Disorder-Induced Phase Transitions in the Transmission of Dielectric Metasurfaces.

Light interaction with disordered materials is both complex and fascinating at the same time. Here, we reveal disorder-induced phase transitions in a dielectric Huygens' metasurface made from silicon nanocylinders that simultaneously support an electric and magnetic dipole resonance. Depending on the degree of positional disorder and the spectral detuning of the two resonances, the phase angle of the transmission coefficient exhibits a clear phase transition from normal to anomalous dispersion. Combined with the considerations of whether the resonances of spectrally detuned particles appear as separated or overlapping, we distinguish four different phase states. We study this phenomenon analytically by employing dipole particles and disclose the entire phase diagram, support our insights with full-wave simulations of actual structures, and corroborate the findings with experimental results. Unveiling this phenomenon is a milestone simultaneously in the growing fields of metamaterial-inspired silicon nanophotonics, photonics in disordered media, and the fundamental physics of phase transitions.

New Twists of 3D Chiral Metamaterials.

Rationally designed artificial materials, called metamaterials, allow for tailoring effective material properties beyond ("meta") the properties of their bulk ingredient materials. This statement is especially true for chiral metamaterials, as unlocking certain degrees of freedom necessarily requires broken centrosymmetry. While the field of chiral electromagnetic/optical metamaterials has become rather mature, the field of elastic/mechanical metamaterials is just emerging and wide open. This research news reviews recent theoretical and experimental progress concerning 3D chiral mechanical and optical metamaterials, with special emphasis on work performed at KIT.

Fast and reliable method to estimate losses of single-mode waveguides with an arbitrary 2D trajectory.

Photonic wire bonds, i.e., freeform waveguides written by 3D direct laser writing, emerge as a technology to connect different optical chips in fully integrated photonic devices. With the long-term vision of scaling up this technology to a large-scale fabrication process, the in situ optimization of the trajectory of photonic wire bonds is at stake. A prerequisite for the real-time optimization is the availability of a fast loss estimator for single-mode waveguides of arbitrary trajectory. Losses occur because of the bending of the waveguides and at transitions among sections of the waveguide with different curvatures. Here, we present an approach that resides on the fundamental mode approximation, i.e., the assumption that the photonic wire bonds predominantly carry their energy in a single mode. It allows us to predict in a quick and reliable way the pertinent losses from pre-computed modal properties of the waveguide, enabling fast design of optimum paths.

Broadband suppression of backscattering at optical frequencies using low permittivity dielectric spheres.

The exact suppression of backscattering from rotationally symmetric objects requires dual symmetric materials where ε r = µ r. This prevents their design at many frequency bands, including the optical one, because magnetic materials are not available. Electromagnetically small non-magnetic spheres of large permittivity offer an alternative. They can be tailored to exhibit balanced electric and magnetic dipole polarizabilities a 1 = b 1, which result in approximate zero backscattering. In this case, the effect is inherently narrowband. Here, we put forward a different alternative that allows broadband functionality: Wavelength-sized spheres made from low permittivity materials. The effect occurs in a parameter regime where approximate duality is met for all multipolar order a n ≈ b n , in a weakly wavelength dependence fashion. In addition, and despite of the low permittivity, the overall scattering response of these spheres is still significant. Scattering patterns are shown to be highly directive across an octave spanning band. The effect is analytically and numerically shown using the Mie coefficients.

Measuring the electromagnetic chirality of 2D arrays under normal illumination.

We present an electromagnetic chirality measure for 2D arrays of subwavelength periodicities under normal illumination. The calculation of the measure uses only the complex reflection and transmission coefficients from the array. The measure allows the ordering of arrays according to their electromagnetic chirality, which further allows a quantitative comparison of different design strategies. The measure is upper bounded, and the extreme properties of objects with high values of electromagnetic chirality make them useful in both near- and far-field applications. We analyze the consequences that different possible symmetries of the array have on its electromagnetic chirality. We use the measure to study four different arrays. The results indicate the suitability of helices for building arrays of high electromagnetic chirality, and the low effectiveness of a substrate for breaking the transverse mirror symmetry.

Optical alignment of oval graphene flakes.

Patterned graphene, as an atomically thin layer, supports localized surface plasmon polaritons at mid-infrared or far-infrared frequencies. This provides a pronounced optical force/torque in addition to large optical cross sections and will make it an ideal candidate for optical manipulation. Here, we study the optical force and torque exerted by a linearly polarized plane wave on circular and oval graphene flakes (single layers of graphene). While the torque vanishes for circular flakes, the finite torque allows rotating and orienting oval flakes relative to the electric field polarization. Depending on the wavelength, the alignment is either parallel or perpendicular to the electric field vector. In our contribution, we rely on a full-wave numerical simulation and also on an analytical model that treats the graphene flakes in a dipole approximation. The presented results reveal a good level of control on the spatial alignment of graphene flakes subjected to far-infrared illumination.

