Lossless Dielectrics Research Articles

Unlocking the out-of-plane dimension for photonic bound states in the continuum to achieve maximum optical chirality

The realization of lossless metasurfaces with true chirality crucially requires the fabrication of three-dimensional structures, constraining experimental feasibility and hampering practical implementations. Even though the three-dimensional assembly of metallic nanostructures has been demonstrated previously, the resulting plasmonic resonances suffer from high intrinsic and radiative losses. The concept of photonic bound states in the continuum (BICs) is instrumental for tailoring radiative losses in diverse geometries, especially when implemented using lossless dielectrics, but applications have so far been limited to planar structures. Here, we introduce a novel nanofabrication approach to unlock the height of individual resonators within all-dielectric metasurfaces as an accessible parameter for the efficient control of resonance features and nanophotonic functionalities. In particular, we realize out-of-plane symmetry breaking in quasi-BIC metasurfaces and leverage this design degree of freedom to demonstrate an optical all-dielectric quasi-BIC metasurface with maximum intrinsic chirality that responds selectively to light of a particular circular polarization depending on the structural handedness. Our experimental results not only open a new paradigm for all-dielectric BICs and chiral nanophotonics, but also promise advances in the realization of efficient generation of optical angular momentum, holographic metasurfaces, and parity-time symmetry-broken optical systems.

Concealed Structural Colors Uncovered by Light Scattering

AbstractUnusual structural colors are demonstrated in thin‐film coatings due to a combination of optical interference and light scattering effects. These vivid colors are concealed under ambient illumination but can be observed when light is reflected from the film surface. The origin of the effect is explored computationally and it is shown that, in thin‐films of lossless dielectrics coated on near‐perfect conductors, incident electromagnetic waves form standing waves. Electric field intensities at the thin film interfaces are maximized for wavelengths that fulfil destructive interference conditions, while nanoscale roughness can enhance scattering at these boundaries. The interplay of these two factors yields vivid, thickness‐dependent colors. This approach increases the repertoire of optical effects and perceived colors in thin coatings. When combined with traditional thin‐film interference colours, dichromatic images with distinctly changing colors can be generated, which can function as a covert, optical security feature.

Some remarks on enhanced total internal reflection

Abstract The regime of total internal reflection of plane wave on the boundary between a lossless dielectrics and the lossy, or gainy, medium is analysed. It is shown that in order to achieve enhanced total internal reflection (TIR), ie magnitude of the reflection factor larger than one, the existence of the second boundary, virtually maybe infinitely far away, is inevitable. Some contradictory results published in literature are discussed.

Fast estimation of propagation constants in crossed gratings

Fourier-based modal methods are among the most effective numerical tools for the accurate analysis of crossed gratings. However, leading to computationally expensive eigenvalue equations significantly restricts their applicability, particularly when large truncation orders are required. The resultant eigenvalues are the longitudinal propagation constants of the grating and play a key role in applying the boundary conditions, as well as in the convergence and stability analyses. This paper aims to propose simple techniques for the fast estimation of propagation constants in crossed gratings, predominantly with no need to solve an eigenvalue equation. In particular, we show that for regular optical gratings comprised of lossless dielectrics, nearly every propagation constant appears on the main diagonal of the modal matrix.

Thin Coatings of Cerium Oxide Nanoparticles with Anti-Reflective Properties

Cerium oxide, in addition to its catalytic properties, is also known for its optical properties such as ultraviolet (UV) radiation filtering and a relatively high refractive index ( n > 2 ), which makes it an excellent candidate for multifunctional coatings. Here, we focus on the optical properties of thin deposits (≲2 μ m) of densely packed C e O 2 nanoparticles, which we assemble using two evaporation-based techniques: convective self-assembly (CSA, a type of very slow blade-coating) to fabricate large-scale coatings of controllable thickness—from tens of nanometres to a few micrometers—and microfluidic pervaporation which permits us to add some micro-structure to the coatings. Spectroscopic ellipsometry yields the refractive index of the resulting nano-porous coatings, which behave as lossy dielectrics in the UV-visible regime and loss-less dielectrics in the visible to infra-red (IR) regime; in this regime, the fairly high refractive index (≈1.8) permits us to evidence thickness-tunable anti-reflection on highly refractive substrates, such as silicon, and concomitant enhanced transmissions which we checked in the mid-IR region.

