Kernel methods in machine learning

We review machine learning methods employing positive definite kernels. These methods formulate learning and estimation problems in a reproducing kernel Hilbert space (RKHS) of functions defined on the data domain, expanded in terms of a kernel. Working in linear spaces of function has the benefit of facilitating the construction and analysis of learning algorithms while at the same time allowing large classes of functions. The latter include nonlinear functions as well as functions defined on nonvectorial data. We cover a wide range of methods, ranging from binary classifiers to sophisticated methods for estimation with structured data.Comment: Published in at http://dx.doi.org/10.1214/009053607000000677 the Annals of Statistics (http://www.imstat.org/aos/) by the Institute of Mathematical Statistics (http://www.imstat.org

On design studies for the future 50 GeV arrays of imaging air Cherenkov telescopes

Arrays of imaging air Cherenkov telescopes (IACTs) like VERITAS, HESS have been recently proposed as the instruments of the next generation for ground based very high energy gamma-ray astronomy invading into 50-100 GeV energy range. Here we present results of design studies for the future IACT arrays which have been performed by means of Monte Carlo simulations. We studied different trigger strategies, abilities of cosmic ray rejection for arrays of 4 and 16 telescopes with 10 m reflectors, equipped with cameras comprising 271 and 721 pixels of 0.25 and 0.15 degree, respectively. The comparative analysis of the performance of such telescope arrays has been done for both camera options, providing almost the same field of view of 4.3 degree. An important issue is the choice of the optimum spacing between the telescopes in such an array. In order to maximize the signal-to-noise ratio in observations at the small zenith angles of 20 degree as well as at large zenith angles of 60 degree, different arrangements of IACT array have been examined. Finally, we present a major recommendations regarding the optimum configuration.Comment: 5 pages, presented at the VERITAS Workshop on TeV Astrophysics of Extragalactic Sources, eds. M. Catanese, J. Quinn, and T. Weekes, to be published in Astroparticle Physic

The influence of dipolar doping on charge injection and transport in small molecular organic semiconductors