Phase-change material-based nanoantennas with tunable radiation patterns.

We suggest a novel switchable plasmonic dipole nanoantenna operating at mid-infrared frequencies that exploits phase-change materials. We show that the induced dipole moments of a nanoantenna, where a germanium antimony telluride (Ge3Sb2Te6 or GST for short) nanopatch acts as a spacer between two coupled metallic nanopatches, can be controlled in a disruptive sense. By switching GST between its crystalline and amorphous phases, the nanoantenna can exhibit either an electric or a balanced magneto-electric dipole-like radiation. While the former radiation pattern is omnidirectional, the latter is directive. Based on this property exciting switching devices can be perceived, such as a metasurface whose functionality can be switched between an absorber and a reflector. The switching between stable amorphous and crystalline phases occurs on timescales of nanoseconds and can be achieved by an electrical or optical pulse.

Efficient mode conversion in an optical nanoantenna mediated by quantum emitters.

Converting signals at low intensities between different electromagnetic modes is an asset for future information technologies. In general, slightly asymmetric optical nanoantennas enable the coupling between bright and dark modes that they sustain. However, the conversion efficiency might be very low. Here, we show that the additional incorporation of a quantum emitter allows us to tremendously enhance this efficiency. The enhanced local density of states cycles the quantum emitter between its upper and lower level at an extremely high rate, hence converting the energy very efficiently. The process is robust with respect to possible experimental tolerances, and adds a new ingredient to be exploited while studying and applying coupling phenomena in optical nanosystems.

Image formation properties and inverse imaging problem in aperture based scanning near field optical microscopy.

Aperture based scanning near field optical microscopes are important instruments to study light at the nanoscale and to understand the optical functionality of photonic nanostructures. In general, a detected image is affected by both the transverse electric and magnetic field components of light. The discrimination of the individual field components is challenging as these four field components are contained within two signals in the case of a polarization resolved measurement. Here, we develop a methodology to solve the inverse imaging problem and to retrieve the vectorial field components from polarization and phase resolved measurements. Our methodology relies on the discussion of the image formation process in aperture based scanning near field optical microscopes. On this basis, we are also able to explain how the relative contributions of the electric and magnetic field components within detected images depend on the chosen probe. We can therefore also describe the influence of geometrical and material parameters of individual probes within the image formation process. This allows probes to be designed that are primarily sensitive either to the electric or magnetic field components of light.

A generalized Kerker condition for highly directive nanoantennas.

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.

Bloch oscillations in plasmonic waveguide arrays.

The combination of modern nanofabrication techniques and advanced computational tools has opened unprecedented opportunities to mold the flow of light. In particular, discrete photonic structures can be designed such that the resulting light dynamics mimics quantum mechanical condensed matter phenomena. By mapping the time-dependent probability distribution of an electronic wave packet to the spatial light intensity distribution in the corresponding photonic structure, the quantum mechanical evolution can be visualized directly in a coherent, yet classical wave environment. On the basis of this approach, several groups have recently observed discrete diffraction, Bloch oscillations and Zener tunnelling in different dielectric structures. Here we report the experimental observation of discrete diffraction and Bloch oscillations of surface plasmon polaritons in evanescently coupled plasmonic waveguide arrays. The effective external potential is tailored by introducing an appropriate transverse index gradient during nanofabrication of the arrays. Our experimental results are in excellent agreement with numerical calculations.

The hybrid concept for realization of an ultra-thin plasmonic metamaterial antireflection coating and plasmonic rainbow.

We report on the design, simulation, fabrication, and characterization of a novel two layer anti-reflective coating (ARC) based on a plasmonic metamaterial and a dielectric. Promoted by the strong material dispersion of the plasmonic metamaterial, our novel concept (called hybrid ARC) combines two possible arrangements for layers in an anti-reflection coating into a single structure; albeit at two different wavelengths. This, however, causes a broadband reduction of reflection that is less sensitive against oblique incidence when compared to traditional antireflective coatings. Furthermore, we show that the current metamaterial on a metal reflector can be used for the visualization of different coloration such as plasmonic rainbow despite its sub-wavelength thickness.

Resonant metasurfaces at oblique incidence: interplay of order and disorder.

