Proceedings Volume 4461

Linear and Nonlinear Optics of Organic Materials

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Proceedings Volume 4461

Linear and Nonlinear Optics of Organic Materials

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Volume Details

Date Published: 7 December 2001
Contents: 7 Sessions, 36 Papers, 0 Presentations
Conference: International Symposium on Optical Science and Technology 2001
Volume Number: 4461

Table of Contents

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Table of Contents

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  • Fundamentals
  • Molecules
  • Measurement Techniques
  • Polymers
  • Poster Session
  • Polymers
  • Thin Films
  • Devices and Photonic Crystals
  • Poster Session
  • Polymers
Fundamentals
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Nonlinear molecular magneto-optics and chiral symmetries
We investigate the role of magnetic contributions to the second-order nonlinear optical properties of chiral materials. A new measurement technique is developed to determine the magnitude of such contributions in solutions of chiral molecules.
Chiral materials in second-order nonlinear optics
Martti Kauranen, Bertrand Busson, Thierry Verbiest, et al.
We review second-order nonlinear optical properties of chiral materials from the viewpoint of new second-order materials. Chiral molecules are noncentrosymmetric and therefore have a nonvanishing electric-dipole-allowed second-order response. The response is nonvanishing even in macroscopic samples with no polar order including isotropic solutions. Magnetic-dipole interactions of chiral materials can be strong. Their contributions to the nonlinear response allow second-order processes even in centrosymmetric materials. The magnetic contributions to the second-order response of thin films of chiral polyisocyanides and polythiophenes are comparable to the electric contributions. The chiral part of the electric-dipole-allowed nonlinearity dominates the second-order response of thin films of a chiral helicenebisquinone. Quasi-phase-matched second-harmonic generation was demonstrated in alternating stacks of the two enantiomers of the helicenebisquinone.
Fundamental limits of susceptibilities
In previous work, we showed that there is indeed a fundamental limit to the off-resonant nonlinear susceptibility in the dipole approximation. This limit is calculated using sum rules. We stress that there are no approximations used in deriving the sum rules and they are equivalent to the Schroedinger equation. As such, the sum rules must be obeyed if quantum mechanics is correct. Over the years, complex quantum expressions have been simplified with two and three state models that consider the competition between excited states, symmetry, and bond length alternation. We can show that while such models are good approximations to the measured susceptibilities, since the matrix elements and energy levels may not be independently adjusted, they serve limited usefulness in understanding when the susceptibility is maximum. A large set of measurements form the literature shows that all molecules fall below the susceptibility horizon calculated buy many are close. Further improvements in susceptibilities can be made with the sum rules as a guide. Significant additional improvements will require radically more creative approaches than are presently used.
Molecular probes for nonlinear optical imaging of biological membranes
Mireille H. Blanchard-Desce, Lionel Ventelon, Sandrine Charier, et al.
Second-harmonic generation (SHG) and two-photon excited fluorescence (TPEF) are nonlinear optical (NLO) phenomena that scale with excitation intensity squared, and hence give rise to an intrinsic 3-dimensional resolution when used in microscopic imaging. TPEF microscopy has gained widespread popularity in the biology community whereas SHG microscopy promises to be a powerful tool because of its sensitivity to local asymmetry. We have implemented an approach toward the design of NLO-probes specifically adapted for SHG and/or TPEF imaging of biological membranes. Our strategy is based on the design of nanoscale amphiphilic NLO-phores. We have prepared symmetrical bolaamphiphilic fluorophores combining very high two-photon absorption (TPA) cross-sections in the visible red region and affinity for cellular membranes. Their incorporation and orientation in lipid membranes can be monitored via TPEF anisotropy. We have also prepared amphiphilic push-pull chromophores exhibiting both large TPA cross-sections and very large first hyperpolarizabilities in the near-IR region. These NLO-probes have proved to be particularly useful for imaging of biological membranes by simultaneous SHG and TPEF microscopy and offer attractive prospects for real-time imaging of fundamental biological processes such as adhesion, fusion or reporting of membrane potentials.
Molecules
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Thienyl-bridged oligomeric squaraines of the indole series: synthesis and optical properties
Walter Grahn, Ulf Lawrentz, Dieter Scherer, et al.
