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Im Juni 2018 sprach der britische Journalist und Redakteur Gary Younge vom „Guardian“ mit Interessierten unter anderem über Waffengewalt in Amerika. Bildinformationen anzeigen
Bei wissenschaftlichen Kolloquien steht der direkte Austausch im Fokus. Bildinformationen anzeigen
An der Universität Paderborn finden in allen Bereichen und zu vielfältigen Themen Kolloquien statt. Bildinformationen anzeigen
Weitere Informationen zu aktuellen Kolloquien und Terminen gibt es im Veranstaltungskalender auf der Webseite der Universität Paderborn. Bildinformationen anzeigen

Wissenschaftliches Kolloquium

Im Juni 2018 sprach der britische Journalist und Redakteur Gary Younge vom „Guardian“ mit Interessierten unter anderem über Waffengewalt in Amerika.

Foto: Universität Paderborn, Adelheid Rutenburges

Wissenschaftliches Kolloquium

Bei wissenschaftlichen Kolloquien steht der direkte Austausch im Fokus.

Foto: Universität Paderborn, Adelheid Rutenburges

Wissenschaftliches Kolloquium

An der Universität Paderborn finden in allen Bereichen und zu vielfältigen Themen Kolloquien statt.

Foto: Universität Paderborn, Adelheid Rutenburges

Wissenschaftliches Kolloquium

Weitere Informationen zu aktuellen Kolloquien und Terminen gibt es im Veranstaltungskalender auf der Webseite der Universität Paderborn.

Foto: Universität Paderborn, Adelheid Rutenburges

Prof. Dr. Jens Förstner

Prof. Dr. Jens Förstner

Institut für Elektrotechnik und Informationstechnik

Professor - Koordinator Studiengang Optoelectronics&Photonics

Theoretische Elektrotechnik (TET)

Leiter - Professor

Sonderforschungsbereich Transregio 142

Vorstand - Professor

DFG Graduiertenkolleg Micro- and Nanostructures in Optoelectronics and Photonics

Mitglied - Professor

Paderborn Center for Parallel Computing (PC2) > Vorstand

Mitglied - Professor

Center for Optoelectronics and Photonics (CeOPP)

Mitglied - Professor

+49 5251 60-3013
+49 5251 60-3524

Während der vorlesungsfreien Zeit nach Vereinbarung. Urlaub vom 8.3.-30.32019.

Pohlweg 47-49
33098 Paderborn
Warburger Str. 100
33098 Paderborn

11/2014 - heute

Vorstandsmitglied PC2 (Rechenzentrum der Uni Paderborn)

04/2014 - heute

Vorstandsmitglied Transregio 142

07/2013 - heute

Vorstandsmitglied CeOPP (Center for Optoelectronics and Photonics Paderborn)

06/2013 - heute

Professur W3 für die Theoretische Elektrotechnik an der Universität Paderborn, Deutschland

04/2007 - 05/2013

Leiter der DFG Emmy Noether Nachwuchs-Forschungsgruppe "Computational Nanophotonics", Universität Paderborn, Deutschland

01/2012 - 03/2012

Gastwissenschaftler in der Gruppe von H. Carmichael, Universität von Auckland


Goldene Kreide für exzellente Lehre


Forschungspreis 2009 der Paderborner Universität

10/2006 - 03/2007

Postdoktorand (wissenschaftlicher Mitarbeiter), Technische Universität Berlin, Deutschland

10/2004 - 09/2006

Postdoktorand (wissenschaftlicher Mitarbeiter), Universität Arizona, Tucson, AZ, USA


Carl-Ramsauer-Preis 2005 für Doktorarbeit


Doktorarbeit "Light Propagation and Many-Particle Effects in Semiconductor Nanostructures", Technische Universität Berlin, Deutschland

10/2000 - 09/2004

Wissenschaftlicher Mitarbeiter (Doktorand), Technische Universität Berlin, Deutschland

04/2000 - 09/2000

Wissenschaftlicher Mitarbeiter (Doktorand), Universität Marburg, Deutschland


Diplom in Physik, Universität Marburg, Deutschland

1997 - 1998

Auslandsstudium an der Universität Kent in Canterbury/England, Empfänger des Europhysics Prize 1998

1994 - 1996

Vordiplom (BSc) in Physik und Informatik an der Philips Universität Marburg

Liste im Research Information System öffnen


Optical transition between two optical waveguides layer and method for transmitting light

M. Hammer, J. Förstner, L. Ebers. Optical transition between two optical waveguides layer and method for transmitting light. 2019.


Intensity surge and negative polarization of light from compact irregular particles

Y. Grynko, Y. Shkuratov, J. Förstner, Optics Letters (2018)

We study the dependence of the intensity and linear polarization of light scattered by isolated particles with the compact irregular shape on their size using the discontinuous Galerkin time domain numerical method. The size parameter of particles varies in the range of X = 10 to 150, and the complex refractive index is m = 1.5 + 0i. Our results show that the backscattering negative polarization branch weakens monotonously, but does not disappear at large sizes, up to the geometrical optics regime, and can be simulated without accounting for wave effects. The intensity backscattering surge becomes narrower with increasing particle size. For X = 150, the surge width is several degrees.

Ultrafast electric phase control of a single exciton qubit

A. Widhalm, A. Mukherjee, S. Krehs, N. Sharma, P. Kölling, A. Thiede, D. Reuter, J. Förstner, A. Zrenner, Applied Physics Letters (2018), pp. 111105

We report on the coherent phase manipulation of quantum dot excitons by electric means. For our experiments, we use a low capacitance single quantum dot photodiode which is electrically controlled by a custom designed SiGe:C BiCMOS chip. The phase manipulation is performed and quantified in a Ramsey experiment, where ultrafast transient detuning of the exciton energy is performed synchronous to double pulse p/2 ps laser excitation. We are able to demonstrate electrically controlled phase manipulations with magnitudes up to 3p within 100 ps which is below the dephasing time of the quantum dot exciton.

Polarization Conversion Effect in Biological and Synthetic Photonic Diamond Structures

X. Wu, F.L. Rodríguez-Gallegos, M. Heep, B. Schwind, G. Li, H. Fabritius, G. von Freymann, J. Förstner, Advanced Optical Materials (2018)

Polarization of light is essential for some living organisms and many optical applications. Here, an orientation dependent polarization conversion effect is reported for light reflected from diamond‐structure‐based photonic crystals (D‐structure) inside the scales of a beetle, the weevil Entimus imperialis. When linearly polarized light propagates along its 〈100〉 directions, the D‐structure behaves analogous to a half‐wave plate in reflection but based on a different mechanism. The D‐structure rotates the polarization direction of linearly polarized light, and reflects circularly polarized light of both handednesses without changing it. This polarization effect is different from circular dichroism occurring in chiral biological photonic structures discovered before. The structural origin of this effect is symmetry breaking inside D‐structure's unit cell. This finding demonstrates that natural photonic structures can exploit multiple functionalities inherent to the design principles of their structural organization. Aiming at transferring the inherent polarization effect of the biological D‐structure to technically realizable materials, three simplified biomimetic structural models are derived and it is theoretically demonstrated that they retain the effect. Out of these structures, functioning woodpile structure prototypes are fabricated.

Unveiling and Imaging Degenerate States in Plasmonic Nanoparticles with Nanometer Resolution

V. Myroshnychenko, N. Nishio, F.J. García de Abajo, J. Förstner, N. Yamamoto, ACS Nano (2018), pp. 8436-8446

Metal nanoparticles host localized plasmon excitations that allow the manipulation of optical fields at the nanoscale. Despite the availability of several techniques for imaging plasmons, direct access into the symmetries of these excitations remains elusive, thus hindering progress in the development of applications. Here, we present a combination of angle-, polarization-, and space-resolved cathodoluminescence spectroscopy methods to selectively access the symmetry and degeneracy of plasmonic states in lithographically fabricated gold nanoprisms. We experimentally reveal and spatially map degenerate states of multipole plasmon modes with nanometer spatial resolution and further provide recipes for resolving optically dark and out-of-plane modes. Full-wave simulations in conjunction with a simple tight-binding model explain the complex plasmon structure in these particles and reveal intriguing mode-symmetry phenomena. Our approach introduces systematics for a comprehensive symmetry characterization of plasmonic states in high-symmetry nanostructures.

Oblique incidence of semi-guided planar waves on slab waveguide steps: effects of rounded edges

L. Ebers, M. Hammer, J. Förstner, Optics Express (2018), pp. 18621-18632

Oblique propagation of semi-guided waves across slab waveguide structures with bent corners is investigated. A critical angle can be defined beyond which all radiation losses are suppressed. Additionally an increase of the curvature radius of the bends also leads to low-loss configurations for incidence angles below that critical angle. A combination of two bent corner systems represents a step-like structure, behaving like a Fabry-Perot interferometer, with two partial reflectors separated by the vertical height between the horizontal slabs. We numerically analyse typical high-index-contrast Si/SiO2 structures for their reflectance and transmittance properties. When increasing the curvature radius the resonant effect becomes less relevant such that full transmittance is reached with less critical conditions on the vertical distance or the incidence angle. For practical interest 3-D problems are considered, where the structures are excited by the fundamental mode of a wide, shallow rib waveguide. High transmittance levels can be observed also for these 3-D configurations depending on the width of the rib.