Understanding the impact of order and disorder is of fundamental importance to perceive and to appreciate the functionality of modern photonic metasurfaces. Metasurfaces with disordered and amorphous inner arrangements promise to mitigate problems that arise for their counterparts with strictly periodic lattices of elementary unit cells such as, e.g., spatial dispersion, and allows the use of fabrication techniques that are suitable for large scale and cheap fabrication of metasurfaces. In this study, we analytically, numerically and experimentally investigate metasurfaces with different lattice arrangements and uncover the influence of lattice disorder on their electromagnetic properties. The considered metasurfaces are composed of metal-dielectric-metal elements that sustain both electric and magnetic resonances. Emphasis is placed on understanding the effect of the transition of the lattice symmetry from a periodic to an amorphous state and on studying oblique illumination. For this scenario, we develop a powerful analytical model that yields, for the first time, an adequate description of the scattering properties of amorphous metasurfaces, paving the way for their integration into future applications.

Combining randomly textured surfaces and photonic crystals for the photon management in thin film microcrystalline silicon solar cells.

Photon management aims at optimizing the solar cell efficiency by, e.g., incorporating supporting optical nanostructures for absorption enhancement. Their geometrical design, however, is usually a compromise since requirements in different spectral domains need to be accommodated. This issue can be mitigated if multiple optical nanostructures are integrated. Here, we present a photon management scheme that combines the benefits of a randomly textured surface and an opaline photonic crystal. Moreover, upon considering the device with an increasing complexity, we show that a structure that respects the mutual fabrication constraints has the best performance, i.e., a device where the photonic crystal is not perfect but to some extent amorphous as enforced by the presence of the texture.

Tunable graphene antennas for selective enhancement of THz-emission.

In this paper, we will introduce THz graphene antennas that strongly enhance the emission rate of quantum systems at specific frequencies. The tunability of these antennas can be used to selectively enhance individual spectral features. We will show as an example that any weak transition in the spectrum of coronene can become the dominant contribution. This selective and tunable enhancement establishes a new class of graphene-based THz devices, which will find applications in sensors, novel light sources, spectroscopy, and quantum communication devices.

Scattering cancellation of the magnetic dipole field from macroscopic spheres.

Based on the scattering cancellation technique we suggest a cloak that allows to conceal macroscopic objects, i.e. objects with an optical size comparable to wavelengths in the visible and whose scattering response is dominated by a magnetic dipole contribution. The key idea in our approach is to use a shell of polaritonic spheres around the object to be cloaked. These spheres exhibit an artificial magnetism. In a systematic investigation, where we progressively increase the complexity of the considered structure, we devise the requirements imposed on the shell and outline how it can be implemented with natural available materials.

Optical properties of a fabricated self-assembled bottom-up bulk metamaterial.

We investigate the optical properties of a true three-dimensional metamaterial that was fabricated using a self-assembly bottom-up technology. The metamaterial consists of closely packed spherical clusters being formed by a large number of non-touching gold nanoparticles. After presenting experimental results, we apply a generalized Mie theory to analyze its spectral response revealing that it is dominated by a magnetic dipole contribution. By using an effective medium theory we show that the fabricated metamaterial exhibits a dispersive effective permeability, i.e. artificial magnetism. Although this metamaterial is not yet left-handed it might serve as a starting point for achieving bulk metamaterials by using bottom-up approaches.

Comparison and optimization of randomly textured surfaces in thin-film solar cells.

Using rigorous diffraction theory we investigate the scattering properties of various random textures currently used for photon management in thin-film solar cells. We relate the haze and the angularly resolved scattering function of these cells to the enhancement of light absorption. A simple criterion is derived that provides an explanation why certain textures operate more beneficially than others. Using this criterion we propose a generic surface profile that outperforms the available substrates. This work facilitates the understanding of the effect of randomly textured surfaces and provides guidelines towards their optimization.

Asymmetric transmission of linearly polarized light at optical metamaterials.

We experimentally demonstrate a three-dimensional chiral optical metamaterial that exhibits an asymmetric transmission for forwardly and backwardly propagating linearly polarized light. The observation of this novel effect requires a metamaterial composed of three-dimensional chiral meta-atoms without any rotational symmetry. Our analysis is supported by a systematic investigation of the transmission matrices for arbitrarily complex, generally lossy media that allows deriving a simple criterion for asymmetric transmission in an arbitrary polarization base. Contrary to physical intuition, in general the polarization eigenstates in such three-dimensional and low-symmetry metamaterials do not obey fixed relations and the associated transmission matrices cannot be symmetrized.

Understanding the electric and magnetic response of isolated metaatoms by means of a multipolar field decomposition.

We introduce a technique to decompose the scattered near field of two-dimensional arbitrary metaatoms into its multipole contributions. To this end we expand the scattered field upon plane wave illumination into cylindrical harmonics as known from Mie's theory. By relating these cylindrical harmonics to the field radiated by Cartesian multipoles, the contribution of the lowest order electric and magnetic multipoles can be identified. Revealing these multipoles is essential for the design of metamaterials because they largely determine the character of light propagation. In particular, having this information at hand it is straightforward to distinguish between effects that result either from the arrangement of the metaatoms or from their particular design.