The third-order optical polarizabilities (gamma) of a new homologous series of oligomeric squaraines of the indole series (monomer-pentamer) and of corresponding monomers were investigated by degenerate four wave mixing (DFWM) at 1064 nm and related to results from two-photon absorption experiments in an energy range from 1.75 to 3.2 eV. Important two-photon contributions can be deduced to vibronic coupling and the influence to (gamma) has been considered within a four-level model. The nature of the subchromophore interaction within the oligomers can succesfully be described as mainly electrostatic within the framework of simple exciton coupling theory.
Tunability of optical nonlinear response through twisting of conjugation paths in push-pull biphenyl compounds
Charge separated molecules with strong zwitterionic character make especially interesting species for quadratic Non Linear Optical (NLO) doped materials because of their large dipole moments and rather large quadratic hyperpolarizabilities. Furthermore, their sensitivity to the dielectric environment brands them as good candidates for checking the validity of formalisms relating geometrical structure and NLO properties. We have carried out an experimental and theoretical study on two phenoxide pyridinium derivatives with different inter ring twist angle, combining UV-visible, IR and NLO spectroscopies supplemented by semi-empirical and density functional calculations. Experimentally, changing the polarity of the solvent allows us to explore only a small range of twist angles. However, changing the pyridinium nitrogen atom from para to meta position gives essentially the same compound but with a larger twist angle. Changes in solvent polarity and position substitution give rise to a combined enhancement of the quadratic response by an order of magnitude, showing experimentally the possibility of tuning NLO properties of chromophores to be used as doping agents in NLO applications.
Nonlinear photoluminescence from multiwalled carbon nanotubes
Visible photoluminescence from multiwalled carbon nanotubes (MWNT) was observed on excitation at 1064 nm. Strong nonlinear behavior of the photoluminescence was shown using power law dependence studies. The nonlinear response in MWNT was further investigated using degenerate four wave mixing. An ultrafast response was observed and the magnitude of the third order optical susceptibility, (chi) , was determined to be in the region of 1.2x10-10 esu. Van Hove singularities in the density of states were identified for the first time in MWNT using optical absorption spectroscopy. Optical transitions between the singularities coincide with the spectral region of the photoluminescence. We propose that a multiphoton absorption process, followed by up conversion luminescence, is responsible for nonlinear photoluminescence in MWNT. Photoluminescence from graphitic particles (GP) was also investigated. This is shown to result mainly from thermal behavior and well-known optical centers. Blackbody radiation was observed in the near infrared region in both materials with MWNT exhibiting lower blackbody temperatures than graphite under the same irradiation conditions.
Third-order optical nonlinearities of organometallics: influence of dendritic geometry on the nonlinear properties and electrochromic switching of nonlinear absorption
Marek Samoc, Mark G. Humphrey, Marie P. Cifuentes, et al.
We have investigated a large number of organometallic structures possessing high second and third order optical nonlinearities. Most third-order NLO experiments were performed with 100 femtosecond light pulses at 800 nm. While investigating structure-property relationships we note an enhancement of the real and imaginary parts of the cubic hyperpolarizability in structures of trigonal symmetries, and, in particular, of dendritic geometries with multipolar charge distribution, due to the presence of charge acceptor groups. Two-photon absorption cross sections achievable in these structures are comparable with the best values reported in the literature. We also show that, for some of the ruthenium alkynyl complexes investigated in this study, it is possible to perform electrochemical switching between two forms of the compound. The nonlinear properties of the two forms were investigated by the technique of Z-scan carried out in situ in an electrochemical cell. The neutral form is essentially nonabsorbing in the infrared but shows two-photon absorption at 800 nm. Upon oxidation the complex becomes infrared absorbing and the oxidized form exhibits an absorption saturation effect at 800 nm. This electrochromic switching of both the linear absorption and the sign of the imaginary part of the third-order susceptibility is found to be reversible and is, therefore, of potential application interest.
Resonant and static cubic hyperpolarizabilities of push-pull dipolar and quadrupolar chromophores: toward enhanced two-photon absorption
Marguerite Barzoukas, Mireille H. Blanchard-Desce
Recent reports of push-pull dipolar and quadrupolar chromophores with enhanced two-photon absorption have generated considerable interest in these two molecular systems. Two photon absorption is related to the imaginary part of the two-photon resonant cubic hyperpolarizability Im[(gamma) ((omega) )]. In this work, we have described both push-pull dipolar and quadrupolar chromophores using multi valence-bond states models based on measurable parameters of the valence-bond forms. We have derived analytical expressions of their non-resonant static cubic hyperpolarizability (gamma) (0) and of Im[(gamma) ((omega) )]. Comparison between the transparency / Im[(gamma) ((omega) )] trade-off and Im[(gamma) ((omega) )] / (gamma) (0) correlation helps understand the advantages and drawbacks of each of these two push-pull systems. Furthermore by understanding how the valence-bond parameters are related to the molecular structure and its environment, it is possible to predict how Im[(gamma) ((omega) )] will be affected by changing either the conjugation size, the donor-acceptor pair or the solvent polarity for both of these push-pull systems. The results of this study suggest common guidelines for the molecular engineering of both the push-pull dipolar and quadrupolar chromophores.
