Organic Photonic Materials and Devices V

Board level guided wave optical interconnects

Chulchae Choi, Yujie Liu, Lei Lin, et al.

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We introduce a fully embedded board level guided-wave optical interconnects for short range optical interconnects. The realization of the fully embedded optical interconnects is achieved by overcome several difficulties such as thermal management of a VCSEL, thin film VCSEL fabrication, effective coupling and materials selection. A novel heat sink structure for cooling is developed to solve thermal management. We fabricated 10micron-thick linear VCSEL arrays and MSM detector array. As a physical layer, flexible optical interconnection layer incorporated with channel waveguides and 45° waveguide mirror couplers are also fabricated using compression molding technique.

Excimer-laser-based multifunctional patterning systems for optoelectronics, MEMS, materials processing, and biotechnology

Kanti Jain

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Over the past few years, there has been an increasing impact of microelectronics fabrication technologies on the realization of structures and spatial patterns necessary for advances in optoelectronics, MEMS, materials processing, and biotechnology. These fabrication technologies accelerate the pace of research by enabling the micro-manipulation and patterning of a variety of organic, inorganic, and biological materials (including new polymers, compound semiconductors, DNA, proteins, and others), developing new synthesis techniques, and producing structures and devices previously not deemed possible. In order to facilitate the exploration of these fields, it is desirable to develop processing techniques and cost-effective, multifunctional systems that can handle a wide variety of substrate materials and geometries, including non-planar surfaces. This paper describes recent advances made in excimer-laser-based patterning, photoablation, and photo-crystallization technologies, focusing on how these technologies address the unique requirements of applications for scientific research and for technology development.

Deoxyribonucleic acid (DNA) cladding layers for nonlinear-optic-polymer-based electro-optic devices

James G. Grote, Naoya Ogata, Darnell E. Diggs, et al.

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Nonlinear optic (NLO) polymer based electro-optic devices have been achieving world record low half wave voltages and high frequencies over the last 2-3 years. Part of the advancement is through the use of relatively more conductive polymers for the cladding layers. Based on the current materials available for these cladding materials, however, the desired optical and electromagnetic properites are being balanced for materials processability. One does not want the solvent present in one layer to dissovle the one deposited underneath, or be dissolved by the one being deposited on top. Optimized polymer cladding materials, to further enhance device performance, are continuing to be investigated. Thin films of deoxyribonucleic acid (DNA), derived from salmon sperm, show promise in providing both the desired optical and magnetic properties, as well as the desired resistance to various solvents used for NLO polymer device fabrication. Thin films of DNA were deposited on glass and silicon substrates and the film quality, optical and electromagnetic properties and resistance to various solvents were characterized.

Optical power limiting properties of two-photon absorbing fluorene and dithienothiophene-based chromophores

Kwang-Sup Lee, Hyun-Kwan Yang, Jong-Hyup Lee, et al.

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We have synthesized efficient two-photon absorbing fluorene and dithienothiophene (DTT) derivatives with electron donors (D) or electron acceptors (A) which are attached symmetrically or asymmetrically at the both end of conjugated links, forming D-π-D and D-π-A structures. The two-photon absorption (TPA) cross-section values (σ) of these chromophores were evaluated by the nonlinear transmission measurements using 8 ns laser pulse, and also by the two-photon-induced fluorescence method with 80 fs pulse laser. The σ values obtained were larger by the nanosecond pulse laser than that by the femtosecond laser. This result suggests that the relative contribution of the excited state absorption becomes more manifested when longer time scale pulses are used. The strong nonlinear absorption including TPA of all these materials induced an efficient optical power limiting (OPL) activity. The trend in OPL behavior agreed well with the size of the TPA intensity.

Photostabilizing effects of lidocaine and tris(8-hydroxy-quinoline) aluminum on organic fluorescent dyes

Wade N. Sisk

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The photostabilization efficacy of lidocaine and tris(8-hydroxy-quinoline) aluminum (Alq3) was determined for methanol solutions of the fluorescent laser dyes 1,3,5,7,8-pentamethyl-2,6-diethylpyrromethene-difluoroborate complex (PM-567) and rhodamine 590 (R590) by evaluation with the , rose bengal (RB). The photostability was measured by noting the decrease in fluorescence with accumulated 532 nm Nd:YAG laser pulses. Rose bengal demonstrated dramatic photostability enhancement upon lidocaine or Alq₃ addition; whereas nominal photostability enhancement was observed for PM-567 and R590 upon lidocaine or Alq₃ addition. A geminate dye-singlet oxygen complex is proposed to explain the disparity in dye photostability enhancement between rose bengal and the laser dyes.

Angular dependence of the emission wavelength in microcavity organic light-emitting diodes

Aleksandra B. Djurisic, Aleksandar Rakic, Marian L. Majewski

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In this work, we have calculated the emission wavelength dependence on the viewing angle for different combinations of metallic mirrors. The dispersion of the optical functions of ten different metals is fully taken into account using Lorentz oscillator model. The metals have been assigned to a function of top (cathode) or bottom (anode) mirror based on their work function. Refractive index dispersion of organic layers, N,N'-disphenyl-N,N'-bis(3-methylphenyl)-1,1'-disphenyl-4,4'-diamine (TPD) and tris (8-hydroxyquinoline) aluminum (emitting layer) is taken into acocutn via Cauchymodel. The change of the emission wavelength with angle has been calculated iteratively to fully take into account wavelength dependence of indices of refraction and phase change. Calculations have been performed for different hole transport materials and different thickness of the emitting layer.

Nondoping red fluorophores for red organic light-emitting diodes

Chin-Ti Chen, Li-Hsin Chan, Hsiu-Chih Yeh, et al.

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One of the first bright (~8,000 cd/m² of maximum electroluminance) and efficient (2.4% of maximum external quantum efficiency) saturated red (coordinates x = 0.66, y = 0.32 of 1931 CIE chromaticity) nondopign OLEDs has been achieved based on novel N-Methyl-bis(4-(N-(1-naphyl)-N-phenylamino)phenyl)maleimide red fluorophore. The developing process in achieving the high performance of the device is presented.

Molecular photonics materials with excited-state intramolecular proton transfer (ESIPT) activity

Soo Young Park, Sehoon Kim

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Synthesis and properties of novel excited-state intramolecular proton transfer (ESIPT) materials recently developed in our group are described. Highly efficient ESIPT in polymeric system has been investigated theoretically and experimentally with a semi-rigid polyquinoline possessing an intramolecular tautomerizable hydrogen bond. Poly(aryl ether) dendrimers of three different generations that are cored with photo-tautomerizable quinoline (QGn, n=1,2,3) were also synthesized and characterized to investigate the effect of dendritic architecture on the ESIPT activity. Stimulated emission and amplified spontaneous emission in these organic materials system are discussed in terms of ESIPT activity.

Luminescent polymer optical fibers: gain spectroscopy and lasing

Takeyuki Kobayashi, Werner J. Blau, Hartwig Tillmann, et al.

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We report our investigation of light amplification and lasing in a glass-clad step-index polymer optical fiber that uses a novel blue-emitting stilbenoid compound. The compound, 1,4-bis(4-diphenylamino-styryl)-benzene, is designed and synthesized specifically for the blue region of the spectrum and has a high quantum yield of 0.85 in polystyrene and a relatively large Stokes shift of ~50 nm. The variable stripe length method is employed to deduce the net gain coefficient of a fiber doped with 0.2-wt. % of the compound. The gain spectroscopy has revealed a broad optical gain exceeding 25 cm^-1 and up to 36 cm^-1 at 494 nm that covers a wide spectral range of about 70 nm when the fiber is transversely photoexcited at 12 mJ/cm². An analysis shows that the saturation effect expected for homogeneously broadened gain accounts for the amplified spontaneous emission output behavior at longer excitation lengths. The linear waveguide loss is measured to be 0.7 cm^-1 at 494 nm. The large gain and the low waveguide loss have allowed the unequivocal demonstration of blue laser emission at 489 nm from the fiber (which is only 1.4 cm in length) in a very low finesse cavity defined by the Fresnel reflections at the fiber-air interfaces. The threshold for lasing is found to be 1.7 mJ/cm². The results presented here will open the door to the study and development of very compact polymer fiber lasers and amplifiers.

