Dielectric Array Research Articles

Equivalence principle algorithm using characteristic basis functions with application to finite periodic arrays including uniaxial dielectrics and conducting objects

In this paper, an equivalence principle algorithm combined with the characteristic basis function (CBF) method has been developed to efficiently analyze the finite periodic arrays composed of uniaxial dielectric materials and conducting objects. Three kinds of the CBFs for each equivalent surface and its surrounding object are simultaneously constructed using multiple plane wave illuminations and singular value decomposition technique, and thus memories related to the scattering and translation operators are significantly saved, while retaining good computational accuracy. Numerical results of a frequency selective conducting array, a uniaxial dielectric cube array, and a periodic array of the composite object including a uniaxial rod and a split ring resonator are given to illustrate good performance of proposed method.

Nearly perfect resonant absorption of TE-polarized light at metal surfaces coated with arrayed dielectric stripes

A quasi-transverse electric (TE) surface wave mode exists at a metal surface coated with an ultrathin high-index dielectric layer. As the coating is in dielectric stripe arrays, nearly perfect absorption of TE-polarized incidence light is observed in simulations, due to resonances of the quasi-surface waves at each segment of the dielectric-coated metal surfaces. In analysis, the Fabry-Perot-like nature of the resonances is clarified, and effects of symmetry on different behaviors of the odd- and even-order resonance modes are discussed. While the absorption peak is tunable, perfect absorption appears near cut-off wavelength of the surface mode.

On the thin film optics of a Cassegrain telescope

Optical interference thin films designed and fabricated for use in a Cassegrain telescope were deposited by ion-beam-assisted deposition. The optical parameters of optical interference filters array and the protection layers of silver reflector were simulated and optimized by using admittance loci analysis. The patterned multispectral assembly containing blue, green, red, near infrared, and panchromatic multilayer high/low alternated dielectric band-pass filter arrays in a single chip was fabricated by photolithography process. The corresponding properties of the films were measured by in situ optical monitor and spectrometry. It was found that the average transmittances were above 90% for the multispectral assembly, with a rejection transmittance of less than 1% in the spectral range of 350–1100nm. The average reflectance of the large area silver mirror (with three layers protective interference coating) is boosted above 99% in visible spectrum. In addition, the equivalent admittance of the three protection layer has been derived.

Tunable enhanced 0th-order transmission in a metal–dielectric hole array covered with a subwavelength liquid crystal layer

We demonstrate tunable, enhanced 0th-order transmission through a metal-dielectric nanohole array device with a subwavelength-thick liquid crystal (LC) layer. The LC filled the nanoholes and formed a subwavelength covering layer, which is then capped by a top cover layer. The wavelength where the transmittance dip associated with the LC occurs is determined by the anisotropic refractive-index component of the LC, which is normal to the surface of the hole array. A low-refractive-index cover layer suppresses unwanted higher-order diffraction, which results in an enhancement of the 0th-order transmission, which is closely related to laterally propagating surface plasmon polaritons. The proposed design is expected to help realize tunable plasmonic devices with high optical transmittance.

Fano resonance induced by mode coupling in all-dielectric nanorod array

We propose an efficient way to realize Fano-type resonances at optical frequencies based on a low-loss dielectric nanorod array. An ultrahigh Q factor (larger than 10000) is numerically demonstrated, which is attributed to the mode interference between the broadband Fabry–Perot (FP) resonance and the narrowband guided mode stemming from coupled quadrupoles. A wide gap region for field enhancement is formed between adjacent rods, making such a structure an ideal platform for related applications such as biological sensing and nonlinear devices.

Millimetre‐wave end‐fired antenna array for active 3D holographic imaging system

A compact Ka-band (26.5–40 GHz) dielectric rod end-fired antenna array for the three-dimensional (3D) active holographic imaging system is presented. Without horn shape feeding, the new antenna array is specially designed with characteristics such as small size, high gain, good radiation pattern, easy realisation, low insertion loss and low mutual coupling. In the imaging system, the spacing of sending and receiving antenna units is 1.5 wavelengths with mutual coupling <−30 dB and a gain of 12–16 dB. The results and analysis provide guidelines for the design of millimetre-wave end-fired antenna arrays.