Measurement Techniques
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White-light-continuum spectroscopy to determine third-order nonlinear optical properties
Ulrich J. Gubler, Raluca A. Negres, Rainer Martin, et al.
The realization of all-optical switching schemes is mostly hindered by the lack of suitable materials with a refractive index change that is large and fast enough. The characterization of the linear and nonlinear optical properties of potential materials is therefore of prime importance. Various characterization methods have been proposed and are employed, yielding different parameters of the nonlinear optical response at the involved laser frequencies. However, in most techniques the resulting nonlinearities are measured only at one point in the spectral dispersion. To generate the whole nonlinear spectra, the laser source has to be tuned over the desired wavelength range and consecutive measurements have to be taken. We propose and demonstrate here a novel technique to measure the nonlinear optical response for a broad wavelength region in a pump-probe scheme that requires no laser tuning. In order to detect the two-photon absorption at several wavelengths simultaneously, we use a white-light-continuum as the probe beam. As the pump beam is held constant, the Kramers-Kroenig transformation can be used to calculate the dispersion of the nonlinear refractive index from the two-photon spectra. By delaying the probe beam relative to the pump beam, the temporal behavior of the nonlinearity can be obtained.
Enhancing the accuracy and precision in hyper-Rayleigh scattering: frequency- and angle-resolved femtosecond nonlinear scattering
Hyper-Rayleigh scattering, or second-order nonlinear incoherent scattering, has become a standard technique for the experimental determination of the molecular second-order nonlinear optical polarizability, or first hyperpolarizability, of nonlinear optical chromophores in solution. We review the different contributions to the hyper-Rayleigh signal and discuss the impact on the accuracy of the resulting value for the first hyperpolarizability. Especially incoherent multi-photon fluorescence and coherent second-harmonic generation deserve our attention. Temporal resolution of the response enables the distinction between the immediate scattering and time-delayed fluorescence. This has been implemented in the Fourier domain, where the fluorescence contribution exhibits a demodulation (a reduction in amplitude) versus the scattering for increasing modulation frequency of the fundamental laser light. By adding the experimental determination of the phase shift as a function of modulation frequency, ana analyzing the demodulation and phase shift simultaneously, the accuracy and the precision of the measurement was increased substantially. In addition, hyper-Rayleigh scattering as a function of incidence angle clearly shows any coherent contribution to the signal. Selected examples show that the analysis as a function of angle and time results in a value that is ultimately completely free of systematic error. A comparative study shows the importance of the improved precision.
Excited-state characterization of nonlinear optical materials through electrofluorescence studies
Techniques to characterize the molecular excited states of nonlinear chromophores are presented. These techniques include linear absorption, fluorescence, and quadratic electroabsorption. The experiments are described and results presented. A new approach, electrofluorescence, where the fluorescence signal is modulated by an applied voltage, is also discussed. Preliminary electrofluorescence results indicate that this approach has merit as a valuable characterization technique.
Nonlinear optical diagnostics of phase transitions in C60-TTF derivatives
Isabella Fuks-Janczarek, Xuan Nguyen Phu, Jean-Michel Nunzi, et al.
A shape-like modulation of the linear electro-optics (LEO) coefficient versus the pressure (p) and the temperature (T) has been observed for the first time. The modulation depth of the LEO (r222) tensor component in the C60-TTF (for He-Ne laser wavelength λ=633 nm) was equal to about 2.6%. Varying the applied hydrostatic pressure up to 15 GPa and the temperature from 4.2 up to 25K. We have unambiguously demonstrated that the observed effect is caused by incorporation of the TTF moiety and absent in the unsubstituted fullerenes varying the applied hydrostatic pressure up to 19 GPa and the temperature from 4.2 up to 150 K. The data obtained gives a possibility for using the discovered effect for contact-less measurements of low temperatures and high pressures.