Enhancement in power conversion efficiency in phthalocyaninebased photovoltaic cell

Chung Yin Kwong, Aleksandra B. Djurisic, Po Ching Chui, et al.

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The devices studied in this work consist of copper phthalocyanine (CuPc) and fullerene (C₆₀) films between indium tin oxide (ITO) coated substrate as anode and aluminum (Al) as cathode. In order to have optimal performance of heterojunction photovoltaic cell, ITO/CuPc and C₆₀/Al contact should be ohmic. Various ITO treatmetns can be used to improve ITO/CuPc contact. We have compared influence of different ITO treatments on the device performance. We have found that ITO treatmetn yields significant improvement in the performance of CuPc/C₆₀ photovoltaic cells. The short circuit current of teh cell fabricated on ITO substrate with optimal treatment is 9 times larger than that of the cell fabricated on untreated ITO substrate, open circuit voltage has been increased by 0.12V, resulting in 12 times improvement in the power conversion efficiency. The performance of phthalocyanine solar cells can be further improved using a mixed layer structure, ITO/CuPc/CuPc:C₆₀/Al, to increase exciton dissociation efficiency. The mixed layer is fabricated by co-evaporating the materials. For the mixed layer structure, short circuit current has been increased two times compared to the p-n heterojunction cell. This results in 0.16% power conversion efficiency under 98mW/cm² AM1 solar irradiation.

Measurement of the optical properties of donor and acceptor derivatized PPV block copolymers

Carl E. Bonner Jr., Jared A. Ledbetter, Clinton M Harris, et al.

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Sulfone and methoxy substituted block co-polymers of poly-phenlyenevinylene (PPV) have been synthesized and the optical properties of both the oligomers and the combined block co-polymers have been examined. The substitution on the phenylene ring leads to small energy shifts in both the optical band gap as well as shifts in the valence and conduction band energies. When the oligomers are combined in a block co-polymer either with or without an aliphatic bridge unit, a space charge field is formed which can be used to quench the luminescence intensity in these materials by separating optically generated excitons and electron-hole pairs. PPV materials with this internal field have potential applications as solar energy converters and photodetectors. The absorption and emission spectra and the time dependence of the emission of donor and acceptor derivatized block co-polymers was measured and the quenching of the luminescence was observed and quantified.

Beyond amorphous organic semiconductors

Jun-ichi Hanna

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Recently it has been discovered that some types of liquid crystals, which believed to be governed by ionic conduction, exhibit a very fast electronic conduction. Their charge carrier transport is characterized by high mobility over 10^-2 cm²/Vs independent of electric field and temperature. Now, the liquid crystals are being recognized as a new class of organic semiconductors. In this article, a new aspect of liquid crystals as a self-organizing molecular semiconductor are reviewed, focused on their basic charge carrier transport properties and discussed in comparison with those of molecular crystals and amorphous materials. And it is concluded that the liquid crystal is promising as a quality organic semiconductor for the devices that require a high mobility.

Single crystal, liquid crystal, and hybrid organic semiconductors

Robert J. Twieg, Y. Getmanenko, Z. Lu, et al.

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The synthesis and characterization of organic semiconductors is being pursued in three primary structure formats: single crystal, liquid crystal and organic-inorganic hybrid. The strategy here is to share common structures, synthesis methods and fabrication techniques across these formats and to utilize common characterization tools such as the time of flight technique. The single crystal efforts concentrate on aromatic and heteroaromatic compounds including simple benzene derivatives and derivatives of the acenes. The structure-property relationships due to incorporation of small substituents and heteroatoms are being examined. Crystals are grown by solution, melt or vapor transport techniques. The liquid crystal studies exploit their self-organizing properties and relative ease of sample preparation. Though calamitic systems tha deliver the largest mobilities are higher order smectics, even some unusual twist grain boundary phases are being studied. We are attempting to synthesize discotic acene derivatives with appropriate substitution patterns to render them mesogenic. The last format being examined is the hybrid organic-inorganic class. Here, layered materials of alternating organic and inorganic composition are designed and synthesized. Typical materials are conjugated aromatic compounds, usually functinalized with an amine or a pyridine and reacted with appropriate reactive metal derivatives to incorporate them into metal oxide or sulfide layers.

Charge transport and charge generation in the bulk and at interfaces: discotic hole conductors versus amorphous hole conductors

Dietrich Haarer, Roland Bilke, Mukundan Thelakkat, et al.

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We describe two methods by which discotic liquid crystalline (LC) triphenylene materials can be sensitized for wavelengths of the visible spectrum and the near IR. One way of doping is more conventional and uses bi-layers between the LC-system and an adjacent pigment layer. The second doping is achieved by a molecular doping with C₇₀. Here the influence of doping on the measured mobilities is much smaller as compared to the influence of doping measured in amorphous systems. We attribute this smaller influence of the C₇₀-doping on the mobility to a partial phase separation occuring between the LC-host material and the C₇₀-guest molecules. In our comparative experiments on doped dimethyl-triphenyldiamines we can show taht the magnitude of the trap depths for hole-trapping has a very large influence on the measured transport parameters.

Charge carrier mobility in highly ordered smectic and discotic mesophases

A. Pecchia, L. Siebbeles, Bijan Movaghar

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The recently published data on phototransit signals in smectic and discotic liquid crystals, have led us to reconsider the old problem of the weak temperature dependence of the mobility in ordered narrow band systems and inthe liquid crystalline phases. We argue that one has to distinguish between currents which are due to light-generated carriers and currents due to band conduction in equilibrium and which can be described using the Kubo-Greenwood formula. We use a first principle band model in an electric field and show how a T-independent mobility can be derived for a single particle which obeys band transport, includes joule energy relaxation, elastic disorder, and agrees with the few carrrier limit of the Kubo formula. The result is essentially a generalized Drude velocity applicable to describe "single particle currents". We also discuss alternative explanations for the observed temperature independent mobilities which are based on hopping with weak disorder and polaron theories.

Modeling of electronic charge transport in smectic liquid crystals

Akira Ohno, Kensuke Kurotaki, Akihide Haruyama, et al.

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We have investigated the hole transport in smectic mesophases by Monte Carlo simulation based on a 2D hopping transport in Gaussian-distributed density of states and time-of-flight experiments. We found that their unique carrier transport properties such as non-Poole-Frenkel type of behavior i.e., field-and-temperature independent mobility, is well explained by the 2D disorder model with a small Gaussian width of 50-60 meV. Furthermore, we found the Pool-Frenkel type of behavior in a biphenyl derivatives and at a low temperaure range below ambient temperature in a therthiphene derviative and determined the Gaussian width to be 100-120meV and 50 meV, respectively. We came to a conclusion that the charge carrier transport in smectic mesophases can be explained by a 2D disorder model with a small Gaussian width of the density of states σ, where a value of σ/kT plays important role to determien its behavior at a given temperature.

Photoconducting discotic liquid crystals

Richard J. Bushby, Owen R. Lozman, Jason C. Bunning, et al.

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Molecular engineering of discotic liquid crystals is discussed with relation to charge carrier mobility and generation efficiency. Steady State DC and Pulsed TOF measurements are reported as viable complementary methods in the determination of charge generation efficiencies. Data for the quantum efficiencies of charge carrier generation in 2,3,6,7,10,11-hexa(hexyloxy)triphenylene and its 1:1 mixture with 2,3,6,7,10,11-hexakis(nonylphenyl)triphenylene (a complementary polytopic interaction (CPI) stabilized compound) are presented. The wavelength, field and temperature dependence of the quantum efficiency of charge carrier generation are discussed in respect of the Onsager theory of geminate recombination.

