Technical Abstract Summaries · 2010. 3. 29. · The photophysics of conjugated polymers is dominated by excitonic effects. Due to the vast level of inhomogeneous broadening, intermolecular

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Contents

Conf. 6182 Photonic Crystal Materialsand Devices III . . . . . . . . . . . . . . . . . . . . 3

Conf. 6183 Integrated Optics, Silicon Photonics,and Photonic Integrated Circuits . . . . 44

Conf. 6184 Semiconductor Lasersand Laser Dynamics II . . . . . . . . . . . . .77

Conf. 6185 Micro-Optics, VCSELs, and PhotonicInterconnects II: Fabrication,Packaging, and Integration . . . . . . . . 119

Conf. 6186 MEMS, MOEMS, andMicromachining II . . . . . . . . . . . . . . . . 144

Conf. 6187 Photon Management II . . . . . . . . . . . . 157

Conf. 6188 Optical Micro- and Nanometrology inMicrosystems Technology . . . . . . . . . 179

Conf. 6189 Optical Sensing II . . . . . . . . . . . . . . . 204

Conf. 6190 Solid State Lasers and Amplifiers II . 244

Conf. 6191 Biophotonics and NewTherapy Frontiers . . . . . . . . . . . . . . . . 271

Conf. 6192 Organic Optoelectronicsand Photonics II . . . . . . . . . . . . . . . . 299

Conf. 6193 Reliability of Optical FiberComponents, Devices, Systems,and Networks III . . . . . . . . . . . . . . . . . 341

Conf. 6194 Millimeter-Wave and TerahertzPhotonics . . . . . . . . . . . . . . . . . . . . . 359

Conf. 6195 Nanophotonics . . . . . . . . . . . . . . . . . . 371

Conf. 6196 Photonics in Multimedia . . . . . . . . . . 400

Conf. 6197 Photonics for Solar Energy Systems . 413

Conf. 6198 Photonics in the Automobile II . . . . . 432

Technical Abstract Summaries

3–7 April 2006 Palais de la Musique et des Congres, Strasbourg, France

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510 nm/517 nm) and oxonine - as an acceptor (abs./em. 600 nm/609nm). Donors and acceptors are randomly distributed inside of zeolitechannels. We investigated series of samples were the ratio of thenumber of donor and acceptor molecules is constant (1:1) but meandistance between them is decreasing. By selectively excitingpyronine at 460 nm, bands from both dyes can be observed, whichmeans that energy transfer from pyronine to oxonine takes place.The higher the loading of the sample, the smaller the donor-acceptordistance, the higher the rate of energy transfer. For steady-statemeasurements this means an increasing ratio of acceptor to donorfluorescence intensity.In time resolved measurements fluorescence intensity decays ofboth donor and acceptor identify energy transfer. The acceptorintensity is first built up before it starts to decay. This intensityincrease becomes faster with increasing loading. For the intensitydecay of the donor, it decreases faster with increasing loading. Aswe perform measurements on multiple crystals the intensity decay ofthe donor should follow stretched-exponential model, regarding toFörster theory. This is the case for our system. Parameter δ instretched-exponential model (or parameter β in Klafter-Blumenformalism) is the parameter of dimensionality. It is equal to 1⁄2, 1/3and 1/6 for three, two and one dimensions correspondingly. Theadmirable fact in our series is that δ is decreasing from 0.46 (thelowest loading) to 0.23 (the highest loading) which means that in thepacked sample energy is transported mostly in one direction, alongthe channel, and almost not between the channels. This is a bigadvantage for photonic antennae, as anisotropic transport of theelectronic excitation energy is important for efficient energymigration. Fast energy migration is crucial, since acceptor dye is tobe reached before other processes, such as spontaneous emissionor radiationless decay, will take place. [2]References:[1] D. Brühwiler, G. Calzaferri, Molecular sieves as host materials forsupramolecularorganization, Microp. Mesop. Mater., 2004, 72, 1-23.[2] K. Lutkouskaya, G. Calzaferri, manuscript in preparation