Time-resolved electric-field-induced second harmonic
One limitation of using electric field induced second harmonic (EFISH) to determine the molecular first hyperpolarizability (beta) of nonlinear optical molecules lies in the fact that part of the second harmonic signal comes from the second hyperpolarizability (gamma) produced by mixing two optical fields with the DC field. In analyzing EFISH results, the second hyperpolarizability contribution of the studied molecules is generally neglected. We present a modified time resolved EFISH technique that allows us, in a single experiment, to determine separately the beta and the gamma contributions. We study para-nitro aniline dissolved in Glycerol, a highly viscous solvent, and apply the DC field via a high voltage pulse with a fast rise time of approximately 40 nsec. As a result, the orientation of the molecules under the applied electric field is slow relative to the build-up of the field, enabling us to directly measure only the DC induced second harmonic (gamma contribution), at the beginning of the HV pulse. The pure beta contribution is determined from the difference between this signal and the conventional EFISH signal at the plateau of the HV pulse. Our result confirm that the gamma contribution is indeed less than 10% of the total.
Polarization holography with bacteriorhodopsin
Polarization holographic gratings convert or transform a polarization state of incident wave front. We have studied the diffraction properties of polarization gratings, which formed with two orthogonal linearly or circularly polarized waves. At a polarization modulation recorded on organic material bacteriorhodopsin, the diffraction efficiency and the diffracted wave polarization strongly depend on the read-out wave. We have practiced several simple theoretical treatments, namely Fraunhofer diffraction integral. The experimental results have been interpreted in terms of these simplified models.
Absorption of polymers for optical waveguide applications measured by photothermal deflection spectroscopy
Andre Knoesen, Liu-Ming Wu
A technique based on photothermal deflection spectroscopy has been developed for measuring the absolute absorption of polymers. The technique utilizes the frequency dependence measurement of the deflection signals to retrieve polymers' thermal properties and experimental parameters that are necessary to convert the deflection signals to the absolute absorption coefficients. All the measurements are done on the same experimental setup and the same sample without the need of obtaining either the materials' thermal properties or the reference absorption coefficients from other sources. This technique has been applied to several polymers and the results are reported.
Polymers
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New developments in optical ordering of NLO dyes in polymers
Michel L. Dumont
Basic mechanisms of optical ordering processes of azo dyes in polymer matrices are now well understood. Photo-Induced Anisotropy (PIA), Photoassisted Electrical Poling (PAEP) and All-Optical Poling (AOP) are produced by an angularly selective trans-cis photoisomerization and by the angular redistribution resulting from this reversible isomerization. They differ by the symmetry of the excitation. We present new theoretical and experimental developments on this subject. Experimentally, the problem is to extend these methods to a panel of NLO molecules wider than azobenzene derivatives. Non-photoisomerizable molecules present very small effects, probably due to a selective photodegradation. A route under investigation is to build structures associating two functions: an efficient NLO function and a diazo group to stir up the polymer. Theoretically, the model for the orientation of rod-like photoisomerisable molecules is well established. We are extending it to 2D and 3D molecules, characterized by their tensorial properties and excited by light of any symmetry (particularly octupolar symmetry). The probability of excitation is formally easy to write, but the angular redistribution process and the successive photoisomerization cycles (saturation of optical pumping) modify the tensorial quantities. In AOP, the competition between centrosymmetric and non-centrosymmetric contributions is often detrimental to the building of a good (chi) .
Experimental data and modeling of X(2) temporal stability of poled chromophore-doped sol-gel materials
Helene Goudket, Michael Canva, Yves Levy, et al.
The relaxation of the second-order nonlinearity of poled chromophore-doped sol-gel thin films has been studied at different temperatures between 63 degree(s)C and 82 degree(s)C (poling temperature: 90 degree(s)C). The process has been monitored through the variations of second harmonic generation (SHG) induced in the poled film by a pulsed nanosecond Nd:YAG laser (1064 nm). The SHG decay curves have then been fitted to bi-exponentials and Kohlrausch, Williams and Watts (KWW) stretched exponentials. The better quality of the KWW fits involves that this model is more appropriate than the bi- exponential model to describe chromophore relaxation in sol- gel thin films, as has been observed for classical all- organic polymers. A numerical analysis of the KWW parameters enables a better understanding of the function meaning. The time constants obtained with both models have been logarithmically plotted in function of temperature. An Arrhenius-type linear fit has been adjusted to this representation, and a value of the relaxation time extrapolated at room temperature. The bi-exponential model yields time constants of 26 and 110,000 years, while the KWW model extrapolates to an 87 years' second-order nonlinearity lifetime with a corresponding activation energy of 49 kcal.mol-1. Decay data gathered at room temperature over a period of 17 months can be fitted with both models and yield time constants of 14 days and 13 years for the bi- exponential, and of 115 days for the stretched exponential. The extrapolated KWW fit can be corrected with a factor of 0.58 to adjust the experimental data. This reveals the possible existence of a very rapid relaxation (less than a minute for our measurement temperatures) that was not measured. Nonetheless, our work shows that second-order nonlinearity in poled chromophore-doped sol-gel thin films is stable for more than a year. This is an encouraging result for future applications in telecom industry, especially as this feature has not yet been studied and optimized in terms of material synthesis.