Electronic transport in self-organizing columnar phases

Irina Shiyanovskaya, Kenneth D. Singer, Virgil Percec, et al.

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This paper presents measurements of time-of-flight charge carrier mobility and thermal optical polarized microscopy on a new class of columnar liquid crystals (LC) self-organized into hexagonal columnar mesophase self-assembled form functionalized dendrons. We discuss the temperature and electric field-independent high hole mobility of 1-3.5×10^-3 cm²/Vs. The anomalous temperature behavior of the mobility is consistent with polaron transport mechanisms. Studies of time-of-flight transients, which reveal evidence for dynamic defects with lifetimes in the range of 10^-6 s, are presented as aspects of charge gneeration and recombination kinetics. We demonstrate the enhancement of electron transport in acene-based dendron LCs by dopign wtih TNF-based dendron molecules.

Polarization holographic data storage using azobenzene polyester as storage material

Emoeke Loerincz, Gabor Szarvas, Pal Koppa, et al.

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Polarization holographic read/write and read only demonstrator systems have been developed using ~2 µm thick azobenzene polyester on a card form media. The thin-film holographic system has practical advantages, e.g. high diffraction efficiency, no cross talk between the holograms, reading in reflection mode, no hardware servo, different wavelengths for writing and reading (non-volatile storage), data encryption possibility, no problem with material shrinkage, etc. The candidate azobenzene polyester has good thermal, room temperature and ambient light stability and good optical properties for the purpose of thin film application. Using thin-film holography the possibilities of multiplexing are limited, however, raw data density as high as 2.77 bit/µm² has been achieved in an optimized Fourier holographic system using high numerical aperture (NA³ 0.74) objective in a 8f arrangement with sparse code modulation and Fourier-filtering at 532 nm. High density polarization holographic demonstrator systems have been developed using ~2μm thick azobenzene polyesters on reflective card form media. FFT computer simulation of the system including saturation model of the material allows optimization of system components including data density and capacity. A raw density as high as 2.77 bit/μm² has been achieved without multiplexing in a compact, portable read/write sytem at 532 nm allowing more than 1000 readout without data loss. A separate read only system working at 635 nm realizes non-volatile readout and allows card exchange at a data density of 1.3 bit/μm². Security level of the presents holographic optical card systems can be further increased by using phase encoded reference beam. Advantageous applications of the proposed encrypted holographic card system are also outlined.

Electro-optic waveguide modulators by the integration of self-assembled superlattices with polymeric and semiconductor materials

Tobin J. Marks, Seng Tiong Ho, Zhifu Liu, et al.

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We describe here the fabrication and characterization of novel organic electro-optic materials composed of self-assembled superlattices. The SAS structures are intrinsically acentric and exhibit large second harmonic generation and electro-optic responses. This approach using SAS electro-optic materials has advantages such as not requiring poling for creating nonlinearity in the films and efficient film growth on a variety of substrates over large areas. Prototype waveguide electro-optic modulators have been fabricated using SAS films integrated wtih low-loss polymeric materials functioning as partial guiding and cladding layers. The waveguide EO modulators are fabricated using a multistep process including e-beam deposition, plasma-enhanced chemical vapor deposition, photolithogrpahy, and reactive ion etching. Electro-optic parameters such as thehalf-wave voltage and the effective electro-optic coefficient, and the velocity mismatch between the optical and radio frequency waves have been evaluated.

Femtosecond two-photon absorptivities of a thiacyanine dye for optical limiting applications

LaQuieta Huey, Carl E. Bonner Jr.

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The intensity dependent index of refraction and the molecular two-photon absorptivity (TPA) of 3,3'-dielthylthia-cyanine iodide (DETCI) has been measured in a range of solvents by the femtosecond z-scan technique. In DMSO, where DETCI is quite soluble, both the two-photon absorptivity and the nonlinear index of refraction are linear functions of concentration. In contrast, the two photon absorptivity and non-linear index change of DETCI in MeOH is an order of magnitude lower and shows signs of saturation at concentrations well below the saturation limit. In high index solvents such as MeOH, the TPA of DETCI is much smaller in lower index solvents such as DMSO. The intensity dependent index change, n₂ is large and relatively insensitive to the index of the solvent.

Optical limiting properties of carbon nanostructure and polymer dispersions

Sean M. O'Flaherty, Robert J. Murphy, Stephanie V. Hold, et al.

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Experimental measurements of optical limiting of nanosecond laser pulses by two distinctly different polymer and carbon nanostructure composite materials dispersed in solution is reported here. The polymer poly(para-phenylenevinylene-co-2,5-dioctyloxy-meta-phenylenevinylene) was used to form exclusive multi walled carbon nanotube and polymer composites. The polymer poly(9,9-di-n-octylfluorenyl-2,7-diyl) was used to form composites consisting of multi walled carbon nanotubes, other clearly defined carbon nanoparticles and polymer. The fabrication technique and material characterization steps are described, where it was found that the carbon nanostructures were stably dispersed in the polymer matrix in both cases. A range of each of these composites was prepared and varied according to carbon nanostructure mass content. The optical limiting experiments were performed using an open aperture Z-scan apparatus with 6 ns gaussian pulses at 532 nm from a frequency doubled Q-switched Nd:Yag laser. In the poly(para-phenylenevinylene-co-2,5-dioctyloxy-meta-phenylenevinylene) and exclusive multi walled carbon nanotube composite either the multi walled carbon nanotubes or the polymer dominates the nonlinear response depending on the relative mass of polymer to nanotube. In the other material saturation of the optical limiting was reached at carbon nanostructure mass percentages in excess of 3.8%, relative to the polymer mass, while the polymer exhibited no response of its own. Furthermore, the scattering of high intensity light from the materials was qualitatively probed and its angular dependence investigated. The nature of the carbon nanostructure inclusions in each material was found to significantly influence the scattering response of the composites.

Reverse-saturable-absorption-based optical limiting properties of indium and gallium phthalocyanines and naphthalocyanines

Sean M. O'Flaherty, Stephanie V. Hold, Yu Chen, et al.

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Experimental measurements of optical limiting resulting from reverse saturable absorption in metallo-phthalocyanine and metallo-naphthalocyanine compounds are reported. Open aperture ns z-scan with Gaussian pulses are employed to investigate the interaction of high intensity light with the novel compounds. Rate equations are used to analytically solve the static state solution that simulates the excited state dynamics resulting from the nonlinear excited state absorption, and this is fitted to the experimental data. General molecular engineering trends relating the optical limiting performance of these compounds to their structural characteristics are explored and discussed.

Organic light-emitting diode materials

Sigurd K. Schrader

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The relation between chemcial structure and functional properties of low-molecular and polymeric conjugated organic materials is discussed with respect to their use in organic light-emitting devices. Examples for emissive and for hole and electron transporting/injecting materials are given. The peculiarities of some nitrogen-containing heterocyclic compounds and their use as electron transporting/hole blocking materials are discussed in more detail. Results of an investigation of the valence electronic structure of a heterocyclic model compound (phenyloxadiazole derivative) at the interface to an inorganic conductive substrate are discussed in comparison to the interface between organic semiconductor/conductive inorganic electrode material as prominent in OLEDs. A short outlook towards future development is given.

Poled polymer thin film gratings studied by near-field second harmonic optical microscopy and far-field optical diffraction

Richard D. Schaller, Richard J. Saykally, Yuen-Ron Shen, et al.

