Carsten Rockstuhl

Karlsruhe Institute of Technology, Carlsruhe, Baden-Württemberg, Germany

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Publications (335)799.55 Total impact

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    Ivan Fernandez-Corbaton, Stefan Nanz, Carsten Rockstuhl
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    ABSTRACT: Toroidal multipoles are attracting research attention, particularly in the field of metamaterials. They are often understood as a multipolar family in its own right. The dynamic toroidal multipoles emerge from the separation of one of the two transverse multipoles into two parts, referred to as electric and toroidal. Here, we establish that the dynamic toroidal multipolar components of an electric current distribution cannot be determined by measuring the radiation from the source or its coupling to external electromagnetic waves. We analytically show how the split into electric and toroidal parts causes the appearance of non-radiative components in each of the two parts, which cancel when summed back together. The toroidal multipoles do not have an independent meaning with respect to their interaction with the radiation field. Their formal meaning is clear, however. They are the higher order terms of an expansion of the multipolar coefficients of electric parity with respect to the electromagnetic size of the source.
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    ABSTRACT: The multipolar decomposition of current distributions is used in many branches of physics. Here, we obtain new exact expressions for the dipolar moments of a localized electric current distribution. The typical integrals for the dipole moments of electromagnetically small sources are recovered as the lowest order terms of the new expressions in a series expansion with respect to the size of the source. All the higher order terms can be easily obtained. We also provide exact and approximated expressions for dipoles that radiate a definite polarization handedness (helicity). Formally, the new exact expressions are only marginally more complex than their lowest order approximations.
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    ABSTRACT: In this paper, we propose, design, theoretically study, and experimentally test a simple periodical array which provides perfect absorptivity when one of its surfaces is illuminated and controllable reflectivity for illuminations of the other side. The proposed structure does not contain any ground plane and relies on the bianisotropic properties of the unit cells forming the absorbing layer. The absence of the ground plane makes the new design suitable for applications where the absorber should not block transmission outside the absorption band. The proposed structure is realized as an array of nonidentical conducting patches imprinted on the two sides of a thin dielectric slab. This ultra-thin ($simlambda/150$, where $lambda$ is the operational wavelength) metasurface absorber is optimized, fabricated, and tested. The test results confirm nearly perfect absorption (from one direction) and controllable reflection (from the other direction) at the resonance frequency, as well as partial transparency outside of the absorption band.
    IEEE Transactions on Antennas and Propagation 07/2015; 63(7):1-1. DOI:10.1109/TAP.2015.2423855 · 2.46 Impact Factor
  • R Alaee, R Filter, D Lehr, F Lederer, C Rockstuhl
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    ABSTRACT: A nanoantenna with balanced electric and magnetic dipole moments, known as the first Kerker condition, exhibits a directive radiation pattern with zero backscattering. In principle, a nanoantenna can provide even better directionality if higher order moments are properly balanced. Here, we study a generalized Kerker condition in the example of a nanoring nanoantenna supporting electric dipole and electric quadrupole moments. Nanoring antennas are well suited since both multipole moments can be almost independently tuned to meet the generalized Kerker condition.
    Optics Letters 06/2015; 40(11):2645-2648. DOI:10.1364/OL.40.002645 · 3.18 Impact Factor
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    ABSTRACT: Optical plasmonic antennas allow for localizing and enhancing light at the nanoscale. To enhance the application opportunities of optical antennas, their quality factor needs to be substantially improved. Here, we numerically and experimentally demonstrate that the resonance of a dipolar metallic disc antenna can be enhanced by a circular grating that obeys the Bragg condition. The supporting grating effectively collects energy from an extended spatial domain and guides it spectrally-selected into the central antenna, leading to a significantly enhanced field intensity at resonance. Accordingly, the quality factor of the antenna is enhanced by at least five times. The approach can be applied to other plasmonic systems, hence constituting an important ingredient to a future plasmonic tool box.
    Optics Express 06/2015; 23(11):14583. DOI:10.1364/OE.23.014583 · 3.53 Impact Factor
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    ABSTRACT: We design by transformation optics, fabricate by three-dimensional direct laser writing, and characterize experimentally polymer-based cloaks for 20 µm wide gold-wire contacts on a silicon wafer. The contact shadowing effect is reduced by 90%.
    CLEO / Optical Society of America, San Jose, California United States; 05/2015
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    M. Albooyeh, R. Alaee, C. Rockstuhl, C. Simovski
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    ABSTRACT: Recently, it has been shown that a metasurface of plasmonic nanospheres deposited on a highly refractive substrate requires a bianisotropic magnetoelectric coupling for its effective description. The effect has been coined substrate-induced bianisotropy. It leads to an asymmetric reflectance similar to bianisotropic metasurfaces. In this work, through a circuit model, we show that such bianisotropy does not necessarily emerge for all substrated metasurfaces. Indeed, we show that the thickness of the metasurface plays a crucial role to encounter substrate-induced bianisotropy. Moreover, by taking advantage of substrate-induced bianisotropy, we present the necessary conditions for the circuit model parameters to compensate the asymmetric reflectance generated by an intrinsically bianisotropic metasurface. We finally express that, in substrated metasurfaces, the asymmetric reflectance and the bianisotropic response are two separate issues albeit with interdependencies.
