Theoretical guidance, provided by quantum and statistical mechanical calculations, has aided the recent realization of electro-optic coefficients of greater than 300 pm/V (at 1.3 microns wavelength). This articles attempts to provide physical insight into those recent results and to explore avenues for the further improvement of electro-optic activity by structural modification, including to values of 500 pm/V and beyond. While large electro-optic coefficients are a necessary condition for extensive practical application of organic electro-optic materials, they are not a sufficient condition. Adequate thermal and photochemical stability, modest to low optical loss, and processability are important additional requirements. This article also examines such properties and suggests routes to achieving improved auxiliary properties.

Structurally related chromophores of different symmetry (dipolar, V-shaped, octupolar) are investigated and compared for elucidation of the combined role of branching and charge symmetry on absorption, photoluminescence and two-photon absorption (TPA). Their design is based on the assembly of one, two or three π-conjugated dipolar branches on a central core. Two series of branched structures obtained from a central triphenylamine core and dipolar branches having different charge-transfer characters are investigated: photophysical properties are studied and TPA spectra are determined through two-photon excited fluorescence experiments using fs pulses in the 700-1000 nm range. Calculations based on time-dependent quantum-chemical approaches, as well as the Frenkel exciton model, complement experimental findings. Experiments and theory reveal that a multidimensional intramolecular charge transfer takes place from the central electron-donating moiety to the periphery of the branched molecules upon excitation, whereas fluorescence stems from a dipolar branch. Symmetry and inter-branch electronic coupling are found to be responsible for amplification of the TPA response of branched compounds with respect to their monomeric analogues. In particular, an enhancement is observed in regions where the TPA bands overlap, and TPA activation is obtained in spectral regions where the dipolar analogue is almost two-photon transparent. Thus, appropriate tuning of the number of branches, of the coupling between them, and modulation of intramolecular charge transfer from core to periphery open the way for substantial improvement of TPA efficiency or TPA induction in desired spectral regions.

Various push-pull azobenzene molecular derivatives showing glassy properties have been processed as thin monomeric amorphous films using spin-coating and thermal evaporation in vacuum. This latter deposition technique yielded homogenous thin films whose surface RMS roughness has been found less than 0.3 Å. Contrarily to spin-coated films requiring external poling for SHG emission, evaporated thin films have proved spontaneously active in second harmonic generation (SHG) when probing at 1.907 μm. Heating above T_g and continuous illumination with an Ar⁺ laser in the sample absorption maximum band caused dramatic and irreversible disappearance of the spontaneous SHG signal. Applying an external static electric field by using corona discharge induced SHG reversible modulation whose sign has been tuned by changing the bias from positive to negative. Films obtained by vacuum evaporation under a 4 kV voltage underwent considerable morphology changes characterized by the formation of stable 250 nm high nanoclusters evidenced by AFM imaging. The dramatic decrease in SHG has been attributed to the formed nanopillars whose growth depends apparently on the substrate polarization state and must obey a centrosymmetric organization.

A series of side-chain electrooptic (E-O) polymers have been prepared by Diels-Alder reaction in a solid state and characterized for their nonlinear optical properties. A synthesized chromophore were easily attached to a pendent anthracenyl moiety functionalized on the poly(methylmethacrylate-co-anthrylmethylmethacrylate) thermally in the bulk films during the poling process without compromising E-O performances. We have also controlled a chromophore concentration to determine its critical loading density at which chromophore-chromophore electrostatic interaction occurs in the polymer matrix. The highest E-O coefficient was 110 pm/V for the 34 wt% of the doped chromophore in the polymer at the wavelength of 1.3 μm. A high loading density of chromophore was obtained without observing a severe phase separation in the polymer matrix by an AFM morphology study. This novel approach provides to demonstrate a strategy for developing highly efficient E-O materials with the full potential of a chromophore.