Tailored UV Emission by Nonlinear IR Excitation from ZnO Photonic Crystal Nanocavities

S.P. Hoffmann, M. Albert, N. Weber, D. Sievers, J. Förstner, T. Zentgraf, C. Meier, ACS Photonics (2018), pp. 1933-1942

Oblique Semi-Guided Waves: 2-D Integrated Photonics with Negative Effective Permittivity

M. Hammer, L. Ebers, A. Hildebrandt, S. Alhaddad, J. Förstner, in: 2018 IEEE 17th International Conference on Mathematical Methods in Electromagnetic Theory (MMET), IEEE, 2018

Semi-guided waves confined in dielectric slab waveguides are being considered for oblique angles of propagation. If the waves encounter a linear discontinuity of (mostly) arbitrary shape and extension, a variant of Snell's law applies, separately for each pair of incoming and outgoing modes. Depending on the effective indices involved, and on the angle of incidence, power transfer to specific outgoing waves can be allowed or forbidden. In particular, critical angles of incidence can be identified, beyond which any power transfer to non-guided waves is forbidden, i.e. all radiative losses are suppressed. In that case the input power is carried away from the discontinuity exclusively by reflected semi-guided waves in the input slab, or by semi-guided waves that are transmitted into other outgoing slab waveguides. Vectorial equations on a 2-D cross sectional domain apply. These are formally identical to the equations that govern the eigenmodes of 3-D channel waveguides. Here, however, these need to be solved not as an eigenvalue problem, but as an inhomogeneous problem with a right-hand-side that is given by the incoming semi-guided wave, and subject to transparent boundary conditions. The equations resemble a standard 2-D Helmholtz problem, with an effective permittivity in place of the actual relative permittivity. Depending on the properties of the incoming wave, including the angle of incidence, this effective permittivity can become locally negative, causing the suppression of propagating outgoing waves. A series of high-contrast example configurations are discussed, where these effects lead to - in some respects - quite surprising transmission characteristics.

Simulation leitungsgeführter Störspannungen von DC-DC-Wandlern

T. Baumgarten, P. Scholz, D. Sievers, J. Förstner, in: Elektromagnetische Verträglichkeit - Internationale Fachmesse und Kongress 2018, 2018, pp. 47

Solving Maxwell's Equations with Modern C++ and SYCL: A Case Study

A. Afzal, C. Schmitt, S. Alhaddad, Y. Grynko, J. Teich, J. Förstner, F. Hannig, in: Proceedings of the 29th Annual IEEE International Conference on Application-specific Systems, Architectures and Processors (ASAP), 2018, pp. 49-56

In scientific computing, unstructured meshes are a crucial foundation for the simulation of real-world physical phenomena. Compared to regular grids, they allow resembling the computational domain with a much higher accuracy, which in turn leads to more efficient computations.<br />There exists a wealth of supporting libraries and frameworks that aid programmers with the implementation of applications working on such grids, each built on top of existing parallelization technologies. However, many approaches require the programmer to introduce a different programming paradigm into their application or provide different variants of the code. SYCL is a new programming standard providing a remedy to this dilemma by building on standard C ++17 with its so-called single-source approach: Programmers write standard C ++ code and expose parallelism using C++17 keywords. The application is<br />then transformed into a concrete implementation by the SYCL implementation. By encapsulating the OpenCL ecosystem, different SYCL implementations enable not only the programming of CPUs but also of heterogeneous platforms such as GPUs or other devices. For the first time, this paper showcases a SYCL-<br />based solver for the nodal Discontinuous Galerkin method for Maxwell’s equations on unstructured meshes. We compare our solution to a previous C-based implementation with respect to programmability and performance on heterogeneous platforms.<br

OpenCL-based FPGA Design to Accelerate the Nodal Discontinuous Galerkin Method for Unstructured Meshes

T. Kenter, G. Mahale, S. Alhaddad, Y. Grynko, C. Schmitt, A. Afzal, F. Hannig, J. Förstner, C. Plessl, in: Proc. Int. Symp. on Field-Programmable Custom Computing Machines (FCCM), IEEE, 2018

The exploration of FPGAs as accelerators for scientific simulations has so far mostly been focused on small kernels of methods working on regular data structures, for example in the form of stencil computations for finite difference methods. In computational sciences, often more advanced methods are employed that promise better stability, convergence, locality and scaling. Unstructured meshes are shown to be more effective and more accurate, compared to regular grids, in representing computation domains of various shapes. Using unstructured meshes, the discontinuous Galerkin method preserves the ability to perform explicit local update operations for simulations in the time domain. In this work, we investigate FPGAs as target platform for an implementation of the nodal discontinuous Galerkin method to find time-domain solutions of Maxwell's equations in an unstructured mesh. When maximizing data reuse and fitting constant coefficients into suitably partitioned on-chip memory, high computational intensity allows us to implement and feed wide data paths with hundreds of floating point operators. By decoupling off-chip memory accesses from the computations, high memory bandwidth can be sustained, even for the irregular access pattern required by parts of the application. Using the Intel/Altera OpenCL SDK for FPGAs, we present different implementation variants for different polynomial orders of the method. In different phases of the algorithm, either computational or bandwidth limits of the Arria 10 platform are almost reached, thus outperforming a highly multithreaded CPU implementation by around 2x.


Radar backscattering from a large-grain cometary coma: numerical simulation

S. Dogra, Y. Grynko, E. Zubko, J. Förstner, Astronomy & Astrophysics (2017)

We numerically simulate the circular polarization ratio of the radar signal backscattered from a large-grain cometary coma and compare the simulation results with the radar measurements for seven comets. We apply the discrete dipole approximation method and a model of random irregular particles. Our results confirm water ice composition of the cm-sized chunks detected by the NASA Deep Impact space probe in the vicinity of the nucleus of Comet 103P/Hartley 2. The index of the power-law size distribution in this case can be constrained to the range n ≈ 3.3–4.3. For the other considered comets the circular polarization ratio can be reproduced with variations of the power index between 2 and 5.

Flexible FPGA design for FDTD using OpenCL

T. Kenter, J. Förstner, C. Plessl, in: Proc. Int. Conf. on Field Programmable Logic and Applications (FPL), IEEE, 2017


Hybrid coupled-mode modeling in 3D: perturbed and coupled channels, and waveguide crossings

M. Hammer, S. Alhaddad, J. Förstner, Journal of the Optical Society of America B (2017), pp. 613-624

The 3D implementation of a hybrid analytical/numerical variant of the coupled-mode theory is discussed. Eigenmodes of the constituting dielectric channels are computed numerically. The frequency-domain coupled-mode models then combine these into fully vectorial approximations for the optical electromagnetic fields of the composite structure. Following a discretization of amplitude functions by 1D finite elements, pro- cedures from the realm of finite-element numerics are applied to establish systems of linear equations for the then- discrete modal amplitudes. Examples substantiate the functioning of the technique and allow for some numerical assessment. The full 3D simulations are highly efficient in memory consumption, moderately demanding in com- putational time, and, in regimes of low radiative losses, sufficiently accurate for practical design. Our results include the perturbation of guided modes by changes of the refractive indices, the interaction of waves in parallel, horizontally or vertically coupled straight waveguides, and a series of crossings of potentially overlapping channels with fairly arbitrary relative positions and orientations.

Spiral modes supported by circular dielectric tubes and tube segments

L. Ebers, M. Hammer, J. Förstner, Optical and Quantum Electronics (2017), pp. 49:176

The modal properties of curved dielectric slab waveguides are investigated. We consider quasi-confined, attenuated modes that propagate at oblique angles with respect to the axis through the center of curvature. Our analytical model describes the transition from scalar 2-D TE/TM bend modes to lossless spiral waves at near-axis propagation angles, with a continuum of vectorial attenuated spiral modes in between. Modal solutions are characterized in terms of directional wavenumbers and attenuation constants. Examples for vectorial mode profiles illustrate the effects of oblique wave propagation along the curved slab segments. For the regime of lossless spiral waves, the relation with the guided modes of corresponding dielectric tubes is demonstrated.

Simulation of Second Harmonic Generation from Photonic Nanostructures Using the Discontinuous Galerkin Time Domain Method

Y. Grynko, J. Förstner, in: Recent Trends in Computational Photonics, Springer International Publishing, 2017, pp. 261-284

We apply the Discontinuous Galerkin Time Domain (DGTD) method for numerical simulations of the second harmonic generation from various metallic nanostructures. A Maxwell–Vlasov hydrodynamic model is used to describe the nonlinear effects in the motion of the excited free electrons in a metal. The results are compared with the corresponding experimental measurements for split-ring resonators and plasmonic gap antennas.

Direction-tunable enhanced emission from a subwavelength metallic double-nanoslit structure

X. Song, N. Wang, M. Yan, C. Lin, J. Förstner, W. Yang, Optics Express (2017)

Controlling light emission out of subwavelength nanoslit/aperture structures is of great important for highly integrated photonic circuits. Here we propose a new method to achieve direction-tunable emission based on a compact metallic microcavity with double nanoslit. Our method combines the principles of Young’s interference and surface plasmon polaritons interference. We show that the direction of the far-field beam can be controlled over a wide range of angles by manipulating the frequency and relative phase of light arriving at the two slits, which holds promise for applications in the ultracompact optoelectronic devices.

