14 research outputs found
Overcoming losses with gain in a negative refractive index metamaterial
On the basis of a full-vectorial three-dimensional Maxwell-Bloch approach we
investigate the possibility of using gain to overcome losses in a negative
refractive index fishnet metamaterial. We show that appropriate placing of
optically pumped laser dyes (gain) into the metamaterial structure results in a
frequency band where the nonbianisotropic metamaterial becomes amplifying. In
that region both the real and the imaginary part of the effective refractive
index become simultaneously negative and the figure of merit diverges at two
distinct frequency points.Comment: 4 pages, 4 figure
Surface plasmon polaritons in generalized slab heterostructures with negative permittivity and permeability
Single-mode operation in the slow-light regime using oscillatory waves in generalized left-handed heterostructures
REDUCED TILLAGE AND COVER CROPS IN ORGANIC ARABLE SYSTEMS PRESERVES WEED DIVERSITY WITHOUT JEOPARDISING CROP YIELD
One of the objectives of the TILMAN-ORG Project is to improve weed management under conservation agriculture (reduced tillage and/or cover crops) in organic arable systems. The shift from ploughing to conservation agriculture should not only maintain crop yield but possibly improve weed community diversity. This paper summarises the results on (1) weed abundance, (2) weed diversity and (3) crop yield obtained in the first year of the project (2012) in 13 trials scattered across Europe
Raumzeitliche, nichtlineare Laserdynamik in MikrokavitÀten, Vollvektorielle Maxwell-Bloch FDTD Simulationen
This work explores the ultra fast and non-linear effects in dielectric micro-cavities and micro-cavity lasers on the basis of full vectorial Maxwell-Bloch finite difference in time domain simulations.
Micro-cavity lasers promise low lasing pump thresholds and ultra fast amplitude modulations.
Micro-cavities with low pump thresholds may function as single photon sources which are required for quantum optic data processing.
For the first time the full three dimensional complete polarisation dynamics during the initial transient of a lasing process in a micro-cavity are calculated and discussed.
So called microdisks and microgears were chosen as the cavity geometries.
In order to simulate the non-linear, ultra short effects in micro cavities, a three dimensional finite element in time domain (FDTD) code was set up, adapted to different types of computer architectures and verified.
The FDTD algorithm describes the temporal evolution of the electromagnetic fields on a Yee type numerical grid by using the Faraday and the generalised Ampere law to calculate the fields at a new time step from the former time step in a leap frog like, second order accurate scheme.
The Maxwell material equations introduce the polarisation into the FDTD algorithm.
It is described for the first time in a semi-classical, self consistent three dimensional formalism by the discretised optical Bloch equations as separate difference equations.
Resonators with distributed feedback (DFB resonators), including one or two dimensional corrugations, pose the first type of micro-resonator for which the numerical method is applied.
This type of resonator is used in so called "plastic lasers" which are entirely made from organics in order to form extremely flexible and shapeable, large area light sources.
The simulations in this work lead to qualitative results which give insight in the type of DFB mechanism, and they give quantitative results for DFB structures which are used in experiments.
The last two chapters of this work deal with disk type lasers with length scales in the range of a few wavelengths.
Starting with smooth shaped dielectric cylinders, cold cavity modes are computed and measured.
The results are then compared to experiments and different theoretical computations from literature.
Subsequently, the influence of a periodic modulation of the cylinder radius - which characterises the microgear laser - with variable depth on the properties of cold cavity modes in smooth shaped disks is studied.
Quantitative investigations are performed on selected modes.
These dielectric disks with a periodic modulation of the radius are called microgears.
The simulations of the initial transient relaxation oscillation of the active material in microdisk lasers visualise the quasi stationary spatial hole burning process.
But this initially static behaviour is immediately replaced by a rotational movement of the electromagnetic mode.
We ascribe this effect to the continuous azimuthal degeneracy of the eigenmodes of the cold cavity and the non-linearity of the combined optical Maxwell-Bloch equations.
The continuous azimuthal degeneracy is removed by the transition to the microgear geometry.
The simulations of lasing in this kind of resonator shows a spatially static hole burning of the inversion from the first relaxation oscillations to the steady state lasing process.
It is thereby demonstrated that the mode decomposition of a cold dielectric cavity is an indication of the properties which are to be expected of a lasing resonator, but proves to be insufficient to predict non-linear effects in micro and macro lasers.Der Fokus dieser Arbeit ruht auf extrem schnellen und nichtlinearen Effekten in dielektrischen MikrokavitĂ€ten fĂŒr kohĂ€rente Strahlungsquellen.
Dieser KavitĂ€tstyp verspricht niedrige Pumpschwellen fĂŒr den Laserbetrieb und ultraschnelle Modulierbarkeit.
MikrokavitÀten mit niedrigen Pumpschwellen stellen mögliche Einzelphotonenquellen dar, die in quantenoptischer Datenverarbeitung ihre Anwednungen finden sollen.
Diese Arbeit stellt zum ersten Mal die komplette dreidimensionale Beschreibung der vollstÀndigen Polarisationsdynamik wÀhrend des Einschwingvorgangs eines Laserprozesses in einer MikrokavitÀt vor. Als KavitÀtsgeometrien wurden sogenannte dielektrische "Microdisks" und "Microgears" ausgewÀhlt.
