Felipe A. Pinheiro

Suppression of Anderson localization of light and Brewster anomalies in disordered superlattices containing a dispersive metamaterial

Light propagation through 1D disordered structures composed of alternating layers, with random thicknesses, of air and a dispersive metamaterial is theoretically investigated. Both normal and oblique incidences are considered. By means of numerical simulations and an analytical theory, we have established that Anderson localization of light may be suppressed: (i) in the long wavelength limit, for a finite angle of incidence which depends on the parameters of the dispersive metamaterial; (ii) for isolated frequencies and for specific angles of incidence, corresponding to Brewster anomalies in both positive- and negative-refraction regimes of the dispersive metamaterial. These results suggest that Anderson localization of light could be explored to control and tune light propagation in disordered metamaterials.

Unconventional Fano effect and off-resonance field enhancement in plasmonic coated spheres

We investigate light scattering by coated spheres composed of a dispersive plasmonic core and a dielectric shell. By writing the absorption cross section in terms of the internal electromagnetic fields, we demonstrate it is an observable sensitive to interferences that ultimately lead to the Fano effect. In particular, we show that unconventional Fano resonances, recently discovered for homogeneous spheres with large dielectric permittivities, can also occur for metallic spheres coated with single dielectric layers. These resonances arise from the interference between two electromagnetic modes with the same multipole moment inside the shell and not from interactions between the various plasmon modes of different layers of the particle. In contrast to the case of homogeneous spheres, unconventional Fano resonances in coated spheres exist even in the Rayleigh limit. These resonances can induce an off-resonance field enhancement, which is approximately 1 order of magnitude larger than the one achieved with conventional Fano resonances. We find that unconventional and conventional Fano resonances can occur at the same input frequency provided the dispersive core has a negative refraction index. This leads to an optimal field enhancement inside the particle, a result that could be useful for potential applications in plasmonics.

Tuning plasmonic cloaks with an external magnetic field.

We propose a mechanism to actively tune the operation of plasmonic cloaks with an external magnetic field by investigating electromagnetic scattering by a dielectric cylinder coated with a magneto-optical shell. In the long wavelength limit, we show that the presence of a magnetic field may drastically reduce the scattering cross section at all observation angles. We demonstrate that the application of magnetic fields can modify the operation wavelength without the need of changing material and/or geometrical parameters. We also show that applied magnetic fields can reversibly switch on and off the cloak operation. These results, which could be achieved for existing magneto-optical materials, are shown to be robust to material losses, so that they may pave the way for developing actively tunable, versatile plasmonic cloaks.

Active magneto-optical control of spontaneous emission in graphene

We investigate the spontaneous emission rate of a two-level quantum emitter near a graphene-coated substrate under the influence of an external magnetic field. We demonstr ate that the application of the magnetic field can substantially increase or decrease the decay rate. We show that a suppression as large as 99% in the Purcell factor is achieved even for moderate magnetic fields. The emi tter’s lifetime is a discontinuous function of |B|, which is a direct consequence of the occurrence of discrete Landau levels in graphene. We demonstrate that, in the near-field regime, the magnetic field enables an unprec edented control of the decay pathways into which the photon/polariton can be emitted. Our findings strongly s uggest that a magnetic field could act as an efficient agent for on-demand, active control of light-matter intera ctions in graphene at the quantum level. The possibility of tailoring light-matter interactions at a quantum level has been a sought-after goal in optics since the pioneer work of Purcell [1], where it was first shown that the environment can strongly modify the spontaneous emission (SE) rate of a quantum emitter. To achieve such objective, several approaches have been proposed so far. One of them is to investigate SE in different system geometries [2‐11]. Advances in nanofabrication techniques have not only allowed the increase of the spectroscopic resolution of molecules in complex environments [12], but have also led to the use of nanometric objects, such as antennas and tips, to modify the lifetime, and enhance the fluorescence of single molecules [13‐16]. The presence of metamaterials may also strongly affect quantum emitters’ radiative processes. Fo r instance, the impact of negative refraction and of the hyperbolic dispersion of some metamaterials on the SE have been investigated [17‐19]. Also, the influence of cloaking devices on t he SE of atoms has been recently addressed [20]. Progress in plasmonics has also allowed for an unprecedented control of radiative properties of quantum emitters [21‐24]. However, the metallic structures usually employed as plasmonic materials are hardly tunable, limiting their application in photonic devices. To circumvent these limitations, graphene has emerged as an alternative plasmonic material due to its extraordinary electronic and optical pr operties [25‐30]. Indeed, graphene hosts extremely confined pla smons, facilitating strong light-matter interactions [27‐ 30]. In addition, the plasmon spectrum in doped graphene is highly tunable through electrical or chemical modification of the charge carrier density. Due to these properties, graphene i s a promising material platform for several photonic applications, specially in the THz frequency range [29]. At the quantum level, the spatial confinement of surface plasmons in graphene has been shown to modify the SE rate [31, 32]. The electromagnetic (EM) field pattern excited by quantum emitters near a graphene sheet [33] further demonstrates the huge field enhancement due to the excitation of surface plasmons. A graphene sheet has also been shown to mediate suband super-radiance between two quantum emitters [34]. Recently, the electrical control of the relaxation pathways a nd SE rate in graphene has been observed [35]. Despite all these advances, the achieved modification in the emitter’s decay rate remains modest so far. Most of the proposed schemes consider emitters whose transition frequencies are in the o ptical/near infrared range, usually far from graphene’s intra band transitions. As a consequence, the effects of graphene on the SE rate are only relevant when the emitter is no more than a few dozen nanometers apart. Here, we propose an alternative mechanism to actively tune the lifetime of a THz quantum emitter near a graphene sheet by exploiting its extraordinary magneto-optical properti es. We show that the application of a magnetic field B allows for an unprecedented control of the SE rate for emitter-graphene distances in the micrometer range. This is in contrast to pre vious proposals, in which the modification of the SE rate was achieved by electrically or chemically altering graphene’s doping level. The fact that we consider a low-frequency emitter enables us to probe the effects of intraband transitions in graphene on the decay rate, which have also been unexplored so far. In summary, our key results are (i) a striking 99% reduction of the emitter SE rate compared to the case where B = 0; (ii) a new distance-scaling law ∝ d −4 e −1/d that is valid for a broad range of distances and magnetic fields; (iii ) a highly non-monotonic behavior of the SE rate as a function of |B|, with sharp discontinuities in the regime of low temperatures; and (iv) the possibility of tailoring the decay chann els into which the photon can be emitted. These findings can be physically explained in terms of the interplay among the different EM modes and of electronic intraband transitions between discrete Landau energy levels in graphene.