6192-41, Session 10Organic nanofibers as new media for lasing,waveguiding, and field enhancementF. Quochi, F. Cordella, A. Mura, G. Bongiovanni, Univ. degli Studi diCagliari (Italy); V. G. Bordo, General Physics Institute (Russia); R.Frese, H. Rubahn, Syddansk Univ. (Denmark)Recent experimental results showed optical waveguiding [1] andlow-threshold random laser action [2] in self-assembled mesoscopicstructures, i.e. needle-like structures [3-5] with a cross section of afew hundreds times some tenth of nanometers and an elongation upto several hundred micrometers. These structures, called ‘organicnanofibers’, are made of differently functionalized molecules andaccordingly possess characteristically different morphologicalshapes and electronic properties [6].In this contribution, combined atomic-force microscopy (AFM) andlasing micro-spectroscopy on single organic nanofibers are used todetermine the correlation between the nanofibers’ structure andnonlinear optical properties. The experimental results show that thinfiber breaks give origin to distributed optical feedback along theneedle axis, from which we infer that random lasing in singlenanofibers is one-dimensional [7]. This determination is supported bythe results of simulations of coherent light propagation in thenanofibers using a transfer-matrix approach. Moreover, by selectingbreakless nanofibers it is possible to eliminate the optical feedbackresponsible for coherent oscillation and single out the process ofwaveguide amplification of spontaneous emission (ASE). From ASEmeasurements, modal gain values as large as 10^3 cm-1 arededuced in para-sexiphenyl single nanofibers [8].We also present numerical calculations that estimate the guidedmodes and confinement within the needles. The cross section for thesimulation is obtained from morphology measurements of thestructures itself by AFM microscopy. We discuss cross-coupling oftwo spatial separated structures, up to a case of rigid coupling,where the system acts like a single waveguide. Certain molecularbasis units result in nanofibers that are triangularly shaped - thenumerical calculations suggest a strong field enhancement whichmight be used for future surface enhanced Raman scatteringmeasurements on nanoaggregates [9]. The numerical results are

compared with the results from a newly developed analytical theory,which predicts linear optical phenomena in a single nanofiber, placedon a substrate or immersed in liquid, as well as arrays of nanofiberson a substrate. The analytical theory is focused on scattering of light,excitation of waveguiding modes and photoluminescence [10].[1] F. Balzer et al., Phys. Rev. B 67 (2003) 115408.[2] F. Quochi et al., Appl. Phys. Lett. 84 (2004) 4454.[3] H. Yanagi, and T. Morikawa, Appl. Phys. Lett. 75 (1999) 187[4] A. Andreev et al., Adv. Mater. 12 (2000) 629.[5] F. Balzer, and H.-G. Rubahn, Appl. Phys. Lett. 79 (2001) 3860.[6] F. Balzer, and H.-G. Rubahn, Adv. Funct. Mater. 15 (2005) 17.[7] F. Quochi et al., J. Phys. Chem. B, in print.[8] F. Quochi et al., to be published.[9] R. Frese et al., to be published.[10] V.G. Bordo et al., to be published.

6192-42, Session 10Long-lived charged states in single-walled carbonnanotubesC. Gadermaier, Politecnico di Milano (Italy); E. Menna, M.Meneghetti, Univ. degli Studi di Padova (Italy); J. W. Kennedy, V. Z.Vardeny, Univ. of Utah (USA); G. Lanzani, Politecnico di Milano (Italy)Single-walled carbon nanotubes are elongated members of thefullerene family that are currently the focus of intensemultidisciplinary study because of their unique physical and chemicalproperties and their prospects for practical applications. Dependingon tube diameter and chiral wrapping angle describing its construc-tion from a graphene sheet, SWNTs can be either semiconducting ormetallic. Recent improvements in sample preparation led to thediscovery of fluorescence in isolated semiconducting SWNTs and tosolid evidences on the role of electron-electron correlation and theexistence of excitons. In particular, it is clear that they are respon-sible for the measured fluorescence, and that the tight bindingapproximation to calculate the electronic band structure is insuffi-cient for describing the features seen in the optical absorptionspectrum of SWNTs.We present the first study of photoexcitation dynamics on the mstime scale, pointing out the presence of long-lived charged states,which modulates the absorption spectrum. Trapping of charges istypical of disordered materials, such as amorphous semiconductorsand conjugated polymers. Our findings demonstrate that a fraction ofthe initially neutral photoexcitations created in semiconductingSWNTs dissociates into charged states.We studied samples of SWNTs grown by the high pressure carbonmonoxide procedure (HiPco). Purified HiPco tubes from CarbonNanotechnologies Inc were functionalized with poly(ethylene glycol)(PEG) chains to improve dispersion in a polymer matrix. Thederivatization was achieved trough amidation of nanotube-boundcarboxylic acids with PEG-amine. The functionalized SWNTs wereembedded in polymethylmethacrylate (PMMA) films that were caston glass cover slips.The PA spectrum a SWNT/PMMA blend with most tubes entangledinto bundles shows a series of narrow positive and negative peaks,which is typical of a modulation, i.e. the result of small changes inlineshape and position of the spectral features. Based on thecomparison with previously studied conjugated polymers however,we suggest that electroabsorption (EA) is responsible for ourspectra. The peak positions of the PA and EA spectra are in goodagreement, but their relative intensity is different, pointing towardsdifferent contribution by the individual nanotube chiralities. More-over, the EA signal extends towards higher energies, i.e. beyond 1.2eV, where PA is essentially zero. that the higher transition energytubes of the ensemble do not contribute to the signal. The spectrahint towards a strong migration of singlet excitons and/or thecharges created from them due to the entanglement of thenanotubes into bundles. This migration is preferentially downhill inenergy, leading to an efficient population of the lower energy tubes.On the other hand, the regular EA signal shows a much largercontribution by the higher transition energy tubes, since there thefield is applied externally and not biased by an accumulation ofcharges at low energy tubes.The occurrence of an electroabsorption signal upon photoexcitationcan be rationalised if one assumes that after excitation charge statesare formed - via dissociation of excitons, as indicated by photocon-