    Physical Review B 05/2015; 91(19). DOI:10.1103/PhysRevB.91.195304 · 3.66 Impact Factor
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    ABSTRACT: The radiative recombination limit described by Shockley and Queisser (SQ) provides the maximum efficiency for the conversion of sunlight into electricity by a solar cell. This maximum efficiency depends only on the radiation balance between the sun, the solar cell, and ambient. This chapter summarizes the theory of energy and solid angle restriction and the theoretical limits of light-trapping and cell efficiency. The calculated efficiency limits of c-Si solar cells with directionally and spectrally selective filters are presented. The gain in annual yield is estimated taking into account the loss in diffuse irradiation for same types of solar cells. The chapter considers the filter systems to be a 1D layer stack (Rugate filter). The chapter reports an experimental realization of a spectrally and directionally selective Bragg-like filter covering the front glass of hydrogenated amorphous silicon solar cells and of a similar filter covering the front of a germanium solar cell.
    Photon Management in Solar Cells, Edited by Ralf Wehrspohn, Uwe Rau, Andreas Gombert, 04/2015: chapter Light‐Trapping in Solar Cells by Directionally Selective Filters: pages 183-207; Wiley‐VCH Verlag GmbH & Co. KGaA., ISBN: 978-3527411757
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    ABSTRACT: Optically active artificial structures have attracted tremendous research attention. Such structures must meet two requirements: Lack of spatial inversion symmetries and, a condition usually not explicitly considered, the structure shall preserve the helicity of light, which implies that there must be a vanishing coupling between the states of opposite polarization handedness among incident and scattered plane waves. Here, we put forward and demonstrate that a unit cell made from chiraly arranged electromagnetically dual scatterers serves exactly this purpose. We prove this by demonstrating optical activity of such unit cell in general scattering directions.
    03/2015; 2(3-3):376-384. DOI:10.1021/ph500419a
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    ABSTRACT: The availability of metamaterials with properties that can be actively tuned is crucial for the future development of various metamaterial-based technologies. Here we show that by using silver nanoparticles equipped with a thermally responsive organic coating a metamaterial is obtained with reversibly switchable properties. The material investigated exhibits dynamic self-assembly resulting from temperature-dependent changes of organic coating shape, which translates to a switchable spatial distribution of the silver nanoparticles. This in turn strongly influences the optical properties of the entire material. The measured optical characteristics of the material are in excellent agreement with theoretical calculations, which allow us to use the latter to predict a dynamically tunable epsilon-near-zero behaviour of the metamaterial. The suggested methodology opens new routes for tunable metamaterials that operate in the visible region and will enable various applications for soft-matter-based optical devices.
    Nature Communications 03/2015; 6:6590. DOI:10.1038/ncomms7590 · 10.74 Impact Factor
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    ABSTRACT: We explore the optical properties of a meta-atom made of plasmonic nanopatches that possess an increasing degree of complexity. We show that if two nanopatches are strongly coupled and have a different geometrical footprint, the meta-atom exhibits a resonant magnetoelectric response, in addition to the anticipated resonant electric and magnetic response. Thus, it behaves similarly as the so-called omega particle, but with the unique advantage that frequency and strength of this magnetoelectric resonance can be independently tuned and modified with respect to the corresponding values of the electric resonance. This allows a metasurface of such meta-atoms to possess widely controlled reflection and transmission coefficients, e.g., the regimes of strongly asymmetric reflectance and perfect absorption become possible. Alternatively, an individual meta-atom of such kind can act as a directive nanoantenna with zero backscattered fields (Huygens' scatterer).
    Physical Review B 03/2015; 91(11). DOI:10.1103/PhysRevB.91.115119 · 3.66 Impact Factor
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    ABSTRACT: Aperture based scanning near field optical microscopes (SNOM) are important instruments to study light at the nanoscale and to understand the optical functionality of many photonic nanostructures. However, there remains a controversy about the influence of individual electromagnetic field components contributing to measured images. This information is of paramount importance to fully understand what has been measured. Here, we develop a self-contained theory for the image formation process in an aperture based SNOM. With this theory, we can concisely explain how the relative contributions of the electric and magnetic field components within detected images depend on the probe geometry, its material composition, and the illumination wavelength. Beyond a mere description of the image formation, we suggest a methodology to reconstruct all vectorial components of the electro-magnetic field from phase-resolved measurements. This paves the way for optical network analysers and opens unprecedented metrological opportunities.
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    ABSTRACT: The spontaneous emission rate of dipole emitters close to plasmonic dimers are theoretically studied within a nonlocal hydrodynamic model. A nonlocal model has to be used since quantum emitters in the immediate environment of a metallic nanoparticle probe its electronic structure. Compared to local calculations, the emission rate is significantly reduced. The influence is mostly pronounced if the emitter is located close to sharp edges. We suggest to use quantum emitters to test nonlocal effects in experimentally feasible configurations.