Molding the flow of light with a magnetic field: plasmonic cloaking and directional scattering

We investigate electromagnetic (EM) scattering and plasmonic cloaking in a system composed of a dielectric cylinder coated with a magneto-optical shell. In the long-wavelength limit we demonstrate that the application of an external magnetic field can not only switch on and off the cloaking mechanism but also mitigate losses, as the absorption cross section is shown to drop sharply precisely at the cloaking operation frequency band. We also show that the angular distribution of the scattered radiation can be effectively controlled by applying an external magnetic field, allowing for a swift change in the scattering pattern. By demonstrating that these results are feasible with realistic, existing magneto-optical materials, such as graphene epitaxially grown on SiC, we suggest that magnetic fields could be used as effective, versatile external agents to tune plasmonic cloaks and to dynamically control EM scattering in an unprecedented way. We hope that these results may find use in disruptive photonic technologies.

Spontaneous emission in the presence of a spherical plasmonic metamaterial

We investigate the spontaneous emission of a two-level atom placed in the vicinities of a plasmonic cloak composed of a coated sphere. In the dipole approximation, we show that the spontaneous emission rate can be reduced to its vacuum value provided the atomic emission frequency lies within the plasmonic cloak frequency operation range. Considering the current status of plasmonic cloaking devices, this condition may be fulfilled for many atomic species so that we argue that atoms with a sufficiently strong transition can be used as quantum, local probes for the efficiency of plasmonic cloaks.

Tunable multiple Fano resonances in magnetic single-layered core-shell particles

We investigate multiple Fano, comblike scattering resonances in single-layered, concentric core-shell nanoparticles composed of magnetic materials. Using the Lorenz-Mie theory, we derive, in the long-wavelength limit, an analytical condition for the occurrence of comblike resonances in the single scattering by coated spheres. This condition establishes that comblike scattering response uniquely depends on material parameters and thickness of the shell, provided that it is magnetic and thin compared to the scatterer radius. We also demonstrate that comblike scattering response shows up beyond the long-wavelength limit and it is robust against absorption. Since multiple Fano resonances are shown to depend explicitly on the magnetic permeability of the shell, we argue that both the position and profile of the comblike, morphology-dependent resonances could be externally tuned by exploiting the properties of engineered magnetic materials.

Adiabatic Charge Pumping through Quantum Dots in the Coulomb Blockade Regime

We investigate the influence of the Coulomb interaction on th e adiabatic pumping current through a quantum dot. Using nonequilibrium Green’s functions techniques, w e derive a general expression for the current based on the instantaneous Green’s function of the dot. We apply th is formula to study the dependence of the charge pumped per cycle on the time-dependent pumping potentials. Motivated by recent experiments, the possibility of charge quantization in the presence of a finite Coulomb rep ulsion energy is investigated.

Probing scattering resonances of Vogel's spirals with the Green's matrix spectral method.

Using the rigorous Greens function spectral method, we systematically investigate the scattering resonances of different types of Vogel spiral arrays of point-like scatterers. By computing the distributions of eigenvalues of the Greens matrix and the corresponding eigenvectors, we obtain important physical information on the spatial nature of the optical modes, their lifetimes and spatial patterns, at small computational cost and for large-scale systems. Finally, we show that this method can be extended to the study of three-dimensional Vogel aperiodic metamaterials and aperiodic photonic structures that may exhibit a richer spectrum of localized resonances of direct relevance to the engineering of novel optical light sources and sensing devices.

Tuning quantum fluctuations with an external magnetic field: Casimir-Polder interaction between an atom and a graphene sheet

Instituto de F´isica, Universidade Federal do Rio de Janeiro,Caixa Postal 68528, Rio de Janeiro 21941-972, RJ, Brazil(Dated: August 18, 2014)We investigate the dispersive Casimir-Polder interactionbetween a Rubidium atom and a suspended graphenesheet subjected to an external magnetic field B. We demonstrate that this concrete physical system allows for anunprecedented control of dispersive interactions at micro and nanoscales. Indeed, we show that the applicationof an external magnetic field can induce a 80%reduction of the Casimir-Polder energy relative to its valuewithout the field. We also show that sharp discontinuities em erge in the Casimir-Polder interaction energyfor certain values of the applied magnetic field at low temper atures. Moreover, for sufficiently large distancesthese discontinuities show up as a plateau-like pattern with a quantized Casimir-Polder interaction energy, in aphenomenon that can be explained in terms of the quantum Hall effect. In addition, we point out the importanceof thermal effects in the Casimir-Polder interaction, which we show that must be taken into account even forconsiderably short distances. In this case, the discontinuities in the atom-graphene dispersive interaction do notoccur, which by no means prevents the tuning of the interaction in ∼ 50%by the application of the externalmagnetic field.