Roberto Fenollosa

Monodisperse silicon nanocavities and photonic crystals with magnetic response in the optical region

It is generally accepted that the magnetic component of light has a minor role in the light-matter interaction. The recent discovery of metamaterials has broken this traditional understanding, as both the electric and the magnetic field are key ingredients in metamaterials. The top-down technology used so far employs noble metals with large intrinsic losses. Here we report on a bottom-up approach for processing metamaterials based on suspensions of monodisperse full dielectric silicon nanocavities with a large magnetic response in the near-infrared region. Experimental results and theory show that silicon-colloid-based liquid suspensions and photonic crystals made of two-dimensional arrays of particles have strong magnetic response in the near-infrared region with small optical losses. Our findings might have important implications in the bottom-up processing of large-area low-loss metamaterials working in the near-infrared region.

A New Dielectric Metamaterial Building Block with a Strong Magnetic Response in the Sub-1.5-Micrometer Region: Silicon Colloid Nanocavities

A new dielectric metamaterial building block based on high refractive index silicon spherical nanocavities with Mie resonances appearing in the near infrared optical region is prepared and characterized. It is demonstrated both experimentally and theoretically that a single silicon nanocavity supports well-defined and robust magnetic resonances, even in a liquid medium environment, at wavelength values up to six times larger than the cavity radius.

All-silicon spherical-Mie-resonator photodiode with spectral response in the infrared region

Silicon is the material of choice for visible light photodetection and solar cell fabrication. However, due to the intrinsic band gap properties of silicon, most infrared photons are energetically useless. Here, we show the first example of a photodiode developed on a micrometre scale sphere made of polycrystalline silicon whose photocurrent shows the Mie modes of a classical spherical resonator. The long dwell time of resonating photons enhances the photocurrent response, extending it into the infrared region well beyond the absorption edge of bulk silicon. It opens the door for developing solar cells and photodetectors that may harvest infrared light more efficiently than silicon photovoltaic devices that are so far developed.

Mirror-image-induced magnetic modes

Reflection in a mirror changes the handedness of the real world, and right-handed objects turn left-handed and vice versa (M. Gardner, The Ambidextrous Universe, Penguin Books, 1964). Also, we learn from electromagnetism textbooks that a flat metallic mirror transforms an electric charge into a virtual opposite charge. Consequently, the mirror image of a magnet is another parallel virtual magnet as the mirror image changes both the charge sign and the curl handedness. Here we report the dramatic modification in the optical response of a silicon nanocavity induced by the interaction with its image through a flat metallic mirror. The system of real and virtual dipoles can be interpreted as an effective magnetic dipole responsible for a strong enhancement of the cavity scattering cross section.

Porous silicon microcavities based photonic barcodes.

The FIB assisted EDX analysis was taken at the Centre for Research in NanoEngineering of the Polytechnic University of Catalunya, Spain. The technical assistance of Dr. T. Trifonov is greatly appreciated. Biological stability tests were performed with the support of the Molecular Epidemiology-Genomics and Health, Centre for Public Health Research, Valencia, Spain. The advice of Dr. F.J. Lopez-Labrador is greatly acknowledged. This work has been partially supported by the Spanish CICyT projects, FIS2009-07812, Consolider CSD2007-046, and PROMETEO/2010/043.

Silicon particles as trojan horses for potential cancer therapy

BackgroundPorous silicon particles (PSiPs) have been used extensively as drug delivery systems, loaded with chemical species for disease treatment. It is well known from silicon producers that silicon is characterized by a low reduction potential, which in the case of PSiPs promotes explosive oxidation reactions with energy yields exceeding that of trinitrotoluene (TNT). The functionalization of the silica layer with sugars prevents its solubilization, while further functionalization with an appropriate antibody enables increased bioaccumulation inside selected cells.ResultsWe present here an immunotherapy approach for potential cancer treatment. Our platform comprises the use of engineered silicon particles conjugated with a selective antibody. The conceptual advantage of our system is that after reaction, the particles are degraded into soluble and excretable biocomponents.ConclusionsIn our study, we demonstrate in particular, specific targeting and destruction of cancer cells in vitro. The fact that the LD50 value of PSiPs-HER-2 for tumor cells was 15-fold lower than the LD50 value for control cells demonstrates very high in vitro specificity. This is the first important step on a long road towards the design and development of novel chemotherapeutic agents against cancer in general, and breast cancer in particular.

Photonic crystal microprisms obtained by carving artificial opals

A method for fabrication of photonic crystal prisms is demonstrated. The procedure is based on micromanipulation techniques, here applied to artificial opals. By means of a microgrinder an opal prism comprising a single crystal (several tens of microns in size) has been carved with three different faces: (111), (110), and (100). The faces were morphologically characterized by scanning electron microscopy and their optical reflectance spectra measured and compared with the theoretical band structure.

Silicon colloids: A new enabling nanomaterial

We have recently developed a new type of silicon structure that we refer to as a silicon colloid. This new material consists of almost perfectly spherical silicon micro- and nanoparticles with a very smooth surface. They are able to trap light very efficiently in a large-span frequency range covering the visible to the far infrared regions. Silicon colloids can be thought of as a completely new material for scientific and technological purposes, with manifold applications covering electronics, photonics, cosmetics, or paints, among others. Here, we report on the synthesis of polycrystalline, amorphous, and porous silicon colloids, as well as their optical properties, some applications concerning light filters, and photonic bonding.

Fabrication of photonic crystal microprisms based on artificial opals

This paper reports a new method for faceting artificial opals based on micromanipulation techniques. By this means it was possible to fabricate an opal prism in a single domain with different faces: (111), (110) and (100), which were characterized by Scanning Electron Microscopy and Optical Reflectance Spectroscopy. Their spectra exhibit different characteristics depending on the orientation of the facet. While (111)-oriented face gives rise to a high Bragg reflection peak at about a/(lambda) equals 0.66 (where a is the lattice parameter), (110) and (100) faces show much less intense peaks corresponding to features in the band structure at a/(lambda) equals 1.12 and a/(lambda) equals 1.07 respectively. Peaks at higher energies have less obvious explanation.