Poster Session
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Third-order nonlinear optical properties of copoly(2,3,5,6-tetrafluoro-1,4-phenylenevinylene-2,5-dialkoxy-1,4-phenylenevinylene)s: a novel class of push-pull substituted PPVs
Gianluca M. Farinola, Tiziana Cassano, Raffaele Tommasi, et al.
A series of copoly(2,3,5,6-tetrafluoro-1,4-phenylenevinylene-2,5-dioctyl oxy-1,4-phenylenevinylene)s containing variable ratios of tetrafluorophenylenevinylene and dialkoxyphenylenevinylene repeating units has been synthesized via the Stille cross-coupling reaction in order to investigate the influence on the (chi) (3) of the simultaneous presence of electron-deficient and electron-rich aromatic monomers in the PPV backbone. Third-order nonlinear optical properties of the resulting materials have been studied in chloroform solutions at (lambda) =1064 nm using the picosecond Z-scan technique. The nonlinear refractive index has been found to be negative, strongly dependent on the copolymer composition, and as high as (-10+/- 2)x10-12cm2/W.
Polymers
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Highly efficient and thermally stable organic/polymeric electro-optic materials by dendritic approach
A series of dendron-modified nonlinear optical (NLO) chromophores and multiple chromophore-containing crosslinkable NLO dendrimers have been developed. The enhancement of poling efficiency (40%) in the dendritic NLO chromophore/polymer guest/host system was obtained due to the significant minimization of intermolecular electrostatic interactions among chromophores by the dendritic effect. Multiple NLO chromophore building blocks can be further placed into a dendrimer to construct precise molecular architecture with predetermined chemical composition. The site-isolation effect, through the encapsulation of NLO moieties by dendrons, can greatly enhance the performance of electro-optic (E-O) materials. A very large E-O coefficient (r33=60 pm/V at 1.55 micrometers ) and high temporal stability (85 degree(s)C for more than 1000 h) were achieved in a NLO dendrimer developed through the double-end functionalization of a 3D shape phenyl-tetracyanobutadienyl (Ph-TCBD)- containing NLO chromophore with thermally crosslinkable trifluorovinylether-containing dendrons.
Remarkable optical properties of dendrimers for laser applications
Akira Otomo, Sonoko Otomo, Shiyoshi Yokoyama, et al.
We have investigated optical properties of dye-encapsulated dendrimers for photonic applications and observed several interesting behavior, such as controlled intermolecular interactions, super-radiance, and supernarrowing laser emission. We confirmed limited inter molecular interactions between encapsulated dyes in fluorescence lifetime measurement of rhodamine B-cored dendrimers with different shell sizes. Also, photochemical stability against singlet oxygen was studied for fluorescent dyes encapsulated in a dendrimer box and the lifetime was improved by 60 times due to the shielding effect of the dendrimer shell. A supernarrowing laser spectrum was observed without using any resonator mirrors due to moderate scattering caused by densely doped dendrimer aggregates in a polymer waveguide. At the optimum operation level, the spectral linewidth is found to be 0.55 nm, which is limited by the resolution of the spectrograph used. Weak scattering from the aggregated dendrimers in the host polymer matrix with gain is responsible for photon localization. However, in contrast to conventional random lasers with strong scattering, weak scattering enables the long amplification path through the waveguide producing a directional beam and a supernarrowing spectrum. The dependence of supernarrowing lasing behavior on the density of the scattering moiety was studied by changing the dendrimer doping ratios in the polymer matrix.