Recent breakthroughs in developing exceptional organic electro-optic (EO) materials are reviewed. Whole series of guest-host polymers furnished with high μβ chromophores have shown large electro-optic coefficients around 100~160 pm/V @ 1.31μm. Moreover, new generation of NLO chromophores based on pyrroline and pyrrolizine acceptors have been designed and synthesized. To go beyond the typical oriented gas model limit for poled polymers, new approach of using nanoscale architecture control and supramoleaular self-assembly has been proved as a very effective method to create a new paradigm for materials with very exciting properties. The approaches of employing Diels-Alder reactions for postfunctionalization and lattice hardening also provide a facile and reliable way to generate high-performance EO polymers and dendrimers. This type of "click" chemistry paves the way to systematically study the relationships between chromophore shape and number density, controlled self-assembly, in addition to provide the material properties needed for multi-layer device fabrication. Finally, a new generation of binary monolithic glasses has been developed that exhibit unprecedented high EO activities through careful manipulation of intricate supramolecular interactive forces for self-assembly. The results obtained from these poled binary organic glass materials (r₃₃ as high as 310 pm/V at 1.31μm) are the highest values ever reported which are >10 times of the commercial lithium niobate crystals. The success of these material developments has recently inspired the exploration of new device concepts trying to take full advantage of the organic EO materials with ultrahigh r₃₃ values.

Sum rules have been shown to impose a fundamental limit on the of nonlinear-optical susceptibility. All of the measured values of the hyperpolarizability and second hyperpolarizability over the last 25+ years, be it on- or off-resonance, fall a factor of 10^3/2 below these limits. Not only is this result scientifically puzzling on a fundamental level; but, has implications on the kinds of practical devices that can be made. In this work, we discuss molecular engineering techniques that aim to break this bottleneck.

We report a systematic investigation on organic-inorganic hybrid sol-gels doped with nonlinear optical chromophores. The host is based on photosensitive sol-gels prepared from precursor of 3-(methacryloxy)propyl trimethoxysilane-zirconium (or aluminum) oxide (MAPTMS-Zr(Al)) hybrid system. The chromophores are incorporated into the hybrid sol-gels as both guest and side-chain. Second harmonic generation experiment has been conducted to optimize the poling parameters for the active sol-gel films and examine the stability of the second-order nonlinear optical coefficients. Electro-optic channel waveguides have been fabricated by direct ultra-violet (UV) exposure, direct blue laser writing, and reverse-mesa methods. Electro-optic effect of the waveguides is measured at 1550 nm. All the results show that the hybrid sol-gels are promising media for electro-optic devices for integrated optics in both performance and fabrication flexibility.

Despite many advantages toward nonlinear optical (NLO) waveguide devices, NLO polymers have not been adopted successfully into practical wavelength converters due to their high absorption losses. Empirical and theoretical understandings about NLO susceptibilities imply the fundamental trade-off between optical absorption and nonlinearity. Our theoretical analysis elucidates the effect of absorption losses on second-harmonic generation, difference-frequency generation, and cascaded wavelength conversion. We compare analytically maximum conversion efficiencies for those NLO processes with several NLO polymers and suggest that the cascaded wavelength conversion is a plausible application of NLO polymers. Furthermore, we found a convincing approach for the development of NLO polymers with the optimum combination of high optical nonlinearity and low material absorption, which leads us to realize efficient polymeric wavelength converters.

We present the results of the combination of two independently valid optimization strategies for the first hyperpolarizability of ionic organic chromophores. The first strategy to enhance the nonlinear optical response, at the molecular level, is the extension of the conjugation path in the chromophore itself. The second strategy, at the supramolecular level, is the bottom-up nano-engineering of an inclusion complex of the chromophore in an amylose helix by self-assembly. We have studied a series of five (dimethylamino)stilbazolium-type chromophores with increasing conjugation length between the (dimethylamino)phenyl donor ring and the pyridinium acceptor ring in combination with four amylose helices of different molecular weights. The first hyperpolarizability of the self-assembled inclusion complexes has been experimentally determined by frequency-resolved femtosecond hyper-Rayleigh scattering at 800 and 1300 nm. These values are compared with experimental values for the free chromophores in solution and with theoretical values. Where experimental values for the hyperpolarizability in solution were lower than theoretically predicted, an enhancement upon inclusion was observed - with the longest chromophore in the best amylose helix showing an enhancement by one order of magnitude. Molecular modelling of the inclusion of the chromophore suggests that the coplanarity of the two rings is more important than all-trans configuration in the conjugation path. The degree of enhancement, however, is not enough to breach the apparent limit of the first hyperpolarizability which is about an order of magnitude below the fundamental limit calculated by Kuzyk. This analysis confirms the determining role of the arrangement of the excited-state energy levels on the nonlinear response.

