We report on theoretical analysis and experimental validation of the applicability of the effective medium approximation
to deeply subwavelength (period ⩽λ/30) all-dielectric multilayer structures. Following the theoretical prediction of the
anomalous breakdown of the effective medium approximation [H. H. Sheinfux et al., Phys. Rev. Lett. 113, 243901
(2014)] we thoroughly elaborate on regimes, when an actual multilayer stack exhibits significantly different properties
compared to its homogenized model. Our findings are fully confirmed in the first direct experimental demonstration of
the breakdown effect. Multilayer stacks are composed of alternating alumina and titania layers fabricated using atomic
layer deposition. For light incident on such multilayers at angles near the total internal reflection, we observe pronounced
differences in the reflectance spectra (up to 0.5) for structures with different layers ordering and different but still deeply
subwavelength thicknesses. Such big reflectance difference values resulted from the special geometrical configuration
with an additional resonator layer underneath the multilayers employed for the enhancement of the effect. Our results are
important for the development of new homogenization approaches for metamaterials, high-precision multilayer
ellipsometry methods and in a broad range of sensing applications.
In this presentation we will discuss the analytical and numerical approaches to modeling electromagnetic properties of geometrically regular subwavelength 2D arrays of random resonant plasmonic particles.
Amorphous metamaterials and metasurfaces attract interest of the scientific community due to promising technological implementations with cost-efficient methods of large-scale chemical nanoparticles synthesis as well as their self-organization. Random fluctuations of the particles size, shape, and/or composition are inevitable not only in the bottom-up synthesis, but also in conventional electron beam and photolithography fabrication. Despite the significant progress in large-scale fabrication, modeling and effective properties prediction of random/amorphous metamaterials and metasurfaces is still a challenge, which we address here. We present our results on analytical modelling of metasurfaces with regular periodic arrangements of resonant nanoparticles of random polarizability/size/material at normal plane-wave incidence. We show that randomness of the polarizability is related to increase in diffused scattering and we relate this phenomenon to a modification of the dipoles’ interaction constant. As a result, we obtain a simple analytical formula which describes diffuse scattering in such amorphous metasurfaces. Employing the supercell approach we numerically confirm the analytical results.
The proposed approach can be easily extended from electrical dipole arrays and normal wave incidence to more general cases of electric and magnetic resonant particles and oblique incidence.
We theoretically investigate general existence conditions for broadband bulk large-wavevector (high-k) propagating waves (such as volume plasmon polaritons in hyperbolic metamaterials) in arbitrary subwavelength periodic multilayers structures. Treating the elementary excitation in the unit cell of the structure as a generalized resonance pole of reflection coefficient and using Bloch’s theorem, we derive analytical expressions for the band of large-wavevector propagating solutions. We apply our formalism to determine the high-k band existence in two important cases: the well-known metaldielectric and recently introduced graphene-dielectric stacks. We confirm that short-range surface plasmons in thin metal layers can give rise to hyperbolic metamaterial properties and demonstrate that long-range surface plasmons cannot. We also show that graphene-dielectricmultilayers tend to support high-k waves and explore the range of parameters, where this is possible, confirming the prospects of using graphene for materials with hyperbolic dispersion. The suggested formalism is applicable to a large variety of structures, such as continuous or structured microwave, terahertz (THz) and optical metamaterials, optical waveguide arrays, 2D plasmonic and acoustic metamaterials.
Metamaterials are artificially designed media that show averaged properties not yet encountered in nature. Among such
properties, the possibility of obtaining optical magnetism and negative refraction are the ones mainly exploited but
epsilon-near-zero and sub-unitary refraction index are also parameters that can be obtained. Such behaviour enables
unprecedented applications. Within this work, we will present various aspects of metamaterials research field that we deal with at our department. From the modelling part, we will present tour approach for determining the field enhancement in slits that have
dimensions in the 104 times smaller than the incident wavelength. This huge difference makes it almost impossible for
commercial software to handle thus analytical approached have to be employed. From the fabrication point of view, various 2D and 3D high resolution patterning techniques are used. The talk will describe the ones available within our group. We will present the electron-beam lithography approach for fabricating nano-antennae to be used in coupling of plasmonics waveguides to/from free space. Also, a 3D technique based on twophoton-polymerisation and isotropic metal deposition to fabricate metal-covered 3D photonic crystals will be discussed. From the measuring side we will present two THz based setups for obtaining material’s characteristics, both in the low as well as in the high THz range, thus having the possibility of describing a material from 0.1 to 10THz.
In this paper we present the whole fabrication and characterization cycle for obtaining 3D metal-dielectric woodpile
structures. The optical properties of these structures have been measured using different setups showing the need of
considering e.g. border effects when planning their use in real-life devices. It was found that the behavior of the
structures close to the edge is very different from the one in the middle. The existence of special features in the former
spectra still needs to be completely understood and explained.
We propose the method of effective parameters retrieval based on the Bloch mode analysis of periodic metamaterials.
We perform the surface and volume averaging of the electromagnetic field of the dominating (fundamental) Bloch mode
to determine the Bloch and wave impedances, respectively. We show that our method is able to retrieve both material
and wave EPs for a wide range of materials, which can be lossy or lossless, dispersive, possess negative permittivity,
permeability and refractive index values. It is simple and unambiguous, free of the "branch" problem, which is an issue
for the reflection/transmission based method and has no limitations on a metamaterial slab thickness. The method does
not require averaging different fields' components at various surfaces or contours. The retrieval of both wave and
material EPs is performed within a single computational cycle, after exporting fields on the unit cells facets or in its
volumes directly from Maxwell's equations solver.
The concept of metamaterials (MTMs) is acknowledged for providing new horizons for controlling electromagnetic
radiations thus their use in frequency ranges otherwise difficult to manage (e.g. THz radiation) broadens our possibility
to better understand our world as well as opens the path for new applications.
THz radiation can be employed for various purposes, among them the study of vibrations in biological molecules,
motion of electrons in semiconductors and propagation of acoustic shock waves in crystals.
We propose here a new THz fractal MTM design that shows very high transmission in the desired frequency range as
well as a clear differentiation between one polarisation and another. Based on theoretical predictions we fabricated and
measured a fractal based THz metamaterial that shows more than 60% field transmission at around 1THz for TE
polarized light while the TM waves have almost 80% field transmission peak at 0.6THz. One of the main characteristics
of this design is its tunability by design: by simply changing the length of the fractal elements one can choose the
operating frequency window. The modelling, fabrication and characterisation results will be presented in this paper.
Due to the long wavelength of THz radiation, the resolution requirements for fabrication of metamaterials are within the
optical lithography range. However, the high aspect ratio of such structures as well as the substrate thickness pose
challenges in the fabrication process.
The measurements were made using terahertz time domain spectroscopy (THz-TDS) that allows us to obtain both the
amplitude and phase of the transmission function. The experimental results are in very good agreement with theoretical
calculations based on finite-difference time-domain simulations.
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