Magnetic hot spots in closely spaced thick gold nanorings

Light-matter interaction at optical frequencies is mostly mediated by the electric component of the electromagnetic field, with the magnetic component usually being considered negligible. Recently, it has been shown that properly engineered metallic nanostructures can provide a magnetic response at optical frequencies originated from real or virtual flows of electric current in the structure. In this work, we demonstrate a magnetic plasmonic mode which emerges in closely spaced thick gold nanorings. The plasmonic resonance obtains a magnetic dipole character by sufficiently increasing the height of the nanorings. Numerical simulations show that a virtual current loop appears at resonance for sufficiently thick nanorings, resulting in a strong concentration of the magnetic field in the gap region (magnetic hot spot). We find that there is an optimum thickness that provides the maximum magnetic intensity enhancement (over 200-fold enhancement) and give an explanation of this observation. This strong magnetic resonance, observed both experimentally and theoretically, can be used to build new metamaterials and resonant loop nanoantennas at optical frequencies.

Demonstration of near infrared gas sensing using gold nanodisks on functionalized silicon

In this work, we demonstrate experimentally the use of an array of gold nanodisks on functionalized silicon for chemosensing purposes. The metallic nanostructures are designed to display a very strong plasmonic resonance in the infrared regime, which results in highly sensitive sensing. Unlike usual experiments which are based on the functionalization of the metal surface, we functionalized here the silicon substrate. This silicon surface was modified chemically by buildup of an organosilane self-assembled monolayer (SAM) containing isocyanate as functional group. These groups allow for an easy surface regeneration by simple heating, thanks to the thermally reversible interaction isocyanate-analyte, which allows the cyclic use of the sensor. The technique showed a high sensitivity to surface binding events in gas and allowed the surface regeneration by heating of the sensor at 150 °C. A relative wavelength shift ∆λ(max)λ(0)=0.027 was obtained when the saturation level was reached.

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.

Enlarging the negative-index bandwidth of optical metamaterials by hybridized plasmon resonances

By exploiting the concept of internal surface plasmon polariton (I-SPP) resonances, which appear at nonsingle metallic film stacks, we have designed a metamaterial showing a negative effective index within a large frequency bandwidth. The designed structure consists of an arrangement of several fishnet layers. By properly adjusting the lattice and the thickness of the dielectric slab of the fishnet, an I-SPP mode can be excited at a certain frequency, giving rise to a negative permeability. Thus, when combining several fishnet layers, each one configured to excite an I-SPP at a slightly different frequency, the coupling among the fishnet layers will cause a plasmon hybridization effect that enables us to extend the negative-index bandwidth.

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.

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.

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.

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.

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.