New nonlinear organic materials are conveniently studied and characterized as thin film samples. The macroscopic nonlinear response described by the susceptibility tensor is closely related to the quality of the sample, ordering of molecules, and other properties of the film. In order to characterize the nonlinearity properly and to access the resulting information, the susceptibility tensor should be accurately determined. Unfortunately, this requires a theoretical model of the nonlinear interaction, where certain assumptions, whose validity is often difficult to verify, must be made. This may compromise the reliability of the results. We use a technique based on the polarization properties of second-harmonic generation to characterize thin films, which facilitates the verification of the assumptions while allowing the determination of the susceptibility tensor. Chiral molecules have no center of symmetry and are therefore naturally very interesting subjects of study in secondorder nonlinear optics. We apply our technique to chiral, anisotropic Langmuir-Blodgett films of helicene molecules to obtain the relative values of the components of their susceptibility tensors and estimates of their accuracy. Usually such samples are assumed to have C₂ symmetry, but we prepare samples having different structures and report results where the symmetry groups C₂ and D₂ can be distinguished. In addition, the orientation of the in-plane symmetry axis that appears in D₂ but is absent in C₂ is determined.

Organic-inorganic hybrid sol-gel materials have attracted increasing attention in recent years as low-cost, rugged materials for integrated optical devices such as optical couplers, splitters, and electro-optic modulators. These materials can be easily processed by spin-coating, wet-etching photolithography, and low-temperature baking. Precise control of waveguide core-cladding refractive indices produces well-confined low-loss propagation and good matching of the absolute refractive index to that of fused silica results in low optical coupling loss to optical fiber. The increased thermal and mechanical stability of these materials, relative to optical polymers, results in numerous packaging options and improved reliability. However organic-inorganic hybrid sol-gel materials have not yet been often used as host of active dopants such as erbium (III) ions for 1550nm optical amplification. This limitation owes primarily to matrix and chelate dominated nonradiative relaxation processes, as high phonon energy OH and OH-like oscillators can bridge off the energy from the excited erbium (III) ions at very high rates. Different strategies have been proposed to protect erbium (III) ions from matrix and chelate quenching, including host and ligand fluorination, and inorganic microstructure shielding. Here we report on our work of encapsulating erbium (III) ions in transparent, refractive index matched, and highly re-dispersible lanthanum phosphate nanoparticles and the work of examining the optical properties of these nanoparticles as active dopants in organic-inorganic hybrid sol-gels adopting 2-methacryloxypropyl trimethoxysilane (MAPTMS) as a precursor. 980nm laser pumped photoluminescence at 1535nm was obtained from solid bulk samples of 300mg La.₉₉Er.₀₁PO₄ nanoparticles doped in 1mL hybrid sol-gel. Thick bulk samples of this composition exhibited exceptional clarity and little trace of nanoparticle scattering effects. The lifetime of the nanoparticle doped hybrid sol-gel composite was measured to be 220μs, indicating an intermediate relaxation rate between that of an erbium organic complex and annealed erbium doped glass. La.₉₉Er.₀₁PO₄ nanoparticle doped hybrid sol-gel films were also prepared and the refractive index was measured to be 1.4966 at 1550nm, which is very close to that of optical fiber and provides a suitable index difference from an undoped and metal oxide tuned sol-gel at 1.4870 to comprise an efficient single-mode waveguide system.

We report on the initial time-resolved luminescense and nonlinear absorption properties of two polythiophenes 3-substituted with chiral charged amino acid-derivatized substituents, POWT and POMT. The photo-physical characterization yielded quantum efficiency typically in the range 0.01 - 0.1, however, with two-photon absorption cross-section better than or similar to a typical two-photon reference chromophore, such as fluorescein. They were tested as conformational sensitive optical probes for the recording of pH-induced conformational changes of synthetic peptides, proteins and samples of protein amyloid fibrils characteristic of amyloid related diseases. Particularly, the POMT polyelectrolyte with the L-enantiomeric side chains is shown to favor this induction of well defined structure as judged by the circular dichroic signal as well as a stronger enhancement of luminescense for the L-form over the D-form complex. Furthermore, time-resolved fluorescense and two-photon induced fluorescence both also showed a difference in the complexation with the D and L form. This shows that the multi-photon excitation path can be an efficient means for chiral recognition of biomolecular complexes. It is demonstrated how the conjugated polyelectrolyte L-POMT can be used to spectrally image the formation of amyloid fibrils of insulin using both one- and two-photon absorption based fluorescence imaging.