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ductivity studies - diffuse and get trapped in the sample, generatingstrong local electric fields, which modulate the absorption. Such aphenomenon has also been observed in conjugated polymers. Wefind a lifetime of the trapped charges in the range of 40 ms.Excitation at two different photon energies, likely reaching differentsets of nanotubes, leads to the same spectra. The lack of selectivityin the excitation points to an efficient migration of the photoexcitedstates, either the singlet excitons or the charges resulting from theirdissociation. The modelling of the PA spectra is consistent with thisidea, describing EA as due to modulation of only the lower gap tubeswhich act as excitation traps.

6192-43, Session 11Pinpointing single excitons in polymeric semiconductorsJ. M. Lupton, Ludwig-Maximilians-Univ. München (Germany)The photophysics of conjugated polymers is dominated by excitoniceffects. Due to the vast level of inhomogeneous broadening,intermolecular interactions and inter- and intramolecular energytransfer, it is often hard to access the intrinsic properties of theelementary photoexcitations in these highly complex macromolecu-lar semiconductors. We have recently pursued the technique of lowtemperature single molecule spectroscopy to gain insight intoelectronic excitation and relaxation pathways of single excitations[1]. We are able to identify individual subunits on the polymer chain,which exhibit a marked temporal dynamics in terms of bothfluorescence intensity as well as emission wavelength [2]. Compari-son of different materials on the single molecule level revealssurprising coincidences in terms of the spectral properties, which aremasked in the ensemble. By comparing oligomeric model com-pounds with short and long polymers the transition from a monomo-lecular excitation to a macromolecular exciton can be probed directly[3]. This allows a counting of chromophores in frequency space onthe polymer chain. Single molecule spectroscopy is a versatiletechnique which can be applied right up into the near UV spectralregion. This allows us to probe the different conformational phasesof the complex material polyfluorene, which impact very strongly onthe photochemical reactivity of the polymer backbone [4]. Further-more, energy transfer along the polymer chain to energetic sinkssuch as dye molecules or fluorenone defects can be probed directly.The ultimate goal of the research is to unravel the elementary stepsoccurring on the single molecule level in molecular semiconductordevices. Progress in this direction was achieved by studying theresponse of the fluorescence of single molecules to external electricfields. The linear Stark effect observed reveals the presence of apermanent polarization on the polymer chain, which can be thoughtof in terms of permanent charges or doping. Such a polarizationnaturally influences both charge and energy transfer in the bulkdevice.[1] J. Muller et al., PRL 91, 267403 (2003).[2] F. Schindler et al., PNAS 101, 14695 (2004).[3] F. Schindler et al., Angew. Chem. 44, 1520 (2005).[4] K. Becker et al., JACS 127, 7306 (2005).

6192-44, Session 11Exciton dispersion in pentaceneM. Knupfer, Fraunhofer-Institut für Werkstoff- und Strahltechnik(Germany); H. Berger, École Polytechnique Fédérale de Lausanne(Switzerland)Molecular solids based upon \pi conjugated molecules have receiveda lot of attention in basic as well as applied research. A large numberof research activities has been devoted to the application of thesematerials in organic (opto-)electronic devices. One focus of manyexperimental and theoretical studies has been, and still is, thedetermination of the electronic excited state properties as they areclosely connected to the applicability of the material under investiga-tion in devices such as organic light emitting diodes or organicphotovoltaic cells. Pentacene is one of the materials that is studiedand applied in the context of organic devices. The dispersion of thelowest lying singlet electron-hole excitation of pentacen alongdifferent reciprocal lattice vectors has been studied directly usingelectron energy-loss spectroscopy. This exciton shows a cleardispersion along the directions with a band width of the order of 100meV. Moreover, the translational symmetry indicated by the excitonband structure does not agree with that reported from diffraction

studies. This might be related to the interaction responsible for thedispersion which is of next nearest neighbor type only.