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Electrical poling induces polar ordering of molecules in a grating that has been holographically inscribed on a thin film of polymer functionalized with azobenzene side chains. Depending on the surface relief amplitude, the resulting χ⁽²⁾ grating, seen by second harmonic generation (SHG) near-field scanning optical microscopy (NSOM), can have a periodic structure significantly different from the topographical image. The far-field linear and SHG diffraction patterns correlate well with the grating structures. Poling of the thin-film grating, which presumably has photo-driven non-uniform material properties within each period, leads to the more complex structure of the χ⁽²⁾ grating.

A half-period surface relief grating formed by bicolor interferences in thin azopolymer films

Aleksandra Apostoluk, Celine Fiorini-Debuisschert, Jean-Michel Nunzi

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All optical poling technique allows orientation of dye molecules in a polymer matrix by purely optical means. The coherent superposition of two beams at fundamental (FUND)and second harmonic (SH)frequencies results in the presence of the polar field E(t) inside the material, able to break the centrosymmetry of the medium. The temporal average cube 3>_t of this field E(t) = Eω(t)+E_2ω(t), is non-zero, which leads to an orientation-selective excitation of molecules and the second order χ⁽²⁾-susceptibility grating is encoded, with a period satisfying the phase matching condition for SH generation. The physical origin of the effect lies in the orientational hole-burning in the initially isotropic distribution of dye molecules (via trans-cis trans photoisomerisation). We investigate other phenomena responsible for the SHG in polymer films like the electron transfer between a polymer matrix (donor of electrons) and a dye molecule (electron acceptor) and processes which accompany polar orientation mechanism like the one-direction photoinduced molecular migration. The stability of the induced polar order after orientation has been also studied and the crosslinkable polymer system has been developed D side chain molecules with ends can thermally react with epoxy groups were randomly inserted in the side groups of the polymer backbone, which leads to the reticulation and enhancement of the rotational stability. After poling the second order nonlinear response exhibits a much longer relaxation time. In the set-up configuration used we monitor non-perturbatively the all optical poling and there is no necessity of taking into account the phase difference between writing beams (fundamental and SH). The periodical modulation of the relative phase provides the desired control on the polar order at the molecular level as well as the possibility of encoding surface relief gratings with a period equal to the half of the period of the χ⁽²⁾ grating.

Photoinduced spontaneous patterning of azopolymer films using light-controlled mass transport

Christophe Hubert, Celine Fiorini-Debuisschert, Paul Raimond, et al.

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During the last decade, large effort has been dedicated towards the miniaturization of devices for photonic applications. One of the key points towards this objective is structurization at the nanoscale level. Recently, one-step photoinscription of surface relief gratings onto azo-polymers was demonstrated: when a polymer film containing azobenzene dyes is irradiated by an interference pattern between polarizaed laser beams at a wavelength near the chromophore absorption band, the film surface undergoes a direct, reversible and controlled topographic modification. Grating modulation amplitudes as large as the film thickness can be obtained. In this work we provide experimental evidence of a new spontaneous light-assisted submicrometer hexagonal patterning process. More particularly, we show that uniform irradaiton of an azo-dye polymer using a singel laser beam wtih normal incidence onto the polymer film surface leads to a self structurization process resulting in the formation of a quasi hexagonal grating. The influence of parameters such as the laser intensity, the irradiation time, the thickness of the polymer film but also the irradiation wavelength is studied towards a better understanding of the process. Full control of the mechanisms at the origin of such process could further permit a complete manipulation of the molecular order and thus enable the generation of other spontaneous complex structures opening the way to the development of new easy-to-set micro and nano-structuration technqiues.

Polymeric resonant structures in the reflection geometry for electro-optic modulation and sensing

Robert L. Nelson, Joseph W. Haus, Brad Birchfield, et al.

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Mach-Zehnder interferometers have been the primary type of architecture for construction of polymeric electro-optic modulators. Recent attention has been given to electro-optic modulators in the reflection geometry as well as modulators that employ a resonant cavity to enhance activity or provide for modulator compactness. Most efforts have focused on the Attenuated Total Refelction (ATR) modulators which utilize a guided surface plasmon mode for providing sharply defined angles of incidence at which intesity modulation can be efficiently achieved. This is also the basis of many sensign devices where in both modulation and sensing an active region is placed adjacent to the metal guide. In this work we focus on alternative optical scheems to the ATR for modulation and sensing as well as the possibility of enhanced ATR activity. Resonant cavities are formed using photonic crystals or leaky wave structures which offer the possibiilty of efficient modulation and sensing.

Infrared optics applications of thin polyaniline emeraldine base films

Edward Bormashenko, Roman Pogreb, Semion Sutovski, et al.

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The use of polyaniline emeraldine base films as antireflection coating for near and middle IR optics elements was studied. The optical quality of ZnSe substrates spin-coated with thin PANI EB layers were studied using a Linnik interferometer. The spectral properties of PANI coated ZnSe plates were investigated in broad IR band with FTIR spectrometer. It was shown that PANI coating allows a significant decrease of Fresnel losses in the near and middle IR bands (1.0-6.25 μm). The coating allowed continuous transmission of high power density of IR radiation produced by CO₂ laser. The transmission coefficient doesn't depend on the incident laser beam power density (up to 3 W/mm²). The laser irradiation damage threshodl of the PNAI EB coating was studied at a wavelength of 1.5 μm and established as high as 0.1 GW/mm² (τ=12 10^-9s) for PANI EB coating wtih a thickness of 150 nm. Microhardness of the PANI EB coated ZnSe plates was established as satisfactory.

What the ultimate polymeric electro-optic materials will be: guest-host, crosslinked, or side-chain?

Cheng Zhang, Hua Zhang, Min-Cheol Oh, et al.

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Material processing and device fabrication of many different electro-optic (EO) polymers developed at USC are reviewed. Detailed discussion is given to guest-host CLD/APCs, crosslinking perfluorocyclobutane (PFCB) polymer CX1, and thermally stable side-chain polymers CX2 and CX3. Excellent EO performance (1.4V at 1.31 μm, 2.1 V at 1.55 μm) was achieved in CLD/APC Mach-Zehnder modulators (2-cm, push-pull). CLD/APCs also possess low optical losses (1.2 dB/cm in slab waveguides and in thick core channel waveguides). However, the guest-host materials only have limited thermal stability (110-132 °C in short term, <60 °C in long term) and require special techniques in device fabrication. The crosslinking polymer CX1 was able to provide long-term stability at 85 oC when fully cured. It also has a low optical loss (comparable to CLD/APCs) before curing and decent EO coefficient when poled at 180 °C. However, after the films were poled at the crosslinking temperatures (200 °C or above), the transmissions of the waveguides and EO activity became very poor due to poling-induced chromophore degradation. By judicial molecular design of both chromophore and monomer structures to suppress thermal motion of polymer segments, we were able to realize the same or even better thermal stability in side-chain polymers CX2 and CX3. Since no curing is needed, devices can be poled at their optimal poling temperatures, and all good properties can be obtained simultaneously. Despite the excellent solubility in chlorinated solvents, these side-chain polymers are resistant to some other organic solvents or solutions such as acetone, photoresist and various UV-curable liquids.

Optically addressable hybrid: photoconducting polymer-liquid crystal panels

Stanislaw Bartkiewicz, K. Komorowska, Andrzej Miniewicz, et al.

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The performance of photorefractive hybrid panels made from photoconducting polymer ane nematic liquid crystal layers are reviewed and discussed. Their properties were studied under pulsed and cw illumination. The dynamic holography experiments with pulsed laser beams give information on the charge mobility in the thin photoconducting layers. The use of panels to visualize the phase objects introducing small distortions to plane wave front (such as e.g. turbulence of air) through the well-known Zernike filtering is also shown. The advantage of such nonlinear Zernike filter over conventional one is that no precise optical adjustment is necessary and the filtering is relatively easy to control by tuning the externally applied field to the modulator and/or varying the incoming light intensity.