We demonstrate that two-photon induced molecular orientation of diarylethene (DE) in thin films of poly-methyl-methacrylate (PMMA) and multi-photon induced anisotropic bleaching of disperse red one (DR1). We confirmed that two-photon absorption was induced in DE by measuring the quadratic dependence of isomerization rate on excitation light power, and we observed two-photon photo-orientation of DE in dichroic absorbance. Disperse red one (DR1) was orientationally bleached in PMMA by polarized multi-photon excitation. Anisotropic refractive index changes result from the nonlinear photobleaching of DR1 was detected by a polarization confocal microscope, and bit-oriented data storage was demonstrated.

Investigations of the enhancement process in two-photon absorption organic dendrimers are presented in order to give further details of the mechanism of enhancement. Organic dendrimers based on the branched octupolar trimer building block molecule are studied by time-resolved fluorescence, transient absorption, and three-pulse photon echo peak shift measurements. The time resolved measurements suggest that the fundamental (spectroscopic) unit of excitation in these dendrimers is larger than the trimer building block molecule. These results suggest that it is possible to build dendrimers with enhanced two-photon absorption properties beyond the trimer molecule situation.

We report on electrooptical modulation with a sub 1Volt sensititivity in a photonic crystal slab waveguide resonator which contains a nanostructured nonlinear optical polymer. The electrooptical susceptibility in the core was induced by high-electric-field poling. A square lattice of holes carrying a linear defect was transferred into the slab, creating a photonic crystal slab based resonator. Applying an external electric modulation voltage to electrodes placed underneath and on top of the assembly leads to a modulation of the transmission at a fixed wavelength. This modulation effect is based on the electronic displacement polarization in a noncentrosymmetric medium (Pockels-effect) and is therefore inherently by more than three orders of magnitude faster than any other reported electrooptic modulation effect in nanophotonics.

We report on the application of poled electro-optic (EO) polymer films in a gap-free, broadband terahertz (THz) system. Using polymer films consisting of 40% Lemke/60% APC (LAPC) as an emitter-sensor pair and a Ti:sapphire regenerative laser pulse amplifier operated at 800-nm-wavelength, we generated and detected transient THz waves, via the optical rectification and EO effect, respectively. We obtained ~12-THz bandwidth from this system with no absorption gaps. The absence of resonant absorption gaps normally seen in THz systems based on crystalline EO materials is attributed to the amorphous form of the polymer films, making our EO polymer emitter-sensor pair advantageous over EO crystals in a gap-free, broadband THz time-domain-spectroscopy (THz-TDS) system. A model has been developed to simulate the spectrum from THz systems and the simulation results were compared with the experimental results. We also report our experiments and simulations for the pulsed THz waves generated by a EO polymer film consisting of 40% DCDHF-6-V/60% APC (DAPC) and detected either by an 80-μm ZnCdTe or a 2-mm ZnTe sensor, with 1300-nm-wavelength pulses from an optical parametric amplifier (OPA). In addition, with the help of our model, we propose employing a wavelength tuning technique to achieve good phase-matching for polymer emitter/sensor pairs, which should lead to very broad bandwidth.

Using modified Teng-Man reflection ellipsometry, very high linear electro-optic coefficients (r₃₃ = 250 - 300 pm/V) have been measured in thin films of poled organic glasses. The glasses consist of two chromophores designed to yield synergistically enhanced orientation during the poling process. The chromophores were ordered by the contact poling method under moderate electric fields of ~ 0.44 MV/cm. Compared to measurements made 1-4 hours after poling, the electro-optic coefficient relaxed to a value about 15% lower in a period of one week and thereafter remained relatively stable at room temperature. We report both standard Teng-Man reflection type measurements made at a 45° angle of incidence as well as a more complete analysis of nonlinear reflection ellipsometric data as a function of angle of incidence and optical bias. The more complete analysis takes into account the properties of the multilayer stack structure of the test samples consisting of glass/ITO/NLO-organic/gold. Limitations of a simple model to analyze Teng-Man reflection data will be discussed, as well as contributions of electrochromism.