Directional Emission from Dielectric Leaky-Wave Nanoantennas

M. Peter, A. Hildebrandt, C. Schlickriede, K. Gharib, T. Zentgraf, J. Förstner, S. Linden, Nano Letters (2017), pp. 4178-4183


Comparison between the physical-optics approximation and exact methods solving the problem of light scattering by ice crystals of cirrus clouds

A.V. Konoshonkin, N.V. Kustova, A.G. Borovoi, H. Okamoto, K. Sato, H. Ishimoto, Y. Grynko, J. Förstner, in: 22nd International Symposium on Atmospheric and Ocean Optics: Atmospheric Physics, SPIE, 2016

In the problem of light scattering by ice crystals of cirrus clouds, two exact methods (FDTD – finite difference time domain and DGTD – discontinuous Galerkin time domain) and the physical-optics approximation are used for numerical calculations of the Mueller matrix in the case of ice hexagonal plates and columns. It is shown that for the crystals larger than 10 μm at the wavelength of 0.532 μm the exact methods and physical-optics approximation closely agreed within three diffraction fringes about the centers of the diffraction patterns. As a result, in the case of random orientation of these crystals, the physical-optics approximation provides accuracy 95% for the averaged Mueller matrix.

Second harmonic generation spectroscopy on hybrid plasmonic/dielectric nanoantennas

H. Linnenbank, Y. Grynko, J. Förstner, S. Linden, Light: Science & Applications (2016)

Fabrication and characterization of two-dimensional cubic AlN photonic crystal membranes containing zincblende GaN quantum dots

S. Blumenthal, M. Bürger, A. Hildebrandt, J. Förstner, N. Weber, C. Meier, D. Reuter, D.J. As, physica status solidi (c) (2016), pp. 292-296

We successfully developed a process to fabricate freestanding cubic aluminium nitride (c-AlN) membranes containing cubic gallium nitride (c-GaN) quantum dots (QDs). The samples were grown by plasma assisted molecular beam epitaxy (MBE). To realize the photonic crystal (PhC) membrane we have chosen a triangular array of holes. The array was fabricated by electron beam lithography and several steps of reactive ion etching (RIE) with the help of a hard mask and an undercut of the active layer. The r/a- ratio of 0.35 was deter- mined by numerical simulations to obtain a preferably wide photonic band gap. Micro-photoluminescence (μ-PL) measurements of the photonic crystals, in particular of a H1 and a L3 cavity, and the emission of the QD ensemble were performed to characterize the samples. The PhCs show high quality factors of 4400 for the H1 cavity and about 5000/3000 for two different modes of the L3 cavity, respectively. The energy of the fundamental modes is in good agreement to the numerical simulations.

Light scattering by ice crystals of cirrus clouds: From exact numerical methods to physical-optics approximation

A. Konoshonkin, A. Borovoi, N. Kustova, H. Okamoto, H. Ishimoto, Y. Grynko, J. Förstner, Journal of Quantitative Spectroscopy and Radiative Transfer (2016), pp. 132-140

The problem of light scattering by ice crystals of cirrus clouds is considered in the case of a hexagonal ice plate with different distributions over crystal orientations. The physical-optics approximation based on (E, M)-diffraction theory is compared with two exact numerical methods: the finite difference time domain (FDTD) and the discontinuous Galerkin time domain (DGTD) in order to estimate its accuracy and limits of applicability. It is shown that the accuracy of the physical-optics approximation is estimated as 95% for the averaged backscattering Mueller matrix for particles with size parameter more than 120. Furthermore, the simple expression that allows one to estimate the minimal number of particle orientations required for appropriate spatial averaging has been derived.

Phase sensitive properties and coherent manipulation of a photonic crystal microcavity

W. Quiring, B. Jonas, J. Förstner, A.K. Rai, D. Reuter, A.D. Wieck, A. Zrenner, Optics Express (2016)

We present phase sensitive cavity field measurements on photonic crystal microcavities. The experiments have been performed as autocorrelation measurements with ps double pulse laser excitation for resonant and detuned conditions. Measured E-field autocorrelation functions reveal a very strong detuning dependence of the phase shift between laser and cavity field and of the autocorrelation amplitude of the cavity field. The fully resolved phase information allows for a precise frequency discrimination and hence for a precise measurement of the detuning between laser and cavity. The behavior of the autocorrelation amplitude and phase and their detuning dependence can be fully described by an analytic model. Furthermore, coherent control of the cavity field is demonstrated by tailored laser excitation with phase and amplitude controlled pulses. The experimental proof and verification of the above described phenomena became possible by an electric detection scheme, which employs planar photonic crystal microcavity photo diodes with metallic Schottky contacts in the defect region of the resonator. The applied photo current detection was shown to work also efficiently at room temperature, which make electrically contacted microcavities attractive for real world applications.

Discrete plasmonic solitons in graphene-coated nanowire arrays

Y. Kou, J. Förstner, Optics Express (2016)

e study the discrete soliton formation in one- and two- dimensional arrays of nanowires coated with graphene monolayers. Highly confined solitons, including the fundamental and the higher-order modes, are found to be supported by the proposed structure with a low level of power flow. Numerical analysis reveals that, by tuning the input intensity and Fermi energy, the beam diffraction, soliton dimension and propagation loss can be fully controlled in a broad range, indicating potential values of the graphene-based solitons in nonlinear/active nanophotonic systems.

Simulations of high harmonic generation from plasmonic nanoparticles in the terahertz region

Y. Grynko, T. Zentgraf, T. Meier, J. Förstner, Applied Physics B (2016)


Light scattering by irregular particles much larger than the wavelength with wavelength-scale surface roughness

Y. Grynko, Y. Shkuratov, J. Förstner, Optics Letters (2016)

We simulate light scattering by random irregular particles that have dimensions much larger than the wavelength of incident light at the size parameter of 𝑋=200 using the discontinuous Galerkin time domain method. A comparison of the DGTD solution for smoothly faceted particles with that obtained with a geometric optics model shows good agreement for the scattering angle curves of intensity and polarization. If a wavelength-scale surface roughness is introduced, diffuse scattering at rough interface results in smooth and featureless curves for all scattering matrix elements which is consistent with the laboratory measurements of real samples.

The role of electromagnetic interactions in second harmonic generation from plasmonic metamaterials

J. Alberti, H. Linnenbank, S. Linden, Y. Grynko, J. Förstner, Applied Physics B (2016), pp. 45-50

We report on second harmonic generation spectroscopy on a series of rectangular arrays of split-ring resonators. Within the sample series, the lattice constants are varied, but the area of the unit cell is kept fixed. The SHG signal intensity of the different arrays upon resonant excitation of the fundamental plasmonic mode strongly depends on the respective arrangement of the split-ring resonators. This finding can be explained by variations of the electromagnetic interactions between the split-ring resonators in the different arrays. The experimental results are in agreement with numerical calculations based on the discontinuous Galerkin time-domain method. (PDF) The role of electromagnetic interactions.... Available from: [accessed Aug 13 2018].

Light scattering by ice crystals of cirrus clouds: comparison of the physical optics methods

A.V. Konoshonkin, N.V. Kustova, A.G. Borovoi, Y. Grynko, J. Förstner, Journal of Quantitative Spectroscopy and Radiative Transfer (2016), pp. 12-23

The physical optics approximations are derived from the Maxwell equations. The scattered field equations by Kirchhoff, Stratton-Chu, Kottler and Franz are compared and discussed. It is shown that in the case of faceted particles, these equations reduce to a sum of the diffraction integrals, where every diffraction integral is associated with one plane–parallel optical beam leaving a particle facet. In the far zone, these diffraction integrals correspond to the Fraunhofer diffraction patterns. The paper discusses the E-, M- and (E, M)-diffraction theories as applied to ice crystals of cirrus clouds. The comparison to the exact solution obtained by the discontinuous Galerkin time domain method shows that the Kirchhoff diffraction theory is preferable.


Coupling Mediated Coherent Control of Localized Surface Plasmon Polaritons

F. Zeuner, M. Muldarisnur, A. Hildebrandt, J. Förstner, T. Zentgraf, Nano Letters (2015), pp. 4189-4193

Subwavelength binary plasmonic solitons

Y. Kou, J. Förstner, Optics Letters (2015)

We study the formation of subwavelength solitons in binary metal-dielectric lattices. We show that the transverse modulation of the lattice constant breaks the fundamental plasmonic band and suppresses the discrete diffraction of surface plasmon waves. New types of plasmonic solitons are found, and their characteristics are analyzed. We also demonstrate the existence of photonic-plasmonic vector solitons and elucidate their propagation properties.

Full Resonant Transmission of Semiguided Planar Waves Through Slab Waveguide Steps at Oblique Incidence

M. Hammer, A. Hildebrandt, J. Förstner, Journal of Lightwave Technology (2015), pp. 997-1005

Sheets of slab waveguides with sharp corners are investigated. By means of rigorous numerical experiments, we look at oblique incidence of semi-guided plane waves. Radiation losses vanish beyond a certain critical angle of incidence. One can thus realize lossless propagation through 90-degree corner configurations, where the remaining guided waves are still subject to pronounced reflection and polarization conversion. A system of two corners can be viewed as a structure akin to a Fabry-Perot-interferometer. By adjusting the distance between the two partial reflectors, here the 90-degree corners, one identifies step-like configurations that transmit the semi-guided plane waves without radiation losses, and virtually without reflections. Simulations of semi-guided beams with in-plane wide Gaussian profiles show that the effect survives in a true 3-D framework.