Enhancement of directivity of dipole antenna by a complex conjugate cylinder

ABSTRACT The directional properties of a dipole antenna are studied by placing it in the vicinity of a complex conjugate cylinder. An exact formal solution for the fields is obtained via the differential equation approach. The far field approximation is obtained by asymptotic analysis. The variation of radiation patterns with the imaginary part of permittivity and/or permeability is observed and the results are compared with the lossless dielectric. It is observed that the directivity of the antenna is enhanced in the presence of the cylinder.

Absorption leads to narrower plasmonic resonances

While it is generally accepted that adding loss dampens and broadens plasmonic resonances, here we find adding losses can actually narrow the linewidth and increase the resonance peak. We show that the scattering cross section of a metal nanoparticle in a lossy dielectric medium is 1 order of magnitude larger than the same nanoparticle in a lossless dielectric. The full width at half-maximum of the scattering spectrum is a quarter of that of the non-absorbing medium. The peak narrowing and increased scattering have benefits for biomarkers where high scattering efficiency and narrow linewidths (for denser multiplexing) are desired. The enhanced scattering and absorption in the surrounding lossy medium are also beneficial for photovoltaic applications.

Designer Perfect Light Absorption Using Ultrathin Lossless Dielectrics on Absorptive Substrates

AbstractOptical absorbers comprised of an ultrathin lossy dielectric film on an opaque metallic substrate are an attractive alternative to lithographically intense metamaterial and nanoplasmonic optical absorbers as they allow for large‐scale, cost‐effective fabrication. However, requiring that the dielectric is lossy and the metallic substrate is highly reflective but not a perfect electric conductor (PEC) limits the wavelength range and materials that can be used to realize strong to perfect light absorption. In this work, we theoretically and experimentally investigate light absorption using ultrathin lossless dielectric films. By choosing proper lossless ultrathin dielectrics and substrates, iridescence free, perfect light absorption is possible over the visible, near infrared (NIR), and short‐wave infrared (SWIR) wavelength ranges with designer absorption properties. The proposed class of ultrathin film absorbers relaxes many constraints on the type of materials used to realize perfect light absorption. The flexibility of our design makes it relevant for many applications specifically in structural coloring, selective thermal emission, thermo‐photovoltaics, photo‐thermoelectric generation, and gas sensing.

Modal expansions in periodic photonic systems with material loss and dispersion

We study bandstructure properties of periodic optical systems composed of lossy and intrinsically dispersive materials. To this end, we develop an analytical framework based on adjoint modes of a lossy periodic electromagnetic system and show how the problem of linearly dependent eigenmodes in the presence of material dispersion can be overcome. We then formulate expressions for the bandstructure derivative $(\partial \omega) / (\partial \mathbf{k})$ (complex group velocity) and the local and total density of transverse optical states. Our exact expressions hold for 3D periodic arrays of materials with arbitrary dispersion properties and in general need to be evaluated numerically. They can be generalized to systems with two, one or no directions of periodicity provided the fields are localized along non-periodic directions. Possible applications are photonic crystals, metamaterials, metasurfaces composed of highly dispersive materials such as metals or lossless photonic crystals, metamaterials or metasurfaces strongly coupled to resonant perturbations such as quantum dots or excitons in 2D materials. For illustration purposes, we analytically evaluate our expressions for some simple systems consisting of lossless dielectrics with one sharp Lorentzian material resonance. Finally, we show how the perturbation of a periodic system with a sharp resonance modifies the bandstructure. By combining several Lorentz poles, this provides an avenue to perturbatively treat quite general material loss bands in photonic crystals.

Broadband terahertz absorption enabled by coating an ultrathin antireflection film on doped semiconductor.

We show that perfect absorption of terahertz wave can be achieved in a compact system where an ultrathin film of lossless dielectric is coated on a doped semiconductor substrate. Due to the nontrivial reflection phase shift at the interface between the two media, strong resonant behavior and the concomitant antireflection occur at wavelengths that are much larger than the thickness of the dielectric film, resulting in strong absorption of the incident wave in a wide frequency range. Using this mechanism, we design a broadband terahertz absorber by coating a Ge film on a highly doped GaAs substrate. We show that such a system not only has a perfect absorption peak, but also exhibits high absorptance (over 0.9) within a fractional bandwidth of over 20%. By varying the free carrier density in the GaAs substrate, the central frequency of the absorption band can be tuned from 1.79 to 2.69 THz. In addition, the absorption performance of the proposed system is shown to be insensitive to both incident angle and polarization. Our results offer a low-cost way for the design of absorption-based THz devices.

