Tunable terahertz absorption in Si/SiO2-graphene multilayers: disorder and magneto-optical effects.

In this work we theoretically investigate the influence of disorder and external perpendicular magnetic field on terahertz (THz) absorption in graphene/SiC cap layers on top of one-dimensional photonic structures. We show that left-handed circularly polarized light absorption can be achieved up to 0.9 and even nearly perfect absorption at magnetic fields over 4 T. It is also demonstrated that multichannel absorption can be obtained, in a broad frequency range, by increasing the disorder strength in the layer thicknesses, outperforming the corresponding periodic structures. Altogether, our results reveal the potentialities of introducing disorder to not only enhance but also to tune absorption in photonic superlattices with graphene under the influence of an external magnetic field, allowing for applications such as THz circular polarization selective sensors and photodetectors.

Theory of optical tweezing of dielectric microspheres in chiral host media and its applications.

We report for the first time the theory of optical tweezers of spherical dielectric particles embedded in a chiral medium. We develop a partial-wave (Mie) expansion to calculate the optical force acting on a dielectric microsphere illuminated by a circularly-polarized, highly focused laser beam. When choosing a polarization with the same handedness of the medium, the axial trap stability is improved, thus allowing for tweezing of high-refractive-index particles. When the particle is displaced off-axis by an external force, its equilibrium position is rotated around the optical axis by the mechanical effect of an optical torque. Both the optical torque and the angle of rotation are greatly enhanced in the presence of a chiral host medium when considering radii a few times larger than the wavelength. In this range, the angle of rotation depends strongly on the microsphere radius and the chirality parameter of the host medium, opening the way for a quantitative characterization of both parameters. Measurable angles are predicted even in the case of naturally occurring chiral solutes, allowing for a novel all-optical method to locally probe the chiral response at the nanoscale.

Enantioselective manipulation of single chiral nanoparticles using optical tweezers.

We put forward an enantioselective method for chiral nanoparticles using optical tweezers. We demonstrate that the optical trapping force in a typical, realistic optical tweezing setup with circularly-polarized trapping beams is sensitive to the chirality of core-shell nanoparticles, allowing for efficient enantioselection. It turns out that the handedness of the trapped particles can be selected by choosing the appropriate circular polarization of the trapping beam. The chirality of each individual trapped nanoparticle can be characterized by measuring the rotation of the equilibrium position under the effect of a transverse Stokes drag force. We show that the chirality of the shell gives rise to an additional twist, leading to a strong enhancement of the optical torque driving the rotation. Both methods are shown to be robust against variations of size and material parameters, demonstrating that they are particularly useful in (but not restricted to) several situations of practical interest in chiral plasmonics, where enantioselection and characterization of single chiral nanoparticles, each and every one with its unique handedness and optical properties, are in order. In particular, our method could be employed to unveil the chiral response arising from disorder in individual plasmonic raspberries, synthesized by close-packing a large number of metallic nanospheres around a dielectric core.

Edge modes of scattering chains with aperiodic order.

We study the scattering resonances of one-dimensional deterministic aperiodic chains of electric dipoles using the vectorial Green's matrix method, which accounts for both short- and long-range electromagnetic interactions in open scattering systems. We discover the existence of edge-localized scattering states within fractal energy gaps with characteristic topological band structures. Notably, we report and characterize edge-localized modes in the classical wave analogues of the Su-Schrieffer-Heeger (SSH) dimer model, quasiperiodic Harper and Fibonacci crystals, as well as in more complex Thue-Morse aperiodic systems. Our study demonstrates that topological edge-modes with characteristic power-law envelope appear in open aperiodic systems and coexist with traditional exponentially localized ones. Our results extend the concept of topological states to the scattering resonances of complex open systems with aperiodic order, thus providing an important step towards the predictive design of topological optical metamaterials and devices beyond tight-binding models.

Electromagnetic energy stored in inhomogeneous scattering systems.

We analytically study the time-averaged electromagnetic energy stored inside scatterers containing inclusions of arbitrary shapes. Assuming the low density of inclusions, we derive the expression for the energy-transport velocity through disordered media without relying on the radiative transfer equation. Moreover, this expression is independent of the shape of scatterers. In addition, we obtain a relation between the dwell and absorption times associated with inclusions by considering the relationship between the internal energy and absorption cross-section. An approximation for the electromagnetic energy stored inside a disordered medium in terms of the transport mean free path and the packing fraction is also derived. This expression suggests that the enhanced electromagnetic energy within the host medium is achieved for inclusions exhibiting negative scattering asymmetry parameters. As a result, disordered media with enhanced backscattering is expected to exhibit large quality factors.

A 50/50 electronic beam splitter in graphene nanoribbons as a building block for electron optics.