Unveiling Nanometer Scale Extinction and Scattering Phenomena through Combined Electron Energy Loss Spectroscopy and Cathodoluminescence Measurements

A. Losquin, L.F. Zagonel, V. Myroshnychenko, B. Rodríguez-González, M. Tencé, L. Scarabelli, J. Förstner, L.M. Liz-Marzán, F.J. García de Abajo, O. Stéphan, M. Kociak, Nano Letters (2015), pp. 1229-1237

Plasmon modes of the exact same individual gold nanoprisms are investigated through combined nanometer-resolved electron energy-loss spectroscopy (EELS) and cathodoluminescence (CL) measurements. We show that CL only probes the radiative modes, in contrast to EELS, which additionally reveals dark modes. The combination of both techniques on the same particles thus provides complementary information and also demonstrates that although the radiative modes give rise to very similar spatial distributions when probed by EELS or CL, their resonant energies appear to be different. We trace this phenomenon back to plasmon dissipation, which affects in different ways the plasmon signatures probed by these techniques. Our experiments are in agreement with electromagnetic numerical simulations and can be further interpreted within the framework of a quasistatic analytical model. We therefore demonstrate that CL and EELS are closely related to optical scattering and extinction, respectively, with the addition of nanometer spatial resolution.

Robust Population Inversion by Polarization Selective Pulsed Excitation

D. Mantei, J. Förstner, S. Gordon, Y.A. Leier, A.K. Rai, D. Reuter, A.D. Wieck, A. Zrenner, Scientific Reports (2015), pp. 10313

The coherent state preparation and control of single quantum systems is an important prerequisite for the implementation of functional quantum devices. Prominent examples for such systems are semiconductor quantum dots, which exhibit a fine structure split single exciton state and a V-type three level structure, given by a common ground state and two distinguishable and separately excitable transitions. In this work we introduce a novel concept for the preparation of a robust inversion by the sequential excitation in a V-type system via distinguishable paths.

A process for the preparation of a population inversion in a quantum system using multi-pulse excitation

A. Zrenner, J. Förstner, D. Mantei. A process for the preparation of a population inversion in a quantum system using multi-pulse excitation. 2015.

How planar optical waves can be made to climb dielectric steps

M. Hammer, A. Hildebrandt, J. Förstner, Optics Letters (2015)

We show how to optically connect guiding layers at different elevations in a 3-D integrated photonic circuit. Transfer of optical power carried by planar, semi-guided waves is possible without reflections or radiation losses, and over large vertical distances. This functionality is realized through simple step-like folds of high-contrast dielectric slab waveguides, in combination with oblique wave incidence, and fulfilling a resonance condition. Radiation losses vanish, and polarization conversion is suppressed for TE wave incidence beyond certain critical angles. This can be understood by fundamental arguments resting on a version of Snell’s law. The two 90° corners of a step act as identical partial reflectors in a Fabry–Perot-like resonator setup. By selecting the step height, i.e., the distance between the reflectors, one realizes resonant states with full transmission. Rigorous quasi-analytical simulations for typical silicon/silica parameters demonstrate the functioning. Combinations of several step junctions can lead to other types of optical on-chip connects, e.g., U-turn- or bridge-like configurations.


Engineering plasmonic and dielectric directional nanoantennas

A. Hildebrandt, M. Reichelt, T. Meier, J. Förstner, in: Ultrafast Phenomena and Nanophotonics XVIII, SPIE, 2014, pp. 89841G-8941G-6

Optical and infrared antennas provide a promising way to couple photons in and out of nanoscale structures. As counterpart to conventional radio antennas, they are able to increase optical felds in sub-wavelength volumes, to enhance excitation and emission of quantum emitters or to direct light, radiated by quantum emitters. The directed emission of these antennas has been mainly pursued by surface plasmon based devices, e.g. Yagi-Uda like antennas, which are rather complicated due to the coupling of several metallic particles. Also, like all metallic structures in optical or infrared regime, these devices are very sensitive to fabrication tolerances and are affected by strong losses. It has been shown recently, that such directed emission can be accomplished by dielectric materials as well. In this paper we present an optimization of nanoscopic antennas in the near infrared regime starting from a metallic Yagi-Uda structure. The optimization is done via a particle-swarm algorithm, using full time domain finite integration simulations to obtain the characteristics of the investigated structure, also taking into account substrates. Furthermore we present a dielectric antenna, which performs even better, due to the lack of losses by an appropriate choice of the dielectric material. These antennas are robust concerning fabrication tolerances and can be realized with different materials for both the antenna and the substrate, without using high index materials.

Simulation of Planar Photonic Resonators

S. Declair, J. Förstner, in: Handbook of Optical Microcavities, Pan Stanford Publishing Pte. Ltd., 2014

Accelerating Finite Difference Time Domain Simulations with Reconfigurable Dataflow Computers

H. Giefers, C. Plessl, J. Förstner, ACM SIGARCH Computer Architecture News (2014), pp. 65-70



Cubic GaN quantum dots embedded in zinc-blende AlN microdisks

M. Bürger, R. Kemper, C. Bader, M. Ruth, S. Declair, C. Meier, J. Förstner, D. As, Journal of Crystal Growth (2013), pp. 287-290

Microresonators containing quantum dots find application in devices like single photon emitters for quantum information technology as well as low threshold laser devices. We demonstrate the fabrication of 60 nm thin zinc-blende AlN microdisks including cubic GaN quantum dots using dry chemical etching techniques. Scanning electron microscopy analysis reveals the morphology with smooth surfaces of the microdisks. Micro-photoluminescence measurements exhibit optically active quantum dots. Furthermore this is the first report of resonator modes in the emission spectrum of a cubic AlN microdisk.

Optimal second-harmonic generation in split-ring resonator arrays

Y. Grynko, T. Meier, S. Linden, F.B.P. Niesler, M. Wegener, J. Förstner, in: Ultrafast Phenomena and Nanophotonics XVII, SPIE, 2013

Previous experimental measurements and numerical simulations give evidence of strong electric and magnetic field interaction between split-ring resonators in dense arrays. One can expect that such interactions have an influence on the second harmonic generation. We apply the Discontinuous Galerkin Time Domain method and the hydrodynamic Maxwell-Vlasov model to simulate the linear and nonlinear optical response from SRR arrays. The simulations show that dense placement of the constituent building blocks appears not always optimal and collective effects can lead to a significant suppression of the near fields at the fundamental frequency and, consequently, to the decrease of the SHG intensity. We demonstrate also the great role of the symmetry degree of the array layout which results in the variation of the SHG efficiency in range of two orders of magnitude.

Collective effects in second-harmonic generation from split-ring-resonator arrays

F.B. Niesler, S. Linden, J. Förstner, Y. Grynko, T. Meier, M. Wegener, in: Conference on Lasers and Electro-Optics 2012, OSA, 2013

We perform experiments on resonant second-harmonic generation from planar gold split-ring-resonator arrays under normal incidence of light as a function of the lattice constant. Optimum nonlinear conversion occurs at intermediate lattice constants.

Light scattering by randomly irregular dielectric particles larger than the wavelength

Y. Grynko, Y. Shkuratov, J. Förstner, Optical Letters (2013), pp. 5153-5156


Whispering gallery modes in zinc-blende AlN microdisks containing non-polar GaN quantum dots

M. Bürger, M. Ruth, S. Declair, J. Förstner, C. Meier, D.J. As, Applied Physics Letters (2013), pp. 081105

Whispering gallery modes (WGMs) were observed in 60 nm thin cubic AlN microdisk resonators containing a single layer of non-polar cubic GaN quantum dots. Freestanding microdisks were patterned by means of electron beam lithography and a two step reactive ion etching process. Micro-photoluminescence spectroscopy investigations were performed for optical characterization. We analyzed the mode spacing for disk diameters ranging from 2-4 lm. Numerical investigations using three dimensional finite difference time domain calculations were in good agreement with the experimental data. Whispering gallery modes of the radial orders 1 and 2 were identified by means of simulated mode field distributions.


Optimization of the intensity enhancement in plasmonic nanoantennas

A. Hildebrandt, M. Reichelt, T. Meier, J. Förstner, AIP AIP Conference Proceedings 1475, 2012

We design the geometrical shape of plasmonic nanostructures to achieve field patterns with desired properties. For this, we combine Maxwell simulations and automatic optimization techniques. By allowing variations of the geometrical shape, which can be based on either boxes or arbitrary polygons, we maximize the desired objective.

Photonic crystal waveguides intersection for resonant quantum dot optical spectroscopy detection

X. Song, S. Declair, T. Meier, A. Zrenner, J. Förstner, Optics Express (2012)

Using a finite-difference time-domain method, we theoretically investigate the optical spectra of crossing perpendicular photonic crystal waveguides with quantum dots embedded in the central rod. The waveguides are designed so that the light mainly propagates along one direction and the cross talk is greatly reduced in the transverse direction. It is shown that when a quantum dot (QD) is resonant with the cavity, strong coupling can be observed via both the transmission and crosstalk spectrum. If the cavity is far off-resonant from the QD, both the cavity mode and the QD signal can be detected in the transverse direction since the laser field is greatly suppressed in this direction. This structure could have strong implications for resonant excitation and in-plane detection of QD optical spectroscopy.