Fluorescence characterization of the ternary system TMQ-PBDBD365-POPOP-dye-doped polystyrene optical fiber under gamma and UV irradiation
Polystyrene dye doped plastic optical fiber was prepared and used to detect gamma and beta radiation from I151 and TeO4 gamma tracers typically used to get images of tumor areas within the human body. Absorption and fluorescence emission of TMQ, PBDBD365, POPOP styrene doped was performed under gamma and UV-irradiation. The fluorescence efficiency of the binary system PBDBD365-POPOP and the ternary TMQ- PBDBD365-POPOP was compared and according to the experimental results it was shown that the presence of the TMQ dye enhance the fluoresce obtained under I151 radiation. Systematic characterization of the binary system was performed as function of primary dye concentration .
Copolymer and dye-doped polymer fiber and fiber preform characterization
We apply standard emission and absorption techniques in conjunction with the side-illumination fluorescence (SIF) technique to characterize both co-polymer and dye-doped polymer fibers and fiber preforms. Highly reproducible co- copolymer and dye-doped polymer fiber fabrication is of great interest for many nonlinear optical applications. However, aggregation and environmental breakdown often affect the quality of the fiber. We quantify these effects in both the bulk and fiber geometries using emission and absorption techniques.
Thin Films
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Second-order nonlinear optical responses of ionically self-assembled films: polycation variations and dianionic chromophores
Patrick J. Neyman, M. Guzy, S. M. Shah, et al.
Ionically self-assembled monolayer (ISAM) films have been shown to spontaneously produce noncentrosymmetric ordering that gives rise to a substantial second order nonlinear optical (NLO) response. Typically, the ISAM films for NLO response are an assemblage of bilayers of oppositely charged polymers whose thickness can be controlled through variation of pH and ionic strength of the immersion solutions. Ordinarily, the NLO-inactive polycation solution parameters are chosen the same as for the NLO-active polyanion solution. Here, we study the effects of varying the polycation solution parameters separately from the polyanion solution. We also investigate the effects of replacing the NLO-active polymer layers with layers of dianionic molecules. Films fabricated exclusively using polyelectrolytes contain some fraction of both randomly oriented and anti-parallel oriented chromophores. We have examined the incorporation of dianionic chromophores into ISAM films in order to increase the net polar orientation of the chromophores and reduce bilayer thickness.
Phase-matched second harmonic generation and nonlinear phase shift in a Langmuir-Blodgett film waveguide
Sigurd K. Schrader, Costel Flueraru, Hubert Motschmann, et al.
Wave-guides have been prepared as y-type Langmuir-Blodgett multilayers from 2-docosylamino-5-nitropyridine (DCANP) on quartz glass substrates. The tensor elements of the LB-films as determined by polarization dependent second harmonic generation (SHG) are (Formula available in paper) The wave-guides were fabricated in a way that the second-order susceptibility changes sign at the nodal plane of the first-order wave-guide mode for s-polarization. In such wave-guides efficient second harmonic generation (SHG) was reached via mode conversion at a fundamental wavelength near 1064 nm. The conversion efficiency reached the extraordinary high value of 8%/W which corresponds to a normalized conversion efficiency of 3600 %/(W cm2). In addition, interferometric measurements have been carried out to study the non-linear phase-shift which the fundamental beam experiences due to non-linear interaction in the wave-guide. From these experiments an apparent intensity-dependent refractive index n2SHG of 2,6 10-13 cm2/W was calculated. This as about 400 times the intensity-dependent refractive as expected from third-order susceptibility of the isotropic material. From that it can be concluded that the main contribution of the intensity-dependent refractive index is connected to cascading of second-order processes.
Structure and optical properties of ultrathin p-phenylene oligomer films on dielectrics
A systematic investigation has been performed in order to determine the unit cells of ultrathin p-nP films grown under surface science conditions on dielectrics. We have employed low energy electron diffraction (LEED) as well as polarization dependent linear absorption spectroscopy on films deposited onto alkali halide (NaCl, KCl, NaF, LiF, KBr) and mica(0001) single crystals at various substrate temperatures. Surface unit cells of the films are determined as a function of chain length n(n=3-6) and deposition parameters. Linear optical spectroscopy reveals a strong dichroism and allows us to distinguish between laying and standing molecules on the substrate. In contrast to the alkali halides, we observe on mica at elevated surface temperatures the growth of single crystalline needles, the orientation of which is controlled by the presence of surface dipoles on the cleaved mica surface.
Devices and Photonic Crystals
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Model study in molecular engineering for nonlinear photonic devices: poly (arylene ethynylene) and poly (arylene vinylene) copolymers
Kieran G. Ryder, Stephen Lipson, Anna Drury, et al.
