All-dielectric reciprocal bianisotropic nanoparticles

The study of high-index dielectric nanoparticles 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 nanoparticles. To further advance the field, it is important to identify versatile dielectric nanoparticles with unconventional properties. Here, we show that breaking the symmetry of an all-dielectric nanoparticle leads to a geometrically tunable magneto-electric coupling, i.e. an omega-type bianisotropy. The suggested nanoparticle exhibits different backscatterings and, as an interesting consequence, different optical scattering forces for opposite illumination directions. An array of such nanoparticles provides different reflection phases when illuminated from opposite directions. With a proper geometrical tuning, this bianisotropic nanoparticle 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.Comment: 7 pages, 6 figure

Validity of effective material parameters for optical fishnet metamaterials

Although optical metamaterials that show artificial magnetism are mesoscopic systems, they are frequently described in terms of effective material parameters. But due to intrinsic nonlocal (or spatially dispersive) effects it may be anticipated that this approach is usually only a crude approximation and is physically meaningless. In order to study the limitations regarding the assignment of effective material parameters, we present a technique to retrieve the frequency-dependent elements of the effective permittivity and permeability tensors for arbitrary angles of incidence and apply the method exemplarily to the fishnet metamaterial. It turns out that for the fishnet metamaterial, genuine effective material parameters can only be introduced if quite stringent constraints are imposed on the wavelength/unit cell size ratio. Unfortunately they are only met far away from the resonances that induce a magnetic response required for many envisioned applications of such a fishnet metamaterial. Our work clearly indicates that the mesoscopic nature and the related spatial dispersion of contemporary optical metamaterials that show artificial magnetism prohibits the meaningful introduction of conventional effective material parameters

Circular Optical Nanoantennas: An Analytical Theory

An entirely analytical theory is provided for describing the resonance properties of optical nanoantennas made of a stack of homogeneous discs, i.e. circular patch nanoantennas. It consists in analytically calculating the phase accumulation of surface plasmon polaritons across the resonator and an additional contribution from the complex reflection coefficient at the antenna termination. This makes the theory self-contained with no need for fitting parameters. The very antenna resonances are then explained by a simple Fabry-Perot resonator model. Predictions are compared to rigorous simulations and show excellent agreement. Using this analytical model, circular antennas can be tuned by varying the composition of the stack

Metasurface-Based Realization of Photonic Time Crystals

Photonic time crystals are artificial materials whose electromagnetic properties are uniform in space but periodically vary in time. The synthesis of such materials and experimental observation of their physics remain very challenging due to the stringent requirement for uniform modulation of material properties in volumetric samples. In this work, we extend the concept of photonic time crystals to two-dimensional artificial structures -- metasurfaces. We demonstrate that time-varying metasurfaces not only preserve key physical properties of volumetric photonic time crystals despite their simpler topology but also host common momentum bandgaps shared by both surface and free-space electromagnetic waves. Based on a microwave metasurface design, we experimentally confirmed the exponential wave amplification inside a momentum bandgap as well as the possibility to probe bandgap physics by external (free-space) excitations. The proposed metasurface serves as a straightforward material platform for realizing emerging photonic space-time crystals and as a realistic system for the amplification of surface-wave signals in future wireless communications.Comment: 21 pages, 3 figure

A simple and versatile analytical approach for planar metamaterials

We present an analytical model which permits the calculation of effective material parameters for planar metamaterials consisting of arbitrary unit cells (metaatoms) formed by a set of straight wire sections of potentially different shape. The model takes advantage of resonant electric dipole oscillations in the wires and their mutual coupling. The pertinent form of the metaatom determines the actual coupling features. This procedure represents a kind of building block model for quite different metaatoms. Based on the parameters describing the individual dipole oscillations and their mutual coupling the entire effective metamaterial tensor can be determined. By knowing these parameters for a certain metaatom it can be systematically modified to create the desired features. Performing such modifications effective material properties as well as the far field intensities remain predictable. As an example the model is applied to reveal the occurrence of optical activity if the split ring resonator metaatom is modified to L- or S-shaped metaatoms.Comment: 5 figures, 1 tabl

Relating localized nanoparticle resonances to an associated antenna problem

We conceptually unify the description of resonances existing at metallic nanoparticles and optical nanowire antennas. To this end the nanoantenna is treated as a Fabry-Perot resonator with arbitrary semi-nanoparticles forming the terminations. We show that the frequencies of the quasi-static dipolar resonances of these nanoparticles coincide with the frequency where the phase of the complex reflection coefficient of the fundamental propagating plasmon polariton mode at the wire termination amounts to $\pi$. The lowest order Fabry-Perot resonance of the optical wire antenna occurs therefore even for a negligible wire length. This approach can be used either to easily calculate resonance frequencies for arbitrarily shaped nanoparticles or for tuning the resonance of nanoantennas by varying their termination.Comment: Submitted to Phys. Rev.

Gouy phase anomaly in photonic nanojets

We investigate in real space amplitude and phase distributions of light in photonic nanojets emerging from micrometer-sized dielectric spheres with a high-resolution interference microscope. Strong localization of light and a Gouy phase anomaly are witnessed. We show that the phase advance of photonic nanojets significantly deviates from a plane wave due to the sudden transition from a converging to a diverging wave front. Understanding such phase anomalies and verifying the presence of photonic nanojets promises to pave the way to prospective applications that may exploit the ability to localize light in spatial domains smaller than the usual resolution limit

Multiple self-healing Bloch surface wave beams generated by a two-dimensional fraxicon

Two-dimensional surface waves are a cornerstone for future integrated photonic circuits. They can also be beneficially exploited in sensing devices by offering dark-field illuminations of objects. One major problem in sensing schemes arises from the individual sensing objects: the interaction of surface waves with an object reduces the field amplitude, and the readout of other objects along the propagation path suffers from this reduced signal. Here we show in two experiments that nondiffracting and self-healing Bloch surface waves can be launched using a Fresnel axicon (i.e., fraxicon). First, we visualize the generation of an array of multiple focal spots by scanning near-field optical microscopy in the infrared. With a second device operating in the visible, we demonstrate the self-healing effect directly using a far-field readout method by placing metallic nanoantennas onto the multiple focal spots of the fraxicon. Our study extends the versatile illumination capabilities of surface wave systems