Engineering high harmonic generation in semiconductors via pulse shaping

M. Reichelt, A. Hildebrandt, A. Walther, J. Förstner, T. Meier, in: Ultrafast Phenomena and Nanophotonics XVI, Proc. SPIE 8260, 2012, pp. 82601L

Paper Abstract High harmonic generation is investigated for a two-band model of a semiconductor nanostructure. Similar to an atomic two-level system, the semiconductor emits high harmonic radiation. We show how one can specifically enhance the emission for a given frequency by applying a non-trivially shaped laser pulse. Therefore, the semiconductor Bloch equations including the interband and additionally the intraband dynamics are solved numerically and the spectral shape of the input pulse is computed via an optimization algorithm. It is demonstrated that desired emission frequencies can be favored even though the overall input power is kept constant. We also suggest special metallic nano geometries to achieve enhanced localized optical fields. They are found by geometric optimization.

Near-field coupling and second-harmonic generation in split-ring resonator arrays

Y. Grynko, T. Meier, S. Linden, F.B.P. Niesler, M. Wegener, J. Förstner, AIP Conference Proceedings 1475, 2012, pp. 128-130

We simulate the linear and nonlinear optical response from split-ring resonator (SRR) arrays to study collective effects between the constituent SRRs that determine spectral properties of the second harmonic generation (SHG). We apply the Discontinuous Galerkin Time Domain (DGTD) method and the hydrodynamic Maxwell-Vlasov model to calculate the SHG emission. Our model is able to qualitatively reproduce and explain the non-monotonic dependence of the spectral SHG transmission measured experimentally for SRR arrays with different lattice constants

Cavity-assisted emission of polarization-entangled photons from biexcitons in quantum dots with fine-structure splitting

S. Schumacher, J. Förstner, A. Zrenner, M. Florian, C. Gies, P. Gartner, F. Jahnke, Optics Express (2012)

We study the quantum properties and statistics of photons emitted by a quantum-dot biexciton inside a cavity. In the biexciton-exciton cascade, fine-structure splitting between exciton levels degrades polarization-entanglement for the emitted pair of photons. However, here we show that the polarization-entanglement can be preserved in such a system through simultaneous emission of two degenerate photons into cavity modes tuned to half the biexciton energy. Based on detailed theoretical calculations for realistic quantum-dot and cavity parameters, we quantify the degree of achievable entanglement.

Collective Effects in Second-Harmonic Generation from Split-Ring-Resonator Arrays

S. Linden, F.B.P. Niesler, J. Förstner, Y. Grynko, T. Meier, M. Wegener, Physical Review Letters (2012), pp. 015502

Optical experiments on second-harmonic generation from split-ring-resonator square arrays show a nonmonotonic dependence of the conversion efficiency on the lattice constant. This finding is interpreted in terms of a competition between dilution effects and linewidth or near-field changes due to interactions among the individual elements in the array.

Convey Vector Personalities – FPGA Acceleration with an OpenMP-like Effort?

B. Meyer, J. Schumacher, C. Plessl, J. Förstner, in: Proc. Int. Conf. on Field Programmable Logic and Applications (FPL), IEEE, 2012, pp. 189-196

Although the benefits of FPGAs for accelerating scientific codes are widely acknowledged, the use of FPGA accelerators in scientific computing is not widespread because reaping these benefits requires knowledge of hardware design methods and tools that is typically not available with domain scientists. A promising but hardly investigated approach is to develop tool flows that keep the common languages for scientific code (C,C++, and Fortran) and allow the developer to augment the source code with OpenMPlike directives for instructing the compiler which parts of the application shall be offloaded the FPGA accelerator. In this work we study whether the promise of effective FPGA acceleration with an OpenMP-like programming effort can actually be held. Our target system is the Convey HC-1 reconfigurable computer for which an OpenMP-like programming environment exists. As case study we use an application from computational nanophotonics. Our results show that a developer without previous FPGA experience could create an FPGA-accelerated application that is competitive to an optimized OpenMP-parallelized CPU version running on a two socket quad-core server. Finally, we discuss our experiences with this tool flow and the Convey HC-1 from a productivity and economic point of view.


Numerical analysis of coupled photonic crystal cavities

S. Declair, T. Meier, A. Zrenner, J. Förstner, Photonics and Nanostructures - Fundamentals and Applications (2011), pp. 345-350

We numerically investigate the interaction dynamics of coupled cavities in planar photonic crystal slabs in different configurations. The single cavity is optimized for a long lifetime of the fundamental mode, reaching a Q-factor of ≈43, 000 using the method of gentle confinement. For pairs of cavities we consider several configurations and present a setup with strongest coupling observable as a line splitting of about 30 nm. Based on this configuration, setups with three cavities are investigated.

Injection currents in (110)-oriented GaAs/AlGaAs quantum wells: recent progress in theory and experiment

H.T. Duc, M. Pochwala, J. Förstner, T. Meier, S. Priyadarshi, A.M. Racu, K. Pierz, U. Siegner, M. Bieler, in: Ultrafast Phenomena in Semiconductors and Nanostructure Materials XV, SPIE, 2011

We experimentally and theoretically investigate injection currents generated by femtosecond single-color circularly-polarized laser pulses in (110)-oriented GaAs quantum wells. The current measurements are performed by detecting the emitted Terahertz radiation at room temperature. The microscopic theory is based on a 14 x 14 k • p band-structure calculation in combination with the multi-subband semiconductor Bloch equations. For symmetric GaAs quantum wells grown in (110) direction, an oscillatory dependence of the injection currents on the exciting photon energy is obtained. The results of the microscopic theory are in good agreement with the measurements.

Intensity dependence of optically-induced injection currents in semiconductor quantum wells

M. Pochwala, H.T. Duc, J. Förstner, T. Meier, in: CLEO:2011 - Laser Applications to Photonic Applications, OSA, 2011

The intensity dependence of optically-induced injection currents in semiconductor quantum wells is investigated numerically. Oscillatory behavior of the electron charge current transients as function of intensity and time is predicted and explained.

Electrong-factor anisotropy in symmetric (110)-oriented GaAs quantum wells

J. Hübner, S. Kunz, S. Oertel, D. Schuh, M. Pochwała, H.T. Duc, J. Förstner, T. Meier, M. Oestreich, Physical Review B (2011), pp. 041301 (R)

We demonstrate by spin quantum beat spectroscopy that in undoped symmetric (110)-oriented GaAs/AlGaAs single quantum wells, even a symmetric spatial envelope wave function gives rise to an asymmetric in-plane electron Land´e g-factor. The anisotropy is neither a direct consequence of the asymmetric in-plane Dresselhaus splitting nor a direct consequence of the asymmetric Zeeman splitting of the hole bands, but rather it is a pure higher-order effect that exists as well for diamond-type lattices. The measurements for various well widths are very well described within 14 × 14 band k·p theory and illustrate that the electron spin is an excellent meter variable for mapping out the internal—otherwise hidden—symmetries in two-dimensional systems. Fourth-order perturbation theory yields an analytical expression for the strength of the g-factor anisotropy, providing a qualitative understanding of the observed effects.

Phonon-assisted decoherence and tunneling in quantum dot molecules

A. Grodecka-Grad, J. Förstner, physica status solidi (c) (2011), pp. 1125-1128

We study the influence of the phonon environment on the electron dynamics in a doped quantum dot molecule. A non-perturbative quantumkinetic theory based on correlation expansion is used in order to describe both diagonal and off-diagonal electron-phonon couplings representing real and virtual processes with relevant acoustic phonons. We show that the relaxation is dominated by phononassisted electron tunneling between constituent quantum dots and occurs on a picosecond time scale. The dependence of the time evolution of the quantum dot occupation probabilities on the energy mismatch between the quantum dots is studied in detail.

Simulation of the ultrafast nonlinear optical response of metal slabs

M. Wand, A. Schindlmayr, T. Meier, J. Förstner, physica status solidi (b) (2011), pp. 887-891

We present a nonequilibrium ab initio method for calculating nonlinear and nonlocal optical effects in metallic slabs with a thickness of several nanometers. The numerical analysis is based on the full solution of the time‐dependent Kohn–Sham equations for a jellium system and allows to study the optical response of metal electrons subject to arbitrarily shaped intense light pulses. We find a strong localization of the generated second‐harmonic current in the surface regions of the slabs.

Application of the Discontinuous Galerkin Time Domain Method to the Optics of Bi-Chiral Plasmonic Crystals

Y. Grynko, J. Förstner, T. Meier, A. Radke, T. Gissibl, P.V. Braun, H. Giessen, D.N. Chigrin, AIP, 2011, pp. 76-78

A simulation environment for metallic nanostructures based on the Discontinuous Galerkin Time Domain method is presented. It is used to model optical transmission by silver bi‐chiral plasmonic crystals. The results of simulations qualitatively and quantitavely agree with experimental measurements of transmitted circular polarization.

Theoretical approach to the ultrafast nonlinear optical response of metal slabs

M. Wand, A. Schindlmayr, T. Meier, J. Förstner, CLEO:2011 - Laser Applications to Photonic Applications (2011)

We present an ab-initio method for calculating nonlinear and nonlocal optical effects in metallic slabs with sub-wavelength thickness. We find a strong localization of the second-harmonic current at the metal-vacuum interface.