Zur Simulation der nichtlinearen, ultrakurzen Effekte in MikrokavitĂ€ten wurde im Rahmen dieser Doktorarbeit ein auf dem Finite Elemente im Zeitraum (FDTD) basierendes Programm fĂŒr dreidimensionale rĂ€umliche Geometrien entwickelt, an verschiedene Rechnerarchitekturen angepasst und getestet.
Der FDTD Algorithmus beschreibt die zeitliche Entwicklung des elektromagnetischen Feldes auf einem numerischen Gitter vom Yee Typ indem er mit Hilfe der Faradayschen und der generalisierten Ampereschen Gleichung die Felder des nÀchsten aus den Feldern des vorhergehenden Zeitpunktes berechnet.
Dieser Algorithmus ist vom "Leap-frog" Typ und seine Genauigkeit ist von zweiter Ordnung.
Durch die Materialgleichung wird die mikroskopische Polarisation, beschrieben in einem semiklassischen Formalismus durch die optischen Bloch-Gleichungen, als zusÀtzliche reele Differenzengleichung in den FDTD Algorithmus eingebettet.
Resonatoren mit verteilter RĂŒckfĂŒhrung (DFB Resonatoren), mit eindimensionaler oder zweidimensionaler Korrugation, stellen die erste Art von Mikroresonator dar auf die die numerische Methode angewandt wird.
Dieser Resonatortyp findet Anwendung in sogenannten Plastiklasern die vollstÀndig aus organischen Materialien bestehen um extrem flexible und formbare, grossflÀchige Lichtquellen zu bilden.
Die numerischen Rechnungen in dieser Arbeit resultieren in qualitativen Ergebnissen die Aufschluss ĂŒber die Art des DFB Mechanismus geben, und quantitativen Resultaten zu DFB Strukturen wie sie in Experimenten verwendet wurden.
Die letzten zwei Kapitel der Arbeit befassen sich mit Scheibenlasern, die Abmessungen im Bereich der LichtwellenlÀnge besitzen.
Ausgehend von glatten dielektrischen Zylindern werden die kalten KavitĂ€tsmoden vermessen und den Ergebnissen anderer Arbeiten gegenĂŒbergestellt.
Die Auswirkungen von periodischen Modulationen des Zylinderradius mit unterschiedlicher StÀrke auf die Eigenschaften von kalten Resonatormoden werden umrissen und quantitativ an einzelnen Moden untersucht.
Diese dielektrischen Scheiben mit periodischer Modulation des Radius werden MikrozahnrÀder genannt.
Die Simulationen des transienten Verhaltens wÀhrend der Relaxationsoszillationen des aktiven Materials in Scheibenlasern zeigen stationÀres rÀumliches Lochbrennen.
Dieses stationÀre Verhalten wird allerdings sehr schnell durch eine Rotationsbewegung des elektromagnetischen Feldes abgelöst.
Wir fĂŒhren dies auf die kontinuierliche azimuthale Entartung der Eigenmoden der kalten KavitĂ€t und dem nichtlinearen Typ der kombinierten optischen Maxwell-Bloch Gleichungen zurĂŒck.
Durch den Ăbergang zur Zahnradgeometrie wird die kontinuierliche azimuthale Entartung der Moden aufgehoben.
Wie zu erwarten zeigen die Simulationen ein stationÀres rÀumliches Lochbrennen, demnach in diesem Resonatortyp, auch noch wÀhrend des eingeschwungenen
statischen Verhaltens des Laserprozesses.
Es zeigt sich, dass die Modenbeschreibung einer kalten dielektrischen KavitÀt zwar einen Eindruck der zu erwartenden Eigenschaften des um eine nichtlineare Materialgleichung erweiterten Resonators gestattet aber nicht ausreichend ist
nicht lineare Effekte vorherzusagen
Efficient sensitivity analysis of lumped elements with respect to finiteâelement geometry changes
Sensitivity analysis enables powerful gradientâbased mathematical programming techniques in the optimization of electromechanical products with respect to electromagnetic compatibility requirements. We present a sensitivity analysis method based on finite element solutions of both a magnetoquasistatic system in a treeâcotree gauge and an electrostatic system. A repeated application of the adjoint method ensures a high computational efficiency, which is showcased in a comparison with an earlier approach. Application examples provided are the optimization of a noise filter and the sensitivity computation of a choke with dispersive magnetic core
Adjoint Technique for Sensitivity Analysis of Coupling Factors According to Geometric Variations
Optimizing the Inductance Cancellation Behaviour in an EMI Filter Design with the Help of a Sensitivity Analysis
Adjoint technique for sensitivity analysis of coupling factors according to geometric variations
© 2017 IEEE. A physical interpretable equivalent electrical network and its parameters are extracted from a 3-D finite-element model formulated by Darwin's approximation to the Maxwell equations. In this paper, we add a sensitivity analysis procedure to the extraction of inductive parasitics, relying upon the adjoint technique for the extracted partial inductances and coupling factors. The resulting algorithm is an efficient way to calculate the sensitivities with respect to a large number of parameters. In an example, we use the gradient information to guide the geometrical optimization of an electromagnetic compatibility filter.status: publishe