Free-standing films based on silicone resins

Dimitris E. Katsoulis, Michitaka Suto, Nobuo Kushibiki

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We report here the preparation of transparent, flexible films from silicone resins by solvent casting techniques. The films exhibit no birefringence, higher than 90% transparency between 350 to 1700 nm and average surface roughness below 1 nm. Thermal analysis shows that the films are stable at temperatures greater than 200 °C (depending upon the starting resin composition). The films are suitable substrates for deposition of various coatings including indium tin oxide (ITO). Transparent conducting ITO electrodes were prepared with ion plating and RF sputtering methods and characterized by microscopy, XPS, absorption spectroscopy and electrical conductivity measurements. Potential applications for silicone resin films exist in the market areas of displays, electronics and energy.

Thermally stable grating comprised of silsesquioxane film fabricated with hot embossing

Kenjiro Hasui, Ikue Takagaki, Okihiro Sugihara, et al.

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Thermally stable grating was fabricated with hot-embossing with the application of sol-gel derived silsesquioxane film. Planar films consisted of phenylsilsesquioxane and methylsilsesquioxane showed thermal stability with respect to the low birefringence, less than 1×10^-3, and no intrinsic absorption from 400 to 1700nm for the prolonged heating at 200°C for 10hours. The refractive index of the film was controllable from 1.49 to 1.56 at 632.8nm wavelength with increasing a molar ratio of the phenyl contents from 0.3 to 1.0, where the index controllability was ±0.0005. A short embossing process, less than 15 minutes, was demonstrated. In-situ mode-line measurement for the embossed grating showed change in refractive index under heating and cooling cycle was linear and reversible. A high thermooptic coefficient (dn/dT) of the films from room temperature to 150°C, which was approximately -2×10^-4 despite of its composition, was stable after subsequent nine thermal cycles. The measurement at the temperature up to 300°C showed streak propagation in the film and the prolonged heating at 150°C for 700 hours indicated no change in refractive index. This material is considered to be suitable for optical device application, since it showed the combined advantages of the both organic and inorganic materials such as embossing capability and high TO coefficient with thermal stability.

Fabrication of multimode polymeric waveguides and micromirrors using UV and X-ray lithography

Joon-Sung Kim, Jae-Wook Kang, Jang-Joo Kim

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Polymeric optical multimode waveguides have been fabricated by UV and X-ray lithography for optical interconnects. A UV-curable epoxy based polymer (SU-8) was used for VU contact printing. The material has low optical loss and is thermally stable up to 200°C. Polymethylacralate (PMMA) was used as the core layer which is a positive x-ray resist and optically transparent in visible and near IR region. The propagation loss of the fabricated multimode waveguides by the UV contact printing method was 0.36 dB/cm. Waveguide losses of 0.33 dB/cm at 633 nm and 0.54 dB/cm at 830 nm was obtained using PMMA waveguides fabricated by x-ray lithography. The losses of micromirrors were as low as 0.43 dB at both wavelengths.

Zero-birefringent polyimide for polymer optical waveguide

Sung-Ho Baek, Jae-Wook Kang, Xiangdan Li, et al.

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A novel zero birefringent and photosensitive polyimide was synthesized. The polymer is soluble in solvents and contains a chalcone group for photo-crosslinking by UV exposure. The glass transition and decomposition temperature of the polymer were 254°C and 430°C before cross-linking. Evolution of the absorption spectra upon UV exposure indicated that the cross-linking reaction is related to the cycloaddition of the double bonds in the chalcone group to form cyclobutane. The photo-crosslinking reaction not only increased the thermal stability, but also induced a refractive index change of the films. The refractive index of the film was reduced upon UV exposure from 1.5862 to 1.5697 for TE mode and from 1.5807 to 1.5697 for TM mode, respectively, resulting in zero birefringence after curing. Loss of p-conjugation in the chalcone group by the crosslinking reaction is supposed to induce the reduction of the refractive indices and orbital change from sp² to sp³ makes the polymer chain be kinked, resulting in decrease of birefringence. The polymer film showed optical loss of 0.41 dB/cm at 1.3 mm and 0.54 dB/cm at 1.55 mm. Zero birefringence and low optical loss combined with photo-processibility of the material are making it an excellent candidate for the high performance waveguide materials.

New applications using single-sideband polymer electro-optic modulators

Jeehoon Han, Byoung-Joon Seo, Harold R. Fetterman, et al.

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New applications of optical single-sideband modulators incorporating multimode interference (MMI) structures have been investigated. These devices have been fabricated using recently developed polymer materials and advanced polymer modulator technologies. We have studied photonic time-stretching, for the purpose of high-speed analog-to-digital conversion, using the single-sideband modulator. This implementation has been shown to almost completely eliminate the power penalty due to the different chromatic dispersion effects, in upper and lower sidebands, without bandwidth limitations. We also have designed a photonic RF phase shifter array based on the single-sideband modulator structure. In order to improve the performance, this integrated planar device has incorporated a novel balancing arm design, low crosstalk optical waveguide crossings and S-bend waveguide structures. Measurements of this configuration showed that our four outputs were independent and had highly linear RF phases with negligible RF power fluctuation. These pulsed and CW applications demonstrate the capability and complexity possible using polymer electro-optics and are expected to significantly contribute to future optical communication systems and to optical/microwave beam steering and transmission.

Influence and utilization of UV-induced refractive index changes of photopolymers for the fabrication of 3D micro-optical elements

Ulrich Streppel, Peter Dannberg, Christoph A. Waechter, et al.

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The exposure of UV-sensitive polymers leads to a saturable and irreversible change of the refractive index up to 0.1 due to the polymerization generated changing material density. This non-linear phenomenon has a strong impact on the structure formation in the UV-assisted fabrication of thick micro-optical elements. E.g., appearing self-focusing effects and time-dependent absorption influence the sidewall geometry, while self-guiding effects have impact on the internal index distribution. Based on a material model, which describes the index change as a function of a set of process parameters, a modified iterative beam propagation algorithm is developed to simulate the structure formation. It is shown theoretically as well as experimentally that the variation of process parameters, e.g. the photo initiator concentration or the initial complex exposure field distribution, offer possibilities to control the structure formation and to make use of the self-organizing tendency. The developed patterning method is optimized for standard contact lithography processes, e.g., in a mask aligner, requiring only low exposure intensities below 10 mW/cm². This enables for performing integral wafer-scale patterning processes, e.g., on optoelectronical substrates. The realization of arrayed on-chip conic light concentrators are presented as an exemplary application.

Electro-optic modulators based on high-temperature polyimides

Sergey S. Sarkisov, Connie Walton, Michael J. Curley, et al.

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We report on the synthesis of high-temperature electro-optic polyimides and the design of the electro-optic modulators based on these materials. Recently developed electro-optic polyimide has a glass transition temperature of 461°C. It can be processed in a thin light-guiding film form a precursor at a temperature of 160°C. DIfferent novel designs of electro-optic modulators are being considered. A single-arm double-mode interferometer configuration is proposed for a thin-film transverse electro-optic waveguide modulator with low driving voltage. The driving voltage could be less than 5V that makes it compatible with conventional silicon-based electronics. We also consider miniature longitudinal modulators integrated with fiber optics in Fabry-Perot configuration. The advantage of the approach is the use of a small size area of the electro-optic film where high optical quality cna be easily achieved. The approach is being testd for polyimides as well as for single-crystal organic films wtih high electro-optic coefficients.

Novel perfluorocyclobutyl (PFCB) polymers containing isophorone-derived chromophore for electro-optic [EO] applications

S. Suresh, Shengrong Chen, Chris M. Topping, et al.

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Perfluorocyclobutyl (PFCB) polymers and copolymers have a unique combination of properties well suited for optical applications such as high temperature stability, precisely controlled refractive index, low moisture absorption, excellent melt and solution processability, a variable thermo-optic coefficient, and low transmission loss at 1300 and 1550 nm. Electro-optical devices from polymers of ring locked polyene chromophores are attractive due to their thermal, mechanical, optical and dielectric properties. Polyene chromophores with highest hyperpolarizability are covalently attached to trifluorovinyl aryl ether containing moieties and copolymerized with other monomers. The resulting polymers display imrpoved thermal stability, solubility and good film forming capabilities.