Numerical investigation of the coupling between microdisk modes and quantum dots

S. Declair, T. Meier, J. Förstner, physica status solidi (c) (2011), pp. 1254-1257

We numerically investigate the coupling between circular resonators and study strong light‐matter coupling of single as well as multiple circular resonators to quantum‐mechanical resonators in two dimensional model simulations. For all cases, the computed resonances of the coupled system as function of the detuning show anti‐crossings. The obtained mode splittings of coupled optical resonators are strongly depending on distance and cluster in almost degenerate eigenstates for large distances, as is known from coupled resonator optical waveguides. Vacuum Rabi splitting is observed for a quantum dot strongly coupled to eigenmodes of single perfectly cylindrical resonators.

Intensity-dependent ultrafast dynamics of injection currents in unbiased GaAs quantum wells

M. Pochwała, H.T. Duc, J. Förstner, T. Meier, physica status solidi (RRL) - Rapid Research Letters (2011), pp. 119-121

The intensity dependence of optically-induced injection currents in unbiased GaAs semiconductor quantum wells grown in [110] direction is investigated theoretically for a number of well widths. Our microscopic analysis is based on a 14 x 14 band k . p method in combination with the multisubband semiconductor Bloch equations. An oscillatory dependence of the injection current transients as function of intensity and time is predicted and explained. It is demonstrated that optical excitations involving different subbands and Rabi flopping are responsible for this complex dynamics.

Transformation of scientific algorithms to parallel computing code: subdomain support in a MPI-multi-GPU backend

B. Meyer, C. Plessl, J. Förstner, in: Symp. on Application Accelerators in High Performance Computing (SAAHPC), IEEE Computer Society, 2011, pp. 60-63


Application of the discontinous Galerkin time domain method to the optics of metallic nanostructures

Y. Grynko, J. Förstner, T. Meier, AAPP | Atti della Accademia Peloritana dei Pericolanti (2011)

A simulation environment for metallic nanostructures based on the Discontinuous Galerkin Time Domain method is presented. The model is used to compute the linear and nonlinear optical response of split ring resonators and to study physical mechanisms that contribute to second harmonic generation.

Oscillatory excitation energy dependence of injection currents in GaAs/AlGaAs quantum wells

H. Thanh Duc, J. Förstner, T. Meier, S. Priyadarshi, A.M. Racu, K. Pierz, U. Siegner, M. Bieler, physica status solidi (c) (2011), pp. 1137-1140

The injection of photocurrents by femtosecond laser pulses in (110)-orientedGaAs/AlGaAs quantum wells is investigated theoretically and experimentally. The roomtemperature measurements show an oscillatory dependence of the injection current amplitude and direction on the excitation photon energy. Microscopic calculations using the semiconductor Bloch equations that are set up on the basis of k.p band structure calculations provide a detailed understanding of the experimental findings.

Method for transmission of information about polarization state of photons to stationary system

J. Förstner, D. Mantei, S.M.. de Vasconcellos, A. Zrenner. Method for transmission of information about polarization state of photons to stationary system. 2011.


Microscopic analysis of charge and spin photocurrents injected by circularly polarized one-color laser pulses in GaAs quantum wells

H.T. Duc, J. Förstner, T. Meier, Physical Review B (2010), pp. 115316-1

The dynamics of charge and spin injection currents excited by circularly polarized, one-color laser beams in semiconductor quantum wells is analyzed. Our microscopic approach is based on a 14x14 k · p band-structure theory in combination with multisubband semiconductor Bloch equations which allows a detailed analysis of the photogenerated carrier distributions and coherences in k space. Charge and spin injection currents are numerically calculated for [110]- and [001]-grown GaAs quantum wells including dc population contributions and ac contributions that arise from intersubband coherences. The dependencies of the injection currents on the excitation conditions, in particular, the photon energy are computed and discussed.

Modeling excitonic line shapes in weakly disordered semiconductor nanostructures

I. Kuznetsova, N. Gőgh, J. Förstner, T. Meier, S.T. Cundiff, I. Varga, P. Thomas, Physical Review B (2010)

Excitonic spectra of weakly disordered semiconductor heterostructures are simulated on the basis of a one-dimensional tight-binding model. The influence of the length scale of weak disorder in quantum wells on the redshift of the excitonic peak and its linewidth is studied. By calculating two-dimensional Fouriertransform spectra we are able to determine the contribution of disorder to inhomogeneous and also to homogeneous broadenings separately. This disorder-induced dephasing is related to a Fano-type coupling and leads to contributions to the homogeneous linewidth that depends on energy within the inhomogeneously broadened line. The model includes heavy- and light-hole excitons and yields smaller inhomogeneous broadening for the light-hole exciton if compared to the heavy-hole exciton, which agrees qualitatively with the experiment.

Tuning quantum-dot based photonic devices with liquid crystals

K.A. Piegdon, S. Declair, J. Förstner, T. Meier, H. Matthias, M. Urbanski, H. Kitzerow, D. Reuter, A.D. Wieck, A. Lorke, C. Meier, Optics Express (2010)

Microdisks made from GaAs with embedded InAs quantum dots are immersed in the liquid crystal 4-cyano-4’-pentylbiphenyl (5CB). The quantum dots serve as emitters feeding the optical modes of the photonic cavity. By changing temperature, the liquid crystal undergoes a phase transition from the isotropic to the nematic state, which can be used as an effective tuning mechanism of the photonic modes of the cavity. In the nematic state, the uniaxial electrical anisotropy of the liquid crystal molecules can be exploited for orienting the material in an electric field, thus externally controlling the birefringence of the material. Using this effect, an electric field induced tuning of the modes is achieved. Numerical simulations using the finite-differences time-domain (FDTD) technique employing an anisotropic dielectric medium allow to understand the alignment of the liquid crystal molecules on the surface of the microdisk resonator.

Self-assembled quantum dots in a liquid-crystal-tunable microdisk resonator

K.A. Piegdon, M. Offer, A. Lorke, M. Urbanski, A. Hoischen, H. Kitzerow, S. Declair, J. Förstner, T. Meier, D. Reuter, A.D. Wieck, C. Meier, Physica E: Low-dimensional Systems and Nanostructures (2010), pp. 2552-2555

GaAs-based semiconductor microdisks with high quality whispering gallery modes (Q44000) have been fabricated.A layer of self-organized InAs quantumdots (QDs) served as a light source to feed the optical modes at room temperature. In order to achieve frequency tuning of the optical modes, the microdisk devices have been immersed in 4 – cyano – 4´-pentylbiphenyl (5CB), a liquid crystal(LC) with a nematic phase below the clearing temperature of TC≈34°C .We have studied the device performance in the temperature rangeof T=20-50°C, in order to investigate the influence of the nematic–isotropic phase transition on the optical modes. Moreover,we havea pplied an AC electric field to the device,which leads in the nematic phase to a reorientation of the anisotropic dielectric tensor of the liquid crystal.This electrical anisotropy can be used to achieve electrical tunability of the optical modes.Using the finite-difference time domain (FDTD) technique with an anisotropic material model, we are able to describe the influence of the liquid crystal qualitatively.

Phonon-mediated relaxation in doped quantum dot molecules

A. Grodecka-Grad, J. Förstner, Journal of Physics: Conference Series (2010)

We study a single quantum dot molecule doped with one electron in the presence of electron-phonon coupling. Both diagonal and off-diagonal interactions representing real and virtual processes with acoustic phonons via deformation potential and piezoelectric coupling are taken into account. We employ a non-perturbative quantum kinetic theory and show that the phonon-mediated relaxation is dominated by an electron tunneling on a picosecond time scale.A dependence of the relaxation on the temperature and the strength of the tunneling coupling is analyzed.

Enhanced FDTD edge correction for nonlinear effects calculation

C. Classen, J. Förstner, T. Meier, R. Schuhmann, in: 2010 IEEE Antennas and Propagation Society International Symposium, IEEE, 2010

The electromagnetic field in the vicinity of sharp edges needs a special treatment in numeric calculation whenever accurate, fast converging results are necessary. One of the fundamental works concerning field singularities has been proposed in 1972 [1] and states that the electromagnetic energy density must be integrable over any finite domain, even if this domain contains singularities. It is shown, that the magnetic field H(, ϕ) and electric field E(, ϕ) are proportional to ∝ (t−1) for  → 0. The variable  is the distance to the edge and t has to fulfill the integrability condition and thus is restricted to 0 < t < 1. This result is often used to reduce the error corresponding to the singularity without increasing the numerical effort [2 - 5]. For this purpose, a correction factor K is estimated by inserting the proportionality into the wave equation. It is shown, that this method improves the accuracy of the result significantly, however the order of convergence is often not studied. In [4] a method to modify the material parameters in order to use analytic results to improve the numeric calculation is presented. In this contribution we will - inspired by the scheme given in [4] - develop a new method to estimate a correction factor for perfect conducting materials (PEC) and demonstrate the improvement of the results compared to the standard edge correction. Therefore analytic results (comparable to [1]) are consequently merged with the scheme in [4]. The main goal of this work is the calculation of the second harmonic generation (SHG) in the wave response of so-called metamaterials [6]. Frequently these structures contain sharp metallic edges with field singularities at the interfaces which have a strong impact on the SHG signals. Thus, an accurate simulation of singularities is highly important. However, the following approach can also be applied to many other setups, and one of them is shown in the example below.

Theory of phonon-mediated relaxation in doped quantum dot molecules

A. Grodecka-Grad, J. Förstner, Physical Review B (2010)

A quantum dot molecule doped with a single electron in the presence of diagonal and off-diagonal carrierphonon couplings is studied by means of a nonperturbative quantum kinetic theory. The interaction with acoustic phonons by deformation potential and piezoelectric coupling is taken into account. We show that the phonon-mediated relaxation is fast on a picosecond time scale and is dominated by the usually neglected off-diagonal coupling to the lattice degrees of freedom leading to phonon-assisted electron tunneling. We show that in the parameter regime of current electrical and optical experiments, the microscopic non-Markovian theory has to be employed.

