamino styryl thiazolium/benzoxazolium compounds are interesting excited state and two-photon absorbers for these applications. In this work, the nonlinear transmissivity of trans-2-[p-(N-methyl-N-(hydroxyethyl)amino)styryl]-N-methylthiazolium iodide (MHAST), trans-2-[p-(N-ethyl-N-(hydroxyethyl)amino)styryl]-N-methylthiazolium hexafluorophosphate (EHAST), trans-2-[p-(N-methyl-N-(hydroxyethyl)amino)styryl]-N-methylbenzoxazolium iodide (MHASBO) at 532 nm has been studied using 6 ns laser pulses. The two-photon induced fluorescence of these compounds at 940 nm has also been investigated. At 532 nm, both MHAST and EHAST exhibit reverse saturable absorption, however, MHASB exhibit saturable absorption. At 940 nm, all of these compounds exhibit two-photon induced up-conversion fluorescence, and the fluorescence intensity varies when the chemical structure changes. These preliminary results suggest that the nonlinear absorption (excited state absorption or two-photon absorption) characteristics of these compounds vary at different wavelengths and vary when their chemical structure changes.

Spectral and time-resolved fluorescence studies of different eumelanins (natural, synthetic, enzymatic) in solution have been carried out by two-photon excitation at 800 nm, using 80 fs pulses with photon flux densities ≤ 10²⁷ cm^-2.s^-1. Whereas all samples show monotonously decreasing absorption between near UV and near IR, their fluorescence behavior indicates strong heterogeneity. With respect to the also measured one-photon excited fluorescence (OPF) of melanin at 400 nm, the overall spectral shape of the two-photon excited fluorescence (TPF) is red-shifted. Both OPF and TPF exhibit three-exponential decay with a shortest component £ 200 ps. As is also shown, the fluorescence properties of melanin are dependent on the micro-environment. This allows the hypothesis, that the process of malignant transformation in skin tissue could be reflected in the fluorescence, provided the melanin in skin is selectively excited. The latter is realized by the described stepwise absorption of two 800 nm photons. In this way, indeed characteristic differences between the TPF spectra of healthy tissue, nevus cell nevi and malignant melanoma have been found.

The nonlinear energy dynamics and charge carrier generation of a model donor-acceptor two-photon dye, AF-380, is investigated using ultrafast nonlinear spectroscopy. This material has been the focus of several investigations due to its large reported two-photon absorption cross-section, the discrepancies between long and short pulse measurements, and its use in microfabrication through holographic two-photon induced photopolymerization. It is believed that a substantial excited state absorbance can account for the difference in two-photon cross section measurements, and furthermore, that the resulting excited state exhibits a lifetime substantially long enough to affect subsequent absorption of longer pump pulses or higher repetition rates. Experimental and theoretical considerations of the electronic dynamics are discussed using a rate analysis and beam propagation code for comparison.

4 -[4 - (Dimethylamino) styryl) - 1 - docosyl pyridinum bromide ](abbreviated as DASPB) is a nonlinear dye which shows strong two-photon absorption (TPA) and subsequent frequency upconversion fluorescence behavior when excited with near infrared radiation. The dye possesses a much larger TPA cross section and much stronger upconversion fluorescence emission than those of common organic dyes (such as rhodamine). In order to increase the concentration of absorptive or fluorescent centers as well as the optochemical and optophysical stability and effective use of this highly nonlinear dye, it has doped in methyl methacrylate - methacrylic acid co-polymer (PMMA-MA). Linear absorption spectrum, single photon induced fluorescence spectrum and two photon induced fluorescence at different wavelength are studied. The nonlinear optical transmission at both linear absorption region and linear transmission region are studied using nano and pico second laser pulses. The optical limiting behavior of the dye is also studied in cw, pico and femto second region. Using Z-scan studies it is found that in linear absorption region, the DASPB doped in PMMA-PA shows saturation absorption at lower intensity and reverse saturated absorption at higher intensity. At transmission region above 800 nm, it has shown the absorption due to two photon induced fluorescence as well as reverse saturation absorption. With experimental results, the molecular TPA cross-sections and TPF cross-section of the sample are calculated.