6192-45, Session 11J-aggregation in alpha-sexithiophene sub-monolayerfilms on silicon dioxideE. Da Como, M. A. Loi, F. Dinelli, M. Murgia, R. Zamboni, M. Muccini,Istituto per lo Studio dei Materialo Nanostrutturati (Italy)The supra-molecular organization in organic semiconductors thinfilms is crucial in order to tune the optical properties and conse-quently the functioning of opto-electronic devices. In particular, thepossibility to obtain a head to tail organization of chromophores canlead to J-aggregates that have appealing optical properties like giantoscillator strength and superradiance.By confocal laser scanning microscopy (CLSM) and photolumines-cence (PL) spectroscopy we studied the supra-molecular organiza-tion in ultra-thin films of a prototype organic semiconductor: alpha-sexithiophene (T6). We demonstrate the presence of J-aggregates inT6 sub-monolayer films on silicon dioxide, that is a ubiquitoussubstrate for photonic and opto-electronic technology.Spatially resolved PL spectroscopy reveals that J-aggregates areformed by molecules lying flat on the substrate [1]. Excitation energydependence of PL shows a red shifted absorption with respect toisolated molecules and a negligible Stokes shift. J-aggregates PLspectrum is characterized by a well resolved Franck-Condonprogression reflecting a pronounced structural ordering and anallowed electronic transition. From time resolved and low tempera-ture PL experiments we infer a quantum yield of the J-aggregatesbetween 0.6 and 1 [2].In multilayer films the T6 molecules are aligned vertically with respectto the substrate and form H-aggregates, which are characterized bypoor emissive properties [1].These observations show that it is possible to achieve control on thenature of the aggregation and therefore to tune the optical proper-ties. Furthermore, the demonstration of spontaneous formation of J-aggregates of pi-conjugated systems on amorphous silicon-basedsubstrates can be relevant for the development of organic-inorganichybrid photonic devices.References[1] M. A. Loi, E. Da Como, F. Dinelli, M. Murgia, R. Zamboni, F.Biscarini and M. Muccini, Nature Mater. 4, 81 (2005).[2] E. Da Como, M. A. Loi, M. Murgia, R. Zamboni and M. Muccini,(submitted)

6192-46, Session 12Organic thin-film optoelectronics devicesT. B. Singh, N. Marjanovic, H. Neugebauer, N. S. Sariciftci, R.Schroediauer, S. Bauer, A. M. Ramil, H. Sitter, Johannes Kepler Univ.Linz (Austria); A. Y. Andreev, Montan Univ. Leoben (Austria)Organic Thin-Film Opto-Electronic DevicesThokchom Birendra Singh1, Nenad Marjanovi´c1, HelmutNeugebauer1,Niyazi Serdar Sariciftci1, Reinhard Schwödiauer 2, Siegfried Bauer2 ,Alberto M. Ramil3, Helmut Sitter3 and Andrei Andreev3,41Linz Institute of Organic Solar Cells (LIOS), Johannes KeplerUniversity Linz, A-4040 Austria; 2Soft Matter Physics(SOMAP),Johannes Kepler University Linz, Austria; 3Semiconductor Physics,Johannes Kepler University Linz, Austria;Most reports on organic field-effect transistors (OFETs) are bygrowing the organic semiconductor on surface treated or untreatedinorganic SiO2 or Al2O3 dielectrics. Because of the very differentphysical nature of the two media, this deposition technique mayresult in a highly disordered interface. In field-effect devices, the firstfew monolayers close to the dielectric interface dominate the chargetransport in the organic semiconductor layers.[1] We have fabricatedOFETs by growing organic semiconductor layers on top of varioustypes of organic insulators and studied the structure-performancerelationship, connecting nanoscale morphology with transportproperties. Hot wall epitaxy (HWE), is used for growing highlyordered organic thin films, such as C60 films. Especially HWE grownC60 based n-channel OFETs exhibit electron mobilities of 1 cm2/Vswith on/off ratio \>105.[2] At high substrate temperature the resultingsemiconductor thin-films are found to be more ordered. This results

Conference 6192: Organic Optoelectronics and Photonics II