Existence and uniqueness theorems in electromagnetic diffraction on systems of lossless dielectrics and perfectly conducting screens

New vector problem of electromagnetic wave diffraction by a system of non-intersecting three-dimensional inhomogeneous dielectric bodies and infinitely thin screens is considered in a quasiclassical formulation as well as the classical problem of diffraction by a lossless inhomogeneous body. In both cases, the original boundary value problem for Maxwell’s equations is reduced to integro-differential equations in the regions occupied by the bodies (and on the screen surfaces). The integro-differential operator is treated as a pseudodifferential operator in Sobolev spaces and is shown to be zero-index Fredholm operator. Uniqueness of solutions is proved under the realistic hypothesis of discontinuity of the dielectric permittivity the boundary of a volume scatterer. This result allowed to establish invertibility of the integro-differential operator in sufficiently broad spaces. For the problem of diffraction on dielectrics and surface conductors, theorem on smoothness of a solution is proved under assumption of data smoothness. The latter implies equivalence between the differential and integral formulations of the scattering problem. The matrix integro-differential operator is proved to be a Fredholm invertible operator. Thus, the existence of a unique solution to both problems is established.

Characterisation of a ground penetrating radar antenna in lossless homogeneous and lossy heterogeneous environments

Directly measuring the radiation characteristics of Ground Penetrating Radar (GPR) antennas in environments typically encountered in GPR surveys, presents many practical difficulties. However it is very important to understand how energy is being transmitted and received by the antenna, especially for areas of research such as antenna design, signal processing, and inversion methodologies. To overcome the difficulties of experimental measurements, we used an advanced modelling toolset to simulate detailed three-dimensional Finite-Difference Time-Domain (FDTD) models of GPR antennas in realistic environments. A semi-empirical soil model was utilised, which relates the relative permittivity of the soil to the bulk density, sand particle density, sand fraction, clay fraction and volumetric fraction of water. The radiated energy from the antenna was studied in lossless homogeneous dielectrics as well as, for the first time, in lossy heterogeneous environments. Significant variations in the magnitude and pattern shape were observed between the lossless homogeneous and lossy heterogeneous environments. Also, despite clear differences in time domain responses from simulations that included only an infinitesimal dipole source model and those that used the full antenna model, there were strong similarities in the radiated energy distributions.

Total transmission of inhomogeneous electromagnetic waves at planar interfaces

We present the total transmission effect of inhomogeneous electromagnetic waves between dissipative media. The total transmission of a $p$-polarized plane wave at the interface between two lossless dielectrics happens at the Brewster angle. However, when a dielectric-conductor interface is considered, the effect cannot be achieved and there is only a minimum, different from zero, of the reflection coefficient. We prove that, by considering an inhomogeneous plane wave, the total transmission can be obtained both at the interface between two dissipative media and at the dielectric-conductor interface.

Self-focusing media using graded photonic crystals: Focusing, Fourier transforming and imaging, directive emission, and directional cloaking

Using numerical simulations, we investigate the realization of self-focusing media using two-dimensional graded photonic crystals and their applications for imaging and non-imaging purposes. The two-dimensional graded photonic crystals consist of spatially varying cylindrical holes drilled in a dielectric host. By controlling the gradient of the refractive index and the thickness of the self-focusing medium, it is possible to obtain either a focusing lens with Fourier transforming capabilities or an imaging lens, which produces inverted images. Non-imaging applications include a simple antenna for directive emission obtained from the focusing lens, whereas a directional cloak is obtained by modifying the imaging lens. Graded photonic crystal based devices work well up to the Bragg frequencies. They are compact, made from lossless dielectrics, and compatible with planar lithographic techniques, so they can find applications in a broad frequency range, even at the optical frequencies.

MODAL ANALYSIS OF MULTILAYER CONICAL DIELECTRIC WAVEGUIDES FOR AZIMUTHAL INVARIANT MODES

By using fleld expansion in terms of the Legendre polynomials and Schelkunofi functions, Maxwell's equations in the spherical coordinate system are cast into a matrix form which lends itself to the analysis of a multilayer conical waveguide. The matrix formulation is then used to obtain an eigen-value problem whose eigen-values are the allowable wave-numbers for propagation in the radial direction. To verify the proposed numerical approach, it is used to evaluate the resonance frequency of a partially fllled spherical resonator. The computed resonance frequencies are then compared with those obtained using commercial software based on the flnite- element method. The computation time is enormously reduced using the semi-analytical method of this work. Although results are shown for lossless isotropic dielectrics, the method is also applicable to conical waveguides made of lossy dielectrics even with negative permittivity.