Ionic materials to improve charge injection in polymer electroluminescent devices

Tae-Woo Lee, Jong Hyeok Park, Ho-Chul Lee, et al.

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We describe here the fabrication and characterization of novel organic electro-optic materials composed of self-assembled superlattices. The SAS structures are intrinsically acentric and exhibit large second harmonic generation and electro-optic responses. This approach using SAS electro-optic materials has advantages such as not requiring poling for creating nonlinearity in the films and efficient film growth on a variety of substrates over large areas. Prototype waveguide electro-optic modulators have been fabricated usgin SAS films integated with low-loss polymeric materials functioning as partial guiding and cladding layers. The waveguide EO modulators are fabricated using a multistep process including e-beam deposition, plasma-enhanced chemical vapor deposition, photolithography, and reactive ion etching. Electro-optic parameters such as the half-wave voltage and the effective electro-optic coefficient, and the velocity mismatch between the optical and radio frequency waves have been evaluated.

Organic electro-optics: exploiting the best of electronics and photonics

Larry Raymond Dalton, Bruce H. Robinson, Robert Nielsen, et al.

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Utilizing guidance from quantum and statistical mechanics, the electro-optic coefficients of organic materials have been increased to values greater than 100 pm/V at telecommunication wavelengths (e.g., to 130 pm/V at 1.3 microns). Electro-optic materials now afford significant advantages in terms of bandwidth and electro-optic activity over inorganic materials such as lithium niobate. Moreover, organic materials have also been found to be quite processable permitting the fabrication, by reactive ion etching and photolithographic techniques, of 3-D active waveguide structures and integration with both VLSI semiconductor electronics and silica fiber optics. Stripline, cascaded prism, and microresonator structures have been fabricated, as have low-optical-loss coupling structures. A number of prototype devices demonstrating superior performance have been produced; however, the long-term, in-field performance of such devices still remains to be evaluated. Nevertheless, significant advances have been made in improving the thermal and photochemical stability of organic materials and in defining the mechanisms that define these stabilities (by testing under accelerated conditions). The role of nanoscale architecture in systematically improving stability of organic electro-optic materials, as well as contributing to enhanced electro-optic activity and reduced optical loss, has been clarified.

Insertion loss reduction in high-speed polymer electro-optic modulators using tapered waveguide, fiber tip lenses, and modification of waveguide structures

De Yu Zang, Golden Shu, Travis Downing, et al.

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Optical insertion loss improvement is needed for high-speed, electrooptic polymer waveguide modulators. The insertion loss is mainly attributed to fiber end-coupling loss caused by mode mismatch between the fiber and the polymer waveguide. Three approaches of reducing end-coupling loss have been pursued: tapered waveguides, fiber tip lenses and modification of the waveguide structure. Tapered waveguides can be accomplished using Reactive Ion Etching (RIE) and shadow or gray-scale mask techniques. Experimental results have shown that the best coupling loss improvement up to 3 dB per end-coupling can be achieved by tapered waveguide. Fiber tip lens technique has been currently investigated for improving end-coupling loss. Preliminary testing results using fiber tip lenses have been encouraging showing a 1.5 - 3.5 dB improvement per end-coupling. The fiber tip lens technique provides a reliable and repeatable approach for loss reduction for high-speed polymer modulators. Modifying a highly asymmetric rib waveguide to a symmetric buried channel waveguide will greatly improve coupling efficiency.

Recent progress in developing highly efficient nonlinear optical chromophores and side-chain dendronized polymers for electro-optics

Jingdong Luo, Sen Liu, Marnie Haller, et al.

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Recent progress in developing high-performance nonlinear optical chromophores and polymers for electro-optics is reviewed. Using the single-mode focused microwave irradiation, a diversified family of 2,5-dihydrofuran derivatives has been synthesized as a new class of tunable electron acceptors. Very large r33 values (128 and 116 pm/V at 1.3 μm) have been demonstrated by doping one of the 2-dicyanomethylen-3-cyano-4,5,-dimethyl-5-trifluoromethyl-2,5-dihydrofuran (CF3-TCF)-based chromophores in poly(methyl methacrylate) (PMMA) and a high T_g polyquinoline (PQ-100), respectively. An excellent long-term temporal stability at 85°C has also been maintained in the PQ system. Two side-chain dendronized NLO polymers have been synthesized. Using a mild, simple, and generally applicable post-functionalization method, highly polarizable chromophores with dendritic modification has been covalently attached to side chains of poly(4-hydroxystryene). This approach provides the combined advantages of achieving better poling efficiency through the dendritic effect and shortening the development time required for E-O dendrimer synthesis. Systematic property comparison between these polymers and other conventional NLO polymers, such as guest-host and simple side-chain polymers, has been performed. Exceptionally high poling efficiency (a very large E-O coefficient of 97 pm/V at 1.3 μm) and good temporal stability at room temperature were dmeonstrated in this dendronized side-chain polymer system.

Material development and processing for electro-optic device systems

Danliang Jin, Raluca Dinu, Timothy Parker, et al.

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New crosslinked clad polymers were developed for electro-optic polymer modulators with special attention paid to properties such as refractive index tunability, optical loss, and conductivity. These cured polymers showed relatively low optical loss at 1550 nm and desirable conductivity. The clads were used to fabricate electro-optic devices having mode profiles closely matched to that of optical fibers in order to reduce insertion loss. A new hardmasking technique was developed to define Mach-Zehnder rib waveguides by photolithography and dry etching with high reliability and surface smoothness. The hardmasking technique demonstrated flexibility in defining waveguides made with electro-optic polymers having different reactivity towards etchant gasses.

Effects of alkyl spacer group length on Vis-NIR absorption behavior in FTC-like guest-host EO polymers

Richard R. Barto Jr., Peter V. Bedworth, Joseph A. Epstein, et al.

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Spectral absorption behavior of a series of FTC-like dyes of varying shape incorporated into amorphous polycarbonate (APC) is characterized by photothermal deflection spectroscopy. Previous Monte Carlo calculations by Dalton and Robinson predict a strong dependence of the macroscopic nonlinear optical susceptibility on the chromophore waist:length aspect ratio in electric field-poled films. This dependence arises from London interactions between chromophores, which are expected to influence the absorption characteristics of the composite both by changing the local polarity of the medium and through dipole interactions. It is expected that these interactions will play a role in the absorption characteristics of unpoled films as well. Of particular interest are the spectral characteristics of the red edge of the main dye electronic absorption peak, and the fine structure in the near-IR, dominated by overtones of fundamental C-H stretching and bending modes. The spectral structure in these key regions can be influenced by inter- and intramolecular interactions and conformational changes in the dye. The near-IR structure, in turn, will dictate absorption loss in optical devices prepared from these materials at key transmission wavelengths (1.3 and 1.55 um). In this study, a homologous series of spacer lengths, ranging from ethyl to hexyl, attached to an FTC-like NLO chromophore, LMCO-46M, is characterized by a combination of photothermal deflection spectroscopy (PDS) and UV-Vis spectroscopy to examine the effects of the molecular environment on near-IR loss at 1090 nm, 1300 nm and 1550 nm.