A nonlinear optical study in poly (arylene ethynylene)(PAE) and poly (arylene vinylene) (PAV) copolymers for optical switching is presented. The principle aim of this work is to establish the polymer properties, which can be exploited in the design of materials useful for optical switching. A systematic study of the polymer parameters in solution and solid state are analysed in view of their performance as nonlinear materials. Insight is gained into the material performance, which is then related back to the structure. Wavelength dependent values of the third order nonlinear absorption coefficient (beta) and the third order nonlinear refraction coefficient (eta) 2, for all four polymers are measured using the Z-scan technique. Suitability for optical switching applications is ascertained from the linear and nonlinear absorption. Two figures of merit are presented, enabling the suitability of the materials for optical switching to be ascertained, in the wavelength region spanning 465nm through 685nm. Molecular mechanical modeling is employed to provide a preliminary view of the polymer structure to aid the understanding of the nonlinear optical and film forming properties of the polymers. Correlations are drawn between the PAE and PAV polymers, enabling the possible synthesis of the ideal material for optical switching devices at visible and near infra red wavelengths, utilizing solid state organic materials.
Electro-optic modulators based on organic single-crystal films
We describe the characterization of electro-optic properties of single crystal thin films of organic material N-(4- nitrophenyl)-(L)-prolinol grown by the plate-guided method. Characterization was performed using the longitudinal a.c modulation technique. The half-wave voltage, the figure-of- merit, and the related electro-optic coefficients were estimated to be 3.24 kV, 98 pm/V and 461(pm/V) and 154(pm/V) respectively.
Time-resolved transmission through a photonic crystal in the complete Fourier domain
Photonic crystals have recently received much attention for their interesting characteristics and potential applications in optical data handling. The top-down fabrication approach by lithographic techniques results in the best photonic bandgaps but is expensive. Self-assembly of spheres provides a simple approach towards the production of photonic bandgap materials. This bottom-up approach is being explored as a viable alternative towards devices. Photonic crystals have been prepared from the deposition of monodisperse latex spheres in water. A photonic bandgap in the near-IR is clearly observed. Tuning of the bandgap can be achieved by varying the sphere diameter. The time- resolved optical transmission of a femtosecond pulse through such colloidal crystals has been studied in the Fourier domain as a function of modulation frequency of the input light. The percent transmission and the phase shift for the transmitted light pulse versus the impinging light pulse have been determined as a function of incidence angle and optical wavelength. This percentage transmission and phase shift relate to the real and imaginary part or sine and cosine Fourier transform of the temporal profile of the transmitted light pulses in the time domain.
Polymer photonic crystal slab waveguides
Manfred Eich, Christian Liguda, Gunnar Boettger, et al.
Finite two-dimensional (2d) polymer photonic crystal (PC) slab waveguides were fabricated from a benzocyclobutene polymer on a low refractive index substrate from Teflon. Square and triangular air hole lattices were realized by electron beam lithography and reactive ion etching. Polarization and wavelength dependent transmission measurements show TE-like and TM-like stop gaps at 1.3micrometers excitation wavelengths and are in good agreement with the calculated data obtained by 2D and 3D finite difference time domain (FDTD) methods. Transmission was suppressed by 15dB in the center of the TE-like stop gap for a PC length of only ten lattice constants. Defect states were induced within the stopgap by lattice modification and theoretically and experimentally characterized.
Polymer single-arm optical waveguide interferometer for detection of toxic industrial materials
We report a novel single-arm double-mode double-order waveguide interferometer being used as a chemical sensor for detection f toxic industrial materials such as ammonia in air. The sensor is based on thin films of polymers poly(methyl methacrylate) and polyimide doped with indicator dyes bromocresol purple and bromothymol blue. These dye- doped polymer materials exhibit a reversible optical absorption in a band near 600 nm being exposed to ammonia in wet air. The rise of absorption is accompanied by the change of the refractive index in near IR region out of the absorption band. The distinguished feature of the sensor is that is uses for reading the change of the refractive index of the dye-doped polymer film the interference of two propagation waveguide modes of different orders. The modes TM0 and TM1 are simultaneously excited in the light- guiding polymer film with a focusing optics and a prism coupler. The modes are decoupled from the film and recombined producing an interference pattern in the face of an output optical fiber. The sensitivity of the sensor to ammonia is 200 ppm per one full oscillation of the signal. We analyze effects of various factors such as polymer composition, light wavelength, ambient humidity and atmospheric pressure on the performance of the sensor. Various design and fabrication issues are also discussed. The problems of particular interest are reduction of losses and sensitivity improvement.
