Jan Fiala

Physics and design possibilities of plasmonic-based fishnet metamaterial structures

Metamaterials (MM) represent a class of artificially-made structures, exhibiting, if properly designed, negative values of effective permittivity and permeability in specific spectral regions simultaneously. Recently, such structures have indeed attracted much attention due to their unique optical behavior not found in nature. These structures offer, e.g. a possibility of practical realization of perfect lenses, possessing a spatial resolution below the wavelength limit. In this contribution, we have focused on theoretical rigorous study on one specific class of MM structures, called fishnets, consisting of a combination of metal and dielectric layers with periodically arranged sub-wavelength holes. Our attempt was to reveal the physics and optimize the fishnet structure by tailoring the geometrical features in order to achieve optimized response in terms of negative refraction indices in particular spectral regions. For that purpose, our in-house 2D rigorous coupled wave analysis (RCWA) software was used for rigorous computing, the results of which were afterwards post-processed in order to retrieve the effective parameters. Using this tool, with the help of our approximate model, enabling more physical insight of wave-coupling processes, numerical simulations of plane-wave excitation of the multilayered nanofishnets have thus been performed. The reflection and transmission coefficients have been calculated and the effective material parameters have consequently been extracted from the obtained data, via the homogenization procedure.

Analysis of subwavelength-patterned plasmonic structures with approximate models

Using approximate analytical models, supported with rigorous comparison, the interaction of optical waves with various classes of plasmonic sub-wavelength-patterned structures, including nanoaperture arrays and layered structures, is studied.

Explanation of extraordinary transmission on 1-D and 2-D metallic gratings

A rigorous numerical analysis is performed in order to discuss and reveal mechanisms responsible for the extraordinary transmission through 1-D and 2-D sub-wavelength arrays of infinite slits and holes, respectively. The investigation is mainly focused on the influence of structural parameters on the resonant enhancements of optical response in the sub-wavelength region. Zero-order transmission maps are calculated in the array periodicity and slit/hole filling factor plane where a behaviour of the existing modes can be clearly resolved. The effect of metallic film thickness is also considered. Identification of mode resonances playing a significant role in the EOT process is verified by studying the corresponding field profiles. It is found that for TE polarization in the case of 1-D periodic metallic array, the TE fundamental mode plays a significant role in the enhanced energy transfer, however, for TM polarization, TEM modes are responsible for energy transfer through one-dimensional infinite slits. Together with waveguide / cavity resonances investigation, surface plasmon-polariton excitations are studied, and their contribution is discussed in the connection to the enhanced transmission. For the case of 2-D array, map of modes, based on the propagation constants and the coefficients of attenuation, is constructed in order to identify the group of those modes strongly contributing to the EOT. Importantly, metal is not considered as a perfect conductor.

Nonlocal resonances in nanoplasmonics: analysis and simulations (Conference Presentation)

Traditionally in plasmonics, the most common approach in analyzing the resonant behavior of light interaction with plasmonic nanostructures has been to apply the local-response approximation (LRA), using – depending on the structure complexity and relation between a characteristic dimension and the interacting wavelength – either (quasi)analytic or numerical approaches. Recently, however, as the characteristic dimensions of such structures have scaled down, it has turned out that more complex models based on the nonlocal response (NOR), or even quantum interaction) of free electrons are desirable, in order to explain novel effects (new resonances, blue spectral shifts). Newly emerging approaches describing the complexity of interactions at nanoscale, connected with emerging new physics, are shown and discussed in this contribution, in comparison with the standard LRA. This reasoning has lately started a rapid increase of interest in developing appropriate nonlocal models. This new field is by no means completed; there are, actually, several nonlocal models existing, based on different starting conditions, and predicting phenomena. These are, however, not always consistent and equivalent. In particular, in our studies, we have concentrated on understanding the interaction and developing a simple model capable of predicting the longitudinal nonlocal response based on the linearized hydrodynamic model, applied to simple structures, such as a spherical nanoparticle. Within our model, we have also shown and compared several alternatives within the approach, with respect to inclusion of the current “damping”, (1) standard model (with a possible increased damping constant), (2) with damping in acceleration, and (3) with liquid-viscosity damping. Also, the extension to generalized nonlocal response model is considered. In parallel, as an alternative (and more general) approach, based on our previous rich experience with Fourier modal methods, we have considered and developed the extension of the rigorous coupled wave analysis technique capable of treating nonlocal response numerically, for more general structures.