Optical loss of photochromic polymer films

Song Yun Cho, Eunkyoung Kim

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Optical loss of polymer films containing photochromic molecules were investigated. Perfluoroalkyl methacrylates containing carbonate or carbamate group were synthesizd to improve optical and adhesion properties of the polymerized film. Transparent polymer films were prepared by photocuring of a solution containing the perfluoroalkyl methacylates and a photoinitiator. Optical loss lower than 0.35 dB/cm at 1550 nm was determined from the photo cured film without photochromophore, for the TE polarization by measuring the transmitted power along the slab waveguide length. Photochromic polymer films were prepared by photocuring of the perfluoroalkyl methacrylates solution containing diarylethenes as a photochromophore. The resultant photochromic films showed reversible color change plus photo induced refractinve index change upon excitation with a light. Optical loss of the photochromic film at colorless state was determined as 0.27 dB/cm. Optical loss of the photochromic film at colored state was similar to that of the colorless state. The photochromic films showed optical and refractive index tunability by light under very low optical loss.

Intermolecular vibronic coupling in self-assembling molecular nanowires of hexabenzocoronene derivatives

Alexander John Fleming, Jonathan N. Coleman, Andreas Fechtenkoetter, et al.

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The vibronic structure of the luminescence and luminescence-excitation spectra of alkyl substituted hexa-peri-hexabenzocoronene (HBC-C_8,2) and hexa(4-n-dodecylphenyl) substituted hexa-peri-hexabenzocoronene (HBC-PhC₁₂) are described and explained in terms of the collective behavior of molecules in a molecular nanowire structure. The low concentration species of HBC-C_8,2(10^-13 M) was found to have a homogeneouly broadeend emission expected for an isolated molecule. At medium (10^-8 M) and high (10^-6 M) concentration the HBC moelcules aggregate into nanowires and a change in the vibronic and electronic structure is observed. The addition of exo-phenyl groups, as in the case of HBC-PhC₁₂, was found to increase the configurational coordinate displacement in the photo-exicted state by increasing the intermolecular vibronic coupling. These result coroelate well with the observed reduced photo-luminescence efficiency of the HBC-PhC₁₂ nanowires.

Hyperstructured molecules for light harvesting

Hiroyuki Sasabe

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Topologically well defined molecules are called 'hyper-structured molecules (HSM)'. We have developed rigid dendrimers as a typical example of HSM which shows the intramolecular energy transfer from the outer shell to the core. These molecules are applicable for the light harvesting such as antenna molecules.

Conditions leading to the formation of polymer thin layers with densely dispersed organic dyes using the vapor transportation method with vacuum technique

Toshiko Mizokuro, Hiroyuki Mochizuki, Noritaka Yamamoto, et al.

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The "vapor transportation method" with vacuum technique, developed previsouly in our laboratory, was used to form polymer thin layers with densely dispersed photochromic dyes on polystyrene (PS), poly(methyl methacrylate) (PMMA), and polymcarbonate (PC) substrates. The organic photochrmoic dye cis-1,2-dicyano-1,2-bis(2,4,5-trimethyl-3-thienyl)ethane (CMTE) was used, and the rate of formation of the CMTE-conatining layer was as follows: PS > PC > PMMA. These observations are important for application of the present method to the formation of organic memory media for optical recording etc. These results also indicate that the formation rate is dependent on not only the chemical composition and the structure of the polymer substrate, but also on the treatment temperature. Optical density measurements of the CMTE-dispersed thin polymer films showed maximum values near the glass transition temperature (g) with increases in temperature of film formation. The T_g values of CMTE-dispersed polymers decreased rapidly after CMTE-dispersal into the polymer matrices, indicating that T_g values of the polymers are important parameters for investigation of the mechanism of formation of CMTE-dispersed layers on polymer substrates using the present method.

Polymers of photoreactive calixarene derivatives for optical applications

Yoko Yamagishi, Toshikuni Kaino, Atsushi Kameyama, et al.

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Calixarenes are materials that possess many significant features, such as making inclusion complexes and high thermal stability, because of their cavity-shaped cyclic structure. We synthesized three types of calixarene polymers using calixarene derivatives containing photo reactive groups. Calixarene polymers were synthesized by exposing UV light to the calixarene derivatives. The characteristics of calixarene polymers wre investigated for optical applications. Polymer thin films were easily fabricated by spin coating on the substrates. All the polymers possessed high T_d. When calixarene derivatives containing acryloyl groups were polymerized with a monomer containing three methacryloyl groups, the refractive index of the polymer film was saturated within a short period of UV exposure time, and the thickness change was very small, less than 0.6%. The propagation loss of the polymer slab waveguide at 1300nm was 0.5dB/cm, which is less or almost the same as that of PMMA at this wavelength. Single-mode and multi-mode sized optical channel waveguides of calixarene polymer were fabricated using the photolithography and oxygen reactive ion etching. The propagation loss of the multi-mode waveguide was calculated to be less than 1.2 dB/cm at 1300nm.

Self-organized filamentation of photoresponsive polymers during UV curing

Ulrich Streppel, Dirk Michaelis, Andreas H. Braeuer

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The exposure of thick UV sensitive polymer layers leads to the formation of self-organized refractive index filaments, which appear after a certain layer thickness, typically in the range of 50 μm. This effect has strong influence on the fabrication of, e.g., high precision micro-optical elements. In particular, diffraction limited micro-lenses with high sag or high aspect ratio on-chip collimators require a perfect index homogeneity. We show that the saturable and irreversible change of the polymer refractive index causes a modulational instability (MI), which finally leads to the creation of filaments. It will be presented that a maximum growth rate for a certain spatial frequency of an initial perturbation exists. This MI gain determines the dynamics of the filament formation. The impact of the exposure conditions, e.g., chemical process parameters, on the homogeneity of the UV cured polymer is discussed for ORMOCER^TM type polymers.

Optical amplification properties of a cyanine-dye-doped DNA-lipid complex fiber

Koki Ishihara, Y. Kumakura, A. S. Haja Hameed, et al.

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A DNA-CTMA optical fiber was prepared by melt-spinning method for the first time. A hemicyanine dye, trans-4-(4-dibutylamino)-styryl)-1-methylpyridinium iodide (DBASMPI) doped DNA-CTMA fiber with core diameter of 1 mm and dye concentration of 3.6 wt% was obtained by soaking it in an aqueous dye solution. Laser (532 nm) pumped amplified spontaneous emission (ASE) at 610 nm was observed in the dye-doped DNA-CTMA fiber. The ASE occured at energy density 50 mW. The results from ASE emphasize that DBASMPI doped DNA-CTMA fiber is appealing as a good candidate for optical amplifiers and superfluorescence sources in a variety of communication and sensor applications.

Preparation and nonlinear optical properties of phthalocyanine nanocrystals

Christian Nitschke, Sean M. O'Flaherty, Michael Kroell, et al.

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Zinc-2, 9,16,23 -tetra- tert-butyl-29H, 31H -phthalocyanines and Zinc-2, 9,16,23-tetrakis-(phenylthio)-29H, 31H-phthalocyanines were recrystallized from an acetone solution to give regular shaped spherical particles of 50nm diameter, confirmed by transmission electron microscopy (TEM) and atomic force microscopy (AFM). A peak broadening of the Q-Band and a shift of the B-Band in the UV-Visible absorption spectrum combined with a significant fluorescence quenching was observed. The z-scan technique was used to investigate the non-linear optical properties and an increase of approximately 200% in the ratio of excited to ground state absorption cross sections in the crystal state was observed indicative of a significant increase in the optical limiting response of the crystals compared to the monomers.

Fluorinated polyimide film replicated using SiO2 mold for fabrication of waveguide

Tsuyoshi Shioda

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A replication process using a SiO₂ mold was demonstrated using a fluorinated polyimide. The replicated fluorinated polyimide film was separated from the mold by soaking in distilled water. The film separation with slow and then successive fast steps was due to the displacment of the weak adhesive layer by water and release of stress at the interface, respectively. Lowering the curing temperature of the fluorinated polyimide, whcih formed the weak adhesive layer at the interface, improved separation without chipping of the stripes of the SiO₂ mold.

