DEVELOPING ELECTROMAGNETIC AND PHOTONIC DEVICES BY USING ARTIFICIAL DIELECTRIC MATERIALS

Transformation-Optics (TO) is a new theoretical tool that allows for designing advanced electromagnetic and photonic devices. TO theory often prescribes material parameters for transformed media that cannot be found in nature. Metamaterials (MMs) were initially used for realization of TO-based devices. However, conventional MMs possess noticeable losses caused by their metallic parts that prevents their utilization in optical range. Alternatively, photonic crystals (PhCs) formed from arrays of low-loss all-dielectric elements can be good substitutes for building TO-prescribed devices. Metasurfaces (MSs) comprised from 2D arrays of dielectric resonators (DRs) have been found as other promising candidates for realizing flat and efficient devices. In our work, we explored incorporation of all-dielectric artificial media in invisibility cloaks, representing the most exciting TO application, wave collimators, and MSs. We studied associated electromagnetic and photonic phenomena and solved engineering problems met at the development of device prototypes. We designed and used anisotropic PhCs composed of rectangular lattice dielectric rod arrays to build up a cylindrical cloak medium realizing prescriptions of TO (Chapter 2). We also formed another cylindrical invisibility cloak by utilizing the self-collimation phenomenon in PhCs without considering TO prescriptions for turning the wave in the cloak medium (Chapter 3). Furthermore, we designed a wave collimator by employing high-anisotropic rectangular lattice dielectric rod arrays with unidirectional near-zero refractive indices (Chapter 4). Then, we studied the resonance and scattering responses of MSs composed of dielectric disks, while altering the periodicity of MSs. Our results demonstrated that periodicity of arrays has significant influence on defining the responses of MSs. (Chapter 5). Increasing lattice constants of dielectric MSs provided us with an opportunity to investigate interactions between lattice resonances (LRs) and dipolar electric and magnetic resonances that affected characteristics of MSs (Chapter 6). We analyzed the formation of Fano responses and wave interference processes in dense MSs to reveal the nature of electromagnetically induced transparency (EIT) that was detected at the frequency of electric dipolar resonance. (Chapter 7)

Spatial dispersion of index components required for building invisibility cloak medium from photonic crystals

The opportunities to use dielectric photonic crystals (PhCs) as the media of cylindrical invisibility cloaks, designed using transformation optics (TO) concepts, are investigated. It is shown that TO-based prescriptions for radial index dispersion, responsible for turning waves around hidden objects, can be dropped if the PhC media support self-collimation of waves in bent crystals. Otherwise, to provide prescribed anisotropy of index dispersion, it is possible to employ PhCs with rectangular lattices. It is found, however, that at acceptable cloak thicknesses, modifications of crystal parameters do not allow for achieving the prescribed level of index anisotropy. This problem is solved by finding the reduced spatial dispersion law for the radial index component, which is characterized by decreased against TO-prescriptions values near the target and increased values in outer layers of the cloak. The cloak utilizing reduced prescriptions for indices is shown to perform almost as efficiently as a TO-based cloak, in terms of both wave front restoration behind the target and reducing the total scattering cross-width of the target

Employing GRIN PC-inspired approach for building invisibility cloak media from photonic crystals

Employing photonic crystals in transformation media requires realizing prescribed anisotropic spatial dispersions of refractive index components. We show that in invisibility cloaks, anisotropy can be provided by using crystals with rectangular lattices, while inspired by GRIN PCs approach can be employed to decrease scattering cross-section

Using self-collimated wave-guiding in invisibility cloaks

We employ the known in photonic crystals phenomenon of self-collimation (SC) for modifying the performance of invisibility cloaks composed of dielectric rod arrays. Incorporation in the cloak design two circular sections, supporting SC, allows for refusing from Transformation Optics (TO) based prescriptions for the cloak medium, which request too challenging material parameters. In addition to SC sections, unidirectional cloak contains two TO-based transition sections with easily realizable parameters of rod arrays. These sections control wave paths between cloak input and output and SC sections. At plane wave incidence, the designed cloak was found to provide as restoration of initial flat wavefront behind the hidden object, so significant reduction of wave scattering by the object

Analog of electromagnetically induced transparency in metasurfaces composed of identical dielectric disks

An analog of electromagnetically induced transparency was revealed in metasurfaces (MSs), composed from identical dielectric resonators of simple cylindrical shapes. It was detected in numerical experiments at optical and microwave frequencies and confirmed by real experiments in the microwave range. The main specific of the observed phenomenon was its appearance at frequencies of electric dipolar resonances (EDRs) in MS elements, when total reflection of incident waves instead of transmission was expected. Investigations of electric field distributions in MSs allowed for detecting several Fano resonances caused by interaction between background radiation defined by incident waves, and radiation produced by oscillations of resonance fields in dielectric particles. The characteristics for EDR changes in phases of resonance oscillations by πradians were found controlling the transitions from constructive to destructive interference between participating wave processes. The onset of destructive interference was marked by sharp jumps by πradians in the spectra of signal phases. Performed analysis revealed that zero signals at Fano resonances, observed in the gaps between resonators, arranged along the electric field direction, could serve as indicators of realizing the conditions necessary for the transparency of MSs. These conditions included the elimination of the presence of background radiation and thus of interaction between trespassing waves and MSs

Near-Zero Refractive Indices and Collimation Effects in Anisotropic Photonic Crystals with Rectangular Lattices