    Optics Letters 11/2014; 39(21). DOI:10.1364/OL.39.006118 · 3.18 Impact Factor
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    ABSTRACT: A double-patterning process for scalable, efficient, and deterministic nanoring array fabrication is presented. It enables gaps and features below a size of 20 nm. A writing time of 3 min/cm(2) makes this process extremely appealing for scientific and industrial applications. Numerical simulations are in agreement with experimentally measured optical spectra. Therefore, a platform and a design tool for upcoming next generation plasmonic devices like hybrid plasmonic quantum systems are delivered. (C) 2014 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.
    Applied Physics Letters 10/2014; 105(14). DOI:10.1063/1.4897497 · 3.52 Impact Factor
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    ABSTRACT: The standard hydrodynamic Drude model with hard-wall boundary conditions can give accurate quantitative predictions for the optical response of noble-metal nanoparticles. However, it is less accurate for other metallic nanosystems, where surface effects due to electron density spill-out in free space cannot be neglected. Here we address the fundamental question whether the description of surface effects in plasmonics necessarily requires a fully quantum-mechanical approach, such as time-dependent density-functional theory (TD-DFT), that goes beyond an effective Drude-type model. We present a more general formulation of the hydrodynamic model for the inhomogeneous electron gas, which additionally includes gradients of the electron density in the energy functional. In doing so, we arrive at a Self-Consistent Hydrodynamic Model (SC-HDM), where spill-out emerges naturally. We find a redshift for the optical response of Na nanowires, and a blueshift for Ag nanowires, which are both in quantitative agreement with experiments and more advanced quantum methods. The SC-HDM gives accurate results with modest computational effort, and can be applied to arbitrary nanoplasmonic systems of much larger sizes than accessible with TD-DFT methods. Moreover, while the latter typically neglect retardation effects due to time-varying magnetic fields, our SC-HDM takes retardation fully into account.
    Nature Communications 08/2014; 6. DOI:10.1038/ncomms8132 · 10.74 Impact Factor
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    ABSTRACT: The coupling of multiple plasmonic resonators that sustain bright or dark modes provide intriguing spectral signatures. However, probing the onset of coupling effects while engaging the resonators with an increasing proximity has not yet been studied experimentally in detail. Nevertheless, this is of utmost importance to bridge the phenomenological understanding with the peculiarities of real-world-samples. Here, we take advantage of the ability to control spatial dimensions of THz metasurfaces deep in the sub-wavelength domain to study different regimes that occur while coupling split-ring-resonators that sustain a bright and a dark mode with increasing strength. We identify the length scales at which the resonators are uncoupled and then enter the regimes of weak, moderate, and strong coupling. It is shown that a strong coupling takes place only at distances smaller than one hundredth of the resonance wavelength. Understanding the features that emerge from such hybridization is important to take advantage of fundamental effects in metamaterials such as classical analogs of electromagnetically induced transparency, lasing spaser, near-field manipulation, and sensing with dark mode resonances.
    Applied Physics Letters 08/2014; 105(8):081108-081108-5. DOI:10.1063/1.4893726 · 3.52 Impact Factor
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    ABSTRACT: The seeded growth of poly(ethylene imine) – gold nanoparticle clusters enables the formation of particle assemblies with tunable optical properties. Clusters with increasing particle sizes, filling factors and assemblies consisting of PEI–gold–silver core shell particles can be synthesized in this way. Profound structural characterization is carried out via TEM imaging and FIB milling which allows visualizing the cross-section of the clusters. Determination of the optical properties was performed via UV-Vis spectroscopy and spectral dark field microscopy of individual particles. Additionally, numerical calculations were carried out based on the Mie theory. The results are in good agreement with the experimental findings and reveal the contribution of different multipoles to the spectra which cannot be resolved by UV-Vis spectroscopy in solution. The isotropic nature and adjustable properties of these clusters could render them versatile building blocks for metamaterials.
    07/2014; 2(31). DOI:10.1039/C4TC01018C
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    ABSTRACT: The unique properties of plasmonic nanostructures have fuelled research based on the tremendous amount of potential applications. Their tailor-made assemblies in combination with the tunable size and morphology of the initial building blocks allow for the creation of materials with a desired optical response. In this respect, it is crucial to synthesize nanoparticles with a defined shape that are at the core of such developments. Moreover, the interaction of individual nanoparticles with an incident electromagnetic field cannot only be influenced by their structure, but in fact, also by their spatial arrangement to each other. To harvest such opportunities, a profound theoretical understanding of these interactions is required as well as concise strategies to create such ordered assemblies. A quantitative evaluation of their optical properties can only be conducted when discrete structures of high uniformity can be achieved. As a consequence, separation steps have to be applied in order to obtain materials of high purity and uniformity. This also allows for an easier structural characterization of the nanoparticles and their assembled superstructures. In this progress report, an overview about the current development in this field of research is provided.
    Particle and Particle Systems Characterization 07/2014; 31(7). DOI:10.1002/ppsc.201300309 · 0.54 Impact Factor

