Plasmonic-molecular systems are gaining interest as a platform to studying strong light-matter coupling effects, owing to small mode volumes of plasmonic modes and relatively small systems sizes (hundreds of atoms) enabling tackling them by the ab initio methods. In our work we use real-time TD-DFT method to study what modifications of the systems occur under strong coupling conditions, including plasmon redshift and molecular absorption quenching. We observe mixed transitions, i.e. ocurring when the initial and excited states are located in different subparts of the system, as well as the drastic modifications of the molecular oscillator when modeling the systems with the coupled oscillators model. We show how atomistic scale effects influence the coupling and how to tailor the polaritonic states to obtain desired features in the system.
Hyperbolic metamaterials (HMMs) gained a lot of interest amongst researchers in recent years due to their novel properties stemming from hyperbolic dispersion, such as anomalous scattering, subwavelength confinement of light or enhancing far-field radiation. In our work, we investigate optical properties of nanostructured HMMs in a form of spherical nanoresonators composed of stacked alternating metal-dielectric layers, which is one way to realize hyperbolic dispersion. Using T-matrix analysis, combined with a quasistatic approach, we explore their unique spectral response to unravel fundamental electromagnetic properties of hyperbolic nanoresonators. The modal structure of hyperbolic nanospheres (HNSs) is richer than those of conventional nanoantennas and can be tuned either with material properties or incident light conditions. We show that, depending on the direction and polarization of incident light, such nanoresonators can exhibit a plasmonic-like response or one with an atypical modal order, with electric and magnetic modes of higher orders appearing at energies below those of lower order modes. We underline how constructive or destructive cross coupling between various electric and magnetic multipoles, determined by the hyperbolic dispersion, influences the overall optical response and enables phenomena absent in the isotropic medium. Such an example is a negative contribution of a given mode to the extinction cross-section, stemming from destructive cross coupling and is an indicator of energy transfer between modes. We show how mode cross coupling (and thus HNS optical properties) change with material properties – varying the metal fill-factor allows for significant tunability from a uniaxial dielectric through a type I or II hyperbolic material to a uniaxial metal. By applying the quasistatic approach we also analyse the origin of the dipolar modes and obtain material-dependent resonance conditions for both electric and magnetic mode. Furthermore, we conclude that magnetic dipolar resonance presence is determined by the hyperbolic disperison, i.e. opposite signs of the ordinary and extraordinary permittivities.
High index dielectric (HID) metasurfaces are gaining interest in nanophotonics due to their highly tunable optical response which stems from supporting both magnetic and electric resonances, and typically low material losses. These characteristics make them viable candidates for a variety of applications, which if based of silicon, could be in principle compatible with CMOS technology. Being geometrical in nature, the sensitivity of resonances of an isolated dielectric nanoresonator to the refractive index of a homogeneous environment is low. Hence, effort is needed to utilize them as sensors of their surrounding. Herein we discuss various physical aspects that govern the spectral response and sensitivity of HID particles and their metasurfaces to changes in their environment. The specific effects under study are the aspect ratio of a single HID antenna, interaction with a substrate, and the effect of interparticle coupling in amorphous metasurfaces. To provide optimal solutions for HID sensors, it is crucial to understand the interplay of the above effects and strive to attain a regime in which they work in tandem to maximize the sensitivity. We utilize the T-matrix method to carry out calculations with explicit HID nanoantennas as well as describe a computationally efficient and accurate T-matrix-based effective model of amorphous metasurfaces to describe their optical response, accounting for multiple scattering effects and the presence of the substrate. Using this approach, we elucidate how the investigated effects shape the sensitivity of a HID sensor and how to combine the various geometrical aspects to design a sensitive device.
The angular distribution of far-field light intensity scattered by a two-dimensional array of nanoparticles depends on the optical properties of individual nanoparticles and their spatial distribution. The latter factor affects the scattered far-field in a two-fold manner. One such effect is the modification of the multipole moments of each individual scatter as a consequence of multiple scattering of light between the nanoparticles. The other one is the distribution of phase of multipolar fields that interfere to form the far-field pattern, which originates from the spatial distribution of the nanoparticles. In this work, we utilize an effective medium model developed within the T-matrix framework to calculate the angle-resolved scattered far-field intensity of disordered arrays composed of high-index nanoparticles. We show that our model may be used to predict the optical spectra including both radiative (far-field interference) effects as well as the multiple scattering effects making it a computationally efficient and accurate approach to model nanoparticle arrays with positional disorder. We utilize the presented model to study the capability to engineer the scattering-to-absorption ratio as well as the scattering directionality via tailoring the spatial distribution of nanoparticles. Control over those properties is sought after in nanoparticle applications such as photovoltaics and affects the efficiency of dielectric metasurfaces.
Directed refraction and strong dispersion, which are characteristic properties of hyperbolic metamaterials, make such materials suitable for performing spatial-spectral transformation. This transformation involves encoding of spatial information with resolution beyond the diffraction limit in the scattering spectrum, which may be collected with standard optics in the far-field. This gives rise to potential applications in compressive super-resolution microscopy in which compressive sensing algorithms are used for reconstruction of super-resolved images from a broadband measurement of an objects spectrum illuminated through the hyperbolic metamaterial. For such imaging to be successful, light, which carries subwavelength information, needs to be coupled to high-k modes of a hyperbolic metamaterial. This is required to observe wavelength-dependent directional propagation, which is necessary for the spatial-spectral transformation. Although it is possible to exploit diffraction on nanoholes in a Cr mask to provide additional momentum, this approach suffers from low transmittance which is especially detrimental for compressive imaging applications. In this paper we analyze dielectric spherical nanoparticle arrays as an alternative broadband coupling method with increased efficiency. Light transmittance and concentration are enhanced through photonic nanojets, localized enhanced _elds with low beam divergence. The finite-element method study of the structure is performed to find wavelength-dependent coupling efficiency as a function of nanoparticle size, material as well as array period and configuration. The analysis with effective hyperbolic dispersion relation is further refined to include a metal-dielectric multilayer.
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