Hyper-Rayleigh scattering (HRS) is used to measure the first-hyperpolarizability (β) of electro-optic (EO) chromophores. One of the inherent concerns in any HRS measurement is the extent to which resonant enhancement contributes to the observed intensity thereby leading to inaccuracies when evaluating chromophore potential for application in electro-optical devices. One way to address this concern is to employ increasingly longer excitation wavelengths far from resonance. However, in charge-transfer-based non-linear optical chromophores, enhanced β generally correlates with a red-shift of the charge transfer absorption band so that even at the longest excitation wavelengths generally employed in HRS studies, resonant enhancement remains an issue. We have adopted an alternative approach in which the wavelength dispersion of the HRS intensity is determined by performing measurements at a variety of excitation wavelengths. This approach allows one to ascertain the role of resonance enhancement thereby allowing for more accurate correlation of improved β with molecular architecture. We report the results of our HRS studies for nine chromophores employing excitation wavelengths ranging from 780 to 1907 nm. Our HRS results demonstrate good agreement with the predictions of density functional theory. This synthesis of experimental and theoretical techniques has resulted in more effective designs for the next generations of electro-optical chromophores.

The successful development of high-performance polymer optical waveguides depends critically on the stability of the materials during device fabrication. Typically, ensuring the stability of the organic chromophores contained in these materials represents the most difficult challenge. We present an overview of the mainly spectrophotometric techniques recently developed in our laboratory that have served as exceptionally useful tools in materials development. In addition, recent results involving the relative stability of various chromophores under exposure to simulated processing conditions are described. These allow for an improved understanding of the effects of variables such as temperature, oxygen concentration, and radiation intensity during fabrication on the performance of polymer optical waveguides.

To improve the reproducibility of passive alignment, large core single mode waveguides are demonstrated, which can be connected to thermally expanded core fibers so as to increase alignment tolerance. It is shown that polymer waveguide with a core dimension of 25 μm x 25 μm and an index contrast less than 0.001 can satisfy single mode condition. As a novel functional device incorporating the large core waveguide, variable optical attenuators (VOA) are designed and fabricated. For the fabrication of the thick core structure, a soft molding process is developed. Due to the small index contrast of the waveguide, efficient attenuation is expected for the smaller electrical power consumption, which is confirmed by 3-dimensional beam propagation method. From the fabricated VOA device, more than 20 dB of attenuation is obtained by applying 20 mW.

Photoinduced electrical switching was investigated using photochromic diarylethenes (DA), which exhibit reversible photo-isomerization reactions with alternative UV/Vis light irradiation. Photo electrical switching could be introduced into DA molecules by connecting with electron conjugative groups (EC). The oxidation potential of the solution containing DA-EC polymer in a closed form was significantly shifted to a lower value compared to that of an open-form, indicating the electron conjugation in the closed form. The photochromic film prepared from the copolymer of DA-EC become dark colored upon exposure to a UV light source and the new absorption band centered at 570-620 nm was observed from the colored film. A solid cell structured as Au/DA-EC polymer/ITO showed characteristic I-V response. Significant increase in the current was observed from I-V plot for the cell after the UV light irradiation. Exposure of the cell to a visible light resulted in significant decrease in current. The electrical property of the film could be reversibly switched in a solid cell by alternatively irradiating the cell with UV and Vis light. The current-voltage (I-V) characteristics of the cell was related to the photochromic reaction from the colored ring-closed form to the bleached ring-open form state. Thus DA-EC polymer structure is a viable model for a photochromic electrochemical switch, to produce a persistent reversible electrochemical switching vis photon mode.

