[Show abstract][Hide abstract] ABSTRACT: We demonstrate a route to design structures exhibiting optical activity in general scattering directions. This means that when the structure is illuminated from an arbitrary direction, the polarization of the field measured in another arbitrary direction is a rotated version of the incident polarization. The rotation angle does not depend on the incident polarization.
Piers 2015 Prague, and Metamaterials 2015 Oxford; 09/2015
[Show abstract][Hide abstract] ABSTRACT: The study of high-index dielectric nanoparticles and nanoantennas currently
attracts a lot of attention. They do not suffer from absorption but promise to
provide control on the properties of light comparable to plasmonic
nanoantennas. To further advance the field, it is important to identify
versatile dielectric nanoantennas with unconventional properties. Here, we show
that breaking the symmetry of an all-dielectric nanoantenna leads to a
geometrically tunable magneto-electric coupling, i.e. a strong bianisotropy.
The suggested nanoantenna exhibits different backscatterings and, as an
interesting consequence, different optical scattering forces for opposite
illumination directions. An array of such nanoantennas provides different
reflection phases when illuminated from opposite directions. With a proper
geometrical tuning, this bianisotropic nanoantenna is capable of providing a
$2\pi$ phase change in the reflection spectrum while possessing a rather large
and constant amplitude. This allows creating reflectarrays with near-perfect
transmission out of the resonance band due to the absence of an usually
employed metallic screen.
[Show abstract][Hide abstract] ABSTRACT: We introduce a definition of the electromagnetic chirality of an object and show that it has an upper bound. The upper bound is attained if and only if the object is transparent for fields of one handedness (helicity). Additionally, electromagnetic duality symmetry, i.e. helicity preservation upon scattering, turns out to be a necessary condition for reciprocal scatterers to attain the upper bound. We use these results to provide requirements for the design of such extremal scatterers. The requirements can be formulated as constraints on the polarizability tensors for dipolar scatterers or as material constitutive relations. We also outline two applications for objects of maximum electromagnetic chirality: A twofold resonantly enhanced and background free circular dichroism measurement setup, and angle independent helicity filtering glasses.
[Show abstract][Hide abstract] ABSTRACT: We report on the investigation of an advanced circular plasmonic nanoantenna
under ultrafast excitation using nonlinear photoemission electron microscopy
(PEEM) under near-normal incidence. The circular nanoantenna is enhanced in its
performance by a supporting grating and milled out from a gold film. The
considered antenna shows a sophisticated physical resonance behavior that is
ideal to demonstrate the possibilities of PEEM for the experimental
investigations of plasmonic effects on the nanoscale. Field profiles of the
antenna resonance for both possible linear polarizations of the incident field
are measured with high spatial resolution. In addition, outward propagating
Hankel plasmons, which are also excited by the structure, are measured and
analyzed. We compare our findings to measurements of an isolated plasmonic
nanodisc resonator and scanning near-field optical microscopy (SNOM)
measurements of both structures. All results are in very good agreement with
numerical simulations as well as analytial models that are also discussed in
[Show abstract][Hide abstract] ABSTRACT: Bonding individual metallic nanoparticles at small separation distances to let them form dimers and making them available in large quantities is a key requirement for various applications that wish to exploit the tremendous enhancement of electromagnetic fields in plasmonic junctions. Although progress has been witnessed in the past concerning the fabrication of dimers mediated by rigid molecular linkers, the exact bonding mechanism remains unclear. Here, we describe the fabrication of a rigid linker molecule and demonstrate its feasibility to achieve dimers made from closely spaced metallic nanoparticles in large quantities. Although the topography of the dimers proofs the success of the fabrication method, we use what we call a hypermethod characterization approach to study the optical properties of dimers from various perspectives. Measuring the surface enhanced Raman scattering signal of the linker molecule enables tracing directly the optical environment it perceives. By reaching a strong field enhancement in the gap of the dimers, we are able to investigate optical and geometrical properties of the linker. Moreover, upon isolation of the dimers, we use single particle extinction spectroscopy to study the optical response of a fabricated dimer directly. Full wave numerical simulations corroborate the experimental results and provide insights into quantities, which cannot be accessed directly in experiments. The ability to fabricate and to characterize rigidly linked nanoparticles will pave the way towards various plasmonic applications such as sensors, photocatalysis, and plexcitonics.
The Journal of Physical Chemistry C 07/2015; DOI:10.1021/acs.jpcc.5b04346 · 4.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Plasmonic nanoantennas can feature a sophisticated spectral response that may be the springboard for a plethora of applications. Particularly, spectrally sharp Fano resonances have been at the focus of interest because of their promising applications in sensing. Usually, the observation of Fano resonances requires nanostructures that exhibit multiple plasmonic resonances such as higher-order multipole moments. We show that similar spectral features can be observed with nanoantennas sustaining solely electric-dipolar resonances. The considered nanoantennas consist of multiple concentric gold nanorings separated by thin dielectric spacers. These nanoantennas host multiple resonances with disparate line widths in the visible and near-infrared. We theoretically and experimentally show that the interference of these resonances causes Fano features and scattering dark states. The electric-dipolar character permits the use of a simplified dense-array theory to predict the response of arrays of such nanoantennas from the electric polarizability of the individual constituents. This paves the way for a simplified design of plasmonic metasurfaces.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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
[Show abstract][Hide abstract] 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 · 2.18 Impact Factor
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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
[Show abstract][Hide abstract] 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 · 3.74 Impact Factor
[Show abstract][Hide abstract] 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
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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 · 3.74 Impact Factor