Study of resonant processes in plasmonic nanostructures for sensor applications (Conference Presentation)

This contribution is focused on the numerical studies of resonant processes in individual plasmonic nanostructures, with the attention particularly given to rectangular nanoparticles and concominant localized surface plasmon resonance processes. Relevant models for the description and anylysis of localized surface plasmon resonance are introduced, in particular: quasistatic approximation, Mie theory and in particular, a generalized (quasi)analytical approach for treating rectangularly shaped nanostructures. The parameters influencing resonant behavior of nanoparticles are analyzed with special interest in morphology and sensor applications. Results acquired with Lumerical FDTD Solutions software, using finite-difference time-domain simulation method, are shown and discussed. Simulations were mostly performed for selected nanostructures composed of finite rectangular nanowires with square cross-sections. Systematic analysis is made for single nanowires with varying length, parallel couple of nanowires with varying gap (cut -wires) and selected dolmen structures with varying gap between one nanowire transversely located with respect to parallel couple of nanowires (in both in-plane and -out-of-plane arrangements). The dependence of resonant peaks of cross-section spectral behavior (absorption, scattering, extinction) and their tunability via suitable structuring and morphology changes are primarily researched. These studies are then followed with an analysis of the effect of periodic arrangements. The results can be usable with respect to possible sensor applications.

Revealing plasmonic interactions in both isolated and arrayed dimers with analytical and numerical approaches

We explore the plasmonic interactions among multiple localized plasmon modes in isolated dimers as well as in their periodic arrays by means of quasi-analytical models. A model based on dipolar approximation was developed for that purpose and extended to periodic array of particles using coupled dipole approximation technique. In order to capture the modes coupling at shorter distances, with the use of the plasmon hybridization theory, we applied its formalism to dimer structures and studied the plasmonic resonances as a function of particle mutual distance. All models predictions are compared with rigorously computed data.

Physics and advanced simulations of photonic and plasmonic structures

In this contribution, we present the main results of our joint scientific theoretical project with the Czech Science Foundation (2010-2013) Physics and advanced simulations of photonic and plasmonic structures, arisen from the cooperation of three laboratories of Czech Technical University in Prague, Institute of Photonics and Electronics of the Academy of Sciences of the Czech Republic, and Brno University of Technology. First, we present the basics of our in-house methods and numerical tools for the analysis of such structures, developed independently within the scope of the project, together with their mutual comparison. Three linear frequency-domain modal three-dimensional (3D) numerical methods developed and adapted for modelling photonic / plasmonic guiding and resonant subwavelength (SW) structures, will be mentioned, namely, aperiodic rigorous coupled-wave analysis (aRCWA) method, bi-directional mode expansion propagation method (BEP) based on the Fourier series (BEX), as well as the finite difference (FD) / finite element (FE)-BEP technique, connecting the eigensolvers with advanced BEP-based scattering matrix algorithm. Subsequently, a special original method suitable for treating nonlinear structures with Kerr nonlinearities, based on the eigenmode expansion (EME), has been developed (NL-EME) and applied, too. These methods, together with several approximate methods, have formed a solid portfolio for subsequent analysis of various photonic and plasmonic subwavelength structures of interest. The project generated several novel and interesting results, introducing novel structure designs in the following areas: novel magnetooptic (MO) guiding structures with non-reciprocal properties, advanced plasmonic structures based on hybrid dielectric plasmonic slot waveguides, nonlinear plasmonic couplers, SW grating structured waveguides, 3D resonant high-Q nanostructures, gain-loss guiding structures as photonic analogues of quantum structures with parity-time (PT)-symmetry breaking. Selected results of modelling of these promising SW structure designs will be presented and discussed, together with a new result based on our recent investigation of the plasmon-soliton interaction.

Theory and Simulations of Enhanced Transmission through Plasmonic Sub-Wavelength Structures

The interaction of an electromagnetic wave with various classes of plasmonic sub-wavelength structures, including the apertures with/without supporting corrugations is theoretically studied and modeled, using several approximate and rigorous numerical approaches.