Publication Stats

5k Citations
799.55 Total Impact Points

Institutions

  • 2014–2015
    • Karlsruhe Institute of Technology
      • Institute for Theoretical Solid State Physics
      Carlsruhe, Baden-Württemberg, Germany
  • 2008–2014
    • Universitätsklinikum Jena
      Jena, Thuringia, Germany
    • The University of Tokyo
      Tōkyō, Japan
    • Waseda University
      • Department of Electrical Engineering and Bioscience
      Edo, Tōkyō, Japan
  • 2005–2014
    • Friedrich-Schiller-University Jena
      • • Institute of Organic Chemistry and Macromolecular Chemistry
      • • Department of Condensed Matter and Optics
      • • Department of Applied Physics
      Jena, Thuringia, Germany
  • 2013
    • The University of Sheffield
      • Department of Materials Science and Engineering
      Sheffield, England, United Kingdom
  • 2012
    • Oklahoma State University - Stillwater
      • School of Electrical and Computer Engineering
      Stillwater, OK, United States
  • 2011
    • California College San Diego
      San Diego, California, United States
  • 2005–2007
    • National Institute of Advanced Industrial Science and Technology
      • Electronics and Photonics Research Institute
      Tsukuba, Ibaraki, Japan
  • 2006
    • Imperial College London
      • Department of Physics
      London, ENG, United Kingdom
  • 2001–2006
    • Université de Neuchâtel
      • Laboratoire Temps-Fréquence (LTF)
      Neuenburg, Neuchâtel, Switzerland