The present work investigates the effect of dipolar doping on charge injection and charge carrier dynamics in organic semiconducting thin films. In this context, the term dipolar doping refers to the dilution or doping of a non-polar matrix molecule with a polar guest. For this purpose, the hole-conductors N,N ’-Di(1-naphthyl)-N,N ’-diphenyl-(1,1’-biphenyl)-4,4’-diamine (NPB) and 4,4-N,N ’-Dicarbazole-1,1’-biphenyl (CBP) will serve as the host molecules. Dopants include Tris-(8-hydroxyquinoline)aluminum (Alq3) and OXD-7 (1,3-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazo-5-yl]benzene. The main focus, however, is on the system NPB:Alq3, which has also been studied extensively in the past [1–3]. In general, doping of (organic) semiconductors is a well known concept to tune conductivity [4] or optimize emitting properties of OLEDs [5]. The specific effect of doping with polar species, however, was not thoroughly investigated so far, although many guest molecules are indeed polar [6]. Because organic molecules are extended objects, their orientation with respect to the substrate surface [7], other molecules in the film [8] or e.g. the direction of light output from OLEDs [9] plays a crucial role in device performance. The key figure of polar molecules in this regard is their permanent dipole moment, arising from the non-uniform charge distribution on the molecule. If this dipole moment does not orient perfectly isotropic, it will lead to the build-up of a giant surface potential (GSP) and thus to a macroscopic dielectric polarization of the organic film. Despite this being a known fact [1, 7, 10], the implications of such high potentials on charge transport and injection within and into an organic layer stack have only been studied recently [3, 7, 11]. Dipolar doping now allows to introduce and tune the GSP in a former unpolar organic material [2]. The concentration dependence of the magnitude of the GSP in dipolar doped systems is the first major part of this work. Additionally, dipolar doping can be utilized to create hole conducting films that exhibit a GSP, which allow to study the impact of film polarization also in the hole conducting layer (HTL) of an OLED. In neat film, a GSP was previously seen mostly for electron conductors [6, 12]. Therefor, the prototypical, hole conducting mixture NPB:Alq3 is investigated at different doping concentrations and with varied substrate material with respect to hole injection and charge transport. The mixtures are studied in single-layer, monopolar devices with only the HTL present, as well as bilayer OLEDs with Alq 3 -doped NPB as HTL and neat Alq3 as electron transport layer, respectively. The latter are treated as metal insulator semiconductor (MIS) structures following and applying our recently developed method of charge extraction by linearly increasing voltage (CELIV) on polar OLEDs [13,14]. Furthermore, ultraviolet photoelectron spectroscopy allows to compare the electrical observations with the energy alignment between contact and doped NPB. For all device types, an optimum in device performance and carrier injection for moderate doping concentrations of about 5% is found. By comparing all different methods with a focus on charge injection barriers, a complex relationship of carrier transport, substrate workfunction, modified injection and the effect of polarization is found. This effectively manipulates charge carrier injection across the metal-organic interface and charge transport in the device.Die vorliegende Arbeit befasst sich mit den Auswirkungen von polarer Dotierung auf Ladungsträgertransport und -injektion in organischen Halbleitern. „Polare Dotierung“ bezieht sich hierbei auf das Verdünnen oder Dotieren einer unpolaren Matrix aus organischen Molekülen mittels polaren Gast-Molekülen. Für diesen Zweck dienen die organischen Lochleiter N,N ’-Di(1-Naphthyl)-N,N ’-Diphenyl-(1,1’-Biphenyl)-4,4’-Diamin (NPB) und 4,4-N,N ’-Dicarbazole-1,1’-Biphenyl (CBP) als Matrix. Dotiert werden sie mit Tris-(8-Hydroxyquinolin) Aluminium (Alq3) und 1,3-bis[2-(4-tert- butylphenyl)-1,3,4-oxadiazo-5-yl]Benzen (OXD-7). Der Fokus dieser Arbeit liegt allerdings auf dem System NPB:Alq3, welches auch vorher bereits gründlich untersucht wurde [1–3]. Im Allgemeinen ist die Dotierung von (organischen) Halbleitern eine etablierte, zentrale Methode zur Optimierung der Leitfähigkeit [4] oder der Emittereigenschaften von organischen Leuchtdioden [5]. Die Folgen einer Dotierung mit polaren Molekülen im Speziellen wurde allerdings bisher noch nicht systematisch untersucht, obwohl viele häufig verwendete Dotanden durchaus polare Moleküle sind [6]. Organische Moleküle sind ausgedehnte Objekte mit komplexen Formen, deren Orientierung zur Substratoberfläche [7], zu anderen Molekülen in der Schicht [8] oder auch zum Emissionsvektor einer OLED [9] einen großen Einfluss auf die Effizienz des Bauteils hat. Die wichtigste Eigenschaft von polaren Molekülen ist in diesem Zusammenhang ihr permanentes Dipolmoment, das sich auf eine ungleichmäßige Verteilung der Elektronendichte im Molekül zurückführen lässt. Falls sich das Dipolmoment nicht vollständig isotrop orientiert, hat es ein makroskopisches Oberflächenpotential (engl. giant surface potential, GSP) bzw. eine dielektrische Polarisation der gesamten Dünnschicht zur Folge. Die Existenz des GSP ist bereits seit einiger Zeit bekannt [1, 7, 10]. Seine Auswirkungen auf den Ladungstransport bzw. -injektion in Bezug auf organische Halbleiter werden erst seit kurzem häufiger untersucht [3,7,11]. Das Konzept der polaren Dotierung erlaubt nun, ein GSP in ursprünglich unpolare organische Matrizen einzubauen [2]. Mittels Dotierung lassen sich auch polare Lochleiter herstellen, die im weiteren Verlauf der Arbeit eine Untersuchung der Auswirkungen des GSP auf die Lochleiterschicht in einer OLED erlauben. In undotierten Schichten wurde ein GSP bisher hauptsächlich in Elektronenleitern beobachtet [6, 12]. Exemplarisch wird dazu das Mischsystem NPB:Alq3 in verschiedenen Konzentrationen und auf verschiedenen Substraten in Bezug auf Lochinjektion und Lochtransport näher untersucht. Es werden sowohl monopolare Bauteile, die ausschließlich Lochtransport aufweisen, als auch vollständige OLEDs verwendet. Bei OLEDs kommt dabei eine im Laufe dieser Arbeit mit entwickelte neue Methode zum Einsatz, die diese OLEDs wie sog. Metall-Isolator-Halbleiter-Dioden (engl. metalinsulator-semiconductor, MIS) behandelt und Rückschlüsse auf die Energiebarriere für Ladungsinjektion erlaubt [13, 14]. Des Weiteren stehen Ergebnisse aus der ultravioletten Photoelektronenspektroskopie zur Verfügung, die einen Vergleich mit der Ausrichtung der Energieniveaus von NPB am Kontakt ermöglichen. Mittels des Vergleichs verschiedener Messmethoden im Bezug auf die Injektionseigenschaften kann der Effekt des GSP auf verschiedene Parameter im Bauteil nachgewiesen werden, deren Zusammenspiel in sämtlichen Bauteiltypen zu einem Optimum bei moderaten Dotierkonzentrationen von ca. 5% führt

