Ashod Aradian

Toy model to describe the effect of positional blocklike disorder in metamaterials composites

We study theoretically the effect of a new type of blocklike positional disorder on the effective electromagnetic properties of one-dimensional chains of resonant, high-permittivity dielectric particles, where particles are arranged into perfectly well-ordered blocks whose relative position is a random variable. This creates a finite order correlation length that mimics the situation encountered in metamaterials fabricated through self-assembled techniques, whose structures often display short-range order between near neighbors but long-range disorder, due to stacking defects. Using a spectral theory approach combined with a principal component statistical analysis, we study, in the long-wavelength regime, the evolution of the electromagnetic response when the composite filling fraction and the block size are changed. Modifications in key features of the resonant response (amplitude, width, etc.) are investigated, showing a regime transition for a filling fraction around 50%.

Cooperative plasmon-mediated effects and loss compensation by gain dyes near a metal nanoparticle

We present the first unified theory, to the best of our knowledge, of the response of a plasmonic nanosphere (NS) assisted by optical gain media, in the case of a NS coated with a layer of optically active dipolar dyes. We obtain the optical coherent response of the core–shell aggregate in terms of its equivalent polarizability composed of the direct response from the NS and the contribution arising from the cooperative coupling between dyes and surface plasmons of the NS. We identify a mechanism of superradiance-like plasmonic aggregate cooperative emission similar to the conventional Dicke effect with reduced intraband relaxation bandwidth due to the loss compensation in the system. The analysis of the aggregate resonances based on the system eigenvalues provides physical insight into the total loss compensation mechanism and resonance frequency shifts.

Artificial magnetism at terahertz frequencies from three-dimensional lattices of TiO2 microspheres accounting for spatial dispersion and magnetoelectric coupling

We employ the generalized Lorentz-Lorenz method to investigate how both magnetoelectric coupling and spatial dispersion influence the artificial magnetic capabilities at terahertz frequencies of the representative case of a metamaterial consisting of a three-dimensional (3D) lattice of TiO2 microspheres. The complex wavenumber dispersion relations pertaining to modes supported by the array, traveling along one of the principal axes of the array with electric or magnetic field polarized transversely and longitudinally (with respect to the mode traveling direction), are studied and thoroughly characterized. One mode with transverse polarization is dominant at any given frequency for the analyzed dimensions, proving that the 3D lattice can be treated as a homogeneous medium with defined electromagnetic material parameters. We show, however, that bianisotropy is a direct consequence of magnetoelectric coupling, and the dyadic expressions of both effective and equivalent material parameters are derived. In particular, we analyze the effect of spatial dispersion on the effective parameters relative to a composite material made by a 3D lattice of TiO2 microspheres with filling fraction around 30% and near the first Mie magnetic dipolar resonance. Finally, we homogenize the metamaterial in terms of equivalent index and impedance, and by comparison with full-wave simulations, we explain the presence of the unphysical antiresonance permittivity behavior observed in previous work.

Multipolar, time-dynamical model for the loss compensation and lasing of a spherical plasmonic nanoparticle spaser immersed in an active gain medium

The plasmonic response of a metal nanoparticle in the presence of surrounding gain elements is studied, using a space and time-dependent model, which integrates a quantum formalism to describe the gain and a classical treatment for the metal. Our model fully takes into account the influence of the system geometry (nanosphere) and offers for the first time, the possibility to describe the temporal evolution of the fields and the coupling among the multipolar modes of the particle. We calculate the lasing threshold value for all multipoles of the spaser, and demonstrate that the dipolar one is lowest. The onset of the lasing instability, in the linear regime, is then studied both with and without external field forcing. We also study the behaviour of the system below the lasing threshold, with the external field, demonstrating the existence of an amplification regime where the nanoparticle’s plasmon is strongly enhanced as the threshold is approached. Finally, a qualitative discussion is provided on later, non-linear stages of the dynamics and the approach to the steady-state of the spaser; in particular, it is shown that, for the considered geometry, the spasing is necessarily multi-modal and multipolar modes are always activated.

Artificial magnetism at terahertz frequencies in 3D arrays of TiO 2 microspheres including spatial dispersion and magnetoelectric coupling

In this work, we study the electromagnetic properties of a metamaterial made by a cubic array of TiO2 microspheres embedded in a host medium at terahertz frequencies. By employing the method reported in [1] by Silveirinha, we are able to consider in the computation the effect of magnetoelectric coupling between TiO2 microspheres, as well as spatial dispersion. After calculation of the dominant mode inside the structure, we show that the effect of spatial dispersion on the effective parameters for this kind of systems is relatively weak. Accuracy of the results and effectiveness of homogenized parameters have been demonstrated by comparison against full-wave simulations for a 5-layer slab of such a metamaterial.

Unified Theoretical Model of Loss Compensation and Energy Transfer for Plasmonic Nanoparticles Coated with a Shell of Active Gain Molecules

One issue in using metallic nanostructures for metamaterial applications at optical frequencies is their high level of losses. A most promising strategy to circumvent this obstacle is loss compensation, where the structures are coupled to active compounds enabled to transfer energy and therefore amplify the desired response. We here present the first unified theory of the response of plasmonic nanoparticles assisted by optical gain media, in the case of a nanoparticle coated with a shell of optically active dipoles (fluorescent molecules or dyes). The mechanism of the losses compensation is based on nonradiative energy transfer (ET) or quenching between the layer of gain elements and nanoparticle [1, 2]. We establish a complete description of the optical response of the system based on Green’s functions, which allows us to investigate high molecular coverage of nanoparticle with either regular or random distribution of dye molecules, taking into account not only the interactions between NP (treated in a multipolar approach) and dye dipoles, but also between dyes molecules, either directly or via the nanoparticle [3–5]. We then obtain the optical response of the core-shell aggregate in terms of its equivalent polarizability composed of the direct response from the nanoparticle and the contribution rising from the energy transfer mechanism. Our numerical calculations reveals that cooperative plasmon-mediated coupling between optically active dyes and metal nanostructure leads to the compensation of plasmon losses and some instability that is resolved either by surface plasmon amplification of stimulated emission (spasing states) or by enhanced absorption in the system.