The chromophore orientation distribution in photoaddressable bis-azo copolymer thin films is investigated as a function of the poling temperature and the chromophore concentration. The first, second, and third order parameters are deduced from the linear electro-optic coefficients and the linear dichroism by free-beam interferometry and angle-resolved spectroscopy, respectively. Absorption spectra obtained by density-functional calculations are compared with the experimentally observed ones and support the picture of differently aligned bis-azo dye molecules in a trans,trans-configuration. The various kinds of ordering in samples poled at different temperatures is confirmed by complementary wide-angle X-ray scattering. Furthermore, we show that a higher thermal stability for applications can be reached by fractionating the polymer, and how such a material can be successfully employed to build a large-area Fabry-Perot-Modulator using the electro-optic effect.

We report on strong photomechanical effects that we have observed with photochromic diarylethene microcrystals. Clear single crystals become colored under UV irradiation, then they jump when the absorbed energy reaches about 10 μJ. A simple model shows that the elastic energy accumulated during the photoreaction is enough to produce the kinetic energy.

Second-order nonlinear optical properties of organic and inorganic materials have been widely investigated. However, it becomes more important to be able to switch the molecules from an 'on' state to an 'off' state, or vise versa. Several switching schemes can be thought of. We will focus our attention to alter the donor properties of metalorganic compounds. Therefore, we will oxidize the metal center of the compound chemically. Since it is more interesting to be able to electrochemically switch the metal center, we developed a combined electrochemistry/hyper-Rayleigh scattering cell to perform in-situ electrochemical switching of the hyper-Rayleigh response.

A novel wavelength selective 2x2 switch based on two microring resonators is proposed. The device consists of two intersecting channel waveguides with each of the two rings coupled to both waveguides. Depending on the control voltage the light of selected wavelength either can propagate through the straight waveguide or can be coupled to one of the rings and from the ring couple to the perpendicular output. Therefore every ring can independently switch one wavelength from any input to the perpendicular output. This wavelength selective switching adds to the traditional switch functionality. Theoretical calculations were carried out to determine optimum parameters of the switch and soft-lithography was used for implementation. Being fabricated from the latest electro-optical polymers this device can provide very high switching speed and low operation voltage.

In this paper, we present a novel device structure for organic electro-optic modulators using transparent conducting oxides (TCOs) as electrodes to substantially reduce the switching voltage, and describe their fabrication. We report two different types of device geometry, a top conducting and a side conducting geometry, and discuss their strengths and weaknesses. We discuss how the voltage and speed performance of such modulators are dependant on the conductivity/optical loss ratio of the TCO electrodes. Our device simulation shows that by appropriately engineering the high TCO conductivity/optical loss ratio, 4-6x lower switching voltage can be achieved while still maintaining high modulation frequencies and low optical loss. We show that certain new TCO materials are capable of achieving the high conductivity/optical loss required for efficient modulation in the 1300-1550 nm wavelength range. We summarize the optical loss characteristics at 1300 nm of different types of thin-film TCO materials grown using different deposition techniques. TCO electrodes based on different types of materials, such as In₂O₃, ZnO, and ITO have been investigated for our device structures. Fabrication issues associated with the deposition of TCO electrodes directly on organic EO materials and our approach to addressing them are discussed. Initial results for organic EO modulators fabricated with TCOs as electrodes are presented, and the performance of these modulators are compared with theoretical modeling results. The new device structures presented here will enable next generation low-voltage organic EO modulators targeting RF photonics applications.

6-Methoxy naphthaldehyde is prepared by the Grignard reaction of 2-bromo 6-methoxy naphthalene with triethylorthoformate. The formation of the compound is confirmed by recording IR and ¹H NMR spectra. The SHG frequency conversion efficiency of this compound was measured by powder technique using Nd: YAG laser. This compound showed NLO property 0.59 times that of urea. The crystal system is found out by single crystal X-Ray diffraction studies and the crystal system found to be orthorhombic and space group P₂₁₂₁₂₁ with cell parameters : a = 15.657(10) Å, b =16.385(4)Å, c = 7.491(15) Å, α = 90^o, β = 90^o, γ = 90^o. Its bulk crystal of the size 39x 41x14mm³ is grown from acetone solvent by slow evaporation technique.