Triphenyl amine-cored multibranched organic glasses for photonic applications

Soo Young Park, Hye Jeong Lee, Jiwon Sohn, et al.

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Strongly fluorescent organic nonlinear optical materials containing triphenyl amine as a core and 1-(2-ethyl-hexane-1-sulfonyl)-4-vinylbenzene or 4-[4-(2-ethyl-hexane-1-sulfonyl)-1-ethyl] styrene group as the arms which were mono-,di-, and tri-substituted at the para position of nitrogen were synthesized. Photorefractivity of these molecular glasses were measured and correlated with their structure and symmetry. Large two-photon absorption cross-section and strong up-conversion fluorescence emission were also observed in these materials.

Picosecond transient absorption studies of dipyridophenazine

Caitriona Creely, John Kelly, A. M. Selmi, et al.

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We report on ultrafast (200fs pulse durations) pump (267nm) and probe (430-700nm) studies of dipyrido -[3,2-a:2’,3’-c] phenazine (dppz) derivatives.These compounds are of interest as their metal polypyridyl complexes are used as photophysical probes for duplex DNA, where the dppz ligand can intercalate between the base pairs. Dppz, Me2dppz and F2dppz were studied to investigate changes occurring in their excited states over 1ns. In each case a transient decay and formation of a new species are seen in the 10 ®100ps region. The nature of these transients is discussed.

Polymeric waveguide fabrication based on mold technology

Okihiro Sugihara, Naomichi Okamoto

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In this work simple fabrication technique of polymeric optical waveguide and its elements in polymer films based on mold technoloy is demonstrated. The large core optical waveguide, relief gratings and fiber guide were produced in polymer films using hot-emboss technique. Moreover, simultaneous formation of channel waveguide with other elements based on the present technology is also reported.

Preparation of gold nanoparticle embedded graphitic carbon nanotubes and their field emission properties

Kyungkon Kim, Sang Hyun Lee, Se Gi Yu, et al.

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We prepared graphitic carbon nanotubes (Au-CPNTs) embedded with gold particles of 5-6 nm in diameter. First, poly(p-phenylenevinylene( (PPV) nanotubes containing Au particles were synthesized by chemical vapro deposition polymerization (CBDP) of α,α'-dichloro-p-xylene, which was carbonized at 850°C. Field emission properties of the Au-CPNTs were found to be excellent revealing a low turn-on electric field (2.1 V/μm) and a high amplification factor (11,500). In contrast, the values for virgin CPNTs were 3.1 V/μm and 4,910, respectively.

Unique determination of the hyperpolarizability tensor ratio through a depolarized hyper-Rayleigh scattering under an external electric field

Jisoo Hwang, Byoungchoo Park, Hye Jeong Chang, et al.

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We analyzed the depolarized hyper-Rayleigh scattering from molecular system with a partial macroscopic polar ordering. It is shown that hyperpolarizability tensor ratio and molecular dipole moment can be determined simultaneously by the depolarization measurement as a function of the external field strength. As an experimental example, we performed a quantitative analysi sof the electric-field dependent depolarized hyper-Rayleigh signal from a poly-γ-benzyl-L-glutamate solution, obtaining the dipole moment and the ratio of hyperpolarizability components as 4.0 Debye and β₃₁₁/β₃₃₃=-0.81.

Holographic photopolymerization for fabrication of electrically switchable inorganic-organic hybrid photonic structures

Rachel Jakubiak, Dean P. Brown, Fatma Vatansever, et al.

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Holography offers a versatile, rapid and volume scalable approach for making large area, multi-dimensional, organic PBGs; however, the small refractive index contrast of organics prevents formation of a complete band-gap. The introduction of inorganic nanoparticles to the structure provides a possible solution. In contrast to the multiple steps (exposure, development and infiltration) necessitated by lithographic-based holography (e.g. photoresists), holographic photopolymerization of monomer-nanoparticle suspensions enables one-step fabrication of multidimensional organic-inorganic photonic band gap (PBG) structures with high refractive index contrast. The PBGs are formed by segregation of semiconductor nanocrystals during polymerization of the polymer network. Addition of CdSe/ZnS polymerization of the highly cross-linked polymer network. Addition of CdSe/ZnS quantum dots or ZnO nanocrystals to the H-PDLCs formulation results in phase segregation of the nanoparticles into the liquid crystal rich lamellae, producing photonic structures with high diffraction efficiencies that may be modulated by application of an external electric field.

Self-assembled materials for photonic devices

J. H. Lalli, B. Lepene, Jean Huie, et al.

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This paper reports developments in the molecular-level self-assembly of materials that may be used in photonic devices. Specifically we present results of layer-by-layer electrostatic self-assembly processes that may be used to rapidly form electrically-conductive and optically transparent films for potential use as claddings for electro-optic modulators or other optical waveguiding structures.

Electro-optic polymer integrated optic devices and future applications

James H. Bechtel, James H. Menders, De Yu Zang

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Recent developments in electro-optic polymer materials and devices have led to new opportunities for integrated optic devices in numerous applications. The results of numerous tests have indicated that polymer materials have many properties that are suitable for use in high-speed communications systems, various sensor systems, and space applications. These result coupled with recent advances in device and material technology will allow very large bandwidth modulators and switches with low drive voltages, improved loss, long-term stabilty, and integration with other microelectronic deices such as MEMS. Low drive voltage devices are very important for space applications where power consumption scales as the square of the modulator half-wave voltage. In addition, we have demonstrated novel dual polymer modulators for mixing RF signals to produce sum and difference frequency modulation on an optical beam. This novel approach allows the suppression of the modulation at the two input RF signals and only the mixing signals remain superimposed on the optical beam. The dual modulator can be used for various encoding and frequency conversion schemes that are frequently used for both terrestrial and space communcations. Another application of polymer integrated optics is in the field of optical sensing for high frequency electric field.

High-bandwidth graded-index plastic optical fiber fabricated by centrifugal deposition method

Jang-Joo Kim, Bu-Gon Shin, Jeong-Ho Park

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High bandwidth graded index plastic optical fiber was fabricated by the centrifugal deposition method. A preform for graded index plastic optical fiber (GI-POF) was fabricated by the centrifugal deposition method. In the method mixture of two monomers having different refractive indices is fed into a rapidly rotating tube in an oven, The mixture forms a thin layer on the tube wall due to the centrifugal force and polymerize simultaneously by thermal reaction. By changing the composition of the feeding monomer mixture as a programmed way, the refractive index of the preform can be controlled in the radial direction as programmed. Methyl methacrylate (MMA) and benzyl methacrylate (BzMA) were used as the monomers. The GI-POF was obtained by thermal drawing of the preform. The refractive index profile of the drawn fiber was the same as the preform. The optical loss of the GI-POF was 120 dB/km and the bandwidth was 3.45 Gbps100m at the wavelength of 650 nm.

Sub-5-fs spectroscopy of several organic systems

Takayoshi Kobayashi

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Transform-limited (TL) visible pulses with as short as sub-5-fs duration have been generated from a noncollinear optical parameteric amplifier and applied to the study of polyacetylene, polydiacetylene, azobenzene, and J-aggregates of porphyrin for optical devices. We also report the first direct observation of structural changes in a complex biological system, the chromophore of bacteriorihodopsin, as it undergoes the trans-cis photoisomerization by monitoring changes in the vibrational spectra of the transition state.

Introduction of multilayered photorefractive polymer composite for the enhancement of recording stability

Won Sun Kim, Jong Woo Lee, Shi-Joon Sung, et al.

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In the present work, multilayered photorefractive polymer composites were introduced in order to enhance the recording stability without sacrificing the recording speed of the photorefractive polymer composites. In the multilayered photorefractive polymer composite, the gain coefficient, diffraction efficiency, and the recording stabilty could be enhanced without sacrificing the response time of the photorefractive polymer composite.

Organic Photonic Materials and Devices V

Volume Details

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Table of Contents