Poster Session
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Design and synthesis of new acceptor molecules for photoinduced electron transfer reverse saturable absorption
Mamoun M. Bader, Taiala Carvalho, Hu Li, et al.
With the advent of efficient and relatively inexpensive pulsed and CW laser systems for both civilian and military applications, the need for adequate eye and sensor protection is becoming increasingly important. While it is possible to filter out harmful wavelengths if the laser frequency is known, the proliferation of frequency agile laser sources underscores the need for smart passive materials that can sense the incident wavelength and provide protection. There has been considerable progress made in recent years in the design of optical power limiting (OPL) materials that can function by a variety of mechanisms, most of which derive their limiting behavior from some type of nonlinear absorption cross-sections of the photo-generated transient species are much greater than the original S0 to S1 transition. In this presentation we will discuss the efficacy of charge transfer species for optical limiting, and the need for more and better electron acceptor species.
In-situ second harmonic generation measurements of the formation of ionically self-assembled monolayers
Charles Brands, Patrick J. Neyman, M. Guzy, et al.
Recently, ionically self-assembled monolayers (ISAMs) of polymers with nonlinear optical chromophores have been shown to spontaneously exhibit a preferential ordering during the deposition process. This gives rise to a substantial second order nonlinear optical (NLO) response. Here, we use this second harmonic generation (SHG) to our advantage in the in situ study of the deposition of ISAMs. Upon immersion in the NLO-active polyelectrolyte solution, the SHG rises sharply over the first minute, demonstrating a rapid adsorption process. Immersion in the NLO-inactive partner polyelectrolyte leads to an instantaneous reduction in the SHG signal. We also show that the adsorption and ordering of a noncentrosymmetric nonlinear optical polymer increases with increasing PCBS concentration. These studies provide deeper understanding of the processes by which noncentrosymmetric order is formed in ISAM films and allows design of improved self-assembled nonlinear optical materials.
Concentration dependence of surface nonlinear susceptibility of vanadyl-porphyrin at silica surfaces
Lorenzo Echevarria, Hector Gutierrez, Veronica Sosa, et al.
Resonant molecular optical second harmonic generation (SHG) has been obtained from [2, 3, 7, 8, 12, 13, 17, 18- Octaethyl-porphinato] vanadyl, (OEP)V=O, adsorbed on the hypotenuse face of a glass prism. Rotational intensity patterns are showed by plotting the second harmonic intensity as a function of the incident polarization angle of the fundamental wave, at a fixed angle of incidence. Evaluation of these patterns by Fourier analysis allowed the determination of the second order susceptibilities components of the adsorbed layers. Assuming a delta distribution function, the mean orientation angles from normal to the surface plane, were obtained for different solution concentrations. Our results indicate an abrupt change in the angle for two concentration ranges.
Comparative study of the nonlinear optical surface response of metal-substituted porphyrins adsorbed on glass
A comparative study of the second harmonic generation at the M(H2,Ni(II),V=O)-porphyrin/glass interface in a total reflection geometry is presented. The state of polarization of the incident pulses was changed by a computer controlled rotation of a quarter wave plate. The rotation patterns obtained from the experimental data were fit to a Fourier series based upon theoretical Fourier coefficients. Complex valued components of the superficial susceptibility tensor are showed. From these values, average tilt angles concentration dependent were obtained. A different behavior for each Metalloporphyrin was observed.
Polymers
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Polycyanurate ester resins with low loss and low birefringence for use in integrated optics
Christian J. Dreyer, Monika Bauer, Joerg Bauer, et al.
In the age of information society and internet the requirements of fast transfers of large data streams for different applications are growing day by day. Killer-applications like teleconferencing, video-on-demand, online-games, virtual reality etc. are waiting in the wings. The optical network technology using the great bandwidth of glass fibre is the most suitable technology for these demands. Not only glass fibre is required, but also a broad range of optical components, such as multiplexers, demultiplexers, optical switches, optical attenuators, splitters and combiners, which are usually produced in silica technology. Polymeric materials are becoming more and more interesting for these applications, since they promise for instance lower power consumption and a reduction of production costs compared to their silica based pendants. Polycyanurate ester resins are a relatively new class of high-performance polymers with outstanding properties, for example high thermal stability, low optical loss, low dielectric constant, good adhesion and outstanding mechanical properties. This paper focuses on optical loss and birefringence of such materials at 1550 nm. The results lead the way to optimization for use in integrated optics and for the production of embedded waveguides and devices.