We report in this article use of two-photon absorption (TPA) as an effective tool for three-dimensional (3D) micro-nano fabrication. According to materials adopted, the research is classified into two categories: photopolymerizable resins for micro-nano structures and devices and photorefractive/photochromic materials for 3D data storage. In each case, fabrications were benefited from the 3D spatial resolution that is intrinsic to TPA processes.

Two-photon absorption phenomena are useful for the generation of short wavelength radiation. When the same atom absorbs IR and visible photons it generates uv/visible energy. Such two-photon absorption phenomena in certain rare-earth ion doped materials have been implemented in our laboratory. The importance of these studies in fibers will be discussed. Infrared quantum counter studies in Eu, Tb, Ho, Er, and Pr doped fibers are proposed for future work.

Eu3+-doped crystalline Si-enriched SiO2 nanocomposite thin films were prepared using Ar sputtering deposition on quartz substrates. By conventional laser spectroscopy the material was characterized in either frequency or time domain. The results show that the doped europium ions are present in trivalent state, as Eu3+. They are distributed in SiO2 matrix and on the boundary surface of c-Si nanoparticles. With increased excitation intensity at 532nm, two-photon absorption (TPA) induced new emission was observed. It is characterized by an additional broadband emission with a peak at 560nm and lifetime of ~ 0.8 s. This feature has been identified as the emission from Eu2+ ions. Further measurements reveal that the observed phenomenon originates from charge transfer process giving rise to a photoinduced transient valence switching from Eu3+ to Eu2+ in this particular material. Free carriers were originally created in the conduction band (CB) of c-Si nanoparticles by TPA, then trapped at the surrounding Eu3+ center due to strong Coulomb interaction. Luminescence of the formed divalent Eu2+ is characterized by a broadband d → f transition with fast decay rate. Degenerate four-wave-mixing experiment further revealed that in undoped sample TPA-created charge carriers in CB were trapped in shallow centers of Si nanoparticles. The trapping has an average time period of 500ps, and the carriers were then released for recombination. In Eu3+ -doped sample, however, the average time period of 500ps was no longer observable. It therefore suggests that the strong Coulomb attraction results in immediately capturing of the created charges by positive Eu3+ center.

We present the results of combined single and two photon linearly polarised time resolved fluorescence anisotropy measurements of the order and motion of a fluorescent probe (rhodamine 6G) in the nematic phase of 4-n-pentyl4'-cyanobiphenyl (5CB). Variation of the excitation polarisation angle (β) with respect to the nematic director yield a set of initial single and two photon anisotropies R(0,β). Single photon R(0,β) measurements yield the and moments of the ground state orientational distribution function. For rhodamine 6G in 5CB these indicate that the inclusion of higher moments ( and above) are necessary to describe the probe ordering within the nematic host.. Two photon R(0,β) measurements however allow the direct measurement of , for rhodamine 6G these yield a value close to theoretical predictions. Two and single photon initial anisotropy measurements are wholly consistent with an approximately Gaussian probe distribution at an angle of 38° to the nematic director with a full width half maximum of c.a. 26°. Variation of β affords the photoselection of both cylindrically symmetric and asymmetric degrees of probe alignment that are sensitive respectively to θ and θ plus φ diffusion in the laboratory (nematic director) frame. Cylindrically symmetric and asymmetric alignment relaxation are observed to be linear but with distinctly different relaxation rates, indicating highly restricted probe motion within the nematic environment.

We report image formation via single and two-photon photoinduced fluorescence changes in a polymeric medium with two-photon fluorescence readout of multiplayer structures. Photoinduced acid generation in the presence of a two- photon fluorescent dye possessing strongly basic functional groups (7-benzothiazolyl-9,9-didecyl-2,2-(N,N- diphenylamino)fluorene underwent protonation upon exposure with UV or near-JR (740 nm fs pulses). Solution studies demonstrate formation of monoprotonated and diprotonated species upon irradiation, each resulting in distinctly different absorption and fluorescence properties. The fluorescence ofthe original, neutral, fluorophore is quenched upon monoprotonation with a concomitant increase in fluorescence at longer wavelengths due to the monoprotonated form. Hence, two-channel two-photon fluorescence imaging provides "positive" or "negative" image readout capability. Results of solution and solid polymer thin films experiments are presented.