Based on the investigation of the multi-terminal conductance of a system composed of two graphene nanoribbons, in which one is on top of the other and rotated by [Formula: see text], we propose a setup for a 50/50 electronic beam splitter that neither requires large magnetic fields nor ultra low temperatures. Our findings are based on an atomistic tight-binding description of the system and on the Green function method to compute the Landauer conductance. We demonstrate that this system acts as a perfect 50/50 electronic beam splitter, in which its operation can be switched on and off by varying the doping (Fermi energy). We show that this device is robust against thermal fluctuations and long range disorder, as zigzag valley chiral states of the nanoribbons are protected against backscattering. We suggest that the proposed device can be applied as the fundamental element of the Hong-Ou-Mandel interferometer, as well as a building block of many devices in electron optics.

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

Using the rigorous Green's 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 Green's 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.

Omnidirectional absorption and off-resonance field enhancement in dielectric cylinders coated with graphene layers.

We investigate electromagnetic scattering and absorption by dielectric cylinders coated with a concentric plasmonic shell at arbitrary incidence angles. Exploiting bulk and surface plasmon resonances in the long wavelength regime, we obtain an analytical condition to achieve wide-angle enhanced absorption for both TE and TM polarizations. By using the Lorenz-Mie theory, we apply this result to investigate electromagnetic absorption in a silicon cylinder coated with a graphene monolayer epitaxially grown on silicon carbide. Our theoretical results show that enhanced absorption occurs for a broad frequency range in the terahertz, and that omnidirectional absorption exists at a frequency in between the bulk and localized surface plasmon resonances. By showing that omnidirectional absorption does not correspond to an extinction resonance, we associate this phenomenon with off-resonance field enhancement in this system, which in turn is explained in terms of Fano resonances in the graphene layer.

Enabling focusing around the corner in multiple scattering media.

We report an alternative experimental setup to laterally focus light at an angle of 90 deg relative to turbid, multiple scattering media, using preprocessing wavefront shaping. We compare the measured image quality to one obtained in the usual configuration for focusing light through turbid media, where focusing occurs behind the scattering sample. We demonstrate that the depth of focus in the lateral configuration is of the same order of the usual transversal one because both setups are designed to operate in the deep Fresnel zone. This result shows that this novel, versatile lateral configuration allows for effectively focusing around corners through multiple scattering samples.

Electromagnetic energy within coated cylinders at oblique incidence and applications to graphene coatings.

We address electromagnetic (EM) wave scattering by an infinite coated cylinder at an arbitrary incidence angle. The time-averaged EM energy stored inside the core-shell cylinder is analytically calculated for TM- and TE-polarized incident plane waves. An analytical expression relating the internal energy to the absorption cross section is derived. As an application, the EM energy inside dielectric cylinders coated with isotropic graphene layers epitaxially grown on silicon carbide (SiC) is studied. We find that off-resonance field enhancement occurs in graphene SiC microshells for TM-polarized terahertz waves, a phenomenon that can be explained in terms of Fano resonances.

Electronic transport, transition-voltage spectroscopy, and the Fano effect in single molecule junctions composed of a biphenyl molecule attached to metallic and semiconducting carbon nanotube electrodes.

We have investigated electronic transport in a single-molecule junction composed of a biphenyl molecule attached to a p-doped semiconductor and metallic carbon nanotube leads. We find that the current-voltage characteristics are asymmetric as a result of the different electronic natures of the right and left leads, which are metallic and semiconducting, respectively. We provide an analysis of transition voltage spectroscopy in such a system by means of both Fowler-Nordheim and Lauritsen-Millikan plots; this analysis allows one to identify the positions of resonances and the regions where the negative differential conductance occurs. We show that transmittance curves are well described by the Fano lineshape, for both direct and reverse bias, demonstrating that the frontier molecular orbitals are effectively involved in the transport process. This result gives support to the interpretation of transition voltage spectroscopy based on the coherent transport model.

Electromagnetic energy within single-resonance chiral metamaterial spheres.

We derive an exact expression for the time-averaged electromagnetic (EM) energy inside a chiral dispersive sphere irradiated by a plane wave. The dispersion relations correspond to a chiral metamaterial consisting of uncoupled single-resonance helical resonators. Using a field decomposition scheme and a general expression for the EM energy density in bianisotropic media, we calculate the Lorenz-Mie solution for the internal fields in a medium that is simultaneously magnetic and chiral. We also obtain an explicit analytical relation between the internal EM energy and the absorption cross section. This result is applied to demonstrate that strong chirality leads to an off-resonance field enhancement within weakly absorbing spheres.

Light transport in chiral and magnetochiral random media.

We present a microscopic approach to study electromagnetic wave propagation in media with broken mirror symmetry. We introduce and calculate the transport mean free path l*C associated with the residual polarization of diffuse light in chiral systems. In chiral media subject to an external magnetic field B, all symmetry requirements exist to create a macroscopic "super" light current in the direction of B that persists even in the absence of a spatial photon density gradient. However, we show that such a current is identically zero in our model. We finally show the existence of a linear magnetotransmission in magnetochiral media.