Anticrossing of Whispering Gallery Modes in microdisk resonators embedded in an anisotropic environment

S. Declair, C. Meier, T. Meier, J. Förstner, Photonics and Nanostructures - Fundamentals and Applications (2010), pp. 273-277

We numerically investigate the behavior of Whispering Gallery Modes (WGMs) in circularly shaped resonators like microdisks, with diameters in the range of optical vacuum wavelengths. The microdisk is embedded in an uniaxial anisotropic dielectric environment. By changing the optical anisotropy, one obtains spectral tunability of the optical modes. The degree of tunability strongly depends on the radial (azimuthal) mode order M (N). As the modes approach each other spectrally, anticrossing is observed, leading to a rearrangement of the optical states.

Reversal of Coherently Controlled Ultrafast Photocurrents by Band Mixing in Undoped GaAs Quantum Wells

S. Priyadarshi, A.M. Racu, K. Pierz, U. Siegner, M. Bieler, H.T. Duc, J. Förstner, T. Meier, Physical Review Letters (2010)

It is demonstrated that valence-band mixing in GaAs quantum wells tremendously modifies electronic transport. A coherent control scheme in which ultrafast currents are optically injected into undoped GaAs quantum wells upon excitation with femtosecond laser pulses is employed. An oscillatory dependence of the injection current amplitude and direction on the excitation photon energy is observed. A microscopic theoretical analysis shows that this current reversal is caused by the coupling of the light- and heavy-hole bands and that the hole currents dominate the overall current response. These surprising consequences of band mixing illuminate fundamental physics as they are unique for experiments which are able to monitor electronic transport resulting from carriers with relatively large momenta.

Microscopic theoretical analysis of optically generated injection currents in semiconductor quantum wells

H.T. Duc, J. Förstner, T. Meier, in: Ultrafast Phenomena in Semiconductors and Nanostructure Materials XIV, SPIE, 2010, pp. 76000S-76000S-9

A microscopic theory that describes injection currents in GaAs quantum wells is presented. 14 × 14 band k.p theory is used to compute the band structure including anisotropy and spin-orbit interaction. Transient injection currents are obtained via numerical solutions of the semiconductor Bloch equations. Depending on the growth direction of the considered quantum well system and the propagation and polarization directions of the incident light beam, it is possible to generate charge and/or spin photocurrents on ultrashort time scales. The dependence of the photocurrents on the excitation conditions is computed and discussed.


Generation of injection currents in (110)-oriented GaAs quantum wells: experimental observation and development of a microscopic theory

M. Bieler, K. Pierz, U. Siegner, P. Dawson, H.T. Duc, J. Förstner, T. Meier, in: Ultrafast Phenomena in Semiconductors and Nanostructure Materials XIII, SPIE, 2009, pp. 721404-721404-13

We have experimentally investigated injection currents generated by all-optical excitation of GaAs/AlGaAs quantum wells excited with 130 fs optical pulses. The currents have been detected via free-space THz experiments at room temperature. Our experiments prove that Coulomb effects strongly influence injection currents. This becomes most prominently visible when exciting light-hole exciton transitions. At this photon energy we observe a pronounced phase shift of the current transients which is due to oppositely oriented heavy-hole and light-hole type contributions. We are currently developing a microscopic theory based on a 14×14 k.p model in combination with the semiconductor Bloch equations to describe the observed features quantitatively. The combined theoretical and experimental approach will allow us to analyze the influence of the bandstructure and interaction effects on the injection current amplitude and current dynamics.

Indirect Dephasing Channel for Optically Controlled Spin in a Single Quantum Dot

A. Grodecka, P. Machnikowski, J. Förstner, in: Advances in Optical Sciences Congress, OSA Technical Digest (CD) (Optical Society of America, 2009), 2009

We show that an optically driven carrier spin undergoes indirect dephasing even in the absence of spin-reservoir coupling and illustrate it for phonon-induced decoherence during optical spin rotation in a single quantum dot.

Coupling Dynamics of Quantum Dots in a Liquid-Crystal-Tunable Microdisk Resonator

J. Förstner, C. Meier, K. Piegdon, S. Declair, A. Hoischen, M. Urbanski, T. Meier, H. Kitzerow, in: Advances in Optical Sciences Congress, OSA Technical Digest (CD) (Optical Society of America, 2009), paper NTuC2, 2009

We experimentally and theoretically investigate microdisk resonators with embedded quantum dots immersed in a liquid crystal in its nematic phase, showing the tunabililty of the photonic modes via external parameters like temperature or electric field.

Anticrossing of Whispering Gallery Modes in Microdisk Resonators Embedded in a Liquid Crystal

J. Förstner, S. Declair, C. Meier, T. Meier, in: AIP Conference Proceedings, AIP Conference Proceedings , 2009, pp. 60-62

We numerically investigate Whispering Gallery Modes (WGM) in a subwavelength microdisk resonator [1] embedded in an uniaxial anisotropic liquid crystal environment. It is shown that the WGMs have anticrossing behavior when modes of different radial mode order M or azimuthal order N approach each other spectrally.

Indirect spin dephasing via charge-state decoherence in optical control schemes in quantum dots

A. Grodecka, P. Machnikowski, J. Förstner, Physical Review A (2009)

We demonstrate that an optically driven spin of a carrier in a quantum dot undergoes indirect dephasing via conditional optically induced charge evolution even in the absence of any direct interaction between the spin and its environment. A generic model for the indirect dephasing with a three-component system with spin, charge, and reservoir is proposed. This indirect decoherence channel is studied for the optical spin manipulation in a quantum dot with a microscopic description of the charge-phonon interaction taking into account its non-Markovian nature.


Theoretical study of phononassisted singlet-singlet relaxation in two-electron semiconductor quantum dot molecules

A. Grodecka, P. Machnikowski, J. Förstner, physica status solidi (c) (2008), pp. 474-478

Phonon-assisted singlet-singlet relaxation in semiconductor quantum dot molecules is studied theoretically. Laterally coupled quantum dot structures doped with two electrons are considered. We take into account interaction with acoustic phonon modes via deformation potential and piezoelectric coupling. We show that piezoelectric mechanism for the considered system is of great importance and for some ranges of quantum dot molecule parameters is the dominant contribution to relaxation. It is shown that the phonon-assisted tunneling is much faster (down to ∼ 6 ps even at zero temperature) in comparison with other decoherence processes. The influence of Coulomb interaction is discussed and its consequences are indicated. We calculate the relaxation rates for GaAs quantum dot molecules and study the dependence on quantum dot size, distance and offset between the constituent quantum dots. In addition the temperature dependence of the tunneling rates is analyzed.

Transition between different coherent light–matter interaction regimes analyzed by phase-resolved pulse propagation

T.H. zu Siederdissen, N.C. Nielsen, J. Kuhl, M. Schaarschmidt, J. Förstner, A. Knorr, G. Khitrova, H.M. Gibbs, S.W. Koch, H. Giessen, Optics Letters (2008)

We present phase-resolved pulse propagation measurements that allow us to fully describe the transition between several light–matter interaction regimes. The complete range from linear excitation to the breakdown of the photonic bandgap on to self-induced transmission and self-phase modulation is studied on a high-quality multiple-quantum-well Bragg structure. An improved fast-scanning cross-correlation frequency-resolved optical gating setup is applied to retrieve the pulse phase with an excellent signal-tonoise ratio. Calculations using the semiconductor Maxwell–Bloch equations show qualitative agreement with the experimental findings.

Phonon-assisted tunneling between singlet states in two-electron quantum dot molecules

A. Grodecka, P. Machnikowski, J. Förstner, Physical Review B (2008)

We study phonon-assisted electron tunneling in semiconductor quantum dot molecules. In particular, singletsinglet relaxation in a two-electron-doped structure is considered. The influence of Coulomb interaction is discussed via comparison with a single-electron system. We find that the relaxation rate reaches similar values in the two cases but the Coulomb interaction shifts the maximum rates toward larger separations between the dots. The difference in electron-phonon interaction between deformation potential and piezoelectric coupling is investigated. We show that the phonon-induced tunneling between two-electron singlet states is a fast process, taking place on the time scales of the order of a few tens of picoseconds.


Line narrowing and hole burning within the homogeneous linewidth: a new wave-mixing effect in two-level systems

J. Förstner, A. Knorr, M. Lindberg, S.W. Koch, Optics Letters (2007)

The interaction of strong low-area pulses with two-level systems shows absorption line narrowing and hole burning within the homogeneous linewidth as a result of nonlinear wave mixing. The wave mixing results from the two-level electronic saturation nonlinearity and occurs, depending on the sign of the pulse area, as a strong absorption enhancement or gain at the transition frequency of the two-level system for resonant excitation.


Interplay of electron-phonon and Coulomb interaction in semiconductor quantum dots

J. Förstner, A. Knorr, J.V. Moloney, physica status solidi (c) (2006), pp. 2389-2392

We theoretically study the biexciton-phonon interaction in strongly confined semiconductor quantum dots. For spectrally narrow single-pulse excitation generation of biexcitonic occupations is only possible via a two-photon cascade, which exhibits renormalized Rabi oscillations and spectrally compressed phononsidebands.