Extraction of Dielectric Constant and Loss Tangent Using New Rapid Plane Solver and Analytical Debye Modeling for Printed Circuit Boards

Dielectric material properties of printed circuit boards (PCBs) are needed by designers working in various areas such as signal integrity, antennas, and embedded RF components. Among many methods to extract the material properties, the full sheet resonance technique is commonly used on PCBs due to its simplicity. The disadvantage of this method is that an analytical equation is used to extract the dielectric constant, which is accurate only for lossless dielectrics. In this paper, a new method is presented to solve the inaccuracy issue of the extraction of the dielectric constant by applying customized electromagnetic simulation based on a new rapid plane solver instead of analytical equations. For PCB dielectrics, the loss tangent tends to be flat over several decades. The dielectric constant then varies as a function of frequency based on the Kronig-Kramers relations. This paper introduces a new Debye type of a model for the complex permittivity of such dielectrics. The parameters of the Debye model can be obtained analytically without requiring any curve fitting. The resulting Debye model can then be easily integrated in SPICE or a finite-difference time-domain simulator.

Reconstruction of the shape and location of arbitrary homogeneous objects using sequentially incidences

AbstractIn this study, a new approach is introduced to reconstruct the location and shape of arbitrary homogeneous scatterers (conductors and lossy or lossless dielectrics) with arbitrary contrast. First, the induction current is reconstructed for radiation of each transmitter. Then, the amplitude of all reconstructed induction currents are summed with each other as a suitable function to estimate the location and shape of the scatterers. The performance of the introduced procedure is verified using some examples. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 1248–1251, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23334

Slow-light enhanced absorption for bio-chemical sensing applications: potential of low-contrast lossy materials

Slow-light enhanced absorption in liquid-infiltrated photonic crystals has recently been proposed as a route to compensate for the reduced optical path in typical lab-on-a-chip systems for bio-chemical sensing applications. A simple perturbative expression has been applied to ideal structures composed of lossless dielectrics. In this work we study the enhancement in structures composed of lossy dielectrics such as a polymer. For this particular sensing application we find that the material loss has an unexpected limited drawback and surprisingly, it may even add to increase the bandwidth for low-index contrast systems such as polymer devices.

Multiple-scattering approach to interatomic interactions and superradiance in inhomogeneous dielectrics

The dynamics of a collection of resonant atoms embedded inside an inhomogeneous nondispersive and lossless dielectric is described with a dipole Hamiltonian that is based on a canonical quantization theory. The dielectric is described macroscopically by a position-dependent dielectric function and the atoms as microscopic harmonic oscillators. We identify and discuss the role of several types of Green tensors that describe the spatio-temporal propagation of field operators. After integrating out the atomic degrees of freedom, a multiple-scattering formalism emerges in which an exact Lippmann-Schwinger equation for the electric field operator plays a central role. The equation describes atoms as point sources and point scatterers for light. First, single-atom properties are calculated such as position-dependent spontaneous-emission rates as well as differential cross sections for elastic scattering and for resonance fluorescence. Secondly, multiatom processes are studied. It is shown that the medium modifies both the resonant and the static parts of the dipole-dipole interactions. These interatomic interactions may cause the atoms to scatter and emit light cooperatively. Unlike in free space, differences in position-dependent emission rates and radiative line shifts influence cooperative decay in the dielectric. As a generic example, it is shown that near a partially reflecting plane there is a sharp transition from two-atom superradiance to single-atom emission as the atomic positions are varied.

Transmission line formulation for the full-wave analysis of two-dimensional dielectric photonic crystals

Using a matrix formulation it is shown how the full-wave analysis of multilayer photonic crystals can be reduced to the analysis of an equivalent multiconductor transmission line (TL). In the special case of single-layer two-dimensional photonic crystals the inductance and capacitance matrices of this equivalent TL provide the propagation characteristics of the crystal under investigation. Specifically, the determinants of these matrices contain the required information for determining propagation constants and consequently the field distribution of TM and TE propagation modes inside the crystal. Comparison of the numerical results presented in this work with those available in the literature verifies the soundness of the TL formulation introduced. Although the numerical results are presented for lossless isotropic dielectrics, the method is applicable to photonic crystals made of lossy and/or anisotropic dielectrics.