Carlos García-Meca

Double-negative polarization-independent fishnet metamaterial in the visible spectrum

We show that a second-order magnetic resonance present in the fishnet metamaterial can be enhanced so as to achieve simultaneous negative permittivity and permeability in the visible range. The double-negative behavior leads to reduced losses in this particular fishnet metamaterial. We also study the stacking of several functional layers, verifying the convergence of the refractive index.

Analysis of Hybrid Dielectric Plasmonic Waveguides

Future ultracompact photonic integrated circuits (PICs) will rely on high-index-contrast dielectric materials, which permit a strong confinement of the optical field in the diffraction limit as well as low propagation losses. This is the case of PICs implemented on a silicon-on-insulator (SOI) platform. To achieve confinement beyond the diffraction limit, plasmonic waveguides (based on metal-dielectric interfaces) have been recently proposed. This new kind of waveguide provides a strong enhancement of the field in the metal-dielectric interface, which is of paramount importance for nonlinear functionalities or sensing. Plasmonic waveguides can also be built on SOI wafers. Thus, it can be reasonably thought that high index contrast as well as plasmonic waveguides can coexist in future ultradense PICs. In this paper, a theoretical and numerical study on the performance of several dielectric and plasmonic waveguides is presented. Thanks to their plasmon-coupled supported modes, ultracompact devices as hybrid ring resonators can be devised and integrated with silicon photonic circuits.

Squeezing and expanding light without reflections via transformation optics.

We study the reflection properties of squeezing devices based on transformation optics. An analytical expression for the angle-dependent reflection coefficient of a generic three-dimensional squeezer is derived. In contrast with previous studies, we find that there exist several conditions that guarantee no reflections so it is possible to build transformation-optics-based reflectionless squeezers. Moreover, it is shown that the design of antireflective coatings for the non-reflectionless case can be reduced to matching the impedance between two dielectrics. We illustrate the potential of these devices by proposing two applications in which a reflectionless squeezer is the key element: an ultra-short perfect coupler for high-index nanophotonic waveguides and a completely flat reflectionless hyperlens. We also apply our theory to the coupling of two metallic waveguides with different cross-section. Finally, we show how the studied devices can be implemented with non-magnetic isotropic materials by using a quasi-conformal mapping technique.

Engineering antenna radiation patterns via quasi-conformal mappings.

We use a combination of conformal and quasi-conformal mappings to engineer isotropic electromagnetic devices that modify the omnidirectional radiation pattern of a point source. For TE waves, the designed devices are also non-magnetic. The flexibility offered by the proposed method is much higher than that achieved with conformal mappings. As a result, it is shown that complex radiation patterns can be achieved, which can combine high directivity in a desired number of arbitrary directions and isotropic radiation in other specified angular ranges. In addition, this technique enables us to control the power radiated in each direction to a certain extent. The obtained results are valid for any part of the spectrum. The potential of this method is illustrated with some examples. Finally, we study the frequency dependence of the considered devices and propose a practical dielectric implementation.

Negative refractive index metamaterials aided by extraordinary optical transmission

We study under which conditions extraordinary optical transmission (EOT) structures can be used to build negative refractive index media. As a result, we present a metamaterial with superimposed EOT and negative index at visible wavelengths. The tailoring process starting from a simple hole array until achieving the negative index is detailed. We also discuss the so-called fishnet metamaterial (previously linked to EOT) under the same prism. Using the ideas put forward in this work, other structures with negative index could be engineered in the optical or visible spectrum.

Analogue Transformations in Physics and their Application to Acoustics

Transformation optics has shaped up a revolutionary electromagnetic design paradigm, enabling scientists to build astonishing devices such as invisibility cloaks. Unfortunately, the application of transformation techniques to other branches of physics is often constrained by the structure of the field equations. We develop here a complete transformation method using the idea of analogue spacetimes. The method is general and could be considered as a new paradigm for controlling waves in different branches of physics, from acoustics in quantum fluids to graphene electronics. As an application, we derive an “analogue transformation acoustics” formalism that naturally allows the use of transformations mixing space and time or involving moving fluids, both of which were impossible with the standard approach. To demonstrate the power of our method, we give explicit designs of a dynamic compressor, a spacetime cloak for acoustic waves and a carpet cloak for a moving aircraft.

Low-loss single-layer metamaterial with negative index of refraction at visible wavelengths

We present a structure exhibiting a negative index of refraction at visible or near infrared frequencies using a single metal layer. This contrasts with recently developed structures based on metal-dielectric-metal composites. The proposed metamaterial consists of periodically arranged thick stripes interacting with each other to give rise to a negative permeability. Improved designs that allow for a negative index for both polarizations are also presented. The structures are numerically analyzed and it is shown that the dimensions can be engineered to shift the negative index band within a region ranging from telecommunication wavelengths down to blue light.

Coaxial plasmonic waveguide array as a negative-index metamaterial.

We propose the use of closely packed deep-subwavelength plasmonic coaxial waveguides that support backward propagating modes at visible frequencies, analogous to those in planar metal-insulator-metal geometries, as negative-index metamaterials. We show through simulation that the propagation properties of the metamaterial are determined by the dispersion relation of the constitutive waveguides. The metamaterial is polarization independent, is uniform in the propagation direction, and has a subwavelength character in the transversal directions. The transmission loss through the structure is also analyzed.

Partial transmutation of singularities in optical instruments

Some interesting optical instruments such as the Eaton lens and the invisible sphere require singularities of the refractive index for their implementation. We show how to transmute those singularities into harmless topological defects in anisotropic media without the need for anomalous material properties.

Multiple extraordinary optical transmission peaks from evanescent coupling in perforated metal plates surrounded by dielectrics

We study numerically and theoretically the optical transmission of nanostructured gold films embedded in dielectric claddings. We show how multiple transmission peaks appear as the claddings thickness increases. These transmission peaks come not only from surface plasmon polariton excitations but also from the excitation of Fabry-Perot modes sustained at the claddings, coupled through the metal, as long as a periodic pattern is milled in the metal film. We propose that this structure could be used as an ultracompact all-optical switch by surrounding the metal film with Kerr nonlinear dielectric layers.