Dense Dielectric Patch Array Antenna With Improved Radiation Characteristics Using EBG Ground Structure and Dielectric Superstrate for Future 5G Cellular Networks

In this paper, a new dense dielectric (DD) patch array antenna prototype operating at 28 GHz for future fifth generation (5G) cellular networks is presented. This array antenna is proposed and designed with a standard printed circuit board process to be suitable for integration with radio frequency/microwave circuitry. The proposed structure employs four circular-shaped DD patch radiator antenna elements fed by a 1-to-4 Wilkinson power divider. To improve the array radiation characteristics, a ground structure based on a compact uniplanar electromagnetic bandgap unit cell has been used. The DD patch shows better radiation and total efficiencies compared with the metallic patch radiator. For further gain improvement, a dielectric layer of a superstrate is applied above the array antenna. The measured impedance bandwidth of the proposed array antenna ranges from 27 to beyond 32 GHz for a reflection coefficient (S11) of less than -10 dB. The proposed design exhibits stable radiation patterns over the whole frequency band of interest, with a total realized gain more than 16 dBi. Due to the remarkable performance of the proposed array, it can be considered as a good candidate for 5G communication applications.

Broadband enhanced transmission in a film-array plasmonic structure through the plasmon coupling effects

Enhanced optical transmission in metal structures has received much attention due to the interesting physics and important applications in optoelectronic devices. Here, we propose and demonstrate the broadband enhanced optical transmission of a cooperative plasmonic structure consisting of double SiO2 films inserted with double parallel nanoparticle arrays composed of metal and dielectric spheres by the three-dimensional finite-difference time-domain (FDTD) method. Based on the bright mode and dark mode rule, the proposed structure shows a greatly enhanced broadband transmission through the hybridization of the plasmon resonant coupling effects of adjacent metal spheres, the surface plasmon waves at the interface between the metal array and the dielectric material and the optical cavity modes formed by the double dielectric films. The full width at half maximum (FWHM) of this broadband optical transmission with a highest transmission up to 85% is more than 400nm. The broadband optical transmission can be efficiently tailored by varying the lattice period of the arrays and the distance between the metal and dielectric arrays. This proposed structure with subwavelength size may provide potential applications in optoelectronic devices such as broadband transparent and conductive devices, slow light devices, and highly sensitive sensors.

Near-field distribution and propagation of scattering resonances in Vogel spiral arrays of dielectric nanopillars

In this work, we employ scanning near-field optical microscopy, full-vector finite difference time domain numerical simulations and fractional Fourier transformation to investigate the near-field and propagation behavior of the electromagnetic energy scattered at 1.56 μm by dielectric arrays of silicon nitride nanopillars with chiral α1-Vogel spiral geometry. In particular, we experimentally study the spatial evolution of scattered radiation and demonstrate near-field coupling between adjacent nanopillars along the parastichies arms. Moreover, by measuring the spatial distribution of the scattered radiation at different heights from the array plane, we demonstrate a characteristic rotation of the scattered field pattern consistent with net transfer of orbital angular momentum in the Fresnel zone, within a few micrometers from the plane of the array. Our experimental results agree with the simulations we performed and may be of interest to nanophotonics applications.

Design of Compact High‐Gain Dielectric Rod Antennas and Arrays in Lossy Medium

ABSTRACTIn this article, compact dielectric rod antennas with relatively high gain are presented, which are fit for use in lossy medium. A quasi‐monopole and a bifilar helix are used to simplify the feeding structure and guarantee high gain. Furthermore, antenna arrays with high gain and low side‐lobe levels are considered. Lower side‐lobe levels are achieved by lossy medium. Then, a C‐band dielectric rod antenna fed by a quasi‐monopole is designed and fabricated, and a Ku‐band antenna array, composed of four dielectric rod antennas fed by a bifilar helix is also displayed. Simulated results illustrate that the gains of the antenna are 9.53 dB and −0.29 dB in free space and in lossy medium, respectively, whereas the gain of the array is 12.2 dB with the side‐lobe level lower than 14 dB. The measured results of the antenna coincide with the simulated results. © 2013 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:2277–2282, 2013