The objective of this work is to design highly anisotropic 2D photonic crystals (PhCs) with near-zero refractive indices (NZRIs). We demonstrate that PhCs, composed of dielectric rods and organized in rectangular lattices can support unidirectional wave propagation with NZRIs along short sides of unit cells. Divergent waves that are incident are collimated as a result of complete suppression of wave propagation along the orthogonal direction. We used MPB and COMSOL software packages for simulating the dispersion diagrams, S21 transmission spectra, and wave patterns. The results show that the observed collimation effect is correlated with flat equi-frequency contours (EFCs) of PhCs, while operating frequency corresponds to the lower edge of the 2nd transmission band of the crystal. We have recorded wave patterns beyond PhC fragment designed for operation in the microwave range as experimental confirmation of the computational results. Fabricated samples were irradiated by divergent TM polarized electromagnetic waves and were made up of ceramic dielectric rods with mm-size diameters. Obtained results extend perspectives of low-loss artificial anisotropic media with NZRI properties for microwave and photonic applications. Collimators can have many potential applications in electromagnetics and photonics, such as enhancing the gain and directionality of antennas’ radiation patterns and connecting waveguides with different widths

Collimation effects controlled by near-zero refractive indices in highly anisotropic dielectric photonic crystals: Simulation and experiment

We demonstrate that 2D photonic crystals (PhCs), composed of dielectric rods and organized in rectangular lattices with precisely determined lattice constants, can support, at both TM and TE polarization of incident waves, unidirectional wave propagation with nearzero refractive indices (NZRIs) along short sides of unit cells. Wave propagation along the orthogonal direction, i.e., along long sides of unit cells, is fully suppressed that results in collimation of incident divergent waves. We show that the observed promising collimation effect is correlated with flat equi-frequency contours of PhCs, while operating frequency corresponds to the lower edge of the 2nd transmission band of the crystal media. For experimental verification of the simulation results, obtained by using full-wave electromagnetic solvers, we have recorded wave patterns beyond PhC fragment designed for operating frequency in the microwave range. Fabricated samples were composed of ceramic dielectric rods with mm-size diameters. The samples were illuminated by divergent TM polarized electromagnetic waves. Obtained results extend perspectives of low-loss artificial anisotropic media with NZRI properties for microwave and photonic applications

Implementing photonic crystals, instead of metamaterials, in the media of transformation optics-based devices

Extending Transformation Optics in optical range is challenging because of losses in metamaterials. We propose, instead, to use dielectric photonic crystals capable of supporting superluminal wave propagation and realizing spatial dispersion of refractive index values. Implementing these materials in cylindrical invisibility cloaks is demonstrated

Specifics of scattering and radiation from sparse and dense dielectric meta-surfaces

Metasurfaces composed of nanosized silicon particles are considered prospective low-loss media for future planar devices with subwavelength thickness, capable of realizing many optical functionalities, including beam steering, focusing, and holography. Previous studies revealed an opportunity to provide directional scattering from silicon metasurfaces at Kerker’s conditions and projected obtaining significantly enhanced intensity of scattering at overlapping of dipolar magnetic and electric resonances in particles at their specific geometries. Although silicon metasurfaces are usually represented by dense arrays, interactions between resonators are often neglected in their analysis, which typically uses metamaterial concepts, assuming that responses of arrays can be represented by responses of single “meta-atoms.” In this work, we investigate cooperative resonance phenomena in dielectric metasurfaces, including interactions between electric and magnetic resonances within single particles and inter-resonator interactions in arrays. First, we analyze the transformation of the responses of single resonators, when their shape changes from a sphere to a cylinder, and then to a disk, and, in particular, describe the specifics of the formation of electric and magnetic dipole modes at a coincidence of resonances. Then, phenomena in arrays are considered, including the effects of arraying on resonator responses and the effects of packing density on metasurface responses. We demonstrate that dense packing causes strong changes of resonances, transverse coupling, and integration of resonance fields, affecting scattering and radiation from metasurfaces. The obtained results are important for understanding the complexity of responses of dielectric metasurfaces and provide guidance for their design and for scattering and radiation control

Extra high-Q resonances and extraordinary transparency in finite fragments of dielectric metasurfaces: Prospects for 5G applications

We investigate the effects of fragmenting metasurfaces (MSs), composed of dielectric disks, on their electromagnetic responses and show that the presence of four abrupt boundaries between finite size structures and free space leads to the formation of new resonance modes. In addition to the characteristic for infinite metasurfaces modes with identical dipolar resonances formed in all unit cells, fragmented metasurfaces can exhibit out-of-phase electric and magnetic responses in neighboring “meta-atoms.” While in-phase responses correspond to field patterns representative for even resonance modes, out-of-phase responses produce a variety of patterns typical for odd resonance modes. These modes are formed as the result of partial reflections of surface waves from boundaries between MS fragments and free space, and their respective responses demonstrate extremely high intensities and Q-factors. Enabled by new responses, a significantly localized wave/matter interaction can be used for enhancing the performance of sensors and absorbers of 5G systems. In addition, we report the detection of extraordinary narrow-band transmission at electric and magnetic dipolar resonances in fragmented MSs that can be used to locally enhance mm-wave signals for 5G communications. As a proof of concept, transmission through a 5 × 5 MS fragment has been experimentally confirmed in the X-band of microwave spectrum