Fernando Ramiro-Manzano

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.

Solar energy harvesting in photoelectrochemical solar cells

Here we study different approaches for increasing energy harvesting in titania based photoelectrochemical solar (PES) cells. We study the light harvesting of PES cells when photonic crystal and photonic sponge architectures are used. We also report on the influence of the surface corrugation of the metal electrode on the harvesting of photocarriers in solar cells

Apollony photonic sponge based photoelectrochemical solar cells

We have developed a quasi-fractal colloidal crystal to localize efficiently photons in a very broad optical spectral range; it has been applied to prepare dye sensitized photoelectrochemical solar (PES) cells able to harvest very efficiently photons from the ultraviolet (UV) and the visible (VIS) regions of the solar spectrum.

Oscillatory vertical coupling between a whispering-gallery resonator and a bus waveguide.

We report on a theoretical and experimental study of the optical coupling between a whispering-gallery type resonator and a waveguide lying on different planes. In contrast to the usual in-plane geometry, the present vertical one is characterized by an oscillatory behavior of the effective coupling as a function of the vertical gap. This behavior manifests itself as oscillations in both the resonance peak waveguide transmission and the mode quality factor. An analytical description based on coupled-mode theory and a two-port beam-splitter model of the waveguide-resonator vertical coupling is developed for arbitrary phase-matching conditions and is successfully used to interpret the experimental observations.

Packing Confined Hard Spheres Denser with Adaptive Prism Phases

We show that hard spheres confined between two parallel hard plates pack denser with periodic adaptive prismatic structures which are composed of alternating prisms of spheres. The internal structure of the prisms adapts to the slit height which results in close packings for a range of plate separations, just above the distance where three intersecting square layers fit exactly between the plates. The adaptive prism phases are also observed in real-space experiments on confined sterically stabilized colloids and in Monte Carlo simulations at finite pressure.

A fully integrated high-Q Whispering-Gallery Wedge Resonator

Microresonator devices which possess ultra-high quality factors are essential for fundamental investigations and applications. Microsphere and microtoroid resonators support remarkably high Qs at optical frequencies, while planarity constrains preclude their integration into functional lightwave circuits. Conventional semiconductor processing can also be used to realize ultra-high-Qs with planar wedge-resonators. Still, their full integration with side-coupled dielectric waveguides remains an issue. Here we show the full monolithic integration of a wedge-resonator/waveguide vertically-coupled system on a silicon chip. In this approach the cavity and the waveguide lay in different planes. This permits to realize the shallow-angle wedge while the waveguide remains intact, allowing therefore to engineer a coupling of arbitrary strength between these two. The precise size-control and the robustness against post-processing operation due to its monolithic integration makes this system a prominent platform for industrial-scale integration of ultra-high-Q devices into planar lightwave chips.

Porous silicon microspheres: synthesis, characterization and application to photonic microcavities

Porous silicon microspheres have been synthesized by chemical vapour deposition of disilane gas. Their spectral signatures are similar to those of electrochemically grown porous silicon, in particular they yield photoluminescence and they show an oxidation behaviour upon their exposure to the open air. The particles are highly spherical and poly-disperse in size with diameters of approximately 0.5 to 5 micrometres, and they have a very smooth surface. Because of these reasons they work as optical microcavities with well defined resonating Mie modes. These modes have been identified in the near-infrared range. They blue-shift considerably whenever the microspheres are in contact with air because of the aforementioned oxidation process.

Layering transitions in colloidal crystal thin films between 1 and 4 monolayers

This paper investigates the sequence of morphological transitions in a nearly hard particle arrangement of colloidal crystals confined in a wedge cell. Earlier studies have shown intermediate particle arrangements (such as the buckling, rhombic, prismatic, and hcp like phases) to account for the drastic changes in the filling fraction values between the triangular and square arrangements. However, to the best of our knowledge, there is no complete description of the transitions between n and n + 1 layers. Here we describe the ordering of the particles for films between 1 and 4 layers. We also present a new intermediate phase hcp(011) to show the transition between n□ → n△ for n > 2 layers.

Role of Edge Inclination in an Optical Microdisk Resonator for Label-Free Sensing

In this paper, we report on the measurement and modeling of enhanced optical refractometric sensors based on whispering gallery modes. The devices under test are optical microresonators made of silicon nitride on silicon oxide, which differ in their sidewall inclination angle. In our approach, these microresonators are vertically coupled to a buried waveguide with the aim of creating integrated and cost-effective devices. Device modeling shows that the optimization of the device is a delicate balance of the resonance quality factor and evanescent field overlap with the surrounding environment to analyze. By numerical simulations, we show that the microdisk thickness is critical to yield a high figure of merit for the sensor and that edge inclination should be kept as high as possible. We also show that bulk-sensing figures of merit as high as 1600 RIU−1 (refractive index unit) are feasible.