A Hollow‐Core Optical Cavity Built in a Three‐Layer Silicon Photonic Crystal

Hollow‐core (HC) optical microcavities differ from conventional ones in that they can strongly confine light in air. HC cavities simultaneously possessing high air–energy ratios (αair), high quality factors (Q), and small mode volumes (Veff) may enable on‐chip low‐concentration molecular sensing, high optical power delivery or storage, and low‐threshold optical oscillation or lasing when filled with materials of desirable nonlinearities. However, designing and fabricating such cavities has been challenging, especially when using only a small number of layers. Here, the design and fabrication of such an HC cavity in a three‐layer silicon photonic crystal (PhC) is reported. It supports a transverse magnetic (TM) cavity mode with αair ≈ 67% and Q ≈ 1.7 × 105 in the 1550 nm wavelength range. The in‐plane confinement is provided by the 2D photonic bandgap (PBG) of a dielectric rod array, while a pair of capping slabs with air–hole PhC patterns confine light in air in the vertical direction. The cavity allows detection of low‐concentration substances that induce refractive index changes as small as around 8.8 × 10−6. The proposed cavity design and its fabrication provide a new approach to three‐dimensional photonic confinement using a structure consisting of only a few layers.

Broadband converging plano–concave lens

A plano-concave lens with source-tailored geometric profile and transformational gradient index is proposed for broadband illumination. Such a design, capable of focusing and collimating the electromagnetic fields, fulfils the functionality of a converging lens and can also achieve a steerable beam and multiple beams efficiently. Nonresonant synthesis with a perforated dielectric plate and dielectric rod arrays is demonstrated for the lensing realization, promising a wide operating frequency band in the practical implementation.

On Wave Propagation Mechanisms in a Dielectric Rod Array

Wave propagation mechanisms in a lossy dielectric rod array are examined using full-wave simulation and homogenization theory. Both approaches show that a slab mode and an air mode propagate in the structure with different phase velocities. The propagation constants, attenuation constants, and field distributions of these two dominant mechanisms are extracted from simulation data and compared to those found from homogenization theory.

Back Cover: Solar cell efficiency enhancement via light trapping in printable resonant dielectric nanosphere arrays (Phys. Status Solidi A 2/2013)

Resonant dielectric structures are a promising platform for addressing the key challenge of light trapping in thin-film solar cells. Grandidier et al. (pp. 255–260) experimentally and theoretically demonstrate efficiency enhancements in solar cells from dielectric nanosphere arrays. Two distinct amorphous silicon photovoltaic architectures were improved using this versatile light-trapping platform. In one structure, the colloidal monolayer couples light into the absorber in the near-field acting as a photonic crystal light-trapping element. In the other, it acts in the far-field as a graded index antireflection coating to further improve a cell which already included a state-of-the-art random light-trapping texture to achieve a conversion efficiency over 11%.

Effect of permittivity on energy band diagrams of dielectric metamaterial arrays

Band diagrams of 2D arrays of dielectric rods are simulated at TE wave incidence in dependence on dielectric permittivity of rod material. It is shown that permittivity defines the variety of propagation modes corresponding to extreme energies of the lowest transmission bands because of interplay between Bragg and Mie resonances. © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:134–137, 2013; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.27219

Solar cell efficiency enhancement via light trapping in printable resonant dielectric nanosphere arrays

AbstractResonant dielectric structures are a promising platform for addressing the key challenge of light trapping in thin‐film solar cells. We experimentally and theoretically demonstrate efficiency enhancements in solar cells from dielectric nanosphere arrays. Two distinct amorphous silicon photovoltaic architectures were improved using this versatile light‐trapping platform. In one structure, the colloidal monolayer couples light into the absorber in the near‐field acting as a photonic crystal light‐trapping element. In the other, it acts in the far‐field as a graded index antireflection coating to further improve a cell which already included a state‐of‐the‐art random light‐trapping texture to achieve a conversion efficiency over 11%. For the near‐field flat cell architecture, we directly fabricated the colloidal monolayer on the device through Langmuir–Blodgett deposition in a scalable process that does not degrade the active material. In addition, we present a novel transfer printing method, which utilizes chemical crosslinking of an optically thin adhesion layer to tether sphere arrays to the device surface. The minimally invasive processing conditions of this transfer method enable the application to a wide range of solar cells and other optoelectronic devices. magnified image False‐color SEM image of an amorphous silicon solar cell with resonant spheres on top.