In this paper the results of study on the nonlinear and lasing output characteristics of an organic polymer system, are presented. To describe the dynamics of light field in the medium, the forced oscillation model of optical multi-stable and lasing system are used. For the matter model, the material with the conjugate structure, such as polydiacetylenes which energy structure can be reduced to a three-level system considered; meanwhile the approach of efficient susceptibility is used to deduce the nonlinear susceptibility. Under these conditions, theoretical analyzing and numerical simulation shows that the system appears special output characteristics and dynamic behaviors, also instability, responding to the incident signal, especially when it satisfies both the gain condition of amplifying and that of multi-stable states. Further, the relationship between the detuning and the conditions of light amplification in this system are given out. Moreover, we find the detuning condition can describe and generalize the "frequency pulling effect", in proper situation.

Photoreactive third-harmonic (TH) generation at 355 nm in diarylethene- polymethylmethacrylate (DE-PMMA) polymer thin films is obtained by either one- or two-photon excitation. TH intensity generated from a DE-PMMA polymer thin film decreases, when it is pumped by either 325 nm or 442 nm, which change molecular structure of DE molecules from open-form (A form) to closed-form (B form). TH intensity recovers to its original intensity level, after all B form DE molecules return to A form induced by 532 nm or 1064 nm laser irradiation. The experimental results reveal that the second-order hyperpolarizability (γ) of A form molecules may be larger than that of B form molecules. Moreover, TH output efficiency is independent of the angle between the pump and probe polarization directions. Those experimental results were explained by using a photoinduced isomerization theory based angular hole burning and angular redistribution mechanisms for two-dimensional structure of DE molecules including the two-photon absorption effect of B form. The simulation results are consistent with those of optical pumping TH experiment.

Current interest in nanophotonics has spurred the synthesis of a variety of molecular rotors--custom-designed molecules attached to a substrate via an axle about which the barrier to rotation is relatively low--and the investigation of their optical and mechanical properties. The dielectric response of molecular rotors possessing a permanent dipole moment, recently measured at frequencies in the kHz range, would be expected to be quite modest at optical frequencies. Nonpolar rotors, in contrast, could potentially exhibit large nonlinearities at optical frequencies, with the most promising rotor candidates being rigid molecules with molecular polarizabilities that are highly anisotropic; a good example is anthracene, for which the diagonal component of the polarizability tensor corresponding to the long axis of the molecule differs from that corresponding to axis normal to the ring plane by 20 ų. Anthracene molecules are constrained to rotate about the principal axis associated with the diagonal component of the molecular polarizability that is intermediate in size. The rotation axes are oriented vertically ("azimuthal rotors") and are attached to a covalent monolayer grid such as that recently reported by Magnera et al. [in Nanostructural Materials: Clusters, Composites, and Thin Films, Moskovits and Shalaev, eds., ACS, 1997, p. 213]. In this configuration, the interaction between the incident laser beam and the induced rotor dipoles dominates the physics; rotor-rotor interactions are negligible. This allows us to calculate the nonlinear refractive index n^I₂ for the system, for which we obtain a relatively modest value of 2.6 × 10^-15 cm²/W.

Single crystals of nonlinear optical material 1-(4-methylphenyl)-3-(4-methoxyphenyl)-2-propen-1-one were successfully grown for the first time by slow evaporation method up to a dimension 25 x 15 x 2 mm³. Optical studies such as UV-Visible, energy band gap, refractive index, second harmonic efficiency have been performed. The UV-Visible spectrum reveals that the crystal is transparent in the entire visible region and absorption takes place in the UV-region. Using UV-Vis data, the energy band gap was found and it shows an energy band gap of 2.7eV for this material. The refractive index was measured using Brewster's angle method. The Kurtz powder second harmonic generation test shows that the compound is a potential candidate for photonic applications. From the I-V measurements the dc conductivity of these crystals has been studied and it is found to be very low. Dielectric constant, dielectric loss and ac conductivity of a grown single crystal have been studied in the frequency range 120Hz to 100 kHz at the room temperature and proper interpretations were drawn. The micro hardness test was carried out and the load dependence hardness was studied.