Our aim has been the design of optimized NLO-phores with very high two-photon absorption (TPA) cross-sections (s2) in the red-NIR region, while maintaining high linear transparency and high fluorescence quantum yield. Our molecular engineering strategy is based on the push-push or pull-pull functionalization of semi-rigid nanoscale conjugated systems. The central building blocks were selected as rigid units that may assist quadrupolar intramolecular charge transfer by acting either as a (weak) donor or acceptor core. Quadrupolar molecules derived either from a phenyl unit, a rigidified biphenyl moiety or a fused bithiophene unit have been considered. Conjugated oligomers made of phenylene-vinylene and/or phenylene-ethynylene units were selected as connecting spacers between the core and the electroactive end groups to ensure effective electronic conjugation while maintaining suitable transparency/fluorescence. The TPA cross-sections were determined by investigating the two-photon-excited fluorescence properties using a Ti:sapphire laser delivering fs pulses. Both the nature of the end groups and of the core moiety play an important role in determining the TPA spectra. In addition, by adjusting the length and nature of the conjugated extensor, both amplification and spectral tuning of TPA cross-sections can be achieved. As a result, push-push fluorophores which demonstrate giant TPA cross-sections (up to 3000 GM) in the visible red, high fluorescence quantum yields and good transparency in the visible range have been obtained.

Barium titanate, Ce-doped barium titanate and La- and Zr-doped lead zirconate titanate thin films were prepared on fused-quartz substrate by laser pulsed deposition. The morphologies and microstructures of the films were examined by x-ray diffraction, scanning electron microscopy and transmission electron microscopy. The barium titanate and Ce-doped barium titanate thin films and Zr-doped lead zirconate titanate film are transparent respectively above 322 and 340 nm; while the corresponding optical band gaps are 3.46, 3.48 and 3.46eV. The two-photon absorptions coefficients for barium titanate and Ce-doped barium titanate thin films are very large, as large as 51.7 and 59.3 cm/GW, respectively. Nonlinear refractive index of PLZT film is 1.3 X 10-6 esu, much higher than organic polymers, CdS-doped glasses and high density Au-dispersed SiO2 composite films. The mechanisms for the strong nonlinear optic susceptibilities and large two-photon absorption appeared in barium titanate and Ce-doped barium titanate and Zr-doped lead zirconate titanate film are discussed. On the basis of first principle calculation the nonlinear optic susceptibility and nonlinear two-photon absorption in BaTiO3 was performed. The theoretical calculations are in good agreement with experimental results.

In this paper we report the observation of nonlinear behavior of Kerr type of developed and bleached photographic film using cw laser sources. The nonlinear characterization is made using the Z-scan technique with Argon and He-Nel lasers beams. The results show that depending on the intensity, wavelength and exposition time the refractive index changes can be reversed or permanent giving new application to this material.

A new formulation of ab initio theory is presented that treats a large molecule in terms of wavefunctions of its constituent molecular subunits (to be called fragments). The method aims to achieve near conventional ab initio accuracy but using a truncated set of fragment orbitals with a consequent drastic reduction of computing time and storage requirement.

Maximization of scan speed and throughput while maintaining resolution is critical for commercialization of two-photon microfabrication technology. We report contrast curves for cationic and free radical two-photon polymerization. An unusual relationship between voxel shape and dose is observed where the voxel ranges from highly asymmetric (aspect ratio of 4:1 or greater) at high dose to nearly spherical low dose (aspect ratio 1:1 or lower). Similar behavior is observed for both types of polymerization systems suggesting that the change is aspect ratio is not a function of the specific type of chemistry. It is found that a simple optical model describing the intensity distribution near the focal point can predict the change in aspect ratio.