Antenna-enhanced Optoelectronic Probing of Carbon Nanotubes

We report on the first antenna-enhanced optoelectronic microscopy studies on nanoscale devices. By coupling the emission and excitation to a scanning optical antenna, we are able to locally enhance the electroluminescence and photocurrent along a carbon nanotube device. We show that the emission source of the electroluminescence can be point-like with a spatial extension below 20 nm. Topographic and antenna-enhanced photocurrent measurements reveal that the emission takes place at the location of highest local electric field indicating that the mechanism behind the emission is the radiative decay of excitons created via impact excitation

Cavity-enhanced Raman Microscopy of Individual Carbon Nanotubes

Raman spectroscopy reveals chemically specific information and provides label-free insight into the molecular world. However, the signals are intrinsically weak and call for enhancement techniques. Here, we demonstrate Purcell enhancement of Raman scattering in a tunable high-finesse microcavity, and utilize it for molecular diagnostics by combined Raman and absorption imaging. Studying individual single-wall carbon nanotubes, we identify crucial structural parameters such as nanotube radius, electronic structure and extinction cross-section. We observe a 320-times enhanced Raman scattering spectral density and an effective Purcell factor of 6.2, together with a collection efficiency of 60%. Potential for significantly higher enhancement, quantitative signals, inherent spectral filtering and absence of intrinsic background in cavity-vacuum stimulated Raman scattering render the technique a promising tool for molecular imaging. Furthermore, cavity-enhanced Raman transitions involving localized excitons could potentially be used for gaining quantum control over nanomechanical motion and open a route for molecular cavity optomechanics

Gas dynamics in the inner few AU around the Herbig B[e] star MWC297: Indications of a disk wind from kinematic modeling and velocity-resolved interferometric imaging

We present near-infrared AMBER (R = 12, 000) and CRIRES (R = 100, 000) observations of the Herbig B[e] star MWC297 in the hydrogen Br-gamma-line. Using the VLTI unit telescopes, we obtained a uv-coverage suitable for aperture synthesis imaging. We interpret our velocity-resolved images as well as the derived two-dimensional photocenter displacement vectors, and fit kinematic models to our visibility and phase data in order to constrain the gas velocity field on sub-AU scales. The measured continuum visibilities constrain the orientation of the near-infrared-emitting dust disk, where we determine that the disk major axis is oriented along a position angle of 99.6 +/- 4.8 degrees. The near-infrared continuum emission is 3.6 times more compact than the expected dust-sublimation radius, possibly indicating the presence of highly refractory dust grains or optically thick gas emission in the inner disk. Our velocity-resolved channel maps and moment maps reveal the motion of the Br-gamma-emitting gas in six velocity channels, marking the first time that kinematic effects in the sub-AU inner regions of a protoplanetary disk could be directly imaged. We find a rotation-dominated velocity field, where the blue- and red-shifted emissions are displaced along a position angle of 24 +/- 3 degrees and the approaching part of the disk is offset west of the star. The visibility drop in the line as well as the strong non-zero phase signals can be modeled reasonably well assuming a Keplerian velocity field, although this model is not able to explain the 3 sigma difference that we measure between the position angle of the line photocenters and the position angle of the dust disk. We find that the fit can be improved by adding an outflowing component to the velocity field, as inspired by a magneto-centrifugal disk-wind scenario.Comment: 15 pages, 13 Figure

A model for the very early Universe

A model with N species of massless fermions interacting via (microscopic) gravitational torsion in de Sitter spacetime is investigated in the limit N->infinity. The U_V(N)*U_A(N) flavor symmetry is broken dynamically irrespective of the (positive) value of the induced four-fermion coupling. This model is equivalent to a theory with free but massive fermions fluctuating about the chiral condensate. When the fermions are integrated out in a way demonstrated long ago by Candelas and Raine, the associated gap equation together with the Friedmann equation predict that the Hubble parameter vanishes. Introducing a matter sector (subject to a finite gauge symmetry) as a source for subsequent cosmology, the neutral Goldstone field acquires mass by the chiral anomaly, resulting in a Planck-scale axion.Comment: 14 pages, 2 figures; some references added; version to appear in JHE