1-(4-methylphenyl)-3-(4- N, N dimethyl amino phenyl)-2-propen-1-one, a chalcone derivative nonlinear optical material has been synthesized by standard method. FT-IR and NMR spectral studies have been performed to confirm the molecular structure of the synthesized compound. The single crystals up to a dimension of 13 x 9 x 3 mm³ were grown by slow evaporation method. The grown crystals were transparent in the entire visible region and absorbs in the UV-region. The refractive index has been measured using a He-Ne laser. The grown crystals have been subjected to single crystal X-ray diffraction studies to determine the crystal structure and hence the cell parameters of the crystal. From this study it is found that this compound crystallizes in orthorhombic system with a space group P2₁2₁2₁ and corresponding lattice parameters are, a = 7.3610(13) Å, b = 11.651(2) Å, c = 17.6490(17) Å. The Kurtz powder second harmonic generation test shows that the compound is a potential candidate for Photonic application. The micro hardness test on these crystals were carried out and the load dependence hardness was observed

Carbyne is a hypothetical carbon allotrope that consists of sp-hybridized carbon atoms in an infinitely-long, one-dimensional (1-D) linear chain. Polyynes, the oligomeric cousins of carbyne, with a dense delocalized-electron framework, could offer groundbreaking electronic properties. We have studied the linear and third-order nonlinear optical properties of both triisopropylsilyl end-capped (TIPS-PY) and phenyl end-capped (p-PY) polyynes containing pure sp-hybridized carbon backbones. Analysis of the TIPS-polyyne UV-vis absorption spectra shows that the absorption gap, Eg, in these materials scales very precisely as a power-law with increasing oligomer length, n, with Eg~n^{-0.379±0.002}. The phenylated polyynes show a similar trend of E_g~n^-0.36±0.01. Ultrafast molecular second-hyperpolarizabilities, γ, were obtained in solution using 800nm, 100fs pulses in a differential optical Kerr effect (DOKE) setup. Polyyne second-hyperpolarizabilities also scale with a power-law, and, surprisingly, with exponents higher than that of any other reported oligomer system, yielding a behavior of γ~n^4.3±0.1 and γ~n^3.8±0.1 for the TIPS-polyynes and phenylated-polyynes, respectively. These findings contrast direct theoretical predictions that increases in gamma with increasing conjugation length for polyynes should be considerably lower than those of polyenes and polyenynes. Furthermore, the combined linear and nonlinear optical results agree with recent theoretical studies on ideal 1-D conjugated systems, suggesting that polyynes display true 1-D behavior.

This paper presents some new study results on the threshold conditions and characteristics of lasing output for an organic polymer optical multi-stable system in which the medium is with pumping and the system acted with inject signal. The dynamics of light field in the medium are described with the extended forced oscillation model of optical multi-stable and lasing system. Moreover, through the theoretical analysis and numerical analogue, the gain threshold condition of amplifying under population reversion is discussed. Meanwhile the characteristics of the lasing output state with inject signal inspected. Further, the relationships of the amplifying multiple of the incident light field with the cavity detuning; and that with the two-photon detuning; also that between the two kinds detuning are found by analyzing the stationary equations. Finally, theoretical and numerical analyzing shows that the lasing frequency characteristic of such an organic polymer system with pumping and inject signal is different from the common laser. These results should be significant in some degree for both theoretical and experimental research, also application purpose.

Three types of the two-photon chromophores, D-π-D, D-π-d(a)-π-D, are used to investigate the mechanism of two-photon-induced polymerization (TPIP), wherein D is the terminal amine group, d and a are an electron-donating group (MeO) and an electron-accepting group (CN) linked with π-conjugated core, respectively. Based on the fact that deactivation of two-photon absorption is through the lowest singlet excited state, we study the process of electron transfer of TPIP between the excited dye and the monomer via one-photon process. In the bulk monomer used for TPIP, the emission spectra of the chromophores were obviously red-shifted and the lifetimes were elongated relative to that in the solvents. This indicates formation of an exciplex between the excited dye and the monomer. Correlated with the quantum calculation, we conclude that even for the symmetrical molecular structure, the intramolecular electron transfer from the donor terminal group to the π-conjugated core of the chromophore takes place before intermolecular electron transfer which leads to formation of exciplex. Consistent with our experimental results obtained from the polymerized rate in TPIP, the totally large electron transfer rate of the D-π-D chromophore is attributed to both efficient intramolecular and intermolecular electron transfer. Moreover, the abstraction of proton from the terminal group produced the amine radical. That is why Et₂Nas the terminal group is more active than n-Bu₂N- for two-photon polymerization. We are expecting that the same mechanism can be realized in two-photon photodynamic therapy.