Optical Experiments on Second-Harmonic Generation with Metamaterials Composed of Split-Ring Resonators

M.W. Klein, C. Enkrich, M. Wegener, J. Förstner, J.V. Moloney, W. Hoyer, T. Stroucken, T. Meier, S.W. Koch, S. Linden, in: Photonic Metamaterials: From Random to Periodic, OSA, 2006

We study optical second-harmonic generation from planar arrays of magnetic split-ring resonators at 1.5 microns resonance wavelength. We obtain by far the largest signals when exciting the magnetic-dipole resonance.

Theory of ultrafast nonlinear optics of Coulomb-coupled semiconductor quantum dots: Rabi oscillations and pump-probe spectra

J. Danckwerts, K.J. Ahn, J. Förstner, A. Knorr, Physical Review B (2006), pp. 165318-165318-18

We investigate the optical properties of a Coulomb-coupled double-quantum dot system excited by coherent light pulses. Basic effects of Coulomb coupling regarding linear and nonlinear optical processes are discussed. By numerically solving the Heisenberg equation of motion we are able to present the temporal evolution of the system’s density matrix for a wide range of coupling parameters. The two main coupling effects in dipole approximation, biexcitonic shift and Förster energy transfer, are investigated and their qualitative and quantitative influence on absorption spectra, Rabi oscillations, and single- and two-pulse excitation is discussed. We present simulated differential transmission spectra to allow for comparison with recent experimental studies.

Optical experiments on second-harmonic generation from metamaterials consisting of split-ring resonators

M.W. Klein, C. Enkrich, M. Wegener, J. Förstner, J.V. Moloney, W. Hoyer, T. Stroucken, T. Meier, S.W. Koch, S. Linden, in: 2006 Conference on Lasers and Electro-Optics and 2006 Quantum Electronics and Laser Science Conference, IEEE, 2006

We discuss second-harmonic generation experiments on planar arrays of magnetic split-ring resonators, using 150 fs pulses at 1.5 mum wavelength. Lithographic tuning reveals by far the largest signals when exciting the magnetic-dipole resonance.


Normal Mode Coupling in Photonic Crystal Nanocavities

J. Förstner, C. Dineen, A. Zakharian, J.V. Moloney, S.W. Koch, in: Frontiers in Optics, OSA, 2005

We numerically investigate strong light-matter interaction and normal mode coupling/splitting in a system composed of a single two-level atom and the localized mode of a small mode volume photonic crystal nanocavity.

Microscopic theory of electron dynamics and time-resolved two-color two-photon photoemission at semiconductor surfaces

A. Zeiser, N. Bücking, J. Förstner, A. Knorr, Physical Review B (2005)

A microscopic description based on the density matrix formalism is developed to describe the dynamics of photoemission of hot electrons at semiconductor surfaces, including the interaction of bulk and surface states. The equations of motion for the electronic occupations and transitions include the interaction with arbitrary optical fields as well as the electron-phonon coupling. Model wave functions are used to qualitatively describe the bulk-surface dynamics and the subsequent time resolved two-photon photoemission s2PPEd spectra. Our results suggest that it is possible to extract energetic and temporal information of the underlying dynamical occupations of the intermediate states from the 2PPE spectra.

Ultrafast quantum kinetics of semiconductor intersubband transitions: polaron signatures and dephasing dynamics

S. Butscher, J. Förstner, I. Waldmuller, A. Knorr, in: 2005 Quantum Electronics and Laser Science Conference, IEEE, 2005, pp. 640-642

The ultrafast intersubband dynamics in a semiconductor quantum well subband system is investigated theoretically. Non-Markovian electron-phonon interaction leads to polaron formation and enhanced dephasing.

Femtosecond Transfer Dynamics of Photogenerated Electrons at a Surface Resonance of Reconstructed InP(100)

L. Töben, L. Gundlach, R. Ernstorfer, R. Eichberger, T. Hannappel, F. Willig, A. Zeiser, J. Förstner, A. Knorr, P.H. Hahn, W.G. Schmidt, Physical Review Letters (2005)

Time-dependent two-photon photoemission spectra are used to resolve the femtosecond dynamics of hot electrons at the energetically lowest surface resonance of reconstructed InP(100). Two different cases are studied, where electrons either are lifted into the surface resonance via a direct optical transition or are captured from bulk states. These data are the first of this kind recorded with a time resolution below 70 fs. The microscopic analysis shows that electron-phonon scattering is a major mechanism for electron transfer between surface and bulk states.

Ultrafast electron-phonon interaction of intersubband transitions: Quantum kinetics from adiabatic following to Rabi-oscillations

S. Butscher, J. Förstner, I. Waldmüller, A. Knorr, Physical Review B (2005), pp. 045314-045314-4

The interaction of electrons with LO phonons provides an important mechanism of optical dephasing and carrier scattering for the two-dimensional electron gas in semiconductor quantum wells. In this paper, the corresponding ultrafast nonlinearities for off-resonant and resonant intersubband excitations are investigated. Quantum kinetic effects of the electron-phonon interaction and the corresponding violation of the microscopic energy conservation yield a qualitative different picture compared to the standard Markovian theory, if the phonon energy is larger than the intersubband-gap energy.

Quantum information processing using Coulomb-coupled quantum dots

J. Danckwerts, J. Förstner, A. Knorr, in: AIP Conference Proceedings, AIP, 2005

A system of two quantum dots coupled by dipole‐dipole interaction is investigated within a density matrix approach. We compute the temporal evolution of the system in the linear and nonlinear optical regime and discuss the possibility of performing basic quantum information gates. The influence of the Förster energy transfer on Rabi oscillations is discussed.

Electromagnetic field structure and normal mode coupling in photonic crystal nanocavities

C. Dineen, J. Förstner, A. Zakharian, J. Moloney, S. Koch, Optics Express (2005)

The electromagnetic field of a high-quality photonic crystal nanocavity is computed using the finite difference time domain method. It is shown that a separatrix occurs in the local energy flux discriminating between predominantly near and far field components. Placing a two-level atom into the cavity leads to characteristic field modifications and normalmode splitting in the transmission spectra.

Phase Evolution of Solitonlike Optical Pulses during Excitonic Rabi Flopping in a Semiconductor

N.C. Nielsen, T.H. zu Siederdissen, J. Kuhl, M. Schaarschmidt, J. Förstner, A. Knorr, H. Giessen, Physical Review Letters (2005)

We demonstrate that the temporal pulse phase remains essentially unaltered before separate phase characteristics are developed when propagating high-intensity pulses coherently on the exciton resonance of an optically thick semiconductor. This behavior is a clear manifestation of self-induced transmission and pulse breakup into solitonlike pulses due to Rabi flopping of the carrier density. Experiments using a novel fast-scan cross-correlation frequency-resolved optical gating (XFROG) method are in good agreement with numerical calculations based on the semiconductor Bloch equations.

Temporal and Spatial Pulse Compression in a Nonlinear Defocusing Material

N.C. Nielsen, T. zu Höner Siederdissen, J. Kuhl, M. Schaarschmidt, J. Förstner, A. Knorr, S.W. Koch, H. Giessen, in: Springer Series in Chemical Physics, Springer Berlin Heidelberg, 2005, pp. 19-21

We investigate the spatiotemporal characteristics of subpicosecond pulse propagation in the nonlinear defocusing regime below the band edge of bulk GaAs. We observe temporal and spatial pulse compression and instabilities.

Kinetic theory of the electron transport in the two photon photo emission at semiconductor surfaces

N. Bucking, A. Zeiser, J. Förstner, A. Knorr, in: 2005 Quantum Electronics and Laser Science Conference, IEEE, 2005, pp. 1929-1931

A theoretical description of ultrafast phonon induced electronic transport between surface and bulk states after optical excitation is presented. In particular, the influence of the electron transfer processes on two photon photo emission is evaluated.

Resonance fluorescence of semiconductor quantum dots: Signatures of the electron-phonon interaction

K.J. Ahn, J. Förstner, A. Knorr, Physical Review B (2005)

Using a fully quantized description of strongly confined electrons interacting with acoustic phonons and the photon field, the nonstationary resonance-fluorescence spectra of a semiconductor quantum dot are investigated. For excitation pulses with durations approaching typical electron-phonon scattering times, the virtual quantum processes yield an observable electron-phonon sideband broadening.


Temporal phase evolution during excitonic Rabi flopping in semiconductors

T. Höner zu Siederdissen, N.C. Nielsen, J. Kuhl, J. Förstner, A. Knorr, H. Giessen, in: International Quantum Electronics Conference and Photonic Applications Systems Technologies, OSA, 2004

Theoretically and experimentally, we investigate temporal phase evolution during Rabi-flopping on the A-exciton resonance in CdSe using a novel fast-scanning XFROG method and observe phase changes smaller than π/2 compared to the slightly-chirped input pulse.

Polaron signatures in the line shape of semiconductor ;intersubband transitions: quantum kinetics of the electron–phonon interaction

S. Butscher, J. Förstner, I. Waldmüller, A. Knorr, physica status solidi (b) (2004), pp. R49-R51

We present a theory of the optical line shape of coherent intersubband transitions in a semiconductor quantum well, considering non-Markovian LO-phonon scattering as major broadening mechanism. We show that a quantum kinetic approach leads to additional polaron resonances and a resonance enhancement for gap energies close to the phonon energy.

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