Beam manipulating by graded photonic crystal slab made of dielectric elastomer actuators

The focusing lens and beam-steering device are implemented using two-dimensional graded photonic crystals with dielectric elastomer tube array embedded in air background. The gradient refractive index could be adjusted by varying the applied voltage on the dielectric elastomer tube so that the tube radius would change accordingly, which consequently alters the filling factor of the constituent material in photonic crystals, and the photonic dispersion band can be modulated. The finite element method is employed to simulate the electromagnetic wave propagation inside the designed device and analyze both the tunable focusing properties and beam-deflecting behavior. The implementation of the proposed device is validated within the lowest band. It is concluded that the proposed innovative device could easily accomplished the applications of focusing and steering electromagnetic fields with nonuniform applied voltage profiles.

Analytical light scattering and orbital angular momentum spectra of arbitrary Vogel spirals

In this paper, we present a general analytical model for light scattering by arbitrary Vogel spiral arrays of circular apertures illuminated at normal incidence. This model suffices to unveil the fundamental mathematical structure of their complex Fraunhofer diffraction patterns and enables the engineering of optical beams carrying multiple values of orbital angular momentum (OAM). By performing analytical Fourier-Hankel decomposition of spiral arrays and far field patterns, we rigorously demonstrate the ability to encode specific numerical sequences onto the OAM values of diffracted optical beams. In particular, we show that these OAM values are determined by the rational approximations (i.e., the convergents) of the continued fraction expansions of the irrational angles utilized to generate Vogel spirals. These findings open novel and exciting opportunities for the manipulation of complex OAM spectra using dielectric and plasmonic aperiodic spiral arrays for a number of emerging engineering applications in singular optics, secure communication, optical cryptography, and optical sensing.

Influence of metallic and dielectric nanowire arrays on the photoluminescence properties of P3HT thin films

The optical properties of organic semiconductor thin films deposited on nanostructured surfaces are investigated using time-resolved two-photon photoluminescence (PL) microscopy. The surfaces consist of parallel aligned metallic or dielectric nanowires forming well-defined arrays on glass substrates. Keeping the nanowire dimensions constant and varying only their spacing from 40 to 400 nm, we study the range of different types of nanowire–semiconductor interactions. For silver nanowires and spacings below 100 nm, the PL intensity and lifetime of P3HT and MDMO-PPV decrease rapidly due to the short-ranged metal-induced quenching that dominates the PL response with respect to a possible plasmonic enhancement of optical transition rates. In the case of P3HT however, we observe an additional longer-ranged reduction of non-radiative losses for both metallic and dielectric nanowires that is not observed for MDMO-PPV. Excitation polarization dependent measurements indicate that this reduction is due to self-assembly of the P3HT polymer chains along the nanowires. In conclusion, nanostructured surfaces, when fabricated across large areas, could be used to control film morphologies and to improve energy transport and collection efficiencies in P3HT-based solar cells.

Reflectivity enhanced two-dimensional dielectric particle array monolayer diffraction

Very high diffraction efficiencies (>80%) were observed from two-dimensional (2-D) photonic crystals made of monolayers of ∼ 490 nm diameter dielectric polystyrene spheres arranged in a 2-D hexagonal lattice on top of a liquid mercury surface. These almost close packed 2-D polystyrene particle arrays were prepared by a self-assembly spreading method that utilizes solvent evaporation from the mercury surface. Two-dimensional arrays transferred onto a dielectric glass substrate placed on top of metal mirrors show diffraction efficiencies of over 30%, which is 6- to 8-fold larger than those of the same 2-D monolayers in the absence of mirrors. A simple single particle scattering model with refraction explains the high diffraction efficiencies